CC BY
19LMI-I-4
Solving Bloch equations to evaluate the energy deposition in silica induced by two-color femtosecond laser pulses
P. González de Alaiza Martínez1, E. Smetanina2, I. Thiele3, B. Chimier1, G. Duchateau4
1- University of Bordeaux-CNRS-CEA, Centre Lasers Intenses et Applications, UMR5107, 351 Cours de la
Libération, F-33405 Talence, France 2- Faculty of Physics, L. V. Lomonosov Moscow State University, Moscow, Russia 3- Department of Physics, Chalmers University of Technology, SE-412 96 Göteborg, Sweden 4- CEA, CESTA, 15 Avenue des Sablieres, CS60001, 33116 Le Barp Cedex, France
Main author email address: guillaume. duchateau@cea.fr
The manufacturing industry widely uses micromachining (from the ^m to mm scale) based on laser ablation in microelectronics and many other high-tech fields [1]. Regarding laser processing of dielectric materials, a large number of applications have emerged including surface processing such as patterning, texturing, and etching, and material removal such as cutting and drilling. In these applications, the laser pulse interacting with the dielectric target first drives electrons into excited states through photo- and impact ionization together with laser heating in the conduction band, and their subsequent relaxation towards the lattice through collisions leads to an effective energy deposition into the material [2]. If this deposited energy exceeds locally a given threshold of the material, then irreversible macroscopic material modifications take place a few picoseconds later.
To further optimize such laser processes by tailoring the electron dynamics and energy deposition, pulse-shaping techniques consisting of complex temporal envelopes, frequency chirps, or a mixture of colors are developed. The use of two-color laser pulses, i.e., two laser pulses with different colors, is of particular interest since specific processes of electron dynamics depend on the photon energy. A high-frequency pulse is efficient for seeding electrons through multiphoton ionization whereas a lower-frequency pulse is advantageous to heat these electrons and then cause material damage by an electron avalanche [3,32]. Since both wavelengths introduce opposite behaviors, by delaying in time the two laser pulses, one expects to find configurations that optimize the energy deposition into the material.
In this talk, the electron dynamics in dielectric materials induced by two-color femtosecond laser pulses is addressed by solving dedicated optical Bloch equations [3]. This model includes photo- and impact ionization, the laser heating of conduction electrons, their recombination to the valence band, and their collisions with phonons. The influence of photon energies, laser intensities, and pulse-to-pulse delay is analyzed. Depending on the interaction process, colors cooperate to excite electrons or drive them independently. For the given laser parameters, an optimal pulse-to-pulse delay is found which enhances significantly the energy deposition into the material, in agreement with experimental observations [4].
[1] K. Sugioka et al, Femtosecond laser 3D micromachining: a powerful tool for the fabrication of microfluidic, optofluidic, and electrofluidic devices based on glass, Lab on a Chip 14, 3447-3458 (2014).
[2] E. G. Gamaly, The physics of ultra-short laser interaction with solids at non-relativistic intensities, Phys. Reports 508, 91-243 (2011).
[3] E. Smetanina et al, Optical Bloch modeling of femtosecond-laser-induced electron dynamics in dielectrics, Phys. Rev. E 101, 063206 (2020)
[4] P. González de Alaiza Martínez et al, Modeling the time-dependent electron dynamics in dielectric materials induced by two-color femtosecond laser pulses: Applications to material modifications, Phys. Rev. A 103, 033107 (2021).
