CC BY
9LM-I-31
Ultrafast laser-matter interactions in solids with tightly-focused
mid-IR laser pulses
Fedor Potemkin1
'Faculty of Physics, M.V. Lomonosov Moscow State University, Leninskie Gory, bld.1/62, Moscow 119991, Russia
The development of high-power mid-IR laser applications requires a study on laser-induced damage threshold in the mid-IR. In this paper, we present a complex comparison of the effect of pulses of different durations (from 0.1-0.2 ps to 1.2 ps) of the near (1.24 ^m) and middle (4.6 ^m) infrared on transparent dielectrics (MgF2) and semiconductors (Si) under tight (NA = 0.85) focusing of laser radiation into the bulk of the material. It was found that with such a change in the wavelength and the effect on the dielectric, the change in the ionization mechanisms is significant. While the rate of field ionization decreases smoothly with the wavelength, the mechanism of heating electrons in the conduction band goes from single-photon (direct and electron-photon-phonon absorption) to multiple photon inverse Bremsstrahlung absorption [1]. In practice, such a change in the heating mechanism leads to a weak dependence of the plasma formation threshold on the pulse duration in mid-IR (4.6 ^m) and a significant impact in the case of the near-IR (1.24 ^m) excitation. In the case of semiconductors, the ionization mechanism remains within the framework of the multiphoton approximation, but an increase in the photon order for excitation by mid-IR pulses leads to a decrease in delocalization processes and losses in the prefocal region.
Fig. 1. Plasma formation threshold as a function of the detuning length of the femtosecond compressor for different solids and pulse durations. a) MgF2 1.24 ^m, 6) MgF2 4.6 ^m, b) Si 1.24 ^m, r) Si 4.6 ^m
In addition, it has been demonstrated that the maximum absorption ~ 50% (and hence the deposited energy density ~ 6 kJ/cm3) under the ultrafast laser excitation in dielectrics (for example, MgF2) is achieved in the case of shortest pulses (~ 100 fs) with no matter to the excitation wavelength, while the deposited energy density higher for near-IR pulses. In semiconductors (for example, Si), greater absorption (~ 80%) is achieved for longer (sub-ps) laser pulses. These results will serve as the basis for the development of modern microprocessing technologies since the creation of modifications in the bulk of a semiconductor in this case does not require complex schemes of solid-state immersion or space-time chirping.
This work was supported by Russian Science Foundation (RSF) (Project No. 17-72-20130).
[1] 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, Article number: 14007 (2020).
