Photon-assisted ultrafast electron-hole plasma expansion in direct band semiconductors

TL;DR

This study uses time-resolved terahertz spectroscopy to observe ultrafast electron-hole plasma propagation in GaAs and InP, revealing coherent and incoherent regimes with speeds up to a fraction of light speed over hundreds of micrometers.

Contribution

It demonstrates the formation and ultrafast propagation of electron-hole plasma in direct band semiconductors under strong photoexcitation, highlighting regimes with speeds up to c/10 and c/25.

Findings

Coherent plasma propagation in GaAs at 20 K reaches c/10.

Incoherent plasma propagation in InP at 20 K reaches c/25.

Phenomena likely occur in most direct band semiconductors under strong photoexcitation.

Abstract

Time-resolved terahertz spectroscopy is used to investigate formation and ultrafast long-distance propagation of electron-hole plasma in strongly photoexcited GaAs and InP. The observed phenomena involve fundamental interactions of electron-hole system with light, which manifest themselves in two different regimes: a coherent one with the plasma propagation speeds up to c/10 (in GaAs at 20 K) and an incoherent one reaching up to c/25 (in InP at 20 K), both over a macroscopic distance > 100 micrometers. We explore a broad range of experimental conditions by investigating the two materials, by tuning their band gap with temperature and by controlling the interaction strength with the optical pump fluence. Our interpretation suggests that the observed phenomena should occur in most direct band semiconductors upon strong photoexcitation with low excess energy.