The combination of the arsenic trisulfide (As2S3) waveguide and titanium diffused lithium niobate (Ti:LiNbO3) waveguide provide us compact and versatile means for transmitting and processing optical signals, which benefits from the high index contrast between these two materials and the electro-optical properties of Ti: LiNbO3. Furthermore, waveguide gain is introduced through selective surface erbium (Er) doping which yields high quality loss-compensated or even amplifying waveguides without disturbing the excellent electrooptical, acoustooptical and nonlinear properties of the waveguide substrate LiNbO3. The integration of these waveguides allows the development of a whole class of new waveguide devices of higher functionality and complexity. As one kind of the hybrid waveguide devices, a new configuration consisting of an As2S3 channel waveguide on top of an Er doped titanium diffused x-cut lithium niobate waveguide has been investigated by simultaneous analytical expressions, numerical simulations, and experimentation. Both simulation and experimental results have shown that this structure can enhance the optical gain, as predicted by the analytical expressions. An As2S3 channel waveguide has been fabricated on top of a conventional Er:Ti:LiNbO3 waveguide, where the higher refractive index As2S3 waveguide is used to pull the optical mode towards the substrate surface where the higher Er concentration yields an improved propagation gain. The relationship between the gain and As2S3 layer thickness has been evaluated and the optimal As2S3 thickness was found by simulation and experimentation. Side integration was applied to reduce the extra propagation loss caused by the titanium diffusion bump. The propagation gain (dB/cm) has been improved from 1.1 to 2 dB/cm. Another hybrid device which combines the As2S3 and LiNbO3 is to make an As2S3 racetrack ring resonator on top of an x-cut y-propagation Er:Ti:LiNbO3 waveguide which is the potential structure for integrated lossless all-path filter. The ring was side-coupled with the Ti:LiNbO3 waveguide and the optical gain was achieved when the 5mm long coupling region where has been diffused with Er in advance pumped by 144mW pump laser. The free spectral range (FSR) of the measured ring response for TM mode is 0.0587nm (7.33GHz) at 1550nm. The roundtrip loss are 4.4dB (2.60dB/cm) when pump on and 5.8dB (3.44dB/cm) when pump off. The optical gain in the Er diffused area is 0.72dB/cm.

Availability of amorphous semiconductors in the form of high quality multilayers provide potential applications in the field of micro- and optoelectronics. Although the misfit problems in amorphous multilayers (AML) are considerably reduced compared to crystalline superlattices, there are still some physical processes (e.g. quantum confinement, diffusion) that are not well understood. Recently chalcogenide glass multilayers were prepared with high quality nanomodulation, which demonstrated their potential for tailoring the optical properties. Moreover studies on amorphous nanolayered chalcogenide structures (ANC) are still at the infant stage. These ANCs are similar to the crystalline superlattices, yet distinct from ideal crystalline superlattices produced by molecular beam epitaxy. The ANCs can be considered as well-correlated layers with good periodicity and smooth interface. They are attractive because of the prominent photoinduced effects, similar to those exhibited by uniform thin films. For example, photoinduced diffusion in short period multilayer systems is important because of its potential applications in holographic recording and fabrication of phase gratings. In spite of its practical usefulness, the mechanism of photoinduced interdiffusion is not properly understood. Since most structural changes are related to atomic diffusion, understanding of the structural transformation must be based on the diffusion process. Moreover, in AML, the process of interdiffusion is not well understood. The main aim of this thesis is to study the photoinduced interdiffusion in Se/As2S3 and Bi/As2S3 nanolayered films. In literature, there are reports about the photoinduced interdiffusion in Se/As2S3 and Bi/As2S3 nanolayered films, but the mechanisms of photoinduced interdiffusion of these elements are not properly understood. Raman scattering and infrared spectroscopy techniques were used to study the photoinduced interdiffusion in Se/As2S3 and Bi/As2S3 nanolayered films by Kikineshi et al, but the results were not satisfactory. The characteristic spectra of components in the multilayer and those of the mixed layer were rather similar. In the present thesis, photoinduced interdiffusion in Se/As2S3 and Bi/As2S3 nanolayered samples are studied by optical absorption spectroscopy, X-ray photoelectron spectroscopy (XPS) and Photoluminescence (PL). The detailed information about the distribution of electronic states in the absorption edge, localized states and the new bonds formed between the components due to photoinduced interdiffusion elucidated from the above studies will give more insight into the mechanism and kinetics of photoinduced interdiffusion. The thesis consists of six chapters. References are given at the end of each chapter. Various general and unique physical properties of amorphous chalcogenides are discussed in Chapter 1. This chapter summarizes the fundamental aspects of amorphous state, such as the structure and its models, electronic band structure, defects as well as the physical properties like d.c conductivity, a.c conductivity, optical absorption, photoconductivity and PL. A more detailed account of the various photoinduced effects are also discussed. Apart from this, similar photoinduced effects observed in other systems like a-Si:H, oxide glasses, Polymers etc are described in brief. Finally, the scope of present investigations is furnished. Chapter 2 has been devoted to photoinduced interdiffusion and related changes in optical properties of nanolayered Se/As2S3 films. It begins with a brief introduction followed by a survey of the earlier work done on these multilayered films. The theory of optical absorption and experimental procedures are discussed. Photoinduced interdiffusion was observed with above band gap light in nanolayered Se/As2S3 films. It is discussed in terms of the