CC BY
9LM-I-9
LASER-MATTER INTERACTION
Specific mechanisms of nonlinear absorption of intense ultrashort mid-infrared laser pulses in transparent semiconductors
V. Gruzdev
Department of Physics and Astronomy, University of New Mexico, 210 Yale Blvd. NE, Albuquerque, NM, 87106, USA Main author email address: vgruzdev@unm.edu
Since discovery of generation of very-high order harmonics by high-power femtosecond laser pulses in semiconductors [1, 2], increasing research efforts are being focused on ultrafast laser-semiconductor interactions at mid-infrared wavelengths. Absorption of laser radiation is the basic effect that triggers a broad range of those interactions. For the semiconductors transparent to low-intensity laser pulses, the traditional models [3-5] suggest domination of nonlinear multiphoton absorption due to inter-band multiphoton or tunneling electron excitation and linear absorption by laser-generated free carriers. However, they miss some special effects characterisitc of the ultrafast mid-infrared laser-semiconductor interactions. First, reduced photon energy (as compared to near-infrared wavelength range) supports generation of free electrons at a near-bottom part of conduction band that favors reduced electron-phonon collision rate. Second, extended wavelengths of the mid-infrared range suggest feasibility of transferring large amounts of pondero-motive energy from laser radiation to electrons via laser-driven electron oscillations. The energy is enough for specific intra-band free-electron excitatiosn even at moderate intensity, but may exceed band gap at higher intenisty. Third, complicated band structure of typical semiconductors suggest feasibility of specific intra-band and inter-band excitations with contributions from multiple energy bands. Finally, the special features of laser-driven free-carrier dynamics suggest significnat contribution of the absorption mechanisms that are considered as not effective at shorter wavelengths of near-infrared radiation.
In this talk, we overview the major mechanisms and specific physical effects of inter-band and intra-band electron excitations produced in typical semiconductors by mid-infrared laser pulses that contain a few (from 3 to 10) opticall cycles. Estimations are reported GaAs, GaP, GaN, and ZnSe at peak intensity varying from 10 GW/cm2 to damage and ablation thresholds. Of major focus are the special features of inter-band electron transitions driven by the laser pulses with significant spectrum width, contributions of multiple bands to the laser-driven free-carrier generation, the free-carrier absorption influenced by inter-conduction-band and inter-valley transitions, and special mechanisms of mid-infrared absorption, e. g., Brunel-type absorption and free-carrier multiphoton absorption. We discuss the fundamental limitations of the traditional models of light absorption and compare theoretical predictions against available experimental data.
This work is supported by Research Technology & Laboratory Directorate / Basic Research Office of the US Department of Defense via Newton Award for Transformative Ideas during the COVID-19 Pandemic No. HQ00342010028 and the Air Force Office of Scientific Research under award number FA9550-15-1-0254.
[1] S. Ghimire, A. D. DiChiara, et al, «Observation of high-order harmonic generation in a bulk crystal,» Nat. Phys. 7, 138 (2011).
[2] S. Ghimire, D. A. Reis, "High-harmonic generation from solids", Nat. Phys. 15, 10-16 (2019).
[3] A. Kaiser, et al, "Microscopic processes in dielectrics under irradiation by subpicosecond laser pulses", Phys. Rev. B 61, 11437 (2000).
[4] S.S.Mao, F.Quere, S.Guizard, X.Mao, R.E.Russo, et al, Appl. Phys. A 79, 1695 (2004).
[5] P. Balling and J. Schou, Rep. Prog. Phys. 76, 036502 (2013).
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