Ultrafast magneto-photocurrents as probe of anisotropy relaxation in GaAs

TL;DR

This study demonstrates that ultrafast magneto-photocurrents in GaAs, detected via THz emission, can serve as a novel probe for anisotropy relaxation dynamics, revealing two distinct relaxation time scales linked to different scattering processes.

Contribution

It introduces a new method using magneto-photocurrents as a probe for anisotropy relaxation in GaAs, applicable to non-centrosymmetric semiconductors.

Findings

Magneto-photocurrents differ from other photocurrents in their dynamics.

Anisotropy relaxation occurs on two time scales: ~10 fs and ~100 fs.

Magneto-photocurrents can probe microscopic anisotropy relaxation processes.

Abstract

We induce ultrafast photocurrents in a GaAs crystal exposed to a magnetic field by optical femtosecond excitation. The magneto-photocurrents are studied by time-resolved detection of the simultaneously emitted THz radiation. We find that their dynamics differ considerably from the dynamics of other photocurrents which are expected to follow the temporal shape of the optical intensity. We attribute this difference to the influence of carrier-anisotropy relaxation on the magneto-photocurrents. Our measurements show that the anisotropy relaxation for carrier densities ranging between $10^{16}$ cm$^{-3}$ and $5 \times 10^{17}$ cm$^{-3}$ occurs on two different time scales. While the slow time constant is approximately 100 fs long and most likely governed by electron-phonon scattering, the fast time constant is on the order of 10 fs and presumably linked to the valence band. Our studies not only help to better understand the microscopic origins of optically induced currents but - being even more important - show that magneto-photocurrents can be employed as novel probe of anisotropy relaxation in GaAs. This technique is applicable to all non-centrosymmetric bulk semiconductors.