Ultrafast photocurrents in MoSe$_2$ probed by terahertz spectroscopy

TL;DR

This study uses terahertz spectroscopy to investigate femtosecond photocurrent dynamics in MoSe$_2$, revealing distinct mechanisms in bilayer and multilayer samples and observing excitonic quantum beats.

Contribution

It demonstrates the use of terahertz emission spectroscopy to differentiate photocurrent mechanisms in 2D MoSe$_2$ and identifies quantum beat oscillations between excitonic states.

Findings

THz radiation generated by out-of-plane currents in multilayer MoSe$_2$

In-plane shift current observed in bilayer MoSe$_2$

Detection of 23 THz oscillations due to excitonic quantum beats

Abstract

We use the terahertz (THz) emission spectroscopy to study femtosecond photocurrent dynamics in the prototypical 2D semiconductor, transition metal dichalcogenide MoSe$_2$. We identify several distinct mechanisms producing THz radiation in response to an ultrashort ($30\,$fs) optical excitation in a bilayer (BL) and a multilayer (ML) sample. In the ML, the THz radiation is generated at a picosecond timescale by out-of-plane currents due to the drift of photoexcited charge carriers in the surface electric field. The BL emission is generated by an in-plane shift current. Finally, we observe oscillations at about $23\,$THz in the emission from the BL sample. We attribute the oscillations to quantum beats between two excitonic states with energetic separation of $\sim100\,$meV.