optical parameters such as bandgap, Urbach edge (Ee) and Tauc’s parameter (B1/2). From the analysis of the optical absorption spectra, it was concluded that the optical bandgap, Ee and B1/2 change with photoinduced interdiffusion. These changes in properties are ascribed to the solid solution formation due to the intermixing of adjacent layers. The photoinduced intermixing of the adjacent layers are obviously related to the photoinduced viscous flow and it depends on the number of excited chalcogen bridge atoms, which determine the local deformations due to the bond switching and displacements. Experimental data of B1/2 and Ee for as prepared samples do not show a clear correlation implied by the Mott-Davis model. It is also observed that the optical parameters can be changed with a change in the Se sublayer thickness. Variations of these optical parameters as a function of modulation period and photoinduced interdiffusion were discussed in terms of the quantum confinement effect and changes in the valence and conduction bands. Chapter 3 deals with the PL studies on as prepared and irradiated samples of Se/As2S3 nanolayered films. The theory of PL, experimental procedures and data analysis are discussed in detail. PL studies were carried out on as prepared and irradiated nanolayered samples of Se/As2S3 films. None of the samples showed PL at 77 K, which clearly indicate that there exists a competitive non-radiative mechanism. We observed a broad PL in the range of 0.8–1.2 eV for as prepared and irradiated samples at 4.2 K. The observed stoke shift in PL is discussed in terms of the strong electron-phonon coupling at the recombination centers. We found that the PL intensity can be increased by several orders of magnitude by irradiating the samples with appropriate wavelengths in the range of the absorption edge. The broadening of luminescence bands takes place either with a decrease in Se layer thickness or with irradiation. The former is due to the change in interface roughness while the latter is due to photoinduced interdiffusion. Deconvolution showed that the PL spectrum consists of five transitions. The deconvoluted peak PL intensity, PL quantum efficiency and full width at half maximum are varying according to the function of sublayer thickness and interdiffusion. All these results indicate the high impact of interdiffusion on the luminescence intensity in the given AML is due to changes of defect states, which in turn are not directly connected to the band structure, i.e., confinement effects are not essential for this type of luminescence. The whole picture is complex due to more complicated carrier relaxation and recombination process, possibly with several interconnected effects, which are not properly understood, but the possibility for tuning the optical parameters of the Se/As2S3 nanolayered films, including the low temperature luminescence, is established. Chapter 4 is on kinetics and chemical analysis of photoinduced interdiffusion in nanolayered Se/As2S3 films. The basic formalism of X-ray photoelectron spectroscopy and in situ optical absorption spectroscopy together with a brief description of the theory and data analysis adopted in the present studies are given. We have studied the kinetics of photoinduced interdiffusion in nanolayered Se/As2S3 film by in situ optical absorption measurements. All previous measurements were performed ex situ, i.e., a film exposed under light irradiation during the measurement was never studied. In situ changes in the transmission spectra were measured, but at a fixed wavelength. Since the measurements were done on a single wavelength, the kinetics of the variation of optical bandgap and Tauc parameter were missing. In short, information has been missing about the metastable changes in the multilayer structure during photoinduced interdiffusion. In situ changes in transmission spectra were recorded over the wavelength range λ=400-1000 nm, and also at fixed wavelengths to understand the changes in absorption coefficient, optical bandgap and Tauc parameter during photoinduced interdiffusion. The in situ optical absorption measurements reveal that the photo darkening in amorphous nanolayered Se/As2S3 film is followed by photoinduced interdiffusion. An increase in disorder during photodarkening and its subsequent decrease during photoinduced interdiffusion was also observed. The observation of photodarkening of Se at room temperature when confined between As2S3 layers suggests that the glass transition temperature of Se shifts to higher temperature. The analysis shows that the atoms, which take part in photodarkening, play a vital role in photoinduced interdiffusion. We used XPS to analyze the new bonds formed between the components due to photoinduced interdiffusion. The XPS results showed that there is a considerable decrease in the As-S, As-As and S-S bonds after photoinduced interdiffusion; As-O and some of the S-S homopolar bonds are retained. There was a considerable decrease in As-S bond followed by an increase in As-Se and S-Se bonds. XPS analysis also shows that during photodiffusion, heteropolar bonds replace homopolar bonds, i.e., the irradiated samples are chemically ordered than the corresponding as prepared samples. Chapter 5 is concerned with the photoinduced interdiffusion in Bi/As2S3 nanolayered films. A brief description about the photoinduced interdiffusion of metals such as Ag, Zn, etc is given in the introduction. The experimental procedures and data analysis are also given. Two sets of samples with different ratios of sublayer thickness (d), d-Bi/d-As2S3 = 1/12 and 1/6 prepared by cyclic thermal evaporation are employed for the present study. A pump probe optical absorption technique was used to study the photoinduced interdiffusion in Bi/As2S3 nanolayered samples. Photoinduced interdiffusion of Bi into As2S3 was observed in both the films. The XPS analysis shows that the as prepared samples contain a large number of wrong As–As bonds and some of the As-As bonds are converted to As-S bonds during irradiation. The XPS analysis also reveals that the Bi is forming only bond with S during photoinduced interdiffusion. Chapter 6 summarizes the essential features of the present work and also points a few possible directions along which further work can be carried out.

