CC BY
16LP-I-13
Optical response of metals to ultrashort laser pulses: A puzzle for optical models
N.M. Bulgakova1, S.A. Lizunov12, V.P. Zhukov134, A.V. Bulgakov12
1HiLASE Centre, Institute of Physics of the Czech Academy of Sciences, Dolni Brezany, Czech Republic
2S.S. Kutateladze Institute of Thermophysics, Siberian Branch of RAS, Novosibirsk, Russian Federation
3Institute of Computational Technologies, Siberian Branch of RAS, Novosibirsk, Russian Federation
4Novosibirsk State Technical University, Physical-Technical Faculty, Novosibirsk, Russian Federation
The interaction of ultrashort laser pulses with solid surfaces involves a wealth of the physical processes, depending on the material kind and laser light properties. Even within the same material family, considerably different modifications of material properties can be achieved when applying similar laser pulses. Improvement in understanding of the physical mechanisms of laser-induced material modification/ablation and contributions of individual processes is of a vital need for further advance of ultrafast laser processing techniques for a broad range of applications, including micro- and optoelectronics, surface nanostructuring, and photovoltaics.
The optical response of metal surfaces to ultrashort laser excitation is one of important topics of laser-matter interaction. It is known that many metals, which are highly reflective at normal conditions, become considerably absorbing during ultrashort laser pulse action but understanding of the dynamic variation of their optical properties remains to be challenging. On the other hand, namely dynamic change of metal reflectivity determines the amount of the energy absorbed by metal and, hence, its post-irradiation evolution. Numerous optical models have been proposed to describe this effect and to link the actually absorbed laser energy with the incident laser energy. This includes variations of the Drude and Drude-Lorenz models, contribution of plasma-like response of free electrons in metal, as well as attempts of ab initio simulations of the optical response (see overview in [1]). However, all these models require adjusting parameters and give a reasonable agreement with experiments for limited ranges of irradiation conditions.
In this talk, the existing models of the optical response of metals will be overviewed with critical assessment of their applicability to the conditions of ultrashort laser irradiation, based on direct comparison with experimental data for several metals. The results of two-temperature modeling (TTM) of femtosecond laser irradiation of a number of metals by fs laser pulses, combined with simulations of dynamic change of optical properties, will be reported and compared with available data on measured reflectivity behavior. Strong interconnection between optical and thermodynamic properties of the irradiated metals will be demonstrated. A collective behavior of free electrons in metals subjected to ultrashort laser action will be discussed. A question about the effect of decoherence of free electrons on metal reflectivity at different laser intensities and pulse durations will be raised.
References
[1] M.V. Shugaev, M. He, S.A. Lizunov, Y. Levy, T. J.-Y. Derrien, V.P. Zhukov, N. M. Bulgakova, L. V. Zhigilei, Insights into Laser-Materials Interaction Through Modeling on Atomic and Macroscopic Scales, In: Advances in the Application of Lasers in Materials Science, Springer Series in Materials Science, Ed. P.M. Ossi, Vol. 274, Chapter 5 (Springer, 2018). - P. 107-148.
