Study of the dynamics and mechanisms of deformation of thin chalcogenide As(Ge)<Subscript>2</Subscript>Se<Subscript>3</Subscript> films by nanoindentation

The mechanical properties of As₂Se₃ and Ge₂Se₃ thin films have been studied by the method of quasi-static nonoindentation. The mechanisms of formation and recovery of the indentations in the studied materials have been analyzed under conditions of their local loading. It has been revealed that the deformation mechanism of the chalcogenide films changes in going from As₂Se₃ to Ge₂Se₃. It has been found that, during deformation of the As₂Se₃ film under the indenter, the accumulation of plastic deformation prevails, and, for the Ge₂Se₃ film, the substantial mechanism is the relaxation of its deformation.     

Giant photoplastic effect in vitreous semiconductors near the rigidity percolation transition

The negative giant photoplastic effect (giant photosoftening) has been experimentally observed in the As-Se system when films obtained by thermal evaporation of As₂₀Se₈₀ chalcogenide glass are irradiated by light from the region of the fundamental absorption edge. Correlation has been found between the photoplastic effect and rigidity percolation transition in the As-Se chalcogenide glass matrix. Such a correlation is not revealed when light irradiation changes the optical properties of these glass films. It has been shown that a nonlinear (non-Hookian) mechanism of the formation of the strain response is realized in the films subjected to the combined action of light and external mechanical loading.     

Photoinduced phenomena in mAs2S3·nAs2Se3 amorphous films and their application in micro-optics technology

Photoinduced phenomena, such as photodarkening, photorefraction, and photodissolution, were studied in different compositions of three-component mAs₂S₃·nAs₂Se₃ amorphous films. The main emphasis was on the effect of photoinduced change of dissolution rate since this effect is the basis of many applications of amorphous chalcogenide films. The results of the investigation were compared with photoinduced effects in binary As₂S₃ and As₂Se₃ films. Advanced micro-optical devices: micro-lens and micro-mirror arrays, diffractive gratings, and photonic bandgap crystals, based on three-component amorphous films which possessed optimal photodissolution characteristics were developed. The primary parameters of the micro-optical devices developed are discussed.     

Photoinduced absorption edge shift of As20Se60Tl20 films

As₂₀Se₆₀Tl₂₀ bulk material was prepared firstly then thermal evaporation method was used to prepare As₂₀Se₆₀Tl₂₀ films. Illumination effect at room temperature for the mentioned films has been studied within the homogeneous glass-forming region. The prepared film samples were amorphous, this is depicted by X-ray chart. Optical properties have been studied using spectrophotometric measurements. The absorption edge was shifted to shorter wavelengths due to light exposure or photobleaching effect. It was found that the extinction coefficient and the optical band gap have opposite character after illumination.     

硫系非晶半导体薄膜中的超快光 Kerr效应

利用飞秒激光超外差光Kerr(OHD-OKE)技术研究了As₂S₃, As₂Se₃, GeS₂, GeSe₂, Ge₂₀As₂₅S₅₅, Ge₂₀As₂₅Se₅₅, Ge₁₀As₄₀S_{20关键词： 全光开关 硫系非晶半导体薄膜 飞秒激光超外差光Kerr(OHD-OKE) 三阶非线性}     

Quantum efficiency of light-induced defect creation in hydrogenated amorphous silicon and amorphous As2Se3

The quantum efficiency (QE) of light-induced metastable defect creation in hydrogenated amorphous silicon (a-Si?:?H) and amorphous As₂Se₃ (a-As₂Se₃) by bandgap and subgap illumination has been deduced from photocurrent measurements. The QE decreases with increasing number of absorbed photons. A higher QE for a-As₂Se₃ than for a-Si?:?H has been observed and this is interpreted in terms of the higher structural flexibility of a-As₂Se₃. We have also found that, for both materials, subgap illumination yields a higher QE than does bandgap illumination.     

Reversible photo-induced volume changes in evaporated As2S3 and As4Se5Ge1 films

Reversible photo-induced volume changes have been observed accompanying reversible optical absorption edge shift produced by successive cycles of band gap illumination and annealing for well-annealed evaporated As₂S₃ and As₄Se₅Ge₁ films. The As₂S₃ film shows an increase in its volume by illumination, while As₄Se₅Ge₁ film shows a decrease. Qualitative discussion has been given in connection with pressure-induced optical constant change.     

Effect of age in vitreous As2Se3 thin films

In this paper are reported the studies of structural changes developed in the photoconductivity response spectrum of amorphous As₂Se₃ thin films with their age. Thin films were prepared by the thermal evaporation of amorphous As₂Se₃. Also reported are the X-ray diffraction studies. An explanation of the phenomenon in terms of structural changes has been attempted.     

