Passive Optical Networks (PON)

With the popularity of broadband services of the terminal users the demands for bandwidth in the access network is rapidly increasing. Rapid increase of global data traffic and massive deployment of new networks are becoming a key environmental, social and economic issue. The access network consumes about 70% of overall network energy. Passive Optical Networks (PON) has been considered to be one of the most promising solutions for access networks due to its immense bandwidth and low cost infrastructure. Wavelength Division Multiplexing Passive Optical Networks (WDM-PON) provides a solution for having longer and larger capacity networks comparing with existing PON systems which can meet the ever increasing bandwidth demand of next generation ac-cess networks. Moreover, the combination of Orthogonal Frequency Division Multi-plexing (OFDM) and WDM-PON technique is a subject of great interest to increase the system capacity and dispersion tolerance. Coherent detection OFDM method has more prominent performance than direct detection method. For the practical implementation of WDM-OFDM-PONs, low-cost Optical Network Units (ONUs) and Optical Line Termi-nals (OLTs) are of most critical importance, in particular, avoiding a wavelength-specific laser source at each ONU. To address this issue, wavelength reuse concepts such as Trav-eling Wave Semiconductor Optical Amplifier, Wideband Traveling wave SOA and Re-flective SOA have been implemented in WDM-OFDM-PON. Among these, RSOA per-forms better than the others. To further reduce the system cost a WDM-OFDM-PON architecture with simplified structure by using Vertical-Cavity Surface-Emitting Laser (VCSEL) as transmitter at OLT and ONU. All these simulations are done using OptiSys-tem 12.0 software.

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The combination of an exponential increase in bandwidth-intensive applications and customer base, has resulted in the rapid increase of fiber networks in the access network segment in recent years. In terms of fiber access technology, the point-to-multipoint passive topology in the form of the Passive Optical Network (PON) has been proven to be beneficial to both customers and operators. Due to rapid increase of global data traffic and bandwidth demands, massive deployment of new network, is becoming a key environmental, social and economic issue. To address this issue, great effort has been ex-pended on researching the high-speed, cost-effective, flexible bandwidth allocation and future-proof Next Generation Passive Optical Network (NG-PON) system. Among these techniques, WDM based PON systems have attracted a great deal of research and devel-opment interest, due to their capability of providing cost-effective way for increasing the overall bit rate and transmission reach of networks.
The access network, also known as the first-mile network, connects the service provider Central Offices (COs) to businesses and residential subscribers. An access net-work is a part of a telecommunications network which connects subscribers to their im-mediate service providers. This network is also referred to as the subscriber access net-work, or the local loop. The bandwidth demand in the access network has been increasing rapidly over the past several years. Active Optical Network (AON), the first based access network has been characterized by a single fiber which carries all traffic to a Remote Node (RN) mainly electrically powered switching equipment such as a switch or a router that is placed close to the end users from the central office. In this AON architecture, later the active node is replaced with a passive optical power splitter/combiner leading to the development of Passive Optical Network (PON).
Optical Orthogonal Frequency Division Multiplexing (OFDM) technique has re-cently been a promising technique in access networks due to its high spectral efficiency and robust dispersion tolerance. OFDM, is a form of signal modulation that divides a high data rate modulating stream placing them onto many slowly modulated narrow band close-spaced sub-carriers, and in this way is less sensitive to frequency selective fading. Furthermore, the OFDM is widely considered as one of the strongest candi-date for WDM-based PON system, owing to its unique advantages of superior toler-ance to chromatic dispersion impairments, dynamic provision of multi-granularity band-width allocation both in time and frequency domains. WDM-OFDM-PON, combining the advantages of WDM and OFDM techniques, can provide higher data rate and more flexible bandwidth allocation for end users. Nonetheless, OFDM modulation modules, consisting of high-speed Digital Signal Processing (DSP) chips, Digital-to-Analog Con-verters (DAC), and E/O modulators, are needed for the generation of optical OFDM sig-nals in WDM-OFDM-PON. These components consume much more energy. Moreover, each OFDM modulation module is fixed for one Optical Network Unit (ONU) group in conventional WDM-OFDM-PON, which causes a rough granularity and wastes a large amount of bandwidth resource since the users do not fully utilize the network capacity all the time. Therefore, it is of great significance to design an energy-efficient and cost effective WDM-OFDM-PON system with high data rates compatible with large num-ber of users. This can be accomplished by using wavelength remodulation methods for bandwidth utilization and VCSELs for cost effectiveness.
Objectives of the Project
The main objectives of this project are:
Simulation and performance analysis of Wavelength Division Multiplexing- Or-thogonal Frequency division Multiplexing- Passive Optical Network (WDM-OFDM-PON) systems for different data rates
Simulation and performance analysis of Wavelength Division Multiplexing- Or-thogonal Frequency Division Multiplexing -Passive Optical Network (WDM-OFDM-PON) systems using wavelength reuse by

Traveling wave SOA (TWSOA)
Wideband TWSOA (WBTSOA)
Reflective SOA (RSOA)

Simulation and performance analysis of bidirectional WDM-OFDM-PON with dif-ferent transmission length and users using VCSELs
Report Outline
This report contains six chapters. Chapter 1 gives an introduction about the rel-evance of the project and also convey the main objectives of the project. The second chapter describes about the theories and literature survey of the project. This chapter begins with the basic access networks and describes the various terms relevant to the project. The terms like PON, WDM, OFDM etc are included. Chapter 3 describes the system model for the design. Here explains the basic block diagram representation of the major network technologies used. Chapter 4 explains the simulation procedures, platform used for simulations and details of each section and subsystem in the simulation process. Chapter 5 deals about the results obtained after the simulations and the major inferences got from these results. Sixth chapter concludes the project with findings followed by references section.
 

CdS Quantum dots: Synthesis and Optical Properties

CdS Quantum dots: Synthesis and Optical Properties Characterization for Solar Cell

 
Abstract— In this work CdS quantum dots were synthesized using Successive Ionic Layer Adsorption and Reaction (SILAR) method. Then a study of the morphology and optical property were made for the application of solar cell. The structural characterization were made by XRD while the optical characterization where done by UV-vis-NIR spectroscopy techniques.
Index Terms—Quantum dots, SILAR
I. INTRODUCTION
Quantum dot sensitized solar cell is an emerging field of photovoltaic in which the absorbing material is a quantum dot. The advantage of using such solar cell is size tunability and increased surface to volume ratio.
In a quantum dot based solar cell the active layer consist of the quantum dot and the scattering layer is formed by the TiO2 layer. The mesoscopic TiO2 when deposited with CdS quantum dot act as an energy harvester and convert the incident photon to electricity. In this work, a model of the photoanode for the solar cell was made with mesoscopic TiO2 layer as scattering layer and quantum dots as absorbing layer. Here instead of ITO a glass slide was used. [1]

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To synthesize a quantum dot various techniques are used. Among them Successive Ionic Adsorption and Reaction (SILAR) method is a cost effective and is used to prepare quantum dot. In a SILAR method the time of reaction or the number of cycles can be controlled. Depending on which the size of the quantum dot varies. Another advantage of this technique is that it can be prepared at room temperature. Also this method provides a close contact between the quantum dots and the oxide layer, so it is an attractive method for the preparation of electrodes in a solar cell. [1]
Cadmium sulfide (CdS) quantum dot is a direct band gap semiconductor. It is a II-VI compound semiconductor that is used for many optoelectronic devices such as solar cell, laser diodes and photoconductors. It is an inorganic semiconductor which has several advantages over conventional dyes. These advantages are band gap tunability, large extinction coefficient (this means that the dark current can be reduced and the overall efficiency can be improved) and multiple electron generation by utilizing hot electrons. [2]
II. EXPERIMENTAL SETUP

Chemicals Required

Titanium dioxide powder (SD Fine-Chem Limited, purity 60%), 2M nitric acid, 0.05M cadmium nitrate, ethanol, 0.05M sodium sulfide hydrate (Sigma Aldrich, assay=60%), methanol.

Preparation of TiO2 layer on glass slide

A paste of titania (TiO2) was prepared from TiO2 powder and nitric acid. The chemicals were added in 2:1 proportion. A thin layer of titania paste was coated on the glass slide using a technique called doctor blade method [3]. In this method, either a glass rod or a microscope slide is used. We have used a microscope slide of thickness 1.45mm to coat the paste. A glass slide of dimension 2cm X 1cm was cut and cleaned. With the help of an adhesive tape, the glass slide is positioned firmly on the work bench. Another advantage of using such tape is that we could define an area to coat the paste and to deposit the quantum dot. Now place the paste on one side of the glass slide, positioning the microscope slide in 45ï‚° spread the paste across the glass slide. Repeat the operation till a reasonably homogeneous layer is formed. After coating heat the paste to 80ï‚°C followed by annealing at 450ï‚°C for 30 min. After sintering the paste is white in color. This provides a better surface for adsorption of the CdS quantum dots since sintering makes the mesoporous films to a continuous network.

Deposition of CdS Quantum Dots

Successive Ionic-Adsorption and Reaction method is commonly used to deposit metal sulphide onto a nanostructured film. CdS quantum dot was deposited onto titania using this method as described in [4]. The first precursor solution used is 0.05M cadmium nitrate (Cd(NO3)2) and the second precursor solution is 0.05M sodium sulphide (Na2S). The bare TiO2 paste is dipped onto the first precursor solution for one minute. The Cd2+ ions have been deposited onto the TiO2 surface. This is then rinsed in an ethanolic solution for one minute and dried under room temperature. It is then dipped in the anionic precursor for one minute and then rinsed in methanolic solution for one minute and allowed to dry at room temperature. This completes one deposition cycle of SILAR. In this work we have performed four deposition cycles of SILAR.
III. RESULT AND DISCUSSION
The CdS quantum dot was deposited on to the surface of TiO2. An obvious color change was observed during the deposition cycle which is shown in Fig.1. The color change was pale yellow to golden yellow. The characterization was done using XRD and UV-vis spectroscopy techniques.

Fig 1: Photograph of glass slides with CdS coating with increasing SILAR cycles

XRD Characterization

Fig 2. shows the obtained XRD pattern for TiO2 (Fig.2a), TiO2/ CdS (Fig 2b.) . From the peak obtained, we confirm that CdS quantum dot was deposited onto the film. Since the peaks at 44.1ï‚°, 51.9ï‚°, 64.3ï‚°, 70.4ï‚° and 72.9ï‚° coincides with the intensity pattern as defined by the JCPDS 10-0454 for the CdS QD. The corresponding miller indices are (220), (311), (400), (331) and (420). From this we conclude that CdS QD was deposited. It belongs to the cubic crystal system and the mineral name is hawleyite. For TiO2 the XRD pattern exactly matches with JCPDS 21-1272. It belongs to tetragonal crystal system and its mineral name is anatase.

Fig.2 : XRD pattern (a) TiO2 (b) TiO2/CdS

Size Characterization

The size characterization was done by non-contact mode AFM (Atomic Force Microscopy). The size of the CdS quantum dot was found to be 25.83nm. the thickness of the deposited layer was calculated to be 29.65nm. Fig 3.

Fig 3 : AFM non-contact mode characterization of CdS quantum dot

UV-vis Characterization

The optical property was characterized using Jasco Spectrophotometer V670. The absorption spectrum is shown in Fig 4. The absorption spectrum for the TiO2 and CdS/TiO2 is shown in Fig 4a. and TiO2/CdS alone is shown in Fig.4b. The absorption peak for CdS is as reported by Antonio et.al [4]. From the absorption spectrum we could observe a shift in the peak indicating CdS QD is being deposited. The absorption peak was observed in the range of 386nm-484nm. For TiO2 the absorption peak was observed at 341nm. In Fig 4b. the inset is the absorption spectrum that was reported in [5]

Fig 4: Absorption spectrum of (a) TiO2 and TiO2/CdS (b) TiO2/CdS

Fig 5: UV-Vis absorption spectra showing increase (~49 %) in absorption due to CdS
Figure 5. depicts the percentage increase in the absorption peak of CdS with respect to TiO2. It was calculated to be a 49.08% increase in the absorption peak.

Determination of Optical Band gap

The DRS (Diffuse Reflectance Spectroscopy) characterization was done to obtain the optical band gap. The optical band gap was calculated by plotting the Tauc plot . It is the plot between energy and absorbance. The optical band gap can be determined by Tauc relation

Where  is the absorption coefficient in cm-1, h is the photon energy in eV and A is a constant. The value of n is given as follows
n = ½ for direct allowed transition
n = 2 for indirect allowed transition
The Tauc plot for TiO2 and TiO2/CdS is shown in Fig 6. TiO2 is an indirect band gap material whereas CdS is a direct band gap semiconductor. The bandgap value of CdS in bulk is given as 2.42eV [5]. From the experiment we calculated the optical band gap to be 2.38eV. Also the absorbance value of the CdS QD is blue shifted. Using the equation

The value of the peak was calculated to be 519.16nm which is within the absorption region.

