Excitonic shift current induced broadband THz pulse emission efficiency of layered MoS2 crystals

TL;DR

This study demonstrates that femtosecond optical pulses induce excitonic shift currents in layered MoS2, enhancing THz emission, with a critical fluence indicating a transition to an electron-hole liquid state at low temperatures.

Contribution

It reveals the role of excitonic shift currents in THz emission from MoS2 and identifies a fluence threshold for a quantum condensate transition.

Findings

Excitonic shift current enhances THz emission at low temperatures.

A critical fluence of 150 μJ/cm^2 marks a transition to electron-hole liquid.

Temperature-dependent behavior aligns with the Varshni model and Debye temperature.

Abstract

Following the ultrafast photoexcitation of a semiconductor, it embodies competing dynamics among photocarriers, many-body transient states of highly energetic excitons, and electron-hole liquid. Here, we show that femtosecond optical pulse excitation induces transient excitonic shift current contributing to stronger THz emission from a single crystalline bulk MoS2 at low temperatures. The control of dominating excitonic shift current is elucidated from excitation density dependent experiments at varying temperatures. A strong decrease in the excitonic contribution beyond a critical fluence of 150microJ/cm^2 is observed at a very low temperature of 20K. This behavior suggests the formation of a new quantum condensate, i.e., the electron-hole liquid, in the regime when the exciton density is overwhelmingly large that the average spacing between exciton pairs is comparable to the exciton radius. Furthermore, the exciton density dependent THz emission at varying temperatures is consistent with the Varshni model and the crystal Debye temperature of 260K.