Photo-induced dynamical changes in refractive index in amorphous As-S films

Compositional trends of the dynamical change in refractive index induced by band-gap illumination and its recoveries by irradiation of lower photon energy are investigated in thin films of As_xS_100?x. The magnitude of the changes is positive when x ? 35, otherwise negative, and has a maximum of 0.03 at x = 43. The experimental results suggest that the dynamical changes originate from trapping of photo-excited carriers, and are essentially related to an optical “stopping effect”.     

Phonon spectroscopy measurements at amorphous films

Phonon spectroscopy measurements were used to examine the scattering of high frequency phonons (300 GHz-1 THz) in amorphous materials. The experiments were done with the use of time and frequency resolved measurements of the phonon transmission behaviour through amorphous single films of different thicknesses. The typical film thicknesses were of the order of 10 nm. In contrast to the pure amorphous semiconductors Si and Ge our experiments show inelastic phonon scattering processes in the case of SiO₂ and SiH. This inelastic phonon scattering also occurs when the pure semiconductors Si and Ge are prepared in an O₂ or H₂ atmosphere, but is missing when the preparation process is done in an N₂ atmosphere. In films of the pure semiconductors a-Si and a-Ge we only found evidence to elastic scattering processes. In further experiments at heated a-SiH samples we could examine the atomical bonded hydrogen to be the center of the inelastic phonon scattering.The measurements and investigations described in this work were done in time of preparing a thesis at: Physikalisches Institut Teil 1, Universität Stuttgart, Pfaffenwaldring 57, D-70569 Stuttgart, Germany     

Transient grating measurements of refractive-index changes in intensively pumped Yb-doped laser crystals

Mechanisms of transient light-induced refractive-index changes in Yb-doped crystals Yb:KYW, Yb:KGW and Yb:YVO₄ were studied. Relative amplitudes and lifetimes of the electronic component of phase gratings caused by the polarizability difference of the excited and unexcited Yb³⁺ ions and the thermal component arising from thermalization of the pump energy were measured. Polarizability difference values of the ground ² F _7/2 and the excited metastable ² F _5/2 levels at the testing wavelength of 633 nm were determined for all crystals. The thermal diffusivity of the samples was estimated for selected directions of the thermal gradient. An athermal direction, along which the thermal component of refraction disappears, was found in Yb:KYW crystal under specific experimental conditions. PACS 42.70.Hj; 42.65.An; 66.30.Xj     

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.     

Characterization of amorphous silicon films by contactless transient photoconductivity measurements

Contactless transient photoconductivity measurements of intrinsic a-Si: H films in the microwave frequency range are presented. The measurements are evaluated quantitatively and the electron drift mobility is determined. It is shown that the influence of the surface on the observed decay behaviour can be neglected. A relation between the long time decay behaviour and the position of the Fermi level is observed.     

Laser deposition of amorphous diamond films from liquid aromatic hydrocarbons

6 H₅CH₃, C₆H₆, and C₆H₅CH(CH₃)₂) to pulsed visible laser radiation of a copper vapor laser (λ=510.6 nm). The X-ray Auger electron spectroscopy (XAES), reflection high energy electron diffraction (RHEED), and Raman analysis are employed to characterize the deposited films. The sp³ fraction in deposited films amounts to 60–70% and depends on the precursor. The average film thickness on a glass substrate is about 100 nm. The films show excellent adherence, are transparent in the visible and have microhardness of 50–70 GPa, as measured by nanoindentor. Received: 28 September 1998 / Accepted: 13 January 1999     

Photo-induced dynamical changes in amorphous As2S3 films

Optical properties of amorphous As₂S₃ films, which have been illuminated well by bandgap light in advance, can be changed dynamically by exposing to less-bandgap light. This dynamical change has been studied in connection with its plausible relation to the reversible photo-induced change. It has been found that these changes have intimate connections with each other, and can be explained by a certain configurational diagram in a coherent fashion.     

Ellipsometric in situ measurements during deposition of amorphous CuxSn100−x films

Amorphous metallic films are produced by quench condensation onto a 4K cold Si substrate under ultra high vacuum conditions. During evaporation the film growth is recorded in situ ellipsometrically. At the same time the mass of the film is measured with quartz microbalances. It is shown that the dielectric constants are dependent on film thickness. Different film models for evaluating the ellipsometric measurements are used and tested. The influence of porosity and interface roughness is taken into account. The dependence of mass density on thickness is mainly understood in terms of island growth. The dependence of the dielectric constant on concentration is discussed with respect to the Faber-Ziman theory.Dedicated to Prof. Dr. G. von Minnigerode on the occasion of his 60th birthday     

Observation of field quenching of photo-induced effects in hydrogenated amorphous silicon

Field quenching phenomena were observed in the photo-induced changes in dark current—voltage and dark low frequency capacitance-voltage characteristics of hydrogenated amorphous silicon (a-Si:H) diodes. The photo-induced changes in photoconductivity of undoped a-Si:H measured in coplanar type samples also depended on the externally applied electric field. The mechanisms of the field quenching were discussed referring to trapping and/or recombination of photogenerated carriers in a-Si:H.     

