CC BY
7ALT'22
LM-I-2
LASER-MATTER INTERACTION
Ultrafast excitation of silicon by mid-IR tightly focused laser radiation
F. V. Potemkin
M.V. LomonosovMoscow State University, Moscow, Russia potemkin@physics.msu.ru
Silicon (Si) is one of the most important materials for modern electronics and photonics. However, due to low bandgap (~1.1 eV) and high refractive index (~3.3), it is challenging to perform the three-dimensional (3D) femtosecond micromachining of its volume. Novel photonics platform for high-speed data transfer and optical memory demands higher flexibility of the silicon modification, including on-chip and in-bulk inscription regimes. These are deepness, three-dimensionality, controllability of sizes and morphology of created modifications. The two-photon absorption[1], aberrations induced by refractive index mismatch, and plasma delocalization in the pre-focal volume[2] drastically spread the energy of the femtosecond pulse across a large volume and drop the deposited energy density below the threshold of micromodification formation[3]. Shifting central wavelength of a driving pulse into the mid-IR can make the process of photoionization[4] dominant in semiconductors, avoid two- and three-photon absorption, and create single-shot femtosecond micromodification in a bulk Si for a broad range of energies. In this paper, we demonstrate that the tightly focused mid-IR femtosecond pulses are capable of micromodification creation due to overcoming deposited energy density threshold, determined by the latent heat of fusion (about 4 kJ/cm3) (see Fig.1).
Fig.1 a) Energy dependence of nonlinear transmission (black dots) and interaction volume (solid red line). The shaded area shows the absorbed energy region. b) Dependence of DED on the laser pulse energy. The dotted line shows the threshold of micromodification formation
In such a regime, we successfully performed single-shot bulk microstruturing of silicon. Using third-harmonic and near-IR microscopy, and molecular dynamics, we demonstrated that there is a low-density region in the center of a micromodification, surrounded by a "ring" with higher density, that could be an evidence of its micro-void structure. The formation of created micromodification could be controlled in situ using third-harmonic generation microscopy. The numerical simulation indicates that single-shot damage becomes possible due to electrons heating in the conduction band up to 8 eV (mean thermal energy) and the subsequent generation of microplasma with an overcritical density of 8.5 x 1021 cm-3. These results promise to be the foundation of a new approach of deep three-dimensional single-shot bulk micromachining of silicon[5].
1. E. V. Zavedeev, V. V. Kononenko, and V. I. Konov, «Delocalization of femtosecond laser radiation in crystalline Si in the mid-IR range,» Laser Physics 26(1), 016101 (2016).
2. E. I. Mareev, K. V Lvov, B. V Rumiantsev, E. A. Migal, I. D. Novikov, S. S. Yu, and P. F.V., «Effect of pulse duration on the energy delivery under nonlinear propagation of tightly focused Cr : forsterite laser radiation in bulk silicon,» Laser Phys. Lett. 17, 015402 (2019).
3. V. V. Kononenko, V. V. Konov, and E. M. Dianov, «Delocalization of femtosecond radiation in silicon,» Optics Letters 37(16), 3369 (2012).
4. E. Migal, E. Mareev, E. Smetanina, G. Duchateau, and F. Potemkin, «Role of wavelength in photocarrier absorption and plasma formation threshold under excitation of dielectrics by high-intensity laser field tunable from visible to mid-IR,» Scientific Reports 10(1), 1-10 (2020).
5. E. Mareev, A. Pushkin, E. Migal, K. Lvov, S. Stremoukhov, and F. Potemkin, «Single-shot femtosecond bulk micromachining of silicon with mid-IR tightly focused beams,» Scientific Reports 2022 12:1 12(1), 1-12 (2022).