Fig 6. Tauc plot of (a) TiO2 (b)TiO2/CdS
IV. CONCLUSION
In this work CdS quantum dot have been synthesized using SILAR method. Its structural characterization was done that confirmed the deposition of the CdS quantum dot on to TiO2 paste. The optical property was characterized and analysed using UV-vis-NIR spectroscopy. The optical band gap was calculated to be 2.38 eV. The size of the quantum dot deposited was calculated to be in nanometer.
REFERENCES
[1] Prashant V Kamat , “Quantum dot Solar cells.The next Big Thing in Photovoltaics” J.Phys.Chem.Lett. 2013, 4, 908-918.
[2] Chang Liu,Yitan Li,Lin Wei,Cuncun Wu,Yanxue Chen,Liangmo MeiandJun Jiao, “CdS quantum dot-sensitized solar cells based on nano-branched TiO2arrays” Nanoscale Research Letters 2014,9.
[3] A. Berni, M. Mennig, H. Schmidt, “Doctor blade method”, Springer.
[4] Antonio Braga,SixtoGimenez, Isabella Concina, Alberto Vomiero and Ivan Mora-Ser, “Panchromatic Sensitized Solar Cells Based on Metal Sulfide Quantum Dots Grown Directly on Nanostructured TiO2 Electrodes”, J. Phys. Chem. Lett. 2011, 2, 454–460.
[5] B. T. Huy, Min-Ho Seo, Jae-Min Lim, Dong-Soo Shin and Yong-Ill Lee, “A Systematic Study on Preparing CdS Quantum Dots” Journal of the Korean Physical Society, Vol. 59, No. 5, November 2011, 3293-3299
 

Optical Properties of Zinc Oxide Thin Films Using Two Dopant

G T Yusuf, MA Raimi, O.E Alaje and AK Kazeem

Abstract
The undoped ZnO, Al doped ZnO and Mg doped ZnO films were deposited by a sol-gel spin coating method onto the glass substrates. 0.3M solution of zinc acetate dehydrates diluted in methanol and deionized water (3:1) was prepared. Equal quantity of Aluminum chloride and tin chloride were added to each solution to serve as dopants. The effect of Aluminum and Magnesium doping on the optical ZnO films was studied. The transparency properties of all thin films are more than 80 % at a visible wavelength of (300-800 nm). The optical band gap of pure ZnO thin film is 3.12ev while the band gap for Al-doped ZnO and Mg-doped films are 3.16eV and 3.26eV respectively. All film parameters changed with dopant types. The variation of optical band gap with doping is well described by Burstein–Moss effect.
Keywords: Band gap; Doping; Films; Transmittance.

Introduction

In this Zinc oxide is an II-VI n-type semiconductor with band gap of approximately 3.3 eV at room temperature and a hexagonal wurtzite structure [1]. Recently, doped zinc oxide thin films have been widely studied for their application as conducting electrode materials in flat-panel displays or solar devices. Unlike the more commonly used indium tin oxide (ITO), zinc oxide is a non-toxic and inexpensive material [1].
Furthermore, pure zinc oxide films are highly transparent in the visible range (light wavelength of 400-700 nm) and have high electrical conductivity. However, non-stoichiometric or impurity (Group III elements or Group IV elements) doped zinc oxide films have electrical conductivities as well as high optical transparent. Non-stoichiometric zinc oxide films have unstable electrical properties at high temperature because the sheet resistance of ZnO thin films increases under either oxygen chemisorptions and desorption [9] or heat treatment in vacuum or in ambient oxygen pressure at 3000C-4000C [27]. Turning to impurity doped ZnO thin films, unlike non-stoichiometric ZnO thin films, impurity doped ZnO thin films possess stable electrical and optical properties. Among the zinc oxide films doped with group II elements such as barium, aluminum, gallium and indium, aluminum-doped zinc oxide (AZO) thin films show the lowest electrical resistivity [11]. Aluminum-doped zinc oxide (AZO) has a low resistivity of 2.4×10-4 Ω cm [11-13], which is quite similar to that of ITO films, which is about 1.2×10-4 Ω cm [14-16] and AZO also shows good optical transmission in the visible and near infrared (IR) regions. Thus, AZO films have been used as transparent conducting electrodes in solar cells [16, 8]. In addition to doping with Group III elements, doping ZnO with Group IV elements such as [9, 10] Ge, Sn, Ti, Si is also a good way to obtain low resistivity transparent materials in order to replace ITO because Ge, Ti, Zr could substitute on the Zn atom site. For example, Sn can serve as a doubly ionized donor with the incorporation of SnO2 as a solute in ZnO and, consequently, provide a high electron carrier concentration. It is, therefore, expected that the Sn doped ZnO (SZO) will have a higher electrical conductivity and better field emission properties compared with undoped ZnO [10].

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A variety of techniques such as DC or RF magnetron sputtering [2], electron beam evaporation [19,20], pulsed laser deposition [21], spray pyrolysis [22,23], chemical vapor deposition [24] and sol–gel processing [25–34,5] have successfully been developed to prepare zinc oxide thin films. Among them, the sol–gel spin coating method is simpler and cost effective. Traditionally, AZO films prepared by this method follow the non-alkoxide route, using metal salts such as acetates, nitrates or chlorides as precursor and dopant, respectively. In addition, organic solvent, such as methanol [20,21], ethanol [16], isopropanol [14], methoxyethanol [11], ethyl glycol and glycerol [10] are widely employed by introducing monoethanolamine (MEA), diethanolamine (DEA) or tetramethyl ammonium hydroxide (TMAH) as stabilizer [10,11,30]. Recently, few studies had reported on the growth of the ZnO thin films with different dopants using sol gel spin coating technique.
Therefore, the aim of this research works however is to study the optical and electrical properties of zinc oxide thin films using different dopants with locally fabricated sol gel spin coating technique.

Experimental

The films have been deposited onto the glass substrates at 400 °C substrate temperature. 0.3M solution of zinc acetate dehydrates diluted in methanol and deionized water (3:1) were prepared and divided into three portions. Aluminum chloride and tin chloride were added to each solution as dopants. A few drops of acetic acid were added to improve the clarity of solution. The concentration of dopants (aluminum chloride AlCl3·6H2O, magnesium nitrate hexahydrate [Mg (NO3)2.6H2O and was 3% and kept constant for all experiments. The starting solutions were mixed thoroughly with magnetic stirrer and filtered by WHATMAN filter paper. The solutions were then spin coated on glass substrates which have been procleaned with detergent and then in methanol and acetone for 10min each using ELA 110277248E/2510E-MT ultrasonic cleaner and then cleaned with de ionized water and heated on hot plate for 600C. The coating solutions were dropped onto the glass substrate which was rotated at 4000rpm 45 each by using Ws- 400 Bz – 6NPP/AS spin coater. After depositing by spin coating, the films were then dried at 3000C for 15minutes in a furnace to evapourate the solvent and remove organic residuals. The optical and electrical properties of the films at each time were investigated. The films were then inserted into a tube furnace and annealed in air at 7500C for 1 hour each. The optical transmission and reflectance of the films were examined by spectrophotometer ranging from 400 to 1000nm. The transmittance T and reflectance R data was used to calculate absorption coefficients of the AZO films at different wavelengths. The relationship between transmittance T, reflectance R, absorption coefficient, α, and thickness d of the film is given by equation (1).
(1)
The absorption coefficient data was used to determine energy band gap, Eg , using equation (2).
(2)
Where is the photon energy, A is a constant thus, a plot of against is a curve line whose intercept on the energy axis gives the energy gap. The band energy gap of the film was then determined by extrapolating the linear regions on the energy axis.
The absorption coefficient,, associated with the strong absorption region of the film was calculated from absorbance A and the film thickness, t, using (3).
(3)
The extinction coefficient, k, was evaluated from (4)
(4)
Where the wavelength of the incident radiation and, t is, is the thickness of the film.
The crystal phase of the films was determined by X-ray diffraction (XRD). The refractive index of the films was determined from the maxima and minima of the reflectance curve.
(5)
Where n is the refractive index, d is the film thickness (nm), is the wavelength (nm) of the incident light, and k is the interference order (an odd integer for maxima and even integer for minima).

Results

The crystal structure of ZnO films was investigated through X-ray diffraction (XRD). The X-ray diffraction spectrum of ZnO, Al-ZnO and Mg-ZnO film annealed at 7500C with prominent reflection planes is shown in figure 1.The peaks in the XRD spectrum correspond to those of the ZnO patterns from the JCPDS data (Powder Diffraction File, Card no: 36-1451) having hexagonal wurtzite structure with lattice constants a=3.24982Å, c=5.20661Å.The presence of prominent peaks shows that the film is polycrystalline in nature. The lattice constants ‘a’ and ‘c’ of the Wurtzite structure of the films were calculated using the relations (6) and (7).
a= √â…“.λ/sin θ(6)
c= λ/sin θ(7)
Figure 2 shows the optical transmittance spectra of ZnO, Al-ZnO and Mg-ZnO thin films in the wavelength range between 300 to 800 nm. The transparency properties of all thin films are more than 80 % at a visible wavelength of (300-800 nm). It is observed that the transmittance varies with dopant types i.e. aluminum and magnesium. The overall spectra shows an emission band with two obvious peaks, where the first peak, the UV peak which also called the emission or near band edge emission contributed to the free exciton recombination [18]. The second broad peak, also known as the green emission corresponds to the recombination of a photon generated hole with an electron in singly ionized [18].

Figure 1: X-ray diffraction patterns for ZnO thin film for aluminum and magnesium dopants
The optical absorbance spectrum measured within the wavelength range of 300–800 nm using a Shimadzu Spectrophotometer is shown in figure 3.

Figure 2: Optical Transmittance of the films for aluminum and magnesium dopants
Approximately, the band gap alteration of the thin film can be deduced from Figure 3. Here, it evidently shows that changes in the absorption edges are in parallel with types of dopant in the thin film. In order to appropriately estimate the optical band gap equation (2) was used. The presence of a single slope in the plot suggests that the films have direct and allowed transition. It is also well known that ZnO is a direct band-gap material [1] and the energy gap (Eg) can thus be estimated by assuming direct transition between conduction band and valance bands. Theory of optical absorption gives the relationship between the absorption coefficients α and the photon energy hν for direct allowed transition as shown in (2) The direct band gap determined using this equation when linear portion of the (αhν)2 against hν plot is extrapolated to intersect the energy axis at α = 0. Plot of (αhν)2 against hν for undoped, Al-doped ZnO and Mg-doped films are shown in figure 3. The optical band of pure ZnO is 3.12ev while the band gap for Al-doped ZnO and Mg-doped films are 3.16eV and 3.26eV respectively. The variation of optical band gap with doping is well described by Burstein–Moss effect [2-5]. For AZO films, compared to pure ZnO films, the contribution from Al3+ ions on substitution sites of Zn2+ ions and Al interstitial atoms determines the widening of the band gap caused by increase in carrier concentration. This is the well-known Burstein–Moss effect and is due to the Fermi level moving into the conduction band. Since doping increases the carrier concentration in the conduction band, the optical band-gap energy increases [2]. Enhancement of band gap thus also ensures that doping was successfully carried out in the ZnO thin films. It is further observed in our present work that an increase in band gap occurs in Mg- doped film as compared with ZnO and Al-ZnO thin films. The absorption properties of the films in UV range are due to the behaviour of ZnO intrinsic optical band gap energy. An absorption coefficient in the UV region significantly varies with types of dopant used. The result suggests improvement in the optical absorption in the UV region with nature of dopant, which provides useful information especially in the optoelectronic devices and device fabrication.
.

Figure 3: Plot of (αhν)2 vs. photon energy (in eV) for aluminum and magnesium as dopants

Conclusions

Transparent conducting thin films (ZnO, Al-ZnO and Mg-ZnO) have been deposited by sol–gel spin coating technique. The optical properties of these films were systematically investigated. X-ray diffraction analysis shows that The peaks in the XRD spectrum correspond to those of the ZnO, Al-ZnO and Mg-ZnO structural patterns is that of hexagonal wurtzite structure with lattice constants a=3.24982Å, c=5.20661Å. The optical transmittance spectra in the wavelength range between 300 to 800 nm shows that all thin films are more than 80 % at a visible wavelength of (300-800 nm). It is observed that the transmittance varies with dopant types i.e. aluminum and magnesium. The optical band of pure ZnO is 3.12ev while the band gap for Al-doped ZnO and Mg-doped films are 3.16eV and 3.26eV respectively. The variation of optical band gap with doping is well described by Burstein–Moss effect.
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Nonlinear Optical Phenomena in the Infrared Range

Various aspects of nonlinear optical phenomena in the infrared range

Nonlinear optics is a branch of optics, which describes the behavior of light in nonlinear media, where the dielectric polarization P responses nonlinearly to the electric field of the light E. This is a very broad concept. In this thesis, we focus our study on three aspects of nonlinear optical phenomena in the infrared wavelength range: the characterization of a mid-infrared ultrashort laser by autocorrelation based on Second Harmonic Generation (SHG), the influence of the beam mode on the interaction between laser and media during nonlinear propagation of femtosecond near-infrared pulses in liquid, and the dynamics of the ablation of solid samples submerged in liquid using a long nanosecond near-infrared laser.
Many energy levels of molecules and lattice vibrations are in mid-infrared wavelength range of 2.5-25 µm. For this reason, this wavelength range is called chemical fingerprint zone. Infrared absorption spectroscopy using light source in this wavelength range has been widely used identify different covalent bonds in many kinds of samples. Besides, by irradiation of an intense and short laser pulse whose wavelength is tuned to the resonance, a specific molecular band absorbs the pulse energy, and specific chemical reaction is excited. For this reason, tunable mid-infrared ultrafast lasers have a lot of potential applications in energy and material science, i.e., the production of alcohol or hydrogen from H2O and CO2, and the development of next-generation solar cells.

