Dual role of longitudinal optical phonons for generation of coherent oscillations in gallium arsenide under optical pumping

TL;DR

This paper introduces a simplified quantum model explaining how ultrafast optical pulses generate coherent LO phonons and coupled modes in GaAs, revealing their simultaneous emergence due to phonon-plasmon interactions and stimulated Raman scattering.

Contribution

It presents a new theoretical framework using a Lindblad-type quantum master equation to explain the generation of coherent phonons and coupled modes in GaAs under optical pumping.

Findings

The model accurately reproduces experimental transient reflection data.

It explains the simultaneous generation of LO phonons and LOPC modes.

The approach clarifies the role of phonon-plasmon coupling and stimulated Raman scattering.

Abstract

We present a novel and simple picture of the generation dynamics of coherent longitudinal optical (LO) phonons and LO-phonon-plasmon-coupled (LOPC) modes by the ultrafast infrared pump-pulses in gallium arsenide (GaAs) employing the low-temperature approximation. LO phonons exhibit a pronounced coupling with plasmons formed by the optically excited electrons in the excited states of GaAs. This coupling results in the coherent oscillation of the LOPC modes in the excited states. The pump pulse also induces stimulated Raman scattering, which generates the coherent LO-phonon oscillation in the ground state. This picture is incorporated into a simplified model, and the time evolution of the density operator is calculated using the Lindblad-type quantum master equation. The theoretical results explain well the reported experimental results on the coherent oscillation of LO phonons and LOPC modes observed through transient reflection measurements. Above all, our model provides a natural reason for the simultaneous manifestation of the LO phonons and the LOPC modes.