Observation of Nonequilibrium Carrier Distribution in Ge, Si and GaAs by Terahertz-pump--Terahertz-probe Measurements

TL;DR

This study investigates the nonequilibrium behavior of carriers in Ge, Si, and GaAs using terahertz pump-probe measurements, revealing unique carrier dynamics and saturation effects at high THz intensities.

Contribution

It provides the first detailed comparison of carrier saturation and dynamics in Ge, Si, and GaAs under intense terahertz excitation, highlighting Ge's unique overpopulation phenomena.

Findings

Strong saturation of free carrier absorption at high THz intensities

Observation of a small increase in absorption in Ge at intermediate energies

Inability of the Drude model to fit Ge's frequency response at short delays

Abstract

We compare the observed strong saturation of the free carrier absorption in n-type semiconductors at 300 K in the terahertz frequency range when single-cycle pulses with intensities up to 150 MW/cm2 are used. In the case of germanium, a small increase of the absorption occurs at intermediate THz pulse energies. The recovery of the free carrier absorption was monitored by time-resolved THz-pump/THz-probe measurements. At short probe delay times, the frequency response of germanium cannot be fitted by the Drude model. We attribute these unique phenomena of Ge to dynamical overpopulation of the high mobility gamma conduction band valley.