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Kyoto University Free-electron Laser (KU-FEL) is an oscillator-type free-electron laser, which works in the mid-infrared wavelength range of 5-13 µm. In temporal domain, the pulses from KU-FEL have a dual-pulse structure. In a macropulse with the duration of a few microseconds, thousands of micropulses sit with the interval of 350 ps between each other. Due to its special lasing dynamics, the wavelength instability of this kind of Free-Electron Laser (FEL) is relatively worse compared with optical lasers, i.e., at the working wavelength of 12 µm, this instability is around hundreds of Gigahertzes, which is comparable to the bandwidth of the vibrational modes. For those potential applications in which resonances are involved, stabilization of the wavelength of KU-FEL is necessary. And before that, we should first know the amount of wavelength instability. Besides, similar to all other ultrashort pulse lasers, micropulse duration of KU-FEL is very important information for applications such as nonlinear optics. For these purposes, in this thesis, we report the measurements of both the duration and wavelength instability of KU-FEL micropulses using the technique of Fringe-Resolved AutoCorrelation (FRAC).
For temporal characterization of ultrashort pulses, standard techniques such as Frequency-Resolved Optical Gating (FROG) and Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER) are invented more than ten years ago, which can give a single-shot measure for both the amplitude and the phase of the electric field, even for the pulses with the durations down to few cycle. Both FROG and SPIDER are spectrum-resolved measurement, for which the 2D array detector (CCD) is required to measure the single-shot spectrum. However, such kind of detectors for the mid-infrared wavelength range is very expensive, and not available in our institute. Under this condition, we perform an autocorrelation measurement of KU-FEL, and try to find the information about pulse duration and wavelength instability for the results.
Autocorrelation is a kind of well-known technique, which is invented more than thirty years ago. It is usually used for a rough estimation of the pulse duration of ultrashort laser pulses. In this thesis, by a systematic study of the influence of the wavelength instability on the signal of FRAC measurement, we first propose a method of measuring the wavelength instability of micropulses of an oscillator-type FEL by FRAC. Besides, we find that, by integrating the FRAC over the delay time, we can measure the duration of an ultrafast pulse, without knowing the chirps in advance. To the best of our knowledge, this finding has not been reported anywhere else, and it can save us from an additional Intensity AutoCorrelation (IAC) measurement.
Both of the above mentioned methods work well when applied to an FRAC measurement of KU-FEL at the wavelength of 12 µm. The durations and the wavelength instability of the microoulses are measured to be ~0.6 ps and 1.3%. This technique can be also applied for characterization of ultrashort pulses at other wavelengths, where 2D array detectors are not easily available, i.e., for the extreme-ultraviolet case.
Since our autocorrelation measurement is based on SHG, which is a second order nonlinear process, good focusablity of the laser beam is required to reach the high intensity at the focus position. To test the focusibility of the KU-FEL, a measurement of M2 factor of KU-FEL is carried out by the 2D knife-edge method before the autocorrelation measurement. The most convenient way to measure the M2 factor of a laser is to measure the beam profile at different distances from the focus by a beam profiler, and analyze the results. The reason why we choose the old-fashioned knife-edge method is still the lack of 2D array detector in this wavelength range. The beam profiles at different distances from the focus are reconstructed from the results of knife-edge scanning in both horizontal and vertical directions. During the data analysis, the beam of KU-FEL is found to have the non-Gaussian beam profile. As a result, the analytical methods developed for Gaussian beams under the knife-edge measurement do not work for our case. Taken the non-Gaussian property of the beam into consideration, some special and original treatments are taken during the data analysis.
With the development of the Ti:sapphire laser and the chirped pulse amplification (CPA) system, high power at the order of Terawatt becomes available at the wavelength of around 800 nm. This has attracted a lot of interests on the studies of nonlinear optics, such as the generations of attosecond pulses, Terahertz radiations, high order harmonics, and supercontinuum spectra. From the beginning of this century, the filamentation induced by femtosecond pulses during propagation in nonlinear media has been a hot topic. During the nonlinear propagation of femtosecond pulses, due to the balance between self-focusing, plasma defocusing, and nonlinear loss, the intense part of the laser beam collapses to a spot with very small diameter, which can propagate for a distance much longer than the Rayleigh length. This phenomenon is called filamentation. Because of the long focal depth of the filamentation, it has many applications such as laser machining, Laser Imaging, Detection and Ranging (LADAR), and long distance Laser-Induced Breakdown Spectroscopy. Besides, strong spectral broadening occurs during filamentation, and the coherent white light is generated at the central part of the beam. This effect is widely used for pulse compression. And for the reason of high time resolution, this coherent white light also serves as a good light source in spectroscopy.
Most of the studies about filamentation have used Gaussian beams as the incident beams. Recently, the axicon lens has made the generation of Bessel beam much easier. Many groups have focused their studies on the filamentation induced by Bessel beams. Compared with Gaussian beams, Bessel beams keep the high on-axis intensity for even longer propagation distance, thus can produce longer filamentation. We perform a comparison study of filamentations generated by Gaussian and Bessel beams. Since the pulses we can use are splitted from a CPA system, which contain the energy of 200 µJ, we choose the liquid as the nonlinear media. Compared with gaseous media, liquid has much larger nonlinear coefficient, so that the nonlinear effect can be observed at much lower incident power, and in a much shorter propagation range. Besides, unlike solid media, we can use the liquid sample for long time during experiment, without worrying about the laser-induced damage. During this experiment, we have confirmed the resistance of Self Phase Modulation during the propagation of Bessel beam, which is also reported in some papers by other groups. The experimental results and qualitative explanations are reported in this thesis.
When an intense laser pulse is focused on the material, plasma is generated. During this process, small portion of the material to be analyzed gets atomized and excited, and emits light. By collecting and analyzing the spectra of the emitted light, we can detect the constituents of the material, or even the relative abundance of each constituent element. This technique is called Laser-Induced Breakdown Spectroscopy (LIBS).
Compared with other similar techniques, LIBS has many advantages, i.e., in principle, it can detect all elements, and can analyze any matter regardless of its physical state, be it solid, liquid or gas. Since during a single shot in the LIBS measurement, the mass of the ablated material is in the range of picogram to nanogram, the LIBS is considered to be non-destructive. Another important advantage of LIBS is the easiness of the sample preparation. For most of the cases, the sample does not require any treatment before LIBS measurement. For this reason, LIBS can be applied for in-situ multi-elemental analysis. And due to its fast analysis time, LIBS can be used for a realtime composition measurement.
Nd:YAG laser at fundamental wavelength (1064 nm) is most often used during LIBS experiments. It has several advantages, i.e., the scattered laser light does not influence the measurement of the visible spectra, and compared with shorter wavelength, laser at this wavelength has better heating effect on the laser-induced plasma.
Compared with LIBS of solid sample in gaseous media, LIBS of solid sample under liquid is more complicated. In such condition, if the single nanosecond pulse is used for ablation, the measured spectra are always deformed and broadened, which is due to the strong confinement of plasma plume in liquid environment. One solution of this problem is to use the double pulses LIBS, during which the first pulse can generate a bubble near the surface of the sample, in which the plasma produced by the second pulse can expand. Another solution is to use the long nanosecond pulses, which have the durations of more than 100 ns. During long pulse LIBS, the diameter of the laser-induced bubble can reach hundreds of micrometers at the trailing part of the pulse, which provides a space with low density for the plasma plume to grow. Compared with the double pulses LIBS, the advantage of the long pulse LIBS is that, it can be applied for the measurement under very high pressure. However, if the double pulses LIBS is applied under such condition, the bubble generated by the first pulse can not grow to a size large enough for the plasma plume generated by the second pulse to expand inside. And as a result, the double pulses LIBS loses its advantage.
In this thesis, we report our experimental study of long pulse LIBS of solid samples under liquid. Two experiments are included. The first one is to optimize the laser focus position, and the second one is to study the influence of solvent temperature on the ablation dynamics. The results of these experiments can help us better understand the dynamics of ablation during long pulse LIBS of solid sample submerged into liquid.
 

Free Space Optical (FSO) Communication

INTRODUCTION
Free Space Optical (FSO) communication involves the transmission of data through a wireless medium using modulated near infrared light beam (with wavelength between 800 nm-1700 nm) [1] as carrier wave. FSO communication links can be used for satellite-to-satellite cross links [2] [3], up-and-down links between space platforms- aircraft, ships, and other ground platforms, and among mobile and stationary terminals within the atmosphere [3]. Light as a medium of communication is not a recent innovation as it was used in the Roman era, where polished metal plates where used as mirrors to reflect sunlight for long-range signaling. A similar sunlight-powered device was used by the U.S. military to send telegraph information between mountain tops in the early 1800’s [4]. Additional optical communication developments occurred during the World War II, and the post-war era experienced further developments in this field fueled by electronic innovations such as the transistors, vacuum tubes and integrated circuits, and most especially the invention of the laser in the early 1960s [1]. The unique characteristics of laser such as its powerful coherent light beam, the possibility of modulating it at high frequency and the low beam divergence has made it the preferred light source for enhanced FSO applications. FSO communication is considered to be one of the key technologies for realizing very-high-speed multi-gigabit-per-second large-capacity communications when fibre optic cable is neither practical nor feasible [4]. FSO communication can be of crucial advantage particularly because of its wireless nature and several applications, making it a viable alternative to the laying of fiber cable underground which is expensive and has environmental consequences. Unlike radio and microwave systems, FSO has higher data rate due to its high carrier frequency, low power requirements, no frequency license required and much smaller packaging [4].
 
In FSO links, atmospheric turbulence is capable of degrading the wave-front quality of a signal-carrying laser beam, resulting in signal loss at the receiver and thereby impairing the link performance [4],[5]. In addition, fog, snow, rain, dust, smoke and other aerosol particles contribute to the attenuation of the signal-carrying beam and eventual degradation of the FSO link. Several studies have been done on the atmospheric turbulence channels of a FSO system [3], [5]. The real performance measure of an FSO communication system is provided by the binary error probability also referred to as Bit Error Rate (BER).FSO communication involve the use of optical amplifiers either as an optical booster or optical preamplification. Amplification is achieved by stimulated emission of photons from dopant ions in the dopant fibre by using a pump laser as used in Erbium Doped Fibre Amplifiers (EDFA) or electrically as used in semiconductor lasers, due to the excitation of ions from a higher energy state to a lower energy state. The excited ions can also decay spontaneously (spontaneous emission) or even through non-radioactive processes involving interactions with phonons of the glass matrix. These last two decay mechanisms compete with stimulated emission reducing the efficiency of light amplification introducing what is known as the Amplified Spontaneous Emission (ASE) noise. Digital Pulse Position Modulation (DPPM) with direct-detection is the preferred modulation technique for FSO communication systems because of the lack of dispersion over the free space channel [2] , none requirement of threshold for detection [3] and the complexity associated with phase or frequency modulation [5]. At the moment, the benefits of FSO communications have not been fully exploited, hence more applied researches are needed at the laboratory to help transfer the performance of FSO optical systems into real life applications.
 
CHAPTER TWO
LITERATURE REVIEW
2.1 FSO – PRINCIPLES AND CLASSIFICATION
A FSO system consists basically of a transmitter, usually a modulated laser or light emitting diode (LED) which produces light for conveying data through space, and a receiver such as a photodetector which receives close to collimated radiation independent of the transmitter pointing concentrated lens [1]. Other components include beam control optics, collection lens, optical amplifier, solar radiation filters and other electronics [6]. In general, FSO systems can be classified into indoor and outdoor FSO communication [4], [7] based on the distance covered for communication and the absence of environmental effects in the indoor FSO link. The basic operating principles of indoor point to point systems are not different from outdoor communication links but the designs are very different to accommodate various requirements. Some research on transmitter and receiver designs for long-range FSO communication systems, [8] like satellite and atmospheric optical communications, have already been reported, which might be beneficial to short-range systems. Recently, many indoor communication systems employ laser diode as light sources [8].
 
2.1.1 INDOOR FSO COMMUNICATION
The indoor FSO communication link can be further classified as point and shoot links which are subdivided into infrared data association (IRDA) and retro-reflect links; and networks which have two types namely the diffuse networks and line of sight networks [4],[7]. Indoor FSO applications are confined to short distances; hence it is appropriate to optical FSO systems that use wide divergence beams rather than narrow beams which are suitable for point to point systems. Such systems are sometimes referred to as optical telepoint systems [7]. Due to the fact that indoor FSO systems are not affected by atmospheric effects, the power budget depends solely on the transmitter launch power, free space loss, and receiver sensitivity. As reported in Mahdiraji and Zahedi [9], the use of infrared frequencies for short-range wireless communications has received extensive interest over the decade, and many potential applications of this technology have been suggested. Some of the applications include portable device such as laptop computers, personal digital assistants, and portable telephones. Many indoor communication systems employing infrared LED wireless links have been reported in [7], [9]. Using pure diffuse link, a high speed and power efficient indoor wireless infrared communication using code combining was reported in Majumdar and Ricklin [4], where a multiple transmitter link design was used with a narrow field of view direction diversity receiver. The design goal was to eliminate the effect of inter symbol interference (ISI) so that power efficient signalling schemes such as DPPM can be employed at a very high data rate.
 
