CC BY
13LMI-I-28
New insights to femtosecond excitation of solids with mid-IR laser fields
F. Potemkin1
1M.V. Lomonosov Moscow State University, Faculty of Physics, Moscow, Russian Federation
Direct fabrication of nanoscale structures in bulk material (dielectric or semiconductor) is of significant importance for miniaturization of photonic and optoelectronic devices. Ultrafast lasers are a promising tool for direct writing of nanometer defects due to highly nonlinear nature of process [1,2]. Being defined as the ratio of total absorbed energy to the laser impact area, deposited energy density (DED) serves as a key parameter in the process of the femtosecond laser pulse energy delivery into the bulk of the solids during microstructuring [3]. Thus, in order to control the morphology of residual modifications inside solids one has to find the ways for increasing absorption of incident laser energy under conditions of laser impact area shrinking. However, due to diffraction limit, the minimal size of the structures obtained under single color excitation is still of the order of l/NA, l is laser wavelength and NA is numerical aperture of the focusing optics. In contrast multi-color laser beams allow for precise control of structuring by dividing the excitation process into two steps [3,4]. Firstly, shorter wavelength pulse (visible or UV for dielectric and near-IR for semiconductor) creates seed electrons. Subsequently infrared (from near to mid-IR) pulse arrives and induces damage through efficient initiation of avalanche ionization. The significant importance of this approach is the capability of reaching high deposition energy density simultaneously with minimal defect sizes since energy of both pulses (visible and IR) is below the damage threshold [6].
In this paper we present our latest experimental and theoretical results aiming on the optimization of laser pulses parameters (energy, wavelength, polarization) and uncovering new features of femtosecond excitation of solids with mid-IR laser fields. We have investigated the effect of laser wavelength on the plasma formation and laser-induced damage threshold under femtosecond excitation of solids (SiO2, MgF2, ZnSe) tuning the wavelength from visible (0.62 |im) to mid-IR (4.4 ^m). For all the samples lowering plasma formation and damage threshold was observed scaling laser driver wavelength to mid-IR. The simulation of the electron plasma density dynamics, via MRE, show that using mid-IR laser pulses with shorter pulse durations leads to significant decreasing of the LIDT threshold that is in excellent agreement with experimental observations. Also we show that the highest deposited energy is reached when the energy of both pulses is close to single color threshold of plasma generation. The use of the second pulse with longer wavelength significantly increases absorbed energy due to more efficient heating of the quasi-free electrons in the conduction band. Elliptical polarization of the long wavelength pulse is additionally increase absorbed energy.
References
[1] R. R. Gattass and E. Mazur, "Femtosecond laser micromachining in transparent materials," Nat. Photonics 2(4), 219-225 (2008).
[2] M. Ali, T. Wagner, M. Shakoor, P. A. Molian, "Review of laser nanomachining," J. Laser Appl. 20(3), 169-184 (2008).
[3] F. V. Potemkin et al. "Overcritical plasma ignition and diagnostics from oncoming interaction of two color low energy tightly focused femtosecond laser pulses inside fused silica," Laser Physics Letters 13(4), 045402 (2016).
[4] F. V. Potemkin et al. "Enhancing nonlinear energy depositioninto transparent solids with an ellipticallypolarized and mid-IR heating laser pulseunder two-color femtosecond impact," Laser Physics Letters 14(6), 065403 (2017).
[5] F. Potemkin et al. "Controlled energy deposition and void-like modification inside transparent solids by two-color tightly focused femtosecond laser pulses." Applied Physics Letters 110(16), 163903 (2017).
