Anastasia S. Gruzdeva

Computer simulation of light propagation through optical coatings with inhomogeneous microstructure

There are presented initial results of investigation of influence of local optical and thermal parameters of thin dielectric films and optical coatings with inhomogeneous micromorphology on light propagation. Linear and nonlinear light propagation accompanied by laser-induced heating have been considered. Modeling is used to investigate the process described by coupled nonlinear equations. Obtained results show critical role of nonlinear phenomena resulting in formation of local maxima of laser intensity inside the films which is followed by highly inhomogeneous local heating of the films.

Formation of shock electromagnetic waves during femtosecond pulse propagation in transparent solids

There is considered formation and propagation of shock electromagnetic waves (SEW) of visible spectral range as possible nonlinear optical phenomenon taking place at laser intensities characteristic of femtosecond laser interaction with transparent solids. Main regularities of SHEW formation are studied on the basis of 1D model of plane-wave propagation in isotropic dielectric with nonlinear optical response. Special attention is paid to influence of color dispersion and absorption on SEW formation and propagation. Necessary conditions for appearing of SHEW are obtained, in particular, threshold amplitude is estimated. There is presented a model for numerical simulation of SHEW formation and propagation influenced by dispersion of linear and nonlinear parts of refractive index. Using the simulation, we studied dynamics of SHEW formation on several first optical cycles of femtosecond laser pulse in transparent medium. Important observed features of SHEW of optical frequency are discussed. Obtained results are considered from the viewpoint of experiments on femtosecond laser interaction, in particular, laser-induced damage.

Resonance increase of high-power laser field with nodule defects in multilayer optical coatings: theory and simulation

There are presented results of computer modeling of laser- field evolution in nodular defects in multilayer optical coatings. It is investigated dynamics of laser-wave propagation through both linear and nonlinear nodules. Formation of field maxima is studied. There are presented theoretical estimates of field amplification for resonant case. The presented results are discussed from the viewpoint of laser-induced damage of optical coatings.

Nonthermal effects in femtosecond laser damage of transparent materials

There are considered non-thermal processes of femtosecond laser-induced damage of wide band-gap transparent materials. Dominating of thermal or non-thermal effects depends on radiation and material parameters among which pulse repetition rate, focal spot size and absorption play key role. Non-thermal mechanisms of damage and ablation can dominate at initiating stage and at low repetition rates. They are attributed to nonlinear electrodynamical processes such as higher harmonic generation of formation of shock electromagnetic waves. Considering interaction of shock electromagnetic wave with a particle in potential well, we derive expression for threshold of laser-induced ionization and delocalization. Thermal mechanisms can dominate at later stages of damage and ablation at repetition rates above 10 kHz. There are also discussed after-heating and non- equilibrium non-thermal processes taking place between initiating and thermal stages. There are considered several mechanisms of laser-induced ionization - multiphoton, tunneling, avalanche ionization, also ionization by higher harmonics and by shock-wave front. Estimations of ionization rates show that the latter two mechanisms can dominate at initiating stage of femtosecond damage and determine critically following ionization processes. Obtained results are compared with experimental data.

Thermal and nonthermal effects in femtosecond laser ablation and damage of transparent materials

There are considered non-thermal and thermal processes of femtosecond laser-induced damage and ablation of wide band-gap transparent materials. Dominating of one or other of them depends on radiation and material parameters among which pulse repetition rate, focal spot size and absorption play key role. Non-thermal mechanisms of damage and ablation can dominate at initiating stage and at low repetition rates (below 10 kHz). They are attributed to nonlinear electrodynamical processes such as higher harmonic generation and formation of shock electromagnetic waves. Considering interaction of shock electromagnetic wave with a particle (single charged particle and a dipole) placed in potential well, we derive expression for threshold of laser-induced ionization and delocalization. Thermal mechanisms can dominate at later stages of damage and ablation at repetition rates above 10 kHz. It is considered possibility of their description within modified two-temperature model. There are also discussed after- heating and non-equilibrium non-thermal processes taking place between initiating and thermal stages. There are considered several mechanisms of laser-induced ionization -- multiphoton, tunneling, avalanche ionization, also ionization by higher harmonics and by shock-wave front. Estimations of ionization rates show that the latter two of them can dominate at the stage of initiating of femtosecond damage and ablation and determine critically following ionization processes. Obtained results are compared with experimental data.

