# Transient transition from free carrier metallic state to exciton insulating state in GaAs by ultrafast photoexcitation

## TL;DR

This study investigates the ultrafast transition in GaAs from a free carrier metallic state to an exciton insulator state using time-resolved optical reflectivity, revealing temperature-dependent phase transitions and exciton dynamics.

## Contribution

It provides the first detailed analysis of the transient metal-insulator transition in GaAs driven by ultrafast photoexcitation, identifying two critical transition temperatures and their associated spectral features.

## Key findings

- Transient reflectivity sign reversal indicates a phase change.
- Photoexcited MIT begins at ~230 K and stabilizes below T2.
- The phase diagram maps temperature and photoexcitation effects on MIT.

## Abstract

We present systematic studies of the transient dynamics of GaAs by ultrafast time-resolved optical reflectivity. In photo excited non-equilibrium states, we found a sign reverse in transient reflectivity spectra $\Delta R/R$ (t $>$ 0), from positive around room temperature to negative at cryogenic temperatures. The former corresponds to a transient free carrier metallic state, while the latter is attributed to an exciton insulating state, in which the transient electronic properties is mostly dominated by excitons, resulting in a transient metal-insulator transition (MIT). Two transition temperatures (T$_1$ and T$_2$) are well identified by analysing the intensity change of the time-resolved optical spectra. We found that photoexcited MIT starts emerging at T$_1$ as high as $\sim$ 230 K, in terms of a negative dip feature at 0.4 ps, and becomes stabilized below T$_2$ associated with a negative constant after 40 ps in spectra. Our results address a phase diagram that provides a framework for MIT through temperature and photoexcitation, and shed light on the understanding of light-semiconductor interaction and exciton physics.