Ultrafast non-linear optical properties of Ge20As25Se55 chalcogenide films

The non-resonant third-order non-linear optical properties of amorphous Ge₂₀As₂₅Se₅₅ films were studied experimentally by the method of the femtosecond optical heterodyne detection of optical Kerr effect. The real and imaginary parts of complex third-order optical non-linearity could be effectively separated and their values and signs could be also determined, which were 6.6 × 10⁻¹² and −2.4 × 10⁻¹² esu, respectively. Amorphous Ge₂₀As₂₅Se₅₅ films showed a very fast response in the range of 200 fs under ultrafast excitation. The ultrafast response and large third-order non-linearity are attributed to the ultrafast distortion of the electron orbitals surrounding the average positions of the nucleus of Ge, As and Se atoms. The high third-order susceptibility and a fast response time of amorphous Ge₂₀As₂₅Se₅₅ films makes it a promising material for application in advanced techniques especially in optical switching.     

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.     

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.     

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.     

Influence of Ge addition on the optical properties of As40Se50Ge10 thin film probed by spectroscopy techniques

The thin films of As₄₀Se₆₀ and As₄₀Se₅₀Ge₁₀ were prepared on glass substrates by thermal evaporation method with thickness 1000 nm. The prepared films were amorphous in nature which was confirmed through X-ray diffraction. The chemical composition and the surface picture were obtained from energy dispersive X-ray analysis and Scanning Electron Microscopy analysis. The transmission data of the two films were collected in the wavelength range 400–1000 nm. The transmission percentage is found to be decreased whereas the absorption coefficient is increased with the Ge addition. The addition of Ge into As₄₀Se₆₀ is found to increase the refractive index and the extinction coefficient of As₄₀Se₅₀Ge₁₀ thin film. The decrease in optical band gap is explained on the basis of increase in density of states and disorderness due to Ge addition. The optical absorption in the film is due to allowed indirect transition, and the homopolar bond density is increased with Ge addition. The Raman shift observed in the two films clearly supports the optical changes due to Ge addition.     

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.     

Femtosecond optical Kerr effect study of Ge10As40S30Se20 film

The results of the femtosecond optical heterodyne detection of optical Kerr effect at 805 nm with the 80 fs ultrafast pulses in amorphous Ge₁₀As₄₀S₃₀Se₂₀ film is reported in this paper. The film shows an optical non-linear response of 200 fs under ultrafast 80 fs-pulse excitation, and the values of real and imaginary parts of non-linear susceptibility χ⁽³⁾ were 9.0×10⁻¹² and −4.0×10⁻¹² esu, respectively. The large third-order non-linearity and ultrafast response are attributed to the ultrafast distortion of the electron orbits surrounding the average positions of the nucleus of Ge, As, S and Se atoms. This Ge₁₀As₄₀S₃₀Se₂₀ chalcogenide glass would be expected as a promising material for optical switching technique.     

Optical limiting in fullerenes, colloidal metal solutions, and semiconductors in the field of pico-and nanosecond pulses of an Nd:YAG laser

Optical limiting of pico-and nanosecond pulsed radiation of a Nd:YAG laser (λ=1064 and 532 nm) is studied in solutions of fullerenes (C₆₀ and C₇₀), semiconductor crystals and films (GaAs, As₂S₃, As₂₀S₈₀, 2As₂S₃/As₂Se₃, and 3As₂S₃/As₂Se₃), and colloidal solutions of metals (Au, Ag, Pt, and Cu). The effect of the particle aggregation in silver solutions on the optical limiting is considered. Optical limiting mechanisms, specifically, reverse saturable absorption, two-photon absorption, and self-action effects, are discussed. Nonlinear absorption coefficients of the studied materials are measured.     

The reversible photostructural change studied by ESR of Mn2+ in As2Se3 films

The reversible photostructural change is studied from a microscopic point of view using ESR of Mn²⁺ doped in As₂Se₃ films. It is observed that the hyperfine structure constant changes reversibly corresponding to the reversible optical absorption edge shift. The result is explained in terms of increasing randomness by photoirradiation.     

Experimental study and configurational-coordinate model for photo-induced effects in amorphous As2Se3 films

The optical absorption of As₂Se₃ thin films is studied in the UV–VIS spectral range and the value of the optical energy gap is determined. The effect of photo-irradiation on the optical absorption and transmission of thin film samples is also investigated. The optical energy gap was found to decrease with photo-irradiation time. The results of photo-irradiation are discussed in correlation with the structural aspects of As₂Se₃. A model is proposed to account for the structural changes, resulting from photo-irradiation, causing the decrease of the energy gap. The effect of γ-radiation on the optical absorption of As₂Se₃ thin films was studied also and no detectable effect on the value of the optical energy gap was observed.     

Structural and optical properties of In35Sb45Se20−xTex phase-change thin films

Physical properties of In₃₅Sb₄₅Se_20−xTe_x thin films with different compositions (x=2.5, 5, 7.5, 10, 12.5 and 15 at %) prepared by electron beam evaporation method are studied. X-ray diffraction results indicate that the as-evaporated films depend on the Te content and the crystallized compounds consist mainly of Sb₂Se₃ with small amount of Sb₂SeTe₂. Transmittance and reflectance of the films are found to be thickness dependent. Optical-absorption data indicate that the absorption mechanism is direct transition. Optical band gap values decrease with increase in Te content as well as with increase in film thickness.     