Kinetics of photo‐darkening and self‐bleaching in amorphous As2S3 and As2Se3 thin films

The kinetics of photo‐darkening of amorphous As₂S₃ and a‐As₂Se₃ thin films follows a single exponential, but the magnitude and the rate of the process is higher in case of As₂S₃. The kinetics of self‐bleaching (dark relaxation) in advance photo‐darkened state follows a stretched exponential (SRE) with different stretching parameter for a‐As₂S₃ and a‐As₂Se₃. Within the J. C. Phillips approach we suppose that photo‐darkening in amorphous As₂S₃ films is, to some extent, accompanied by changes in short‐range order interactions, while photo‐darkening of amorphous As₂Se₃ is accompanied rather by changes in Coulomb interactions. The self‐bleaching process reduced the magnitude of photo‐darkening up to 45% and 60% for amorphous As₂S₃ and As₂Se₃ films, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Spectral distribution of steady-state photoconductivity in amorphous As2Se3

The spectral distribution of steady-state photoconductivity has been examined for a series of bulk and evaporated samples of amorphous As₂Se₃. All bulk samples, irrespective of preparation technique or sample treatment, show a distinct shoulder in the photocurrent spectral distribution at energies near 1.4 eV. This feature is interpreted as spectroscopic evidence for the existence of a well-defined defect level in the gap of amorphous As₂Se₃. Evaporated As₂Se₃ films do not show any structure in the spectral response.     

Thickness distribution of thin amorphous chalcogenide films prepared by pulsed laser deposition

Amorphous chalcogenide thin films were prepared from As₂Se₃, As₃Se₂ and InSe bulk glasses by pulsed laser deposition using a KrF excimer laser. Thickness profiles of the films were determined using variable angle spectroscopic ellipsometry. The influence of the laser beam scanning process during the deposition on the thickness distribution of the prepared thin films was evaluated and the corresponding equations suggested. The results were compared with experimental data.     

Photo-induced nonthermal melting of amorphous chalcogenides observed by pump and probe x-ray absorption spectroscopy

A new kind of melting phenomenon which is not based on thermal excitation has been observed. X-ray absorption spectroscopy (XAS) experiments under optical pumping provide a “snap-shot” information on the local structure under excitation. We have studied the local structure of chalcogenide glasses such as vitrious selenium and As₂Se₃ under optical excitation and confirmed the local melting phenomenon under light illumination at low temperature. The photo-induced nonthermal melting (PNM) in chalcogenide glasses is interpreted as the result of pairing of excited lone pair electrons during the illumination. Trapped states in this photo-assisted metastable phase result in a local structural disorder which is partially quenched at room temperature. The increased short-range disorder causing Coulomb repulsion is the origin of red shift of the absorption coefficient known as the photodarkening effect. We found that the bond alternation of chalcogens occurs during the photo-excitation.     

Variation of Raman spectrum after optical excitation in amorphous As2Se3 at 1.6°K

Several experiments, involving electronic states have shown that amorphous As₂Se₃ is modified at low temperature by the action of not very strongly absorbed light (α～10²cm^?1). This paper presents Raman scattering results before and after illumination. The shape of the Raman spectrum is changed by illumination: its high energy side has grown near 260 cm^?1. This light induced modification of the vibrational states is discussed within the framework of a previous qualitative model involving changes in local atomic configuration.     

Effect of nonlinear refraction and two-photon absorption on the optical limiting in amorphous chalcogenide films

This paper reports on the results of investigations into the nonlinear optical characteristics of chalcogenide films (As₂S₃, As₂₀S₈₀, 2As₂S₃/As₂Se₃, 3As₂S₃/As₂Se₃). The nonlinear refractive indices and two-photon absorption coefficients for these films are measured using the Z-scan technique at wavelengths of a picosecond Nd: YAG laser (λ=1064 and 532 nm). The optical limiting due to Kerr-type nonlinearities is analyzed.     

Photo-induced effects on structural and optical properties of Ga15Se81Ag4 chalcogenide thin films

Thin films with thickness of 400 nm have been obtained from the Ga₁₅Se₈₁Ag₄ ternary chalcogenide glass prepared by the melt quenching technique. The behavior of several optical constants has been studied from absorption and reflection spectra as a function of photon energy in the wavelength region 400–1200 nm. The amorphous nature of the sample was examined by X-ray diffraction and non-isothermal DSC measurements. Thin films were illuminated by shining white light using 1500 W tungsten lamp with different exposure time. The ambient temperature during the illumination process was controlled and kept at 348 K, selected by DSC thermogram. Analysis of the optical absorption data shows that the rule of non-direct transition predominates. It is found that the optical band gap decreases by increasing the illumination time. It has also been observed that the value of absorption and extinction coefficients increases while the refractive index decreases by increasing the illumination time from 0 to 150 min. The decrease in optical band gap is explained on the basis of the change in nature of the films, from amorphous to crystalline state, with increase of the illumination time.     

