Ultrafast nonlinear response of GaAs under high pressures

Applying hydrostatic pressure leads to dramatic changes in the band structure of semiconductors. In particular, it enables continuous tuning of the bandgap energy. Thus, optical pump-probe spectroscopy under high pressures allows the study of nonlinear optical response in the vicinity of the interband resonance.
Here, we study ultrafast carrier dynamics in gallium arsenide (GaAs) at pressures up to 3 GPa. Our setup employs a femtosecond Ti:sapphire laser which provides pulses with the spectrum centered around 1.55 eV with 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 with CsI used as pressure transmitting medium. The studied sample 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 pressure, 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 increasing pressure this relaxation becomes slower as demonstrated in fig. 2. Finally, for pressures above 2 GPa, the pump-probe signal changes its sign and the relaxation component vanishes completely. The nonlinear response appears only around zero pump-probe delay and its origin is attributed to the negative real part of the third-order susceptibility of GaAs [2]. We interpret this phenomenon as a shifting of the bandgap energy above the excitation spectrum of our experiment. This assignment is in good consistency 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].
In the future we plan to study the response of the photoexcited electron-hole plasma in GaAs using optical pump / THz probe spectroscopy under pressure. The all-reflective optics of our experimental setup enables us to couple coherent THz radiation pulses from the free-electron laser at the HZDR into a diamond anvil cell.
[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).