Theoretical treatment of ultrashort pulse laser processing of transparent materials: What is energetically and mechanically meaningful?

Bulgakova, N.M., Zhukov, V.P., Meshcheryakov, Y.P. and Kazansky, P.G. (2012) Theoretical treatment of ultrashort pulse laser processing of transparent materials: What is energetically and mechanically meaningful? ICEPA-8: Conference on Photo-Excited Processes and Applications, Rochester, United States. 12 - 17 Aug 2012.

Abstract

Ultrashort laser pulses are a powerful tool for modifying the structure and properties of transparent materials. Depending on material properties and irradiation conditions, a wide variety of modifications can be induced such as surface and bulk periodic structures (nanogratings), densification with associated refractive index change, microvoids and void chains, phase transitions, etc. This gives rise to numerous technological applications based on 3D photonic structures in bulk optical materials (waveguides, Bragg gratings, Fresnel zone plates, rewritable optical memories, and others). Among transparent materials, optical glasses are of prime importance for optoelectronics and photonics due to their relatively low cost, processability, and possibility of governing refractive index and inducing optical anisotropy. The physics behind laser-induced glass modifications is extremely rich and involves the multiplicity of the consecutive processes initiated by radiation absorption during the laser pulse and extending to millisecond time scales when the final structure becomes “frozen” in the glass matrix. While tremendous achievements have been made toward laser-writing techniques and assembling integrated optics, the physical mechanisms underlying glass modifications have not been fully understood. The exigency of controllable generation of desired structures requires deeper insight into of the mechanisms and spatiotemporal dynamics of laser-induced glass transformations.
In this report, we will review the physical processes and mechanisms responsible for various forms of glass modification. Different approaches for modeling ultrashort laser pulse propagation in transparent materials will be critically assessed. The dynamics of laser-induced creation of free electron plasma inside bulk glass will be analyzed, depending on the irradiation conditions. A contradictory issue on the free electron density generated in glass materials upon laser irradiation will be addressed with reviewing the existing theoretical results and experimental evaluations based on application of the Drude theory. The results of modeling will be presented obtained on the basis of the Maxwell’s equations supplemented with the equations describing electron plasma generation and the laser-induced electric current. We will demonstrate that the model allows following important features of laser beam propagation in the regimes of tight focusing and dense electron plasma generation when unidirectional approximations such as the non-linear Schrödinger equation do not provide adequate description. Based on this model we have studied spatiotemporal dynamics of laser beam propagation with self-focusing, free electron generation, and plasma-induced defocusing on the example of fused silica glass under particular irradiation regimes employed for laser direct writing. As a result, the geometry of the laser energy absorption zone is determined and the glass temperature is mapped which may be foreseen at the end of electron – glass matrix relaxation. This, in turn, allows estimating the laser-induced stress levels and making conclusions on the routes of glass modification. Finally, based on the performed analysis, we consider the energy balance, matching the free electron energy and temperature with several threshold values (melting, plastic deformation, material failure with void formation, sublimation).
*This research is supported by Marie Curie International Incoming Fellowship grant of the corresponding author, No. 272919.

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