A portable transceiver for indoor FSO link was reported by Jiang et al [8] .The system employs a transmitter of eye safe infrared LEDs and a receiver of photodiode arrays with multi channel trans-impedance-summer architecture. The received signal achieved a BER of 10-4 at a plane of 2 m away from the transmitter, even at a point 50 degree off the transmitter’s vertical axis. The bit rate of the transceiver was up to 40 Mbit/sec in an indoor non-directed infrared FSO link to be extended to 100 Mbit/s using LEDs with higher cut frequency.

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2.2.2 OUTDOOR FSO COMMUNICATION
Unlike the indoor FSO communication link, the outdoor link covers a long distance over 500 m [7] using the atmosphere as its natural medium of communication. It is unlikely that long distance point-to-point systems will exceed the 4-5 km range due to atmospheric channel degradation. Outdoor point-to-point systems use high power lasers that operate in the Class 3B eye safety band to achieve optimum power link budget, particularly if high bit rate (e.g., 155 Mb/s) is required [7]. In order to achieve an improved power budget, an experiment was carried out by Heatley et al [8] to achieve a trial 155 Mb/s link that cover a 4 km distance between Imperial College and University College London. The aim of the experiment was to minimize the overall propagation loss focussing on the free space loss and the receiver sensitivity as little could be done to reduce the atmospheric losses. A low power laser coupled to an erbium doped fibre amplifier (EDFA) was used as the source; two astronomical telescopes (Schmidt-Cassegrain, 20 cm aperture) were used at both the source and receiver ends, and an Avalanche photodiode (APD) was used as receiver. The result of the experiment was that the diameter at the receiver was reduced to 0.5 m which corresponds to 8 dB free space loss from a beam diameter of 2 m (20 dB free space loss). The major problem encountered was maintaining the beam alignment which depends on temperature changes. Although the APD receiver is relatively costly, it helped to improve the receiver sensitivity and they are generally used for long distance systems.
FSO is well established for intersatellite and deep-space communications [3] but it can also be used in ground-to-space link, Unmanned-Aerial-Vehicle (UAV) to ground link, and among mobile and stationary terminals within the atmosphere [10]. New areas of application include quantum key, traffic and telematics [6]. It has been predicted that FSO can be fruitfully utilized as alternative for last-mile problem [3], [10], [11]. According to Majumdar and Ricklin [4], a group of researchers reported the design and development of acquisition, tracking and pointing subsystem for UAV to ground free space optical communications link. The communication link was developed from a UAV to stationary ground stations located at Wrightwood, California and Maui, Hawaii. The range of the UAV was 50 km. the downlink laser transmitter wavelength was 1550 nm and had power of 200 mW for developing a 2.5 Gbps data rate communication for a BER of 10-9. Furthermore researchers have presented results from experimental demonstrations using a very lightweight optical wavelength communication without laser in space (LOWCAL) [4] between ground based telescope and a space shuttle. The uplink/downlink established at 852 nm wavelength and 852 nm signal beam. Some of the specifications for the experiment were: range of 640 km, data rate of 10 kbps, telescope diameter of 0.6 m, modular weight of 2-4 kg and retro-reflector area of 70-180 cm2. For the downlink, differential circular polarization keying (DCPK) format was used while Frequency Shift keying was used for the uplink. Both the downlink and uplink achieved a BER 2.2 FSO- TECHNOLOGICAL ADVANTAGES AND CHALLENGES
Theoretically, the wavelength range of the near infrared electromagnetic spectrum (800 nm and 1700 nm [1]) used in optical communication implies it offers extremely high bandwidth, hereby providing higher data rates compared to other communication media such as the radio (1 mm-100 Mm ) and microwave (1 mm-1 m) systems [1]. Further, this technology requires no spectrum licensing requirements i.e. traffic free bands, no mutual interference between the FSO systems (high spatial selectivity of the beam), no Fresnel Zone requirement and difficult to eavesdrop on transmitted data [4], [6], [10]. The integration of the optical wireless and fibre is a research area been worked on by my many researchers [4]. This integration is possible because the two optical technologies offer high—speed optical bandwidth to meet market needs. They both use the same optical transmission wavelength (800 nm – 1700 nm) [1]. In addition the two optical technologies both share the same system components and can transmit digital information using a range of protocols. The business advantages of FSO communications for network extensions include the reduction of cost incurred on fibre-optic cable and other associated costs, as well as time for deployments [4].
Despite its potentials, FSO communication link is affected by atmospheric attenuation owing to aerosol particles such as fog, haze, rain and snow which causes fluctuations in both the intensity and the phase of the received light signal hereby limiting the availability of FSO for a given transmission range and increasing the systems bit error rate [5]. These includes the fine mode with diameter, less than 2.5 µm, the accumulation mode with particle diameters 0.1 µm µm and the coarse mode with diameter > 2.5 µm [12]. These particles can grow in size in regions of high humidity, and shrink by evaporation as humidity decreases. The effect of atmospheric aerosols in the channel on laser beam propagation can be determined using the Mie theory which depends critically on wavelength and particle size [13]. The dimensionless size parameter illustrates the nature of the Mie calculation as it gives the relationship between the particle size and radiation wavelength. This is given mathematically in equation 2.1[13] as
where is wave number, is particle radius and is particle wavelength. Table 2.1 adapted from Ricklin et al [13] show some sources of aerosol in the atmosphere. Aerosol particles can be classified into three modes based on the diameter of the particles. Table 2.2 adapted from O’Brien et al [6] shows the attenuations caused by rain, snow and fog.
Table 2.1 Estimates of particles smaller than 20 micron radius emitted into or formed in the atmosphere (106 metric tons/year) (adapted from [13])

Natural sources

Soil and rock debris* 100-500

Forest fires and slash-burning debris* 3-150

Sea salt 300

Volcanic debris 25-150

Particles formed from gaseous emissions

Sulphate from H2S 130-200

Ammonium salts from NH3 80-270

Nitrate from NO8 60-430

Hydrocarbons from plant exudations 75-200

Subtotal 773-2200

 

Man-made sources

Particles (direct emissions) 10-90

Particles formed from gaseous emissions

Sulphate from SO2 130-200

Nitrate from NO8 30-35

Hydrocarbons 15-90

Subtotal 185-415

Total 958-2615

*Includes unknown amounts of indirect man-made contributions.
Table 2.2 Attenuations caused by rain, snow and fog (adapted from [6])

 

Attenuation

Visual range

Clear weather

0.2 ÷1 dB/km

10 ÷25 km

Rain

3 ÷9 dB/km

2 ÷4 km

Snow

7 ÷12 dB/km

1 ÷2 km

Fog haze

30 ÷80 dB/km

200÷500 m

Heavy fog

300 dB/km

50 m

 
Extinction is a term which describes the attenuation of a laser beam as it passes through a medium containing atoms, molecules, and particles. As reported in Ricklin et al [13], Goody and Yung defined the fundamental law of extinction as that of Lambert, which states that “the extinction is linear in both intensity of radiation and in the amount of matter, provided that the physical state (i.e., temperature, pressure, composition) is held constant”. Intensive researches [3], [4], [5], [13] have been conducted on the effects of atmospheric losses, most especially atmospheric scintillation, on FSO communication and some solutions have been proffered to reduce these effects.
 
2.2.1 PROPAGATION LOSS
This atmospheric loss is associated with the distance covered by the laser beam. According to Prokes [12], free space propagation loss can be expressed as shown in equation 2.2
[dB] (2.2)
where L is the link path distance, is the beam divergence full angle and is the diameter of transmitting circular aperture. For the Gaussian beam and a sufficiently long link distance ( >>) the additional gain is = 3.7 dB. In Heatley et al [7], it was reported that for a point-to-point system that operate with a slightly diverging beam, the free space propagation loss would be 20 dB whereas in an indoor system using wide angle beam, the free space loss would be 40 dB or more. At very short link distances, the total transmitted power is detected at the receiver because the beam spot diameter at the receiver position is lower than the diameter of the receiver lens [12].
2.2.2 PHYSICAL OBSTACLES
Physical obstructions such as birds, insects, tree limbs, buildings or other factors can temporarily or permanently block the laser line-of-sight [6], [13]. Platform/building motion due to wind, differential heating and cooling, or ground motion over time can result in serious misalignment of fixed-position laser communication systems [13]. Proper planning and site measurements are ways of avoiding this effect [6].
2.2.3 ABSORPTION AND SCATTERING
Molecular absorption process which is wavelength dependent is a major factor in beam attenuation. At wavelengths greater than 1µm, the effect of molecular extinction can be negligible as atoms couple weakly with electromagnetic field [4]. Furthermore, molecular absorptions at these wavelengths are due to absorption of incident radiation with only minor scattering contributions [4]. Aerosol scattering effect is caused by rain, fog, mist and snow. This effect accounts for the degradation in quality of service experienced during snow, rain, fog and mist as reported in a detailed measurement take by [7] over a period of one year in both rural and urban areas. The results also reported in Heatley et al [7], showed a similar trend but with rather less variability between seasons and higher average attenuations. For many molecules, the absorption spectra have been measured experimentally in the laboratory and the respective extinction ratios of specific molecules have been made available for evaluation [4]. Molecular absorption can be minimized by appropriate selection of the optical wavelength [4]. It has also been suggested in [7] that the attenuation effects due to scattering can be minimized by reducing the link range and/or reducing the optical power budget. The attenuation caused by scattering in decibel scale A10, scat is given by the product of the atmospheric attenuation coefficient α10, scat and the link distance in kilometres. The mathematical expressions are shown in equations (2.3), (2.4) and (2.5) [12].
[dB/km] (2.3)
Where is the particle size distribution coefficient defined as:
[dB] (2.5)
2.2.4 ATMOSPHERIC SCINTILLATION
Scintillation is caused by solar energy heating up small air pockets inhomogenously, thereby creating varying refractive index along the FSO link [7]. This results in the scattering of laser beams at various angles along the propagation path and a resultant fluctuation in both the intensity and phase of the received light [5], [7].Atmospheric scintillation is less significant at distances less than 500 m [7], [12], but degrades performance of a FSO link at ranges of the order of 1km or longer [5]. The intensity I of an optical wave propagating through turbulent atmosphere is a random variable. The normalized variance of optical wave intensity, referred to as the scintillation index, is defined by equation (2.6) [12]
where the angular brackets denote an ensemble average. The scintillation index indicates the strength of intensity fluctuations. For weak fluctuations, it is proportional and, for strong fluctuations, it is inversely proportional to the Rytov variance for a plane which is shown in equation (2.7) [12]
where is the refractive-index structure parameter. This parameter is dependent on temperature, humidity, atmospheric pressure, altitude and wind strength [12]. Beam wander is another occurrence in an atmospheric channel which causes similar effects as atmospheric scintillation. This is characterized by the deflection of the entire laser beam by optical tubules of larger diameter than the beam, resulting in a random movement of the light beam about the target point.
Atmospheric scintillation is a major impairment of FSO communications systems, as it can produce large transient dips in the optical signal. It has been studied extensively with various theoretical models already proposed to describe the signal fading [3], [5], [11]. In addition, several communication techniques have been described to mitigate the signal fading effect [5], [6], [12], [13]. In [3], the error performance of terrestrial FSO links were modelled as PPM/Poisson channels in turbulent atmosphere. The scintillation effects were modelled as lognormal for weak turbulence and as exponential for heavy turbulence. In Kiasaleh [11], the performance of a direct-detection, APD-based PPM FSO communication system in atmospheric turbulence was characterized. Here the weak turbulence link was investigated by modelling the received signal as a log-normal random process and also as a negative exponentially distributed received signal intensity. The binary PPM was used as the modulation scheme for the system. It was assumed that the receiver thermal noise is non-negligible and the average signal intensity was large enough to justify as Gaussian approximation. It was concluded that the performance of the APD-based PPM FSO system was severely affected by turbulence and that the optimum APD gain must be used to avoid excessive APD noise at the receiver. It was also concluded that the negative exponential channel scintillation affected the systems performance with only large signal power capable of influencing the performance. Zhu and Kahn [5] employed the statistical properties of signal fading, as a function of both temporal and spatial coordinates as an approach of mitigating turbulence-induced intensity fluctuation. In spatial domain technique, at least two receivers are used to collect the signal light at different angles. In temporal domain techniques, only one receiver is used. Here signal-by-signal maximum likelihood detector (ML) is used to optimize performance when the receiver knows only the marginal statistics of the fading while maximum-likelihood sequence detection (MLSD) is used when the receiver knows the temporal correlation of the fading. The investigation showed that BER has greater degradation when the standard deviation of the turbulence induced fading is large. Furthermore, the diversity reception with the two receivers can improve the performance than a single receiver. According to Prokes [12], the receiver lens area causes an integration of various intensities incident on particular parts of the lens. It was reported in [12] that optical scintillations can be reduced by increasing the collection area of the receiver lens. This phenomenon is known as aperture averaging and the aperture averaging factor for a spherical wave is shown in equation (2.8) [12]
where is the power scintillation index and is the Rytov variance for the spherical wave. An experiment was carried out by Prokes [12] to investigate the effect of the aperture averaging factor on the power scintillation index using two different refractive index structure parameters for the calculation. The result showed that the influence of both the lens diameter and refractive-index structure parameter on the scintillation level was relatively large.
The power link budget of a FSO communication link was given in [12] based on statistical analysis of the atmospheric attenuation. Figure 2.1 shows a power level diagram of FSO deployed at a distance of about 1 km.
According to figure 1, the total received optical power, was given by Prokes [12] in equation (2.9) as
where is the mean optical power of a laser diode, includes the coupling loss between the laser and the transmitter lens and the attenuation loss in the lens, is the beam attenuation due to propagation loss, includes random losses caused by atmospheric phenomena (scattering and turbulence), and represents the coupling lens between the receiver lens and photodiode and the attenuation and reflection at the lens.
2.2.5 AMPLIFIED SPONTANEOUS EMISSION (ASE)
Optical amplifiers are important components in communication links, as they are used to boost output power levels in the order of one Watt and higher. Due to the atmosphere medium used in FSO, only optical booster and pre-amplification schemes can be used [14]. Apart from the desired optical signal amplification, the optical booster amplifier constitutes a source of background radiation that can exceed the background radiation from the sun. This radiation is called amplified spontaneous emission (ASE). The ASE power spectral density is represented in equation (2.10) [14] as
where is one photon energy, denotes the optical amplifier gain, and is the amplifier’s noise figure which ideally should be 3 dB. Equation (2.10) is valid for a single spatial mode (including polarization modes); in multimode applications has to be multiplied by the number of emitted modes to arrive at the total ASE power spectral density [14].
The effect of ASE radiation in optical booster and preamplification in FSO communication link was investigated by [14] using on/off keying (OOK) as the modulation technique. In the case of the booster amplifier, considering the fact that the maximum on-axis gain of a central unobscured optical transmit antenna with respect to an isolator radiator equals where is the diameter of the telescope optics and is the optical transmit wavelength .The value 0.81 is the optimum value of the fraction of incident power that can be coupled to an optical fibre from an incident plane wave. On inserting into equation (2.10), the ASE power spectral density generated by the booster amplifier and coupled to the receiver is given in equation (2.11) [14] as
[W/Hz] (2.11)
where is the transmitter gain, is the transmitter noise figure and and are the transmitter and receiver telescope diameter respectively. Also the signal power coupled to the receiver was given in equation (2.12) [14] as
[W] (2.12)
where is the transmit optical power at the output of the booster amplifier. Figure 2.2 [14] shows the general setup of an optically boosted FSO system.

