Laser-induced modifications of transparent crystals and glasses: photo-excitation thermodynamics mechanical response

Bulgakova, N.M. (2011) Laser-induced modifications of transparent crystals and glasses: photo-excitation thermodynamics mechanical response. Photonica 2011, Belgrade, Serbia. 28 Aug - 01 Sep 2011.

Abstract

Ultrashort pulsed laser radiation applied to inorganic dielectric materials by focusing either on the surface or toward the bulk depth induces, depending on the irradiation conditions, different kinds of modifications such as formation of surface and volume periodic structures (nanogratings), densification and refractive index changes, formation of micro- and nanovoids, phase transitions such as amorphisation of crystalline materials, etc. Such modifications are already used in optoelectronics and photonics (optical signal transmission. optical beam manipulation, 3D writing of information, etc.) and also in microfluidics and the list of potential applications is rapidly expanding. In spite of growing technological applications of transparent materials and their paramount promises for using in optoelectronics and photonics. The fundamentals of laser-induced excitation of transparent crystalline materials and glasses have not sufficiently studied. This is conditioned by both the complexity of the processes in the laser-excited dielectrics and a great variety of compound transparent materials when even a small variation in atomic composition and structure changes dramatically material response to laser excitation. Successful development of applications based on laser-induces micro- and nanomodifications of transparent materials requires deep understanding of the whole chain of the intricate processes initiated in dielectrics by femtosecond laser pulses and extending up to millisecond time scale with formation of permanent mechanically deformed and/or chemically modified states.
This Lecture is dedicated to an outline of the processes taking place in transparent crystals and glasses under the action of ultrashort laser pulses. The spatiotemporal dynamics of laser-induced modifications will be analyzed which start from material photoionization with creation of seed free electrons followed by electron avalanche multiplication. This results in dense plasma formation accompanied by changing optical response of the laser-excited region toward metallization. The effects of self-focusing counteracted by scattering from the laser produced plasma upon propagation of the laser beam inside the bulk of a transparent material will be discussed The aspects of warm dense matter and plasma collective behaviors will be touched. Special attention will be given to the thermal and mechanical effects which have yet been poorly studied for materials excited by ultrashort laser pulses. Swift heating of the photo-excited region occurring at picosecond timescale and corresponding pressure rise result in generation of the thermoelastic waves which, depending on the heating level and heat localization, can either completely dissipate, or lead to significant plastic deformations of the material, or even to mechanical damages in the form of micro- and nanovoids in the energy-release zone. The dynamics of the laser-induced processes will be illustrated by the results obtained in the frames of several modeling approaches. including those based on the non-linear Schrodinger equation, the Maxwell equations, the drift-diffusion approach, and the thermoelastoplastic stress analysis. The timescales of manifestation of the different processes will be defined, indicating the possibility of applying different modeling representations at different time spans.
A number of phenomena will be considered in detail such as charging of the dielectric surfaces culminating in Coulomb explosion, crater shaping, density redistribution upon waveguide writing; energy clamping effects. Finally, a short summary of several important experimental observations will be given which require explanations and adequate descriptions.

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