It is well known that thermally evaporated a-As2S3 thin films are prone to oxidation when exposed to an ambient environment. These As2O3 crystals are a major source of scattering loss in sub-micron optical integrated circuits. Magnetron sputtering a-As2S3 not only produces films that have optical properties closer to their equilibrium state, the as-deposited films also show no signs of photo-decomposed As2O3. The TM propagation loss of the as-deposited As2S3-on-Ti:LiNbO3 waveguide is 0.20 plus/minus 0.05 dB/cm, and it is the first low loss hybrid waveguide demonstration. Using the recipe developed for sputtering As2S3, a hybrid Mach-Zehnder interferometer has been fabricated. This allows us to measure the group index of the integrated As2S3 waveguide and use it in the study of the group velocity dispersion in the sputtered film, as both material dispersion and waveguide dispersion may be present in the system. The average group index of the integrated As2S3 waveguide is 2.36 plus/minus 0.01. On-chip optical amplification was achieved through thermal diffusion of erbium into X-cut LiNbO3. The net gain measured for a transverse magnetic propagation mode in an 11 μm wide Er:Ti:LiNbO3 waveguide amplifier is 2.3 dB plus/minus 0.1 dB, and its on-chip gain is 1.2 plus/minus 0.1 dB/cm. The internal gain measured for a transverse electric propagation in an 7 μm wide Er:Ti:LiNbO3 waveguide amplifier is 1.8 dB plus/minus 0.1 dB and is among the highest reported in the literature. These gains were obtained with two 1488 nm lasers at a combined pump power of 182mW. In order to increase further the on-chip gain, we have to improve the mode overlap between the pump and the signal. This can be done by doping erbium into As2S3 film using multi-layer magnetron sputtering. The Rutherford backscattering spectroscopy shows that the doping of Er:As2S3 film with 16 layers of erbium is homogeneous, and Raman spectroscopy confirms no significant amount of Er-S clusters in the sputtered film. The deposition method was used to fabricate an Er:As2S3 waveguide, and the presence of active erbium ions in the waveguide is evident from the green luminescence it emitted when it was pumped by 1488 nm diode laser.

Chalcogenide glasses have attracted considerable attention due to their infrared transparency, low phonon energy, and high non linear optical properties. They have been explored as promising candidate for optical memories, gratings, switching devices etc. Because of their low phonon energy and high refractive indices, now a days these are used for high efficiency fibre amplifiers. Nevertheless, the availability of amorphous semiconductors in the form of high quality multilayers provides potential applications in the field of micro and optoelectronics. Among amorphous multilayers, chalcogenide multilayers are attractive because of the prominent photoinduced effects. Studies in chalcogenide amorphous multilayer have been directed towards two phenomena. One is photoinduced interdiffusion in short period multilayer systems which finds potential applications in holographic recording and fabrication of phase gratings . The other is photo darkening or photobleaching which is also known in thick films. These multilayers exhibit prominent photoinduced effects, similar to those exhibited by uniform thin films. In spite of its practical usefulness, the mechanism of photoinduced interdiffusion is not properly understood. Since most structural transformations are related to atomic diffusion, understanding of the structural transformation must be based on the diffusion process. The main aim of this thesis is to study the photoinduced diffusion in Sb/As2S3 multilayered films and (As2S3)1-xSbx thin films. In literature, there are reports about the photoinduced interdiffusion in Se/As2S3 and Bi/As2S3 multilayered films, but the mechanisms of photoinduced interdiffusion of these elements are not very clear. Raman scattering and infrared spectroscopy techniques have been used to study the photoinduced interdiffusion in Se/As2S3 and Bi/As2S3 multilayered films by Malyovanik et al. (M. Malyovanik, M. Shiplyak, V. Cheresnya, T. Remeta, S. Ivan, and A. Kikineshi, J. Optoelectron. Adv. Mater. 5, 397 (2003). But many questions remain unanswered. The characteristic spectra of components in the multilayer and those of the diffused layer were rather similar. In the present thesis, photoinduced interdiffusion in Sb/As2S3 multilayered samples are studied by Fourier Transform Infrared spectroscopy (FTIR) at room and low temperature and X-ray photoelectron spectroscopy (XPS). The photoinduced effects in (As2S3)1-xSbx thin films are studied by FTIR, XPS and Raman Spectroscopy. The detailed information about the distribution of electronic states in the absorption edge, localized states and the new bonds formed between the components due to photoinduced interdiffusion elucidated from the above studies will give more insight into the mechanism and kinetics of photoinduced interdiffusion. The thesis consists of seven chapters. References are given at the end of each chapter.