CC BY
3The 30th International Conference on Advanced Laser Technologies
ALT'23
LM-I-7
Laser-induced extreme state of matter in silicon: the way to create and to diagnose
Fedor Potemkin
119991, Russia, Moscow, Faculty of Physics, Leninskie Gory, bld.1/62 potemkin@physics.msu.ru
The field of silicon photonics is a rapidly developing area of research due to the fact that modern microelectronics are primarily based on silicon, making the development of photonic circuits in semiconductors crucial [1]. Silicon's small band gap (~1.1 eV) and high number of free electrons also limit the use of commercially available lasers. Moreover, due to the presence of a sufficiently large number of free electrons, even when exposed to ultra-short laser pulses with a wavelength greater than 1100 nm, microplasma is efficiently generated, which effectively increases the laser impact area, leading to a drop in the deposited energy density (DED).
In the first part of this study, the way to control and to increase the DED inside the silicon sample has been proposed. Mid-IR (more than 4 um) ultrashort laser pulse excitation is one the most convenient way to increase the deposited energy density which is enough for bulk microstructuring of silicon [2]. Two- and three- photon absorption cross section in silicon becomes negligible in mid-IR that leads to field-driven laser energy absorption localization in space.
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(a) (b) (c)
Fig. 1 (a) DED into the bulk Si as a function of laser energy of tightly focused (NA=0.86) mid-IR (X=4,6 |im) femtosecond (x=160 fs) laser pulse. Blue dashed line indicates the DED threshold for micromodification formation. (b) DED into the bulk Si under two color excitation by a pair of tightly focused (NA=0.5) mid-IR (X=4,6 |im) and near-IR femtosecond laser pulses as a function of mid-IR laser pulse energy compared to single pulse mid-IR excitation. (c) Intensity map of changes in the frequency of coherent phonons in silicon, depending on the delay between the pump and probe mid-IR (X=4,6 |im) laser pulses.
Using tightly focused (NA=0.86) mid-IR (A,=4,6 |m) femtosecond (x=160 fs) laser pulse excitation the DED more than 5 kJ/cm3 was achieved that allow to modify the bulk Si in single pulse regime (see Fig.1 a). It was demonstrated that adding the near-IR laser field at the wavelength of 1,24 |m with the energy much less than the plasma formation threshold energy makes it possible to weaken the numerical aperture of the focusing optics down to 0,5 as well as to increase the DED value up to 8 kJ/cm3(see Fig.1 b).
Such a huge impact on the silicon makes it possible to create the extreme state of matter inside it and the phase transitions can be occurred. Thus, in the second part of this study the dynamics of the impact of mid-IR-range (A=4.6 ^m) femtosecond laser pulses on bulk silicon under tight focusing conditions (NA = 0.5) was reconstructed for the first time. Initially, the femtosecond pulse energy is absorbed by the laser-induced plasma, with a lifetime of approximately 160-320 fs (depending on the laser pulse energy). The energy transfer from the plasma to the atomic subsystem occurs on a sub-ps timescale, which generates a shock wave and excites coherent phonons on a sub-ps scale. The shift of atoms in the lattice at the front of the shock wave results in a cascade of phase transitions (Si-X => Si-VII => Si-VI => Si-XI => Si-II), leading to a change in the phonon spectra of silicon [3] (see Fig.1 c).
This work in part of phase transitions investigations was supported by Russian Science Foundation (Project № 23-73-00039) and Russian Foundation for Basic Research (Project № 21-32-70021) in part of research on phonons.
[1] Bogaerts, W.; Chrostowski, L. Silicon Photonics Circuit Design: Methods, Tools and Challenges. Laser Photonics Rev. 12, 1700237 (2018).
[2] Mareev, E.; Pushkin, A.; Migal, E.; Lvov, K.; Stremoukhov, S. Single-shot femtosecond bulk micromachining of silicon with mid-IR tightly focused beams. Sci. Rep. 12, 7517 (2022).
[3] Mareev, E.; Obydennov, N.; Potemkin, F. Dynamics of the Femtosecond Mid-IR Laser Pulse Impact on a Bulk Silicon. Photonics, 10, 380 (2023).
