Andrei M. Mekryukov

Photo-structural transformation of chalcogenide glasses under non-linear absorption of laser radiation

Copyright (c) 1997 Elsevier Science B.V. All rights reserved. The optical properties of commercial and experimental arsenic sulfide glasses were studied during and after exposure to pulsed Nd:YAG laser radiation at 1.064 and 1.318 μm. Photo-induced losses (absorption and scattering) are produced in chalcogenide glasses by high-power laser radiation, the with photon energy hn<E g . This phenomena is only observed if the irradiance is greater than a value of about 10 8 W/cm 2 . Such exposure results in both reversible and irreversible structural transformations of these glasses in the exposed region. The reversible darkening is similar to the known effect of photo-structural transformations. It is connected with non-linear excitation to upper intrinsic levels of glass when it exposed to photons with energy less than glass bandgap. The irreversible contribution is due to the photo-induced localized destruction of bulk material, which is not completely erased by annealing near the glass transition temperature. Damage sites are responsible for absorption and scattering in this case. It is believed that a process of laser damage is caused by the thermal instability of structural irregularities randomly distributed in glass, which absorb laser radiation.

Laser system for investigation of nonlinear properties of optical materials

An Nd:glass laser system is described that enables smoothly varying the pulse duration of single-frequency laser radiation in the range from 0.2 to 30 ns and focusing the beam to spot sizes from 0.4 ?m at high reproducibility of spectral and spatial-temporal parameters of radiation. This laser system is used to examine the nonlinear photoinduced processes and laser-induced damage in chalcogenide glasses under high-power laser radiation in the near-IR spectral range. It is found that photoinduced losses (absorption and scattering) are produced in chalcogenide glasses by high-power laser radiation with photon energy hv < Eg . The exposure results in both reversible and irreversible structural transformation of these glasses in the exposed region. The reversible darkening is similar to the known effect of photostructural transformations in thin films exposed to photons with energy higher than the glass bandgap. The irreversible contribution is connected with the photoinduced localized destruction of bulk material. It is believed that the process of local destruction is connected with the nonlinear excitation of the structural irregularities randomly distributed in the glass.

Laser-induced changes of refractive index of lead-silicate glasses

The mechanisms of photo-induced changes of refractive index of the (Tau) (Phi) lead- silicate glasses (analogous with the SF glasses from Schott catalog) under the effect of high power laser radiation with quantum energy less bandgap have been studied. It is shown that the laser-induced color centers results in increase of refractive index into the exposed bulk during the laser pulse action. This leads to considerable redistribution of irradiance and decrease of laser radiation brightness even in the case of optical elements less 1 mm thickness. The observed effect may be connected both with radiation-induced dilation of matter and heating of interaction region owing to absorption of radiation by color centers. Comparing the kinetics of refractive index change of the glass after exposure by laser pulse at 0.53 micrometers and the kinetics of color centers decay allows us to draw a conclusion about heat character of observed changes.

Color centers in lead-silicate glasses and their influence on accumulation effect

The accumulation effect under multi-photon generation of color centers at 532 nm has been investigated in lead silicate glasses. The mechanism of excitation and color centers formation has been studied. It was founded that a long-wave carriers mobility boundary (minimal energy at which there takes place generation of electron-hole pairs) is disposed much above (> 5.6 eV) a fundamental absorption edge of the glass matrix (approximately 3.5 eV). Excitation occurs through virtual levels located in the fundamental absorption region as a result of the three-photon process. Dependency of the accumulation effect on the color centers generation efficiency and its concentration is studied. It is shown that existing models based on multi- photon accumulation of color centers can not account for the observed regularities. In the work possible mechanisms of under study phenomena are discussed.

Intrinsic optical breakdown of glasses

In this paper the influence of multiphoton absorption, self-focusing, absorbing impurity, mode composition, and focusing conditions of laser radiation on the regularities and the absolute values of breakdown threshold of glasses have been studied. The conditions necessary for the realization of intrinsic optical breakdown have been determined. It is shown that under focusing of the neodymium laser radiation in spots of 0.4 - 0.8 micrometers diameters statistical spread in breakdown threshold values, temporal and size dependence are absent. Breakdown thresholds for alkali silicate glass K8 and fused quartz KY1 are similar and are almost 1013 W/cm2. On the basis of the obtained results a new model of intrinsic optical breakdown mechanism has been proposed.

Investigation of optical parameters of silicate glasses in case of before-threshold effect of laser radiation

In this work, the processes resulting in optical parameters change of glasses was investigated during the beforethreshold laser irradiation. Residual changes of refractive index resulting in beam contraction and intensity growth of laser radiation in the caustic center were found to be conditioned by the production of mechanical stresses in the sample bulk. Under multiple laser radiation with h(nu)

Role of self-focusing in laser-induced breakdown of water caused by nano- and picosecond pulses

An experimental investigation of laser-induced breakdown of water and subthreshold phenomena using pulsed laser radiation in the nanosecond to picosecond region has been performed. It has been shown that self-focusing and suspended particles have a strong influence on laser-induced breakdown of water. A significant decrease in transmittance for an input irradiance 1 to 2 orders of magnitude less than the laser-induced breakdown threshold of water as observed, as well as a strong spot size dependence of this threshold. Besides the investigated processes result in a breakdown threshold of water for small spot sizes that is nearly 10 times larger than that of glass due to a sharp increase in scattering from inhomogeneities formed as a result of self- focusing. For large spot sizes the breakdown threshold of water is considerably less than for the same glasses due to breakdown initiated by suspended particles.

Photostructural transformation of chalcogenide glasses under nonlinear absorption of laser radiation

The photoinduced change of transmittancy of chalcogenide glasses was investigated under exposure to the laser radiation at 1.064 and 1.318 micrometer. It was established that two- photon absorption in these glasses arises under exposure to radiation with irradiances of greater than or equal to 108 W/cm2. It results in irreversible structural transformation of the glasses in the interaction region. This process has threshold character over a narrow range of radiation irradiance (plus or minus 0.1Ithr) and results in appearance of a small-dispersion phase in the bulk glass. Formation of such regions has fluctuating character and causes the appearance of strong light scattering. The effect observed is similar phenomenologically to the known effect of photo-structural transformations in the thin films under exposure to near bandgap light. However, in contrast to the last, the emerged changes of the glass properties are not erased completely by the annealing at the glass transition temperature.

Photoinductional change of silicate glasses optical parameters at two-photon laser radiation absorption

The mechanisms of photo-inductional change of the alkali-silicate and lead-silicate glasses optical parameters upon exposure to the intense laser radiation with Eg/2 < h(nu) < Eg (Eg is the glass matrix ionization potential) has been studied. It is shown that under these conditions the color centers accumulation and fundamental luminescence are observed owing to two-photon ionization of a glass matrix. The saturation value of additional absorption is dependent on the irradiance of laser radiation. It is defined by the dynamic equilibrium between two-photon absorption and one-photon discoloration of color centers. This effect may be used for bulk holograms recorded in the colored glasses. It was found that the process of two-photon generation of charge carriers affects the increase of absorption in a wide spectral range as well as the change of the refractive index of glass. These effects essentially decrease the brightness of the laser radiation passing through the medium, even for samples less than 1 mm thick. On the basis of the analysis of the characteristics of fundamental and impure luminescence, the kinetics of color centers formation and decay, and the kinetics of refractive index change under conditions of two-photon glass matrix excitation, the available mechanisms of photo-inductional changes of glass optical parameters are offered.