Laser annealing study of PECVD deposited hydrogenated amorphous silicon carbon alloy films

The influence of carbon content on the crystallization process has been investigated for the excimer laser annealed hydrogenated amorphous silicon carbon alloy films deposited by Plasma Enhanced Chemical Vapour Deposition (PECVD) technique, using silane methane gas mixture diluted in helium, as well as for the hydrogenated microcrystalline silicon carbon alloy films prepared by PECVD from silane methane gas mixture highly diluted in hydrogen, for comparison. The study demonstrates clearly that the increase in the carbon content prevents the crystallization process in the hydrogen diluted samples while the crystallization process is enhanced in the laser annealing of amorphous samples because of the increase in the absorbed laser energy density that occurs for the amorphous films with the higher carbon content. This, in turn, facilitates the crystallization for the laser annealed samples with higher carbon content, resulting in the formation of SiC crystallites along with Si crystallites.     

A new class of photo-induced phenomena in siloxane films

The light induced atomic desorption effect, known as LIAD, is observed whenever Pyrex cells, coated with siloxane films and containing alkali atoms, are illuminated. LIAD is a non-thermal phenomenon and it can be observed even with very weak light intensities. We show that the simultaneous contribution to the photo-emission of atoms adsorbed both at the film surface and within the film must be taken into account in order to fit the experimental data. We demonstrate that both the desorption efficiency and the diffusion coefficient of the alkali atoms embedded in the dielectric film depend on the desorbing light intensity. These features characterize a new class of photo-induced phenomena whose analysis gives new insights in the comprehension of the atom-surface interaction and of the atom-bulk diffusion and opens interesting perspectives for applications. Received 27 April 2000 and Received in final form 15 September 2000     

Laser heterodyne measurement of small arbitrary displacements

One-dimensional displacements of a moving body are dectected from the phase-modulated beat signal of light, generated by two successive ultrasonic modulators. Measurements of triangular and random displacements are demonstrated using a movable reflection mirror. For a sinusoidal motion its amplitude is measured down to 2.2 Å.     

Mid-infrared heterodyne phase-sensitive dispersion spectroscopy in flame measurements

We present the first demonstration of heterodyne phase-sensitive dispersion spectroscopy (HPSDS) for in situ, non-intrusive and quantitative CO₂ concentration measurements in flames. Dispersion spectroscopy retrieves gas properties by measuring the refractive index in the vicinity of a molecular resonance. The HPSDS scheme features a significant diagnostic advantage of the intrinsic immunity to laser power fluctuations caused by beam steering, thermal radiation and soot scattering in combustion environments, and thus no extra calibration process is required. In this work, we described the spectroscopic fundamentals for measuring heterodyne phase signals in flames. As a proof of principle, we used a mid-infrared interband cascade laser (ICL) near 4183?nm to exploit the strong CO₂ transitions in the R-branch of the v₃ fundamental band. The HPSDS signals of four CO₂ lines, R(76), R(78), R(80) and R(82), were measured in CH₄/air flames to obtain CO₂ concentrations at different equivalence ratios (Φ?=?0.8–1.2), yielding a good agreement with the simultaneous laser absorption measurements using the same ICL. With its immunity to laser power fluctuations verified experimentally, the HPSDS sensor was successfully implemented to measure CO₂ concentrations in C₂H₄/air sooting flames (Φ?=?1.78–2.38). Laser dispersion spectroscopy proves to be a promising and alternative diagnostic tool for combustion measurements.     

Laser induced crystallization of as-deposited amorphous Ge2Sb2Te5 films

Crystallization is induced by pulsed laser irradiation of s-deposited amorphous Ge2Sb2Te5 films. Changes of the irradiated areas have been analyzed with the reflectivity contrast. As laser fluences increasing,the reflectivity contrast increases from 0% - 2% to 14% - 16%, which indicates the structure of as deposited films transforms from amorphous to crystalline phases. The process of crystallization driven by the movement and rearrangement of atoms is described. And also the influence of the pulse duration on the threshold of crystallization is discussed, the results show that a lower threshold of crystallization can be produced for as-deposited films irradiated by the laser with short pulse duration. However, by the laser with long pulse duration, crystallization can only be formed with a higher threshold. The crystallization of films by irradiation of laser pulses is studied by Raman spectra.     

The mechanism of light induced reversible structural changes in amorphous antimony selenide films

Abstract

Photo– and thermoinduced phase transitions amorphous (a)? crystalline (c) in Sb_XSe_1-X (0.4≤x≤0.7) films are studied. For a→ c transition a new crystalline phase is found. The photostructural transitions by pulsed laser excitation are of thermal origin. The measured kinetics and threshold intensities are consistent with the proposed photothermal process model.     

Dynamics of refractive-index changes and two-beam coupling in resonant media

The dynamics of light-induced change in the refractive index of a resonant medium are examined. For illumination with weak fields, the two relevant relaxation times are T ₁, the population lifetime and T ₂, the dipole-dephasing time. The response time of the index change is determined by the slower relaxation time of the medium which is usually the time T ₁ taken by the excited system to relax back to its thermal equilibrium value. Illumination with two beams of the same frequency that intersect within the medium leads to the formation of a volume grating in the medium that is spatially local. Hence there is no exchange of energy between the beams that write the grating, each beam merely reducing the absorption experienced by the other beam. Illumination with a moving, spatially periodic intensity pattern leads to gain for one beam and additional absorption for the other beam as they propagate through the medium. A complete set of coupled equations describing the intensities and phases of the beams undergoing non-degenerate two-wave mixing in the resonant media is derived using third-order perturbation theory, and the solutions are studied numerically. In particular, the two-beam coupling constant for intensity is shown to depend on the frequency difference between the two beams and on the pressure-induced collisional relaxations in the system.