OSE Seminars
Non-Perturbative Modeling of High-Intensity Interaction of Long-Wavelength Ultrashort Laser Pulses with Wide-Band-Gap Solids
Presented by Professor Vitaly Gruzdev, Department of Physics & Astronomy, UNM
Recent progress in mid-infrared ultrafast laser systems has made available femtosecond laser pulses (typical pulse width 100 fs) at wavelengths from 2 to 6 micrometers and peak irradiance as high as few tens of TW/cm2. The increased laser wavelength of infrared and mid-infrared spectrum range delivers reduced photon energy (below 05. eV) that is favorable for tunneling regime of laser-induced generation of free carriers. Under those conditions, the traditional models of laser-radiation absorption may fail because novel physical effects come into play.
By critical overview of the traditional numerical and analytical models, we demonstrate that the range of laser parameters characteristic of mid-infrared ultrashort laser pulses (pulse width 4-20 cycles; peak irradiance 10 TW/cm2 and higher; broad spectrum) is not adequately treated by those models. To fix this gap, we develop a simple time-dependent model to consider interaction of electron sub-system of wide-band-gap semiconductors and dielectrics with few-cycle (from 4 to 20 cycles per pulse) high-intensity laser pulses. To account for strong laser-induced distortion of the electron sub-system and its energy spectrum, we employ the Keldysh-type non-perturbative approach based on the concept of laser-driven electron oscillations. Analysis of experimental data suggest that duration of an oscillation cycle is substantially smaller than electron-particle collision time. Based on those estimations, we also consider low-collision-rate model of ultrafast laser-induced heating of the free carriers (e. g., electrons in the conduction band). Overall approach incorporates laser-induced modifications of band gap by ponderomotive potential of the laser-driven oscillations. It is shown that the oscillatory mechanism may dominate over other mechanisms of band-gap modification in wide-band-gap solids.
Among direct outcomes of the model is the time-dependent rate of nonlinear absorption. The non-perturbative time-dependent model of electron excitation delivers scaling with the basic laser and material parameters. Based on our simulations, we arrive at some novel ultrafast laser effects including formation of indirect-gap transient energy bands, suppression of free-carrier generation at the leading edge of laser pulses; cycle-averaged generation of bias-free photocurrent; and formation of multi-domain non-equilibrium energy distribution of the free carriers in the conduction band.
11:00 am, Friday, September 28, 2018
Room 103, Center for High Tech Materials
Science and Technology Park - South Campus
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