Optical booster amplifier GTX, FTX

Amplified data signal & transmit booster ASE

Using Erbium-doped booster amplifiers with specifications, telescope diameters, and a communication distance of , Winzer et al [14] arrived at which is of the order of the magnitude of the background radiation per mode produced by the sun. It also was reported that the booster ASE constitutes the dominating background radiation term up to communication distances of 600 000 km. In the optically preamplified FSO communication system, (see Figure 2.3) the received signal passes through polarization filter and optical bandpass which reduces the detected preamplifier ASE power. The preamplification introduces additional noise terms such as the shot noise, ASE-shot beat noise, signal-background beat noise, signal-ASE beat noise, background-preamplifier ASE beat noise, background-background beat noise and the ASE-ASE beat noise. It was reported by Winzer et al [14] that up to a link distance of 1000 km the beating of the signal and the transmit booster ASE dominates all other noise terms, causing the signal-to-noise ratio (SNR) to become independent of R. The communication quality does not in this case increase with decreasing communication distance. It was thus concluded that the optical booster ASE has significant impact on the performance of a FSO communication link, especially at short link distances [14].

In Winzer et al [14], the use of the pointing, acquisition and tracking (PAT) system (see figure 2.4) to reduce the optical booster ASE at the receiver was presented. The proposed patent-pending PAT system eliminates the need for the using separate power or hardware for beacon lasers, taking care of alignment procedures between the beacon-laser and the transmit or receive telescopes, and splitting off a certain fraction of the information carrying data signal for PAT purposes [14].The booster ASE is applicable to the PAT system because the ASE spectrum exceeds that of the data signal by orders of magnitude and also the ASE has the same spatial modes as the data signal.
 
Figure 2.4 The PAT system retrieving pointing/tracking information from ASE emitted by the booster amplifier at the transmitter [14]
Phillips et al[2] carried out an analysis of the optically preamplified intersatellite pulse position modulation (PPM) receiver employing maximum likelihood detection (MLD) using Gaussian approximation (GA) and Chernoff bound (CB) techniques. The results from the calculations carried out at a wavelength of and bit rate of 25 Mb/s showed that this mentioned receiver configuration is approximately 1.5 dB more sensitive than the optically preamplified OOK non-return-to zero (NRZ) signalling. This method is proposed to have future implementation in future laser intersatellite communication systems. This paper has been reviewed in my report, with special interest in the BER evaluation.
2.2.6 EYE SAFETY
FSO systems involve the emission of high power optical power which can be unsafe, especially if operated incorrectly [7]. As a result of this, laser safety standards have been established and classified based on the amount of power emitted by the transmitter source. Table 2.3 (adapted from [7]) summarizes the principal classifications.
Table 2.3 Laser safety classifications for point-to-source emitter (adapted from [7])

Structural and Optical Properties of Pulsed Laser

Structural and Optical Properties of Pulsed Laser Deposited ZnO/TiO2 and TiO2/ZnO Thin Films

R. K. Jain, Praveen K. Jain

 
Abstract. ZnO/TiO2 and TiO2/ZnO thin films have been deposited on single crystal Si (100) substrate using pulsed laser deposition (PLD) technique in order to improve structural and optical properties of ZnO and TiO2 thin films. It was observed that the deposition of TiO2 film prior to ZnO, exhibited higher crystallinity along (002) diffraction peak, small compressive strain and stress and thereby rendering better optical properties as compared to ZnO films deposited directly on Si substrates. On the other hand, TiO2 thin film deposited on Si substrate exhibited pure anatase phase while the use of ZnO buffer was found to improve the crystallinity of TiO2 thin film. The photoluminescence spectra showed that TiO2 and ZnO buffer layers enhanced ultraviolet emissions of the ZnO and TiO2 thin films to a larger extent, respectively.
Keywords: ZnO, TiO2, Optical properties, Photoluminescence
PACS: 78.66.Hf, 78.55.Et, 68.37.Ps
Introduction
ZnO is suitable for the production of light emitting devices and a promising candidate for the next generation of electronic devicesdue to its wide band gap (3.37 eV) and large exciton binding energy (60 meV)[1]. ZnO thin films play an important role in solid-state display devices, solar cells and exciting acoustic waves at microwave frequencies[2]. Titanium dioxide (TiO2) is one of the most important semiconductors with high photocatalytic activity, non-toxicity, stability in aqueous solution, and is relatively inexpensive. The excellent photocatalytic property of TiO2 is due to its wide band gap and long lifetime of photogenerated holes and electrons [3-4]. It has been reported that the deposition of ZnO or TiO2 thin films on Si substrates at elevated temperature leads to increase in oxygen vacancies as the surface Si atoms easily capture oxygen atom from ZnO or TiO2, which deteriorates the quality of the these films [5]. So it is required to improve various properties of ZnO and TiO2 films for their potential applications. In the present study, a systematic investigation has been performed in order to improve the structural and optical properties of these films using buffer layers. ZnO and TiO2 are chosen as a buffer layer material on the basis of following considerations: (a) Both are wide-band-gap materials, (b) both exhibit high chemical and thermal stability, (c) both have high refractive indices, high transmittance in the visible region and intense absorption in the ultraviolet band and (d) both are low cost material.

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EXPERIMENTAL DETAILs
ZnO and TiO2 thin films have been deposited on Si (100) substrate by ablating high purity (99.9%) ZnO and TiO2 ceramic target using pulsed laser deposition (PLD) technique. The KrF excimer laser with wavelength of 248 nm was used for deposition. The pulse repetition rate was 10Hz with laser fluence of about 2–3Jcm−2. The target to substrate distance, working O2 pressure and deposition temperature were kept 35mm, 50 mTorr, and 500°C respectively. The thickness of the film and buffer layer was measured using cross section FE-SEM and found to be ~200 nm and 50 nm, respectively. The phase and orientation of as-grown thin films were characterized by X-ray diffractometry (Bruker AXS D-8 Advance Diffarctometer) using CuKα (λ=1.5407 Å) radiation. The surface morphology was examined using atomic force microscope (NTMDT: NTEGRA model). Absorption spectra have been taken using UV-VIS-NIR spectrophotometer (Varian Cary 5000) and PL study was performed using photoluminescence spectrometer (Perkin Almer LS-55).
RESULTS AND DISCUSSION
XRD pattern reveled that ZnO thin film grown on Si (100) substrate was preferentially oriented along the c-axis with a hexagonal wurtzite structure and the use of TiO2 buffer layer increases crystallinity along (002) diffraction peak as shown in Figure 1. On the other hand, TiO2 thin film exhibit pure anatase phase and crystallinity was improved along (004) plane by inserting the ZnO buffer layer between substrate and TiO2 thin film. The improved crystallinity of thin film using buffer layer resulted from the mismatch in thermal expansion coefficient between ZnO and TiO2, which is smaller than that of between ZnO and Si or TiO2 and Si. The lattice mismatch between ZnO and Si (1 0 0) are 40%, whereas for their counterparts i.e. between ZnO and anatase-structured TiO2 are 14% [6]. Therefore, the decrease of lattice mismatch is another reason for the improved crystallinity.
The crystallite size calculated using Scherer’s formula is shown in Table 1. The strain along the c axis, zz is given by the following equation [7]:
(1)
where c is the lattice parameter of the strained ZnO films calculated from x-ray diffraction data and c0 is the unstrained lattice parameter of ZnO. The lattice mismatch between film and substrates can result in varying degrees of stress during the growth process of thin films. The results show that the compressive strain is present in all fabricated ZnO and TiO2 films, which is derived from lattice mismatch between substrates and films owing to increase in crystallite size, and the stress is decreased with the buffer layer. Figure 2 shows the AFM image of the deposited thin films. The grain size and average surface roughness increases when buffer layer is used due to enhancement in crystallinity.
Figure 3 shows the room temperature PL spectra of ZnO and TiO2 thin films grown on Si substrate with and without buffer layer. The ZnO film deposited on Si (100) substrate exhibits strong ultraviolet emission peak along with weak green–yellow emission band. The ultraviolet emission of ZnO films is generally considered to be resulted from recombination of free exciton, whereas the green emission is mainly resulting from oxygen vacancies [8]. The PL spectra of TiO2 thin film deposited on Si (100) substrate shows a broad emission band from 390 to 450nm and there are two emission peaks superimposed on the broad emission band. The peak before 350nm (~3.5eV) is ascribed to direct electron-hole recombination which should be equal to or slightly bigger than the TiO2band gap. The emission band from 390 to 450nm (corresponding to 3.2–2.75eV) arises from indirect band gap and surface recombination processes. Further observation indicates that there are two small peaks at the wavelength range from 460 to 500 nm. These PL signals are attributed to excitonic PL, which mainly result from surface oxygen vacancies and defects of the films. It is observed that ZnO thin film deposited on the TiO2 buffer layer shows stronger ultraviolet emission, as compared to ZnO thin film grown without buffer layer, with no visible emission. The absence of visible emission shows the defect free formation of film. Similarly, the use of ZnO buffer layer also removes the oxygen defects emission peak of TiO2 thin film.
The enhanced ultraviolet emission from ZnO thin films grown on TiO2 buffer layer is also probably connected with fluorescence resonance energy transfer
(FRET) between ZnO and TiO2. After the excitation of electron–hole pairs in TiO2 layer, the energy is easily transferred to ZnO films due to resonance effect [9] as a result, the band gap emission of ZnO is enhanced.
From optical absorption spectra of ZnO and TiO2 thin films, It is observed that ultraviolet absorption edge of ZnO and TiO2 film with buffer layer has a red-shift, compared with ZnO and TiO2 thin film grown on bare Si (100) substrate. The value of direct band gap was found to be 3.29 and 3.24 eV for ZnO thin films grown on Si substrate without and with TiO2 buffer layer, respectively. On the other hand, the value of indirect band gap was found to be 3.24 and 3.19 eV for TiO2 thin films deposited on Si (100) substrate without and with ZnO buffer layer. The decrease in optical band gap of the films could be related to the enhancement in crystallite (grain) size leading to a smaller number of grain boundaries. On the other hand the compressed lattice will provide a wider band gap because of the increased repulsion between the oxygen 2p and the zinc 4s bands [10].
CONCLUSISON
ZnO, TiO2, ZnO/TiO2 and TiO2/ZnO thin films on Si (100) substrate were prepared by pulsed laser deposition technique. XRD and AFM result demonstrate that the crystallinity of ZnO and TiO2 thin films are considerably improved by using TiO2 and ZnO buffer layer, respectively. Compared with PL of ZnO thin film, UV intensity of ZnO grown on TiO2 buffer layer has increased about two fold. Similarly, the ZnO buffer layer improved the UV emission of TiO2 thin film. The band gap of ZnO and TiO2 thin film grown on buffer layer found to decrease due to improved crystallinity.
REFERENCES
[1] X. Teng, H. Fan, S. Pan, C. Ye, G. Li, Materials Letters 61 (2007) 201–204.
[2] G. C. Yi, C. R. Wang, W. I. Park, Semicond. Sci. Technol 20 (2005) S22.
[3]X. Zhang, F. Zhang, K. Y. Chan, Material Chemistry Physics 97 (2006) 384.
[4]A. B. Bodade, A. M. Bende, G. N. Chaudhari, Vaccum 82 (2008) 588.
[5] X. M. Fan, J. S. Lian, Z. X. Guo, H. J. Lu, Appl. Surf. Sci. 239 (2005) 176
[6] L. Xu, L. Shi , X. Li , Applied Surface Science 255 (2008) 3230–3234
[7] H. C. Ong, A. X. E. Zhu, and G. T. Du, Applied Physics Letter 80 (2002) 941.
[8] Y. Zhang, B. Lin, Z. Fu, C. Liu, W. Han, Optical Materials 28 (2006) 1192.
[9] H.Y. Lin, Y. Y. Chou, C. L. Cheng, Y. F. Chen, Optical Express 15 (2007) 13832.
[10] R. Ghosh, D. Basak, S. Fujihara, Journal Applied Physics 96 (2004) 2689.
 