Finite-difference time-domain modeling of laser beam propagation and scattering in dielectric materials

Modified finite-difference time-domain (FDTD) approach to numerical investigation of propagation of laser pulses and beams in transparent materials is presented. In contrary to traditional FDTD technique, it is based on description of wave propagation by wave equation. To take into account important material properties, presented approach can include integrating of a set of coupled nonlinear equations including equations describing dispersion of linear and nonlinear parts of refractive index, linear and two-photon absorption. Developed technique is illustrated with several examples including scattering of laser radiation by diffraction grating (sine relief) and focusing by a dielectric cylinder. There are discussed problems of calculation stability, control of errors, decreasing of required computation time and memory. Application of developed approach to modeling of micro-optical elements for modern areas of optoelectronics is considered.

Interaction of shock electromagnetic waves with transparent materials: classical approach

There is discussed a theoretical model of initiating of laser-induced damage and ablation of transparent materials by femtosecond pluses based on properties of shock electromagnetic wave (SHEW). Advantages of this model are increased efficiency of SHEW-induced ionization and possibility of effective straight action of SHEW front on ions at crystal-lattice points. It is presented simplified description of SHEW-induced processes within approach of classical mechanics and electrodynamics: atoms are described as dipoles with certain ionization energy interacting with SHEW in potential well formed by crystal lattice. There are considered possibilities of laser-induced ionization by higher harmonics appearing during SHEW formation and point- defect formation and delocalization of ions at crystal points by SHEW. Obtained results and predictions are compared with experimental data and shown to be capable of explaining many observed regulations of femtosecond laser interactions with transparent media.

Light scattering by rough dielectric surface

Numerical modelling is applied to investigation of scattering of plane linearly polarized monochromatic wave by sine variations of dielectric surface relief. The modelling is based on finite-difference time-domain technique. Results of modelling include 1) space distribution of scattered light, 2) dependence of field amplification on ratio of roughness amplitude to laser wavelength, and 3) dependence of field amplification on ratio of roughness period to laser wavelength. Obtained results show that for TE polarization a) transmitted signal is more sensitive to roughness parameters than reflected one, b) there is narrow resonance in dependence of amplitude of scattered field on laser wavelength and roughness period, c) dependence of amplitude of scattered field on roughness amplitude is described by parabolic function for small values of relief amplitude. Depending on relief amplitude and period, scattering by sine roughness can result in formation of inhomogeneous space field distribution consisting of periodic field maxima inside dielectric or formation of homogeneous distribution such that both transmitted and reflected signals are close to plane wave. We consider the following applications of obtained results: 1) possibility to develop a new technique for in-situ surface roughness charactensation, 2) possible mechanisms of feedbacks during laser-induced formation of surface ripples, and 3) anti-reflection effect.

Laser-induced instabilities in optical materials induced by low-absorbing microinclusion and laser-induced damage

There are reviewed and summarized several theoretical models describing various mechanisms of developing of laser-induced temperature and field instabilities in both absorbing and nonabsorbing microinclusions. Most attention is paid to application of the models to investigation of laser-induced damage. General criterion for evaluation of damage threshold is deduced from presented models and discussed.

Evolution of rough dielectric surface in high-power laser field

This paper is devoted to investigation of some aspects of laser-driven evolution dielectric surface under action of high-power laser radiation. Main problems to be considered are (1) electrodynamic processes at the surface initiating laser- induced variations of surface relief; (2) possible mechanisms of formation of feedbacks between scattering and heating; (3) possible waveguide mechanisms of formation of surface ripple patterns. In the first case we investigate initiating of laser-induced deformation of dielectric surface through light scattering by both single relief defects and periodic relief modulation. Our goal is to study possibility of formation of local maximums of light intensity resulting in local heating of the surface. To consider the third problem we investigate positive feedbacks accompanying laser-surface interaction through mutual influence of electrodynamic and thermal processes. Obtained results show how the mentioned processes and feedbacks can influence laser-induced surface modification. Computer modeling is widely used to investigate all the mentioned problems. The presented results are discussed from the viewpoint of laser ablation and surface laser-induced damage of transparent optical materials.