Effects of free carriers and acceptor-bound excitons on the exciton dephasing in strained GaAs films: A study with spectrally- and time-resolved four-wave mixing

Abstract

This research aims to improve the knowledge of coherent exciton dynamics in strained bulk semiconductors, paving the way for more efficient applications in optoelectronics and quantum technologies. We studied the coherent exciton dynamics in a 470-nm thick gallium arsenide (GaAs) film using non-degenerate three-beam Four Wave Mixing (FWM) at 14 Kelvin with ~150 fs laser pulses. The film exhibits energy splitting between heavy-hole (Xh) and light-hole (Xl) excitons, attributed to the presence of biaxial tensile strain, which varies across the film. Strain calculations and spectrally resolved FWM measurements at three different regions are used to examine how strain levels impact the dephasing times of Xh and Xl excitons. The strain in the film also causes splitting in the acceptor-bound exciton energy transition (A0X), which -when aligned with the Xl exciton energy- prolongs the Xl dephasing time. Heterodyne FWM (HFWM) measurements in photon echo (PE) configuration demonstrate that the exciton transitions are predominantly homogeneously broadened. They also reveal distinct beat structures for Xh - Xl excitons under different polarization conditions, resulting from a combined effect of excitation-induced dephasing (EID), phase space filling (PSF), and biexciton formation (BIF). The experimental studies are reproduced and supported by numerical calculations with the optical Bloch equations (OBEs) for a 10-level model. This work contributes to a deeper understanding of the exciton dynamics in strained-bulk semiconductors.