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Extraordinary Laser-induced Excitations in Glasses: Analysis and Theory

Final Report Summary - ELEGANT (Extraordinary Laser-induced Excitations in Glasses: Analysis and Theory)

The project objectives were comprehensive experimental and theoretical studies of interaction of ultrashort laser pulses with optical glasses in order to reveal mechanisms of formation of extraordinary structures induced by laser radiation inside the bulk glass. During the period of the contract we have made the comprehensive analysis of the whole route of modification of transparent solids irradiated by single ultrashort laser pulses focused into material bulk, starting from laser energy absorption via creation free electron plasma up to microsecond timescale when absorbed laser energy dissipates and structural change is imprinted in material matrix. For this aim, an opto-thermoelastoplastic model was developed consisting of two linked parts. The first part is based on Maxwell’s equations supplemented with hydrodynamic equations of generated free electron plasma and the second part involves classical model of thermo-elastoplastics to follow material elastic and deformative motion.
Following is an outline of the main project results according to the four project tasks:

(1) To disclose the nature of formation of volume nanogratings in fused silica. The comparative modelling of irradiation conditions of fused silica with relatively low and high laser pulse energies has been performed for getting insight into excitation levels of matter created by single pulses for irradiation regimes particular for nanograting formation (low energy) and strong modification (high energy). Spatiotemporal dynamics of free electron density evolution and laser beam propagation have been studied. Simulations on plasma wave evolution in the laser-excited region were performed which have shown that plasma waves (both the fundamental Langmuir wave and the Tonk-Dattner modes) cannot be responsible for formation of periodic structures inside the material volume (Fig. 1). In view that the Tonk-Dattner hypothesis of the project was not supported by modelling, an extensive analysis of the simulation results on electron plasma parameters was performed and a new concept of nanograting formation has been put forward based on ionization-scattering instability development. An experimental verification of the new concept was proposed in the form of pump-probe irradiation with a week probe beam acting at different angles to the pump beam. Thermophysical, optical and mechanical properties of variety of glasses and transparent crystalline materials were analysed and a list of materials which may be suitable for nanograting imprinting was proposed. From this list the experiments have been performed for ULE glass and nanograting formation was found in this material. Further materials from the list are planned for checking in future.
(2) To find mechanisms responsible for anisotropy of waveguide writing dependent on the direction of laser beam scanning. The discoverer of laser writing anisotropy Prof. Peter Kazansky explained the effect by the presence of the pulse front tilt (PFT) in ultrashort laser beams originated from both pulse generation and manipulation in optical lines. To get understanding how pulses with PFT interact with matter, the above model was extended to account for violation of cylindrical symmetry of laser beams (‘laser knife’ geometry). The spatiotemporal coupling was introduced to the laser beam spatiotemporal shape in the form of the PFT and wave front rotation in collaboration with one of the discoverers of the PFT, Prof. S. Akturk (Istanbul Technical University, Turkey). First comparative simulations of laser energy absorption for tilted and untilted pulses have shown strong effect of localized absorption enhancement in the case of tilted pulses. The simulation results have qualitatively been verified experimentally.
(3) To describe bubble chains formation in glasses. Thermoelastoplastic modelling was performed to elucidate dynamics of material relocation under stresses induced by laser energy coupling into localised glass volume. By modelling and comparison with available experimental data, it has been shown that, for laser-induced bubble formation, the TPa pressures assumed widely in literature are not necessary. Furthermore, at the threshold regimes of bubble formation, the laser-generated stresses only slightly exceed the material tensile strength (~80 MPa in modelling against 48.3 MPa of strength value for fused silica). Important is that in such regimes matter reaches the melting point where its strength drops by several orders of magnitude. The simulations have supported a phenomenological scenario of bubbles formation proposed by the project authors in preliminary studies before the project start (published in Opt. Express 19, 18989 (2011)). The simulations have revealed an intricate evolution of material density after swift laser heating similar to that observed in experiments. It has been shown that glass is deformed irreversibly already at sub-nanosecond timescale while experimentally long-living elastic waves are monitored which cannot cause additional material damage. The final modification structure obtained by modelling is in reasonable agreement with observed structures (Fig. 2). As a conclusion, the developed model can be used as a predictive tool for material modification that however requires further joint studies.
(4) To develop a concept of laser-induced modification diagrams for transparent materials. The diagram is proposed which matches the energy and density of free electrons excited in transparent materials for reaching different levels of heating (e.g. annealing, softening, melting, and sublimation points). The analysis based on this diagram has helped to resolve contradictions existing between a number of pump-probe measurements of free electron density and simulations. The diagram based on the laser-energy balance with accounting thermodynamic state of laser-excited matter unambiguously indicates the actual levels of free electron density and energy achieved in the regimes widely used for direct laser writing. The concept can be applied for any material kind.

Although not all goals have been achieved to the moment in view of the risky program proposed by the project, the performed studies, the developed modeling tools, and established collaboration promises further joint progress in the field of laser material processing. Thus, several other problems were studied and experimental verifications were proposed for future research (double wavelength irradiation for overcoming the laser intensity clamping effect; using an inert gas jet to eliminate “self-vegetation” on soda-lime glass during laser processing; quasi-1D thermal conduction upon laser crystallization of amorphous silicon films). A summary of these works are attached (file other.pdf) while the most of activity described in (1) – (4) have been published or prepared for publishing (list of publications below).