30 January 2012 Formation dynamics of femtosecond laser-induced phase objects in transparent materials
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Abstract
Ultrashort pulse lasers offer the possibility to structure the bulk of transparent materials on a microscale. As a result, the optical properties of the irradiated material are locally modified in a permanent fashion. Depending on the irradiation parameters, different types of laser-induced phase objects can be expected, from uniform voxels (that can exhibit higher or lower refractive index than the bulk) to self-organized nanoplanes. We study the physical mechanisms that lead to material restructuring, with a particular emphasis on events taking place on a sub picosecond to a microsecond timescale following laser excitation. Those timescales are particularly interesting as they correspond to the temporal distances between two consecutive laser pulses when performing multiple pulse irradiation: burst microprocessing usually involves picosecond separation times and high repetition rate systems operate in the MHz range. We employ a time-resolved microscopy technique based on a phase-contrast microscope setup extended into a pump-probe scheme. This methods enables a dynamic observation of the complex refractive index in the interaction region with a time resolution better than 300 fs. In optical transmission mode, the transient absorption coefficient can be measured for different illumination wavelengths (400 nm and 800 nm). The phase-contrast mode provides qualitative information about the real part of the transient refractive index. Based on the study of those transient optical properties, we observe the onset and relaxation of the laser-generated plasma into different channels such as defect creation, sample heating, and shockwave generation. The majority of our experiments were carried out with amorphous silica, but our method can be applied to the study of all transparent media.
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A. Mermillod-Blondin, A. Mermillod-Blondin, A. Rosenfeld, A. Rosenfeld, R. Stoian, R. Stoian, E. Audouard, E. Audouard, } "Formation dynamics of femtosecond laser-induced phase objects in transparent materials", Proc. SPIE 8247, Frontiers in Ultrafast Optics: Biomedical, Scientific, and Industrial Applications XII, 82470Q (30 January 2012); doi: 10.1117/12.914048; https://doi.org/10.1117/12.914048
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