Mechanisms for Optical Limiting

Chapter 2
2.1. Reverse Saturable Absorption
In the mid 1960s shortly after the invention of the laser, many researchers were investigating dyes for potential application to Q-switching of the laser cavity. For this application, dyes were sought that would bleach to transparency under intense illumination (saturable absorbers). Guiliano and Hess [2a] in 1967 were investigating vat dyes and their modified cousins and noted some examples that not only did not bleach to transparency but instead darkened at high intensities. This was the first recognition of the property of reverse saturable absorption (RSA).

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Reverse saturable absorption generally arises in a molecular system when the excited state absorption cross section is larger than the ground state cross section. The process can be understood by considering a system that is modeled using three vibronically broadened electronic energy levels, as shown in figure 2.1. The cross section for absorption from the ground state 1 is 1. 2 is the cross section for absorption from the first excited state 2 to the second excited state 3. The lifetime of the first excited state is 2 (seconds).

Figure 2.1: Three level and Four level models for RSA
As light is absorbed by the material, the first excited state begins to become populated and contributes to the total absorption cross section. If 2 is smaller than 1, then the material becomes more transparent or ‘bleaches’ i.e. it is a saturable absorber.
If 2 is larger than 1, then the total absorption increases, and the material is known as a reverse saturable absorber. This behavior is shown in figure 2.2

Figure 2.2: Plot of the incident intensity versus the transmitted intensity of a typical three level RSA material.
The change in intensity of a beam as it propagates through the material is:
, (2.1)
Where z is the direction traversed, NT is the total number of active molecules per area in the slice dz, N2 is the population of level 2 and the population of level 3 has been neglected.
Initially, the material obeys Beer’s law when 2 is unpopulated, and the transmission is constant as the incident fluence is increased. The slope is given by. At a sufficiently high fluence, however, the first excited state 2 becomes substantially populated and in the limit of complete ground state depletion the slope again becomes constant at the new value of. The optical limiting action is not truly limiting, as the fluence, which is transmitted, is still increasing with increasing incident fluence, but it does so more slowly. If the ratio 2/1, is sufficiently large, however, the new transmission will be small and in a properly designed system the dynamic range of the sensor will be greatly extended. The three level diagrams describe the simplest case for RSA materials but can generally only be applied for subnanosecond pulses and under circumstances such that transitions from the second excited state are negligible.
The energy states involved in three level materials usually consists of singlet states and the transitions are all allowed. The transition cross sections are therefore large, but a disadvantage is that de-excitation is rapid (2 is small). This necessitates larger intensities for long pulses to activate the nonlinearity through populating the excited electronic state. Fortunately, on longer timescales in some systems, significant intersystem crossing to other states can occur from the first excited state. In this case the five level diagrams shown in figure 2.1 is applicable. The excited state 4 is usually a triplet or other long-lived state, and for long pulses it can act as a metastable state that accumulates population during the pulse. The lifetime of 4 gives an indication of the maximum pulse width for which the material is efficient to act as an optical limiter. Pulses with duration longer than the metastable state allow some of the metastable molecules generated by the leading edge of the pulse to decay to the ground state before the trailing edge have passed, thereby reducing the RSA. In most systems, 3 and 5 are very small and significant populations of 3 and 5 do not accumulate. Therefore, N3 and N5 can be set to zero, considerably simplifying the dynamical equations describing. The equations representing the full five level models are given below by:
(2.2)

(2.3)
(2.4)
(2.5)
(2.6)
(2.7)
and
(2.8)
Where h is the energy per photon, I is the intensity of the pulse and stimulated emission has been neglected. The latter assumes that optical coupling to the excited states is well above the bottom of the vibronic manifolds and that relaxation from the optically-coupled states to the bottom of the manifolds occurs on a time scale that is much shorter than the pulse duration. To completely understand the response of an RSA device, these equations must be solved as the pulse propagates through the material. The material parameters necessary to solve the equations are 1, 2, 4, 2, 4 and 24. For optimum optical limiting performance, certain parameters need to be maximized. The ratio of the excited state absorption to the ground state, 2/1, 4/1 should be large to minimize the transmission of the limiter at high incident intensity.
For maximum efficiency, the lifetime of the triplet state (2) and the intersystem crossing rate l/24 should be large to populate the triplet state and maintain the population throughout the pulse. By the mechanism of RSA we get better performance on optical limiting.
2.2. Two-Photon Absorption (TPA):
Two-photon absorption (TPA) can also be used in a manner similar to RSA to construct optical limiters. In contrast with reverse saturable absorption, TPA is an instantaneous nonlinearity that involves the absorption of photon from the field to promote an electron from its initial state to a virtual intermediate state, followed by the absorption of a second photon that takes the electron to its final state. Since the intermediate state for such transitions is virtual, energy need not be conserved in the intermediate state but only in the final state. The mechanism of TPA can be thought of in terms of the three level RSA model for the case where the lifetime of the intermediate state approaches zero and the ground state absorption is extremely low (highly transparent). The intensity of the beam as it traverses the material is:
(2.9)
Where z is the linear absorption coefficient and  is the TPA coefficient which is related to the imaginary part of (3) by the equation (SI units):
(2.10)
Here,  is the circular frequency of the optical field, n0 is the linear index of refraction, and c is the speed of light in vacuum. The solution to the propagation equation for = 0 (transparent material at low intensities) is given by
(2.11)
Where L is the sample length. This clearly demonstrates that the output intensity decreases as the input intensity increases, exactly the behavior that is desired for an optical limiter. The strength of this reduction is explicitly dependent on the TPA coefficient, the incident intensity and the sample thickness.
For TPA, the material response is of the order of an optical cycle and is, therefore, independent of the optical pulse length for a fixed intensity. The device will respond virtually instantaneously to the pulse. On the other hand, because of the limited magnitude of  in existing materials, high intensities are required to realize significant TPA. Since the intensity is essentially the energy density divided by the pulse duration, short pulses are required to achieve limiting with TPA for energy densities that may be high enough to damage an optical sensor.
2.3. Free-Carrier Absorption:
This type of limiting occurs in semiconductor materials. Once carriers are optically generated in a semiconductor, whether by single photon or two-photon absorption, these electrons (holes) can be promoted to states higher (lower) in the conduction (valence) band by absorbing additional photons. This process is often phonon assisted, although depending on the details of the band structure and the frequency of the optical excitation, it may also be direct. The phonon assisted phenomenon is referred to as free-carrier absorption, and it is analogous to excited-state absorption in a molecular system. It is clearly an accumulative nonlinearity, since it depends on the buildup of carrier population in the bands as the incident optical pulse energy is absorbed. Free-carrier absorption always plays some role in the operation of a semiconductor limiter, if the excitation process results in the generation of significant free carrier populations in the bands. While it certainly contributes to the limiter performance and its inclusion is important in the precise modeling of the response of such devices, just as in the case of TPA, its importance typically pales in comparison with nonlinear refractive effects, whether the carriers are generated by single photon or two photon transitions.
2.4. Nonlinear Refraction
Optical limiters based on self focusing and defocusing form another class of promising devices. The mechanism for these devices may arise from nonlinear refraction associated with carrier generation by either linear or two photon absorption in a semiconductor. Both self focusing and defocusing devices operate by refracting light away from the sensor as opposed to simply absorbing the incident radiation. Compared to strictly absorbing devices, these limiters can, therefore, potentially yield a larger dynamic range before damage to the limiter itself.
Figure 2.3 (a) shows the typical device configuration for a self defocusing limiter, while figure 2.3 (b) shows a similar device based on self focusing. A converging lens is used to focus the incident radiation so it passes through the nonlinear medium. This lens provides optical gain to the system, allowing the device to activate at low incident intensities. The output passes through an aperture before impinging on the detector. At low input levels, the nonlinear medium has little effect on the incident beam, and the aperture blocks an insignificant portion of the beam, thus allowing for a low insertion loss for the device. When nonlinear refraction occurs, however, the nonuniform beam profile within the medium results in the generation of a spatially nonuniform refractive index. This acts as either a negative or positive lens, depending on the sign of the refractive nonlinearity, causing the incident beam to either defocus or focus.

Figure 2.3: (a) Typical self defocusing optical limiter configuration (b) Typical self focusing optical limiter configuration.
In a properly designed system, this self lensing results in significant energy blocked by the system aperture, thereby protecting the sensor. The location of the nonlinear medium is critical to the operation of the refractive limiting device. A self-focusing limiter works best if the nonlinear medium is placed approximately a Rayleigh range before the intermediate focus of the device. When the focusing lens is induced the effective focal length of the device is reduced, and hence a larger beam appears at the exit aperture. For a self-defocusing material, the optimum geometry is approximately one Rayleigh range after the focus. This geometry dependence can be exploited to determine not only the sign of the nonlinear refraction in a given medium, but the magnitude as well. This is the principle behind the so-called Z-scan technique, which has been pioneered by Van Stryland and coworkers [2b,2c].
The technique consists of moving the nonlinear medium through the focal region of a tightly focused beam while measuring the transmittance through an aperture placed in the far field of the focal plane. When the medium is far before the focal plane, no self-lensing occurs. As the medium approaches the focal plane, the high intensity begins to induce a lens in the medium. For a negative nonlinearity, this lens tends to collimate the beam, thereby increasing the transmittance through the aperture. Near the focal plane, even though the intensity is highest, the influence of the induced lens is minimized, resulting in a transmittance comparable to the linear transmittance. This is similar to placing a thin lens at the focus of a beam; this results in minimal effect on the far field beam pattern. As the sample is moved beyond the focal plane, the negative lens tends to increase the beam divergence, resulting in a decrease in the aperture transmittance. As the medium is moved still farther from focus, the intensity again becomes weak enough that the induced lensing is negligible. This sequence results in a change in transmittance with a characteristic peak, followed by a null, followed by a valley as the sample is moved from the input lens, through focus, toward the output lens. For a positive nonlinearity, the pattern consists of a valley, a null, and then a peak. Thus, the sign of the nonlinearity is readily determined. While nonlinear absorption has been neglected in this discussion, if present, it must also be accounted for. This is readily done by removing the aperture in the limiter and collecting all the light transmitted by the nonlinear material. This measurement is then insensitive to nonlinear refraction. The response in this case is a valley symmetrically located about the focal plane. It should be noted that nonlinear absorption and induced scattering cannot be distinguished by this technique. The general shape of the Z-scan for a positive index change, negative index change, and a nonlinear absorber or scatterer is shown in figure 2.4
.
Figure 2.4: Schematic representation of z-scan results for a negative refractive nonlinearity (dashed curve) and a positive refractive nonlinearity (dotted curve). Both curves have been corrected for absorption. The solid curve shows the result of removing the aperture from the measurement apparatus and collecting all the transmitted light, thus isolating the nonlinear absorption [1e].
2.5. Induced Scattering
Scattering roots from interaction of light with small centers which may be physical particles or simple interfaces sandwiched between non-excited and excited molecular groups. The size of the scattering centers determines whether the scattering will be quite directional or reasonably uniform. Transmission of a medium, for a given solid angle, decreases when scattering centers are induced in the medium by an optical signal. Therefore, this phenomenon of scattering induced by optical signal may be applied to manufacture of optical limiters for sensor protection. Optical limiters based on induced scattering are usually focused on liquid media, as the phenomenon is usually reversible in these media. That is to say, the liquid in the excited state can return to equilibrium with ease in the absence of chemical or structural decomposition. However, in solids, usually irreversible decomposition processes generate the scattering centers which can lead to degradation in the device’s linear operation.
When light is incident on a particle, the electric charges within the particle oscillate due to its interaction with the electric field. Radiations are then caused by the oscillation. In 1899, Lord Rayleigh originally presented the analytic expression and theory of the elastic scattering of light from particles with dimensions smaller than the wavelength of light. Rayleigh scattering is the name given to the phenomenon. This applies only to particles whose dimensions are quite smaller than the wavelength of light or which are non-absorbing. However, in 1908, Mie developed a theory for particles with dimensions comparable to the wavelength of light or greater [2d]. The transmitted intensity equations of the Mie scattering are notably more intricate than of Rayleigh scattering. In Mie scattering, a bigger percentage of the scattered radiation is in forward direction as the size of the scattering particles increases, implying that limiting based on Mie scattering will not be as effective as Rayleigh scattering.
2.6. Photorefraction
Two devices, namely coherent-beam excisor and the beam fanning limiter based on the photorefractive effect are used to limit coherent optical radiation. Materials showing photorefraction should have a nonzero χ(2). The traditional photorefractive mechanism is based on the photorefractive crystal which possesses deep levels that can be excited optically to generate free charge in the conduction or valence band. In a material showing photorefraction, when two coherent beams interfere, additional mobile charge are generated at the peaks of the intensity pattern than at the valleys through photoexcitation of the deep levels of the crsytal. These charges which are photoexcited at the peaks diffuse into the valleys ensuing a variation of charge spatially, in correspondence to the material’s interference pattern. These charges results in an electrostatic space-charge field which gives rise to a change in refractive index through the electro-optic effect in a properly oriented crystal. Energy coupling and energy exchange can then be achieved between the two beams through the grating generated, which is 90 degrees phase shifted from the intensity of the photon field.
A high intensity coherent beam when incident singly on a photorefractive crystal, the energy can be coupled into a large amount of low intensity scattered beams. Fields with new wave vectors are generated inside the crystal by the scattering of the incident beam at the crystal imperfections. The photorefractive gratings are then produced by the interference of the incident field with these scattered fields. Optical signal can later be coupled from the incident beam to the scattered beams through diffraction from these gratings. The light gets preferentially scattered to one side of the crystal as there is a preferred direction of energy transfer for photorefractive gratings which is determined by the direction of the c-axis of the crystal and the charge carriers’ sign. This photorefractive beam fanning phenomenon can be quite efficient in reducing the intensity of the transmitted beam. Construction of an optical limiter using this beam fanning process has been demonstrated by Cronin-Golomb and Yariv [2e].
The photorefractive excisor is another device which provides a weak seed beam to interfere with the incident beam. It is assembled to protect the sensor in such a way that the photorefractive grating produced by the interference of the primary beam with the seed beam at high intensities couples energy from the strong incident beam to the weak seed beam. The speed and efficiency of the device is thus improved.
2.7. Summary
All of the nonlinear phenomena discussed above can be used for optical limiting, and figure 2.5 schematically illustrates the application of some of these processes. Figure 2.5 (a) depicts the use of induced absorption, such as reverse saturable absorption, two-photon absorption, and free-carrier absorption. Figures 2.5(b) and 2.5(d) represent, respectively, a self-defocusing limiter, self-focusing limiter, and an induced scattering limiter. Finally, figures 2.5(e) and 2.5(f) illustrate a photorefractive beam fanning limiter and a photorefractive excisor device. While it is often the case that any given material will exhibit multiple nonlinear properties, for simplicity the effects of each individual process have been separately depicted in figure 2.5.
Figure 2.5: Some optical limiters based on different mechanisms (a) an induced absorption limiter (b) Self defocusing limiter (c) Self focusing limiter (d) Induced scattering limiter (e) Beam fanning limiter (f) Photorefractive excisor device [1e].
 