Photoelectron spectra of As4S3, P4S3, P4Se3 and As4O6

The He(I) UV photoelectron spectra of As₄S₃, P₄S₃, P₄Se₃ and As₄O₆ are reported in this paper. The spectra of As₄S₃ and the isostructural molecules P₄S₃ and P₄Se₃ comprise seven broad but clearly defined peaks, whilst the spectrum of As₄O₆ consists of six peaks and two shoulders. By comparing the spectra with one another and with the photoelectron spectra of other arsenic and phosphorus compounds, the character and symmetry species of the highest-energy occupied molecular orbitals of the molecules have been assigned.     

Photosensitive post tuning of chalcogenide Te20As30Se50 narrow bandpass filters

We present an experimental study on the photosensitive properties of narrow bandpass filters based on a chalcogenide Te₂₀As₃₀Se₅₀ (TAS) spacer. The transmittance curve of single TAS layer was shifted towards long wavelength direction after 2 h exposure by Xenon arc lamp. The refractive index and extinction coefficient were both increased together with a red shift of optical gap. A maximum 1.7% photo-induced effect was observed. Narrow band filters constituted by TAS and cryolite were manufactured by electron beam deposition. The transmittance spectrum of the filter during the exposure by a wide band source was in situ measured and the resonant wavelength was observed to turn longer gradually till saturation. A spatially localized central wavelength change up to 5.7 nm was finally obtained. The stability of the photo-induced effect was studied and some comments were given at the end of this paper.     

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.     

Photoplasticity of As2Se3 films investigated with combined nanoindentation and AFM methods

Thin As₂Se₃ amorphous chalcogenide films have been studied by nanoindentation and atomic force microscopy in darkness and under illumination by band-gap light. The combination of these two methods has been used to study the peculiarities of the photoplastic effect in amorphous semiconductors. It has been shown by multiple loading indentation experiments that a non-linear mechanism of the formation of the strain response is realized in the As₂Se₃ films subjected to the combined action of light and external mechanical loading. We have observed that light illumination alters the internal friction of the films and their shear modulus. These observations have been considered in the frame of the two-phase model of chalcogenide glasses. Some arguments in favor that the self-organization processes take place in the structure of irradiated film are given.     

Reversible and athermal photo-vitrification of As50Se50 thin films deposited onto silicon wafer and glass substrates

Photo-vitrification of As₅₀Se₅₀ thin films deposited onto silicon wafer and glass substrates has been studied using X-ray diffraction, far-infrared, and differential infrared spectroscopies. The optical study of this photo-amorphization effect has been carried out by two different methods enabling the determination of the average thickness and refractive index of a wedge-shaped thin film. The refractive-index behaviour of the as-evaporated, crystallized, and photo-vitrified As₅₀Se₅₀ films is analyzed within the single-oscillator approach. The optical-absorption edge is described using the non-direct transition model, and the optical energy gap is calculated. In the course of the vitrification of an As₅₀Se₅₀ thin film deposited on a silicon substrate the photo-oxidation of the film has been additionally detected and arsenic trioxide micro-crystals were formed on the surface of the film. Such oxidation has not been observed with As₅₀Se₅₀ films deposited on glass substrates, which demonstrates that the photo-vitrification phenomenon depends also on the type of substrate. Finally, it is concluded from the optical study that a reversible photo-darkening effect accompanies the photo-induced vitrification phenomenon. Received: 3 August 1998 / Accepted: 13 January 1999 / Published online: 7 April 1999     

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}     

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.     

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.     

Influence of composition on electrical and optical properties of new chalcogenide thin films from Ge-Se-Tl system

Bulk Ge₂₀Se_80−xTl_x (x ranging from 0 to 15 at%) chalcogenide glasses were prepared by conventional melt quenching technique. Thin films of these compositions were prepared by thermal evaporation, on glass and Si wafer substrates at a base pressure of 10⁻⁶ Torr. X-ray diffraction studies were performed to investigate the structure of the thin films. The absence of any sharp peaks in the X-ray diffractogram confirms that the films are amorphous in nature. The optical constants (absorption coefficient, optical band gap, extinction coefficient and refractive index) of Ge₂₀Se_80−xTl_x thin films are determined by absorption and reflectance measurements in a wavelength range of 400-900 nm. In order to determine the optical gap, the absorption spectra of films with different Tl contents were analyzed. The absorption data revealed the existence of allowed indirect transitions. The optical band gap showed a sharp decrease from 2.06 to 1.79 eV as the Tl content increased from 0% to 15%. It has been found that the values of absorption coefficient and refractive index increase while the extinction coefficient decreases with increase in Tl content in the Ge-Se system. These results are interpreted in terms of the change in concentration of localized states due to the shift in Fermi level. DC electrical conductivity of Ge₂₀Se_80−xTl_x thin films was carried out in a temperature range 293-393 K. The electrical activation energy of these films was determined by investigating the temperature dependence of dc conductivity. A decrease in the electrical activation energy from 0.91 to 0.55 eV was observed as the Tl content was increased up to 15 at% in Ge₂₀Se_80−xTl_x system. On the basis of pre-exponential factor, it is suggested that the conduction is due to thermally assisted tunneling of the carriers in the localized states near the band edges.