Second-harmonic imaging microscopy for time-resolved investigations of transition metal dichalcogenides

TL;DR

This paper demonstrates the use of second-harmonic generation microscopy as a versatile, non-invasive tool for time-resolved, large-area investigations of the structure and transient properties of transition metal dichalcogenides, especially monolayers