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2Laser swelling at femtosecond nanostructuring of material
A. Afanasiev, N. Sapogova, M. Sveshnikova, A. Pikulin, N. Bityurin*
Institute of Applied Physics Russian Academy of Science, Nizhniy Novgorod
Investigation of the effect of a pair of femtosecond laser pulses, one of the fundamental frequency (FF) and the other of the second harmonic (SH) of a Ti:sapphire laser, on polymer surfaces coated with a layer of colloidal polystyrene microparticles 1 ^m in diameter, when the sum frequency is in the bandgap of the material, reveals that the maximum depth of the ablation craters is achieved when the SH pulse comes 133-200 fs before the FF [1]. Further investigation studying the volume change of the irradiated areas rather than the crater depth shows that decrease in volume (effective ablation) occurs only when the SH comes first. In opposite case one observes the increase in volume that is effective swelling despite the presence of a crater (see Fig. 1).
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Fig. 1. Change in the volume of material under the sphere as a result of irradiation by a pair of different-colored laser pulses depending on the delay between them. Negative delay - the second harmonic pulse comes first.
This phenomenon is addressed by the hydrodynamic model for laser swelling [2] where the evolution of surface layers of a glassy material heated by a laser pulse above the glass transition temperature and cooled by heat diffusion is considered as the flow of a stretchable viscous fluid. The strong dependence of viscosity on temperature and pressure leads to the appearance of a hump with reduced density. New solutions of the model have been received and the results are compared to the above experimental data.
This work was supported by the Russian Science Foundation under project No. 22-19-00322.
[1] A. Afanasiev, I. Ilyakov, B. Shishkin, N. Bityurin, Nanopatterning of the dielectric surface by a pair of femtosecond laser pulses of different color through a monolayer of microspheres, Optics Express, 31(8), 12423-12432 (2023).
[2] N. Bityurin and N. Sapogova, Hydrodynamic model for laser swelling, Preprint ArXiv 2409.06370 (2024).