Quantum Optical Model Nonintegrability & Quantum Fluctuation

Nonintegrability and quantum fluctuations in a quantum optical model

Nilakantha Mehera and S. Sivakumarb

 
Abstract
Integrability in quantum theory has been defined in more than one ways. Recently, Braak suggested that a quantum system is integrable relating the number of parameters required to specify the eigenstates and the number degrees of freedom (both discrete and continuous). It is argued that the dependence of uncertainty product of suitable operators on the atom-field interaction strength is distinctly different for the integrable and nonintegrable cases. These studies indicate that the uncertainty product is able to identify the nonintegrable systems from the integrable ones in the context of this new definition.
Introduction
A classical dynamical system with n degrees of freedom (DOF) is integrable, Liouvillean integrable to be precise, if there are equal number of suitable constants of motion (COM) that have vanishing Poisson bracket among themselves and with the Hamiltonian1.. Otherwise, the system is nonintegrable. While this definition is based on a sound mathematical footing, the situation in quantum dynamics is not very clear, essentially arising from the difficulty in defining or identifying DOF in quantum theory2. One possibility is define integrablity by the existence of sufficient number of observables which commute with the Hamiltonian and pair-wise commute among themselves. However, this is wrought with difficulties as it may not be possible to arrive at classical limits of some quantum systems. One such example is the case of a single two-level atom interacting with a single mode of the electromagnetic field. The former is a discrete DOF (finite dimensional Hilbert space) and the later is a continuous DOF (infinite dimensional Hilbert space). While the continuous DOF, namely, the electromagnetic field, has a proper classical limit, the two-level atom does not have a suitable classical limit.

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According to a new definition introduced by Braak, a system is integrable if the number of parameters required to specify the eigenstates of the Hamiltonian is equal to the sum of the number of discrete DOF and continuous DOF2. This definition does not involve the existence of constants of motion, though all such cases are covered by this definition. In this new definition of integrability, some of the nonintegrable systems based on the Liouvillean definition become integrable. A simple example of such a system is the Rabi model describing the interaction between a two-level atom and a single mode of the electromagnetic field with Hamiltonian3;
(1)
Here, are Pauli matrices, is the atomic transition frequency, () denote the annihilation (creation) operators of field with frequency. is the atom-field coupling constant. This Hamiltonian has only one COM, namely itself. Since there are two DOF, the field and the two-level atom, the Hamiltonian is nonintegrable in the sense of Liouville. However, exploiting the parity symmetry in , the system has been shown to be integrable2. Another interesting case is the rotating wave approximation of , yielding the well known Jaynes-Cummings model4,5:
(2)
This Hamiltonian has two COM, the Hamiltonian itself and the operator for the number of excitations . Existence of these two COMs renders the Hamiltonian integrable. The eigenstates are labelled by two parameters,an integer n corresponding to the number of excitations and the total energy. Also, both the Hamiltonians and exhibit level-crossings of the eigenvalues as the interaction strength g is varied, which is an indication that the models are integrable2. Level-crossing refers to the phenomenon where in the eigenvalues depend on the interaction strength g in such a way that the eigenvalues corresponding to two different eigenstates become degenerate at a specific values of g and reverse their order for other values of g.
Nonintegrable Model :
An interesting modification to to make it nonintegrable is to break the 2 symmetry by adding and this leads to;
(3)
Within the scope of the Braak’s definition of integrability, this model is integrable only when ε is an integral multiple of ω/2. This is also borne out by the existence of level crossings as shown in Fig.1. This criterion is sufficient for nonintegrability. We assume resonance . For the results presented here, it is assumed that =1 and ω=1. In Fig. 1, the absence or presence of level-crossing indicates respectively the nonintegrability or integrability of the Hamiltonian .
Fig.1.Energy level (En) as a function of g for different 𝜀. Level crossing occurs if 𝜀=0 and 0.5 indicating integrability. No level crossing if 𝜀=0.3, indicating nonintegrability. Inset shows larger view of level crossing.
A pertinent question in this context is to know those features that distinguish a nonintegrable atom-field system from an integrable one. One answer to this query appears to be that uncertainty product of a pair of suitably defined operators show markedly different characteristics. Since the system is nonintegrable, it is formidable to construct an analytical solution. Therefore, extensive numerical experimentations have been carried out and the results are presented here which support the claim stated above.
Nonintegrability being a feature of the Hamiltonian, it is natural to expect that the eigenstates carry signatures revealing this feature. To explore this, we define two self-adjoint operators of the two-level atom,
, ,
where () is the atomic raising (lowering) operator. The commutation relation implies that the value of the product of uncertainties lies between 0 and 1/2. The uncertainty relation of above operators is
.
whereis expectation value in any eigenstate. In Fig. 2, the uncertainty product is plotted as a function of the atom-field interaction strength for different values of 𝜀: 𝜀= 0, 0.5 and 1.0 corresponding to the integrable case and a few other values of 𝜀 corresponding to nonintegrable case. It is seen that as the parameter g is increases, the uncertainty product attains its maximum allowed value of ½ for the integrable cases. On the other hand, for the nonintegrable cases the uncertainty product falls below the limit of ½. In order to establish that the uncertainty product is very sensitive to the nature of the the integrable and nonintegrable cases, the plots corresponding to values of 𝜀 very close to integrable cases have been chosen.

Fig.2.Uncertainty product () as a function of the atom-field coupling constant g. Different plots correspond to different values of 𝜀: integrable cases: 𝜀=0, 0.5 and 1.0, nonintegrable cases: 𝜀=-0.01,0.01, 0.49,0.51,0.2 and 0.4. In any plot, the uncertainty is plotted for the eingenstates corresponding to the first fifty eigenvalues.
For instance, in the second row in Fig. 2, the sudden change in the nature of uncertainty product as 𝜀 assumes values 0.49 (nonintegrable), 0.5 (integrable) and 0.51 (nonintegrable) respectively are shown. In order to bring out the features more clearly, the probability distribution of the uncertainty products in different eigenstates are shown in Fig 3 corresponding to the respective figures in Fig. 2. The sharply peaked probability distribution indicates integrability.
Fig.3. Probability distribution of the uncertainty product for all the eigenstates for a particular value of g, chosen to be 1.2 here. Any higher value of g yields the same results.
Summary
Identification of nonintegrability in an interacting atom-field system is possible by the concentration of uncertainty product near a particular value as the atom-field interaction strength is increased. This feature seems to be related closely to the nonintegrability, also supported the absence of level crossings. This feature has been found to be able to identify nonintegrability in many other models that have been studied. In essence, suitable uncertainty product is able to identify nonintegrability, which is often difficult to establish analytically or numerically. Nevertheless, our analyses raise some important questions for which answers are to be found: Is it possible to arrive at the existence of this feature using only the definition of nonintegrability used here? Given a Hamiltonian, how to identify the correct observables whose uncertainty product will concentrate as the interaction strength is increased? How to extend this idea if the number of atoms is larger?
References:

M.V. Berry and M. Tabor, Proc. R. Soc. A 356, 375 (1977).
D.Braak, Phys.Rev.Lett. 107, 100401(2011).
I. I. Rabi, Phys. Rev. 49, 324 (1936); 51, 652 (1937).
E. T. Jaynes and F.W. Cummings, Proc. IEEE 51, 89 (1963).
C.Gerry and P.L. Knight, Introductory Quantum Optics (Cambridge University Press, UK, 2005).

 

Optical and Surface Studies of α-Al2O3 Powders

X-Ray Diffraction, Optical and surface studies of α-Al2O3 powders synthesized via single step solution combustion method
ABSTRACT
α-Al2O3 powders were synthesized at 500 ⁰C via solution combustion synthesis (SCS) technique using urea as an organic fuel. The sample was characterized by X- ray diffraction (XRD), Optical spectroscopy and X-ray photoelectron spectroscopy (XPS) without any further thermal treatment. XRD study reveals that sample crystallized directly in the hexagonal α-Al2O3 phase from combustion reaction. Average crystallite size of 37.6 nm was calculated using Debye-Scherrer’s formula. A band gap of 5.68 eV was estimated using diffuse reflectance spectra. Under various UV excitations (260 nm and 400 nm), the sample exhibits a strong emission peak at 693 nm. For surface investigation X-ray photo electron spectroscopy of sample was carried out. XPS survey scan of α-Al2O3 reveals that no other impurity phases were present in the as synthesized sample which supports the results obtained from XRD. Further to understand the chemical states of Al and O, core level spectra of Al-2s and O-1s were studied.

INTRODUCTION

Among all the known crystallographic phases of alumina, α-Al2O3 is the only stable phase. It represents a ceramic material with a large number of technological importances. This is mainly due to its versatile properties, such as: high melting point, thermal shock resistance, excellent mechanical strength at room temperature and high temperature, large band-gap, hardness and abrasion resistance, chemical inertness [1]. These extra ordinary properties are responsible for α-Al2O3 to be used in various applications such as spark-plugs, ballistic armours [2], abrasives, bioceramics [3], cutting tools [4], electronic components and substrates [5], thermo luminescent dosimeters [6], refractory materials, composite materials [7]. Moreover the compounds and composites of α-Al2O3 also have wide range of applications in various industrial areas such as high-density ceramics [8, 9], biocompatible ceramics [10], and thermal barrier coatings with low thermal conductivities [11, 12]. The high temperature-resistant of Al2O3 coatings have various applications in space and energy production technologies [13]. Since 1961 polycrystalline transparent alumina (Al2O3) has found various optical applications [14]. Single phase α-Al2O3 nanopowders are also important component for solid state fabrication of yttrium aluminum garnet (YAG) transparent laser ceramics [15, 16].
There are several techniques used for the synthesis of α-Al2O3. In literature there are reports available for the synthesis of single-phase α-Al2O3 powders using urea [1, 17–19], carbohydrazide [20] or hydrazine [21] as fuels, without any further heat treatments. Several authors have reported two step method for the synthesis of α-Al2O3 such as reverse micelle [22], sol–gel processing [23], flame spray pyrolysis [24] which require calcinations at 1000–1100 ⁰C to obtain completely phase pure α-Al2O3.