Design of highly birefringent chalcogenide glass PCF: A simplest design

In this article, a new simplified structure of a highly birefringent chalcogenide As₂Se₃ glass photonic crystal fiber (PCF) is designed and analyzed by using fully vectorial finite element method. The effective indices, confinement loss, birefringence, and chromatic dispersion of fundamental polarized mode are calculated in the proposed PCF for a wide wavelength range. To maintain the polarization in chalcogenide As₂Se₃ glass PCF, we enlarged two of the central air holes and reduced two transverse air holes for achieving high birefringence. This helps in creating an effective index difference between the two orthogonal polarization modes. It is also shown that As₂Se₃ glass PCF provides lower chromatic dispersion and less confinement loss compared to silica PCF of the same structure in wavelength range 1.3 to 1.8 μm and hence such chalcogenide As₂Se₃ glass PCF have high potential to be used in dispersion compensating and birefringence application in optical communication systems. In addition to this, the polarization mode dispersion (PMD) result of the proposed PCF is also reported.     

Structural,optical and electrical properties of laser irradiated Pb doped Ga–Se chalcogenide thin films

Using the Transverse Electrical Excitation at Atmospheric Pressure (TEA) nitrogen laser, we had irradiated the amorphous thin films of Ga₁₀Se₈₁Pb₉ chalcogenide glass and the results have been discussed in terms of the structural aspects of Ga₁₀Se₈₁Pb₉ glass. The observed changes are associated with the interaction of the incident photon and the lone-pairs electrons which affects the band gap. The X-ray structural characterization revealed the amorphous nature of as prepared films and polycrystalline nature of the laser irradiated films. The optical band gap of these thin films is measured by using the absorption spectra as a function of photon energy in the wavelength region 400–1200 nm. It is found that the optical band gap decreases while the absorption coefficient increases with increasing the irradiation time. The decrease in the optical band gap has been explained on the basis of change in nature of films, from amorphous to polycrystalline state, with the increase in exposure time. The dc conductivities and activation energies of these thin films are measured in temperature range 303–403 K. It has been found that the activation energy in Ga₁₀Se₈₁Pb₉ chalcogenide thin films decreases whereas the dc conductivity increases at each temperature by increasing the irradiation time.     

Investigation of nonlinear optical parameters of zinc based amorphous chalcogenide films

Third order nonlinear optical properties of amorphous Zn_x–S_y–Se_100−x−y chalcogenide films have been investigated using single beam transmission z-scan technique at 1064 nm of Nd:YAG laser. Measurement of optical properties of amorphous Zn_x–S_y–Se_100−x−y chalcogenide films prepared by thermal evaporation technique has been made. X-ray diffraction patterns of chalcogenide films confirm the amorphous nature. Optical band gap (E_g) has been estimated using Tauc's plot method from transmission spectra that is found to decrease with increase in content due to valence band broadening and band tailing the system. Nonlinear refractive index (n₂), nonlinear absorption coefficient (β) and third order nonlinear susceptibility (χ³) of chalcogenide films have been estimated. Self-focusing effect has been observed in closed aperture and reverse saturable absorption in open aperture scheme. Limiting threshold and dynamic range have been calculated from optical limiting studies. The increase in nonlinearity with increase in Zn content has been observed that is understood to be due to decrease in band gap on Zn doping. High nonlinearity makes these films a potential candidate for waveguides, fibers and two photon absorption in optical limiters.     

Dispersion characteristic of hexagonal and square lattice chalcogenide As2Se3 glass photonic crystal fiber

In this paper, we report a chalcogenide As₂Se₃ glass photonic crystal fiber (PCF) for dispersion compensating application. We have used the improved fully vectorial effective index method (IFVEIM) for comparing the dispersion properties (negative and zero dispersion) and effective area in hexagonal and square lattice of As₂Se₃ glass PCF using different wavelength windows. It has been demonstrated that due to their negative dispersion parameter and negative dispersion slope in wavelength range 1.2-2.5 μm, both lattice structures of As₂Se₃ glass PCFs, with pitch (Λ = 2 μm), can be used as dispersion compensating fibers. Further, design parameters have been obtained to achieve zero dispersion in these fibers. It is also shown that As₂Se₃ glass PCF provides much higher negative dispersion compared to silica PCF of the same structure, in wavelength range 1.25-1.6 μm and hence such PCF have high potential to be used as a dispersion compensating fiber in optical communication systems.     

Low frequency Raman spectra of chalcogenide glasses

Low frequency (< 30 cm^-1) Raman scattering lineshapes for the chalcogenide glasses As₂S₃ and As₂Se₃ have been interpreted in terms of a model which explicitly includes acoustic (∼ ω2 density-of-states) and optic (rigid-layer like) vibrational modes.