Ultrafast nonlinear response of GaAs under high pressures


Ultrafast nonlinear response of GaAs under high pressures

Braun, J. M.; Schneider, H.; Helm, M.; Pashkin, A.

Applying hydrostatic pressure leads to dramatic changes in the band structure of semiconductors. In particular, it enables a continuous tuning of the bandgap energy of a given sample. Here we study ultrafast carrier dynamics in gallium arsenide (GaAs) at pressures up to 3 GPa. The optical pump-probe spectroscopy traces the nonlinear response in the vicinity of the bandgap. Thus, we are able to observe the changes in the ultrafast response caused by tuning the bandgap energy across the spectrum of the femtosecond probe pulse.
Our setup employs a femtosecond Ti:sapphire laser which provides pulses with the spectrum centered around 1.55 eV and a FWHM of ~0.1 eV. The experiment is performed in a non-collinear reflection geometry (fig. 1) where the pump and probe beams are focused by an all-reflective Schwarzschild objective to a ~5μm spot on the sample. Pressure is applied in a diamond anvil cell (DAC) with CsI used as pressure transmitting medium in order to ensure a good contact between the sample and diamond anvil. The sample studied is a semi-insulating GaAs crystal with thickness of about 50μm. The pump pulse with energy of 0.03 nJ induces changes in the optical reflectivity of the sample which is probed by the delayed weaker probe pulse.
In agreement with a previous study at ambient and low pressures, pumping induces an increase in reflectivity which decays on a timescale of hundred picoseconds and is assigned to the recombination of the photogenerated charge carriers [1]. With pressure increase, this relaxation becomes slower as shown in fig. 2. Finally, for pressures above 2 GPa, the pump-probe signal changes its sign and the relaxation components vanish completely. The nonlinear response appears only around zero pump-probe delay and probably originates from the negative real part of the thirdorder susceptibility of GaAs [2]. We interpret this phenomenon as a shifting of the bandgap energy above the excitation spectrum of our experiment. This assumption is in good agreement with the known bandgap pressure coefficient of 0.11 eV/GPa in GaAs leading to a complete transparency of the sample for pump and probe pulses at pressures above 2 GPa [3].
[1] D. G. McLean, M. G. Roe, A. I. D’Souza, P. E. Wigen, Appl. Phys. Lett. 48, 992 (1986).
[2] A. A. Said, M. Sheik-Bahae, D. J. Hagan, T. H. Wei, J. Wang, J. Young, E. W. Van Stryland, J. Opt. Soc. Am. B 9, 405 (1992).
[3] A. R. Goñi, K. Strössner, K. Syassen, M. Cardona, Phys. Rev. B 36, 1581 (1987).

Keywords: pump-probe spectroscopy; recombination dynamics; high pressure; diamond anvil cell; gallium arsenide; GaAs

  • Poster
    Joint AIRAPT-25th & EHPRG-53rd International Conference on High Pressure Science and Technology, 30.08.-04.09.2015, Madrid, Spain

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Publ.-Id: 22907