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In the present paper, we report the optical and surface properties of as synthesized α-Al2O3 powder by the low temperature solution combustion synthesis (SCS) technique. Urea was used as an organic fuel for combustion because it has proven to be the best fuel for combustion of aluminum nitrate [1, 17].

EXPERIMENTAL

α-Al2O3 powders were synthesized by low temperature solution combustion synthesis (SCS) using urea as a fuel. The starting materials for the synthesis of α-Al2O3 were high-purity aluminum nitrate nonahydrate [Al (NO3)3.9H2O] and urea (H2NCONH2) from Merck Chemicals, India. The reagents were weighed according to the chemical reaction given in equation (1) in the molar stoichiometric ratio of 2:5. The oxidizer to fuel ratio was calculated using the oxidizing and reducing vacancies of reactants in equation (1). For a complete combustion reaction the ratio of oxidizer () and fuel () should be unity, because at this ratio maximum heat is produced.

The weighed reactants were mixed in agate mortar by adding few drops of deionized water at room temperature till the solution transform into a transparent viscous gel. The gel was transferred to a Borosil beaker and then introduced to a preheated muffle furnace around 500 ⁰C. The gel undergoes rapid dehydration with evolution of large amount of gasses and burns with an incandescent flame yielding voluminous white product. The whole combustion process was completed within 2-3 minutes. The beaker was then taken out and the resultant product was grounded into a fine powder and was characterized without any further treatment. The crystalline structure and particle morphology of the combusted powders were investigated using a standard diffractometer (Bruker D8 Advance) in the θ –2θ geometry with scanning step of 0.02⁰ and Cu Kα radiation (λ=1.5406 Å). Diffused reflectance spectrum was recorded using ISR assembly attached with Shimadzu UV-2600 Double beam spectrophotometer in the region 190-1400 nm. The spectral features like photoluminescence excitation and emission (in phosphorescence mode) spectra were measured using a Cary-Eclipse spectrofluorometer (Shimadzu) equipped with a Xenon lamp used as an excitation source. The X-ray photoelectron spectroscopy (XPS) measurement was performed using Omicron energy analyzer (EA-125) with Al Kα (1486.6 eV) X-ray source. The background vacuum in the analyzer chamber was of the order of 10-10 Torr during the XPS measurement. All these characterization were carried out at room temperature.

RESULTS AND DISCUSSION

3.1 XRD
The crystal structure and phase purity of the as synthesized Al2O3 powders were analyzed using the X-ray diffraction (XRD) technique. Fig.1. shows the XRD patterns of as synthesized Al2O3 powders recorded in a wide range of Bragg angle 2θ (20° ≤ 2θ ≤ 90°). All the observed diffraction peaks can be indexed with the hexagonal phase of bulk α-Al2O3 referenced in the JCPD’s file no. 71-1123 with space group R. No other impurity peaks were observed in the as synthesized powder neglecting the presence of any other phase other than α-Al2O3. These XRD patterns were in good agreement with the earlier published reports by Robert Ianos et al. [1] and Laishram et al. [17] for the α-Al2O3 phase. The lattice parameter calculated from XRD pattern were (a = b= 4.755 Å, c =12.985 Å) which were very close when compared with the unit cell of α-Al2O3 (a = b= 4.761 Å, c =12.99 Å, JCPDs file No. 71-1123).
The crystallite size was calculated using Debye-Scherer formula [25]

where D is the crystallite diameter, λ is the wavelength of x-ray source used (Cu Kα = 0.1506 nm), is the full width at half maxima (FWHM) of an individual peak at 2θ (where θ is the Bragg angle) and is characteristic of the instrument broadening.

Fig.1. XRD pattern of as synthesized α-Al2O3 powders at 500 ⁰C along with the stick patterns for the JCPDS file no. 71-1123
Three most intense peaks were selected for the calculation of particle size and calculated particle size for α-Al2O3 was 37.6 nm.
3.2 Spectral Study
Fig. 2 shows the diffuse reflectance and the absorption spectra of α-Al2O3. Barium sulfate (BaSO4) compound was used as a reference standard during the measurement. In both spectra a sharp band around 220 nm is observed which corresponds that light having this particular wavelength was absorbed.

Fig.2. The diffuse reflectance and absorption spectra of the α-Al2O3 powders.
Calculation of bandgap.
Kubelka–Munk (K–M) [26] theory was used for the calculation of bandgap of α-Al2O3 powders using diffused reflectance (DR) spectrum. In a DR spectrum, the ratio of the light scattered from a thick layer of sample and an ideal non-absorbing reference sample is measured as a function of the wavelength λ, [26, 27]. The relation between the DR of the sample, scattering coefficient (S) and absorption coefficient (K) is given by

where is the Kubelka–Munk or remission function.
The linear absorption coefficient α and the band gap of a material is related through the well-known relation known as Tauc relation [28]:
3
where hν is the photon energy and C1 is a constant of proportionality. When incident light is illuminated at 60⁰, the material scatters perfectly in a diffuse manner then absorption coefficient K becomes equal to 2α i.e. . Considering the K-M scattering coefficient S as constant with respect to wavelength, and using equations (2) and (3), the following expression can be written:
4
Obtaining the value of from Eq. 2 and plotting versus, the value of is obtained by extrapolating the linear fitted regions to
Figure 3 shows the square of the optical absorption times the photon energy as a function of photon energy for α-Al2O3 powders. In the present case (α-Al2O3), the band gap was estimated around 5.68 eV. Aguilar et al. [29] calculated an optical energy band gap of 5.63 eV for Al2O3 films deposited on quartz substrate.

Fig.3. Energy bandgap calculation of α-Al2O3 using K-M functions.
Photoluminescence
Fig. 4 (a) shows the photoluminescence excitation (PLE) spectra of α-Al2O3 recorded at an emission wavelength of 695 nm, the excitation spectra consists of a broad band centered at 400 nm. Fig. 4 (b) shows the PL emission spectra of α-Al2O3 monitored at excitation wavelengths of 260 nm and 400 nm respectively. An intense peak at 693 nm is observed. Similar results were also observed by Kaplyanskiiet al. [30] and Nagabhushana et al. [31] for α-Al2O3. Kaplyanskiiet al. [30] suggest that this emission peak may be due to crystal lattice belonging to the α phase of Al2O3.

Fig.4. Photoluminescence spectra of as synthesized α-Al2O3 (a) excitation recorded at λemm = 695 nm and (b) emission recorded at λext = 260 nm and 400 nm.
3.3 Surface Studies
In material science, X-ray photoelectron spectroscopy (XPS) has proved to be a powerful analytical technique that can be used to study the elemental composition and the oxidation states.
Figure 5 shows the X-ray photoelectron spectroscopy (XPS) survey scan of the α-Al2O3 powders. The XPS survey scan of the α-Al2O3 indicates that only Al, O and C are present in the sample corresponding to their binding energies. Carbon was the only impurity present in the sample which was expected. The positions of various photoemission peaks are marked in the survey scan corresponding to the elements present in the as synthesized sample. To further understand the chemical states of Al and O ions in α-Al2O3 powder we have further performed the detailed scan for O-1s and Al-2s core spectra. The value corresponding to C 1s peak (284.6 eV) was used as a reference for spectrum analysis.

Fig.5. Survey Scan of as synthesized α-Al2O3 powder.
Figure 6 shows the XPS detailed scan for the O-1s core level. The raw data was fitted with combined Gausssian – Lorentzian functions. The fitted peak shows only one prominent peak which is centered at 529.70 eV and is attributed to the Al-O bonding in the α-Al2O3 structure. Figure 7 shows the narrow scan for the Al 2s core level. Only one peak is observed after fitting which is centered at 118.95 eV. These narrow scan spectra of O-1s and Al-2s shows that all the O2- ions are bonded to Al3+ ions in the sample. Thus the chemical state of Oxygen and Aluminum is -2 and+3 respectively in the lattice. Rotole et al. [32] observed O-1s peak at 530.68 eV and Al-2s peak at 118.93 eV for standard α-Al2O3. The difference in the binding energies may be due to the highly insulating nature of the sample.

Fig. 6 XPS core level spectra of O 1s in α-Al2O3 powder.

Fig. 7 XPS core level spectra of Al 2s in α-Al2O3 powder.

Conclusion

In summary, the α-Al2O3 powders were successfully prepared by low temperature solution combustion method with metal nitrate reactants and urea as organic fuel. The XRD results confirm that hexagonal phase of α-Al2O3 could be obtained directly by combustion method at 500 ⁰C without any further treatment. The band gap of sample was calculated using diffused reflectance spectra and it was found estimated 5.68 eV. Under UV excitations, the powders exhibit a strong emission peak around 693 nm. XPS results show that as synthesized powders were free from impurities. The core level spectra of Al-2s and O-1s reveals that chemical state of Al and O is +3 and +2 respectively in α-Al2O3.
ACKNOWLEDGMENTS
The authors humbly acknowledged Director, UGC-DAE CSR Indore for providing experimental facility. The authors are thankful to Dr. M. Gupta for XRD measurements. The authors are grateful to Mr. A. Wadikar for helping in XPS measurements.
REFRENCES

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Optical Absorption of a Gallium Arsenide Semiconductor

Abstract

In this report, an investigation into the light absorption of a direct band gap semiconductor, GaAs is recorded. By subjecting the GaAs semiconductor to a range of infra-red wavelengths and then comparing the light transmission through the semiconductor to the light transmission when no semiconductor was present, the following experimental values were obtained: the characteristic photoreflectance; the characteristic band gap energy; and the characteristic absorption edge width. The first of these was obtained by the analysis of data when the semiconductor was transparent, and the two energy values by analysis of the Urbach tail region for said GaAs semiconductor. The absorption coefficient as a function of wavelength will also be determined. This general area of research facilitates the use of complex circuitry to make possible the creation, and thus application, of copious electrical equipment, from digital watches to racing cars and beyond.

Introduction and theory

Somewhat disputedly, the effects of what are now known as semiconductors appear to have been first noted by physicist Thomas Johann Seebeck in 1821[1]. However, the first record of the significant consequence of these materials’ existence is often attributed to Michael Faraday for his 1833 account of how a compound of silver sulphide decreases in resistance when heated, contrary to other metals previously tested[2]. Similarly, in 1873, Willoughby Smith noted the decreasing of resistance of selenium rods in the presence of strong light[3]. The understanding of the theory of semiconductors made great progress after JJ Thomson’s discovery of the electron in 1897[4], when electron-based conduction was proposed, enabling a much better understanding of these semiconductors. A while later, shortly following Bernhard von Gudden’s 1930 theory that semiconductor conductivity came from small concentrations of impurities, the theory of band gaps was formulated by Alan Herries Wilson, thus confirming Gudden’s finding[5]. The ensuing advancements in both solid state and quantum physics led to a much more fundamental understanding of semiconductors and was instrumental in the development the physics used in this investigation.

A semiconductor is defined as a substance that is capable of conducting electricity, but only under certain conditions (prescribed by the material properties of said substance), which renders a semiconductor effective in its primary use of controlling electrical current levels.[6] Semiconductor theory relies on a combination of the aforementioned disciplines of solid state and quantum physics which when combined and applied, give band theory. Band theory states that, in semiconductors, electrons’ energy levels are sufficiently close to each other as to form so-called ‘energy bands’, each formed of many energy levels, all very close to each other[7]. Consider three varieties of substance, namely insulators, conductors and semiconductors. In conductors, such as most metals, the energy band gap,
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Wavelength_up = uicontrol(‘Style’,’pushbutton’,’String’,’Up’,…

    ‘Position’,[100 120 70 40],’Callback’,@go_up);

Wavelength_down = uicontrol(‘Style’,’pushbutton’,’String’,’Down’,…

    ‘Position’,[100 20 70 40],’Callback’,@go_down);

s.Rate = 1000

s.NumberOfScans=500;

nout = [51 102 204 153]; % decimal sequence for forward motion

% This is the callback function for the toggle button.

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% ‘hObject’ is the handle for the uicontrol calling the function.

    function wavelength(hObject,eventdata)

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        while hObject.Value == hObject.Max

            for m=1:3

            for n=1:4

                input_data=dec2binvec(nout(n),8);

                % high state=1 low state=0

                outputSingleScan(s1,input_data);

                % outputs the data in output_data to the device

                pause(0.3)

            end

            end

            [data,timestamps] = startForeground(s);

            V(b) = (mean(data(:,1)));

            f(b)=(5/3)*b+(800-(5/3))

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        plot(f,V,’xb-‘);

        end

        figure

       dlmwrite(‘E:\Project\data\V.txt’,V);

        dlmwrite(‘E:\Project\data\f.txt’,f);

    end

% These are the callbacks for the pushbuttons.

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    function go_up(hObject,eventdata)

        nout = [51 102 204 153];

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    function go_down(hObject,eventdata)

         nout = [153 204 102 51];

     end

end