While laser irradiation of materials is employed in a number of applications [1], its theoretical description remains limited in accuracy, slowing down the emergence of breakthroughs of this field. The main difficulty of this field is the great diversity of interplaying phenomena excited by pulsed laser light in materials [2], such as photo-ionization, free-carrier absorption, impact ionization, three-body phenomena, quasi-particle emission, screening, the limited knowledge on the electron-phonon coupling, etc. For example, even widely employed materials (e.g., silicon) are still not fully described by first-principle approaches, e.g., due to the important contribution of three-body interactions during their processing by the intense laser light [3].

In view of constructing a comprehensive picture of the interaction, a number of theoretical approaches have been proposed. While a part of the community develops simplified but comprehensive models for addressing the real-space and real-timescale of the experiments [2], another part focuses at describing the effects taking place at the ultrafast nanoscale from an ab-initio perspective [1,4].

In this seminar, I will revise the earliest moment of interaction of ultrashort laser pulse with band gap materials using first-principle approaches such as the time-dependent density functional theory (TDDFT) and the Floquet theory. Using high power computation, these enable the study of the dynamics of electrons upon irradiation by an ultrashort laser pulse, beyond the perturbative regime [5] and using realistic band structures.

In a first part, by performing TDDFT computations shorter than the electron-phonon coupling time of the material, I will expose advantages and limits of widely employed electron excitation models (such as Keldysh and Drude theories) [6]. I will also challenge the results obtained from TDDFT to experimental observations obtained at the Charles University on high harmonic generation [7].

A second part will be devoted to explore the loss of symmetry induced by the laser light at intensities of the order of 10¹³⁻¹⁴ W/cm². Since the crystal’s symmetry defines its optical properties [5], band structure, and other mechanical and thermal properties, a modification of the electron’s symmetry by laser-dressing [6] is expected to have far-reaching consequences on the transient optical response of solids [8], on the generation of high harmonics [9], and on the study of non-equilibrium electron-phonon coupling [10]. The regime of intensity I identified to enable such situation is close to materials damage regime where a large community of application groups and engineers is operating on a daily basis [11].

[1] Derrien, T. J.-Y.; Levy, Y. & Bulgakova, N. M. Chap. 1 in Ultrafast Laser Nanostructuring - The Pursuit of Extreme Scales, Springer, 2022.

[2] Shugaev, M. V. et al., MRS Bulletin, 2016, 41, 960-968

[3] Derrien, T. J.-Y. et al. Applied Physics A, 2014, 117, 77

[4] Apostolova, T. et al. Tools for investigating electronic excitation: experiment and multi-scale modelling. Universidad Politécnica de Madrid. Instituto de Fusión Nuclear Guillermo Velarde, 2021.

[5] Boyd, R. W. Nonlinear Optics. Academic Press, 2008.

[6] Derrien, T. J.-Y. et al., Physical Review B, 2021, 104, L241201.

[7] Suthar, P. et al., Communications Physics, 2022, 5, 288.

[8] Yamada, S. & Yabana, K. Physical Review B, 2020, 101, 165128.

[9] Apostolova, T. & Obreshkov, B. The European Physical Journal D, 2021, 75, 267.

[10] Sentef, M. A.; Ruggenthaler, M. & Rubio, A. Science Advances, 2018, 4, eaau6969.

[11] Gnilitskyi, I.; Derrien, T. J.-Y. et al., Scientific reports, 2017, 7, 8485.