Ultrafast Terahertz Probe of Transient Evolution of Charged and Neutral Phase of Photoexcited Electron-hole Gas in Monolayer Semiconductor

TL;DR

This study uses ultrafast terahertz spectroscopy to explore how photoexcited electron-hole pairs in monolayer MoS2 form excitons and decay, revealing two distinct decay processes with different lifetimes.

Contribution

It provides the first detailed observation of long-lived dark exciton states and their dynamics in monolayer MoS2 using ultrafast THz probe spectroscopy.

Findings

Identification of two decay components: 20 ps and several ns.

First observation of long-lived dark exciton states.

Decay dynamics depend on temperature and pump fluence.

Abstract

We investigate the dynamical formation of excitons from photoexcited electron-hole plasma and its subsequent decay dynamics in monolayer MoS2 grown by chemical vapor deposition using ultrafast pump and terahertz probe spectroscopy. Different photoexcited electron-hole states are resolved based on their distinct responses to THz photon and decay lifetime. The observed transient THz transmission can be fit with two decay components: a fast component with decay lifetime of 20 ps, which is attributed to exciton life time including the exciton formation and subsequent intraexciton relaxation; a slow component with extremely long decay lifetime of several ns due to either localized exciton state or a long live dark exciton state which is uncovered for the first time. The relaxation dynamics is further verified by temperature and pump fluence dependent studies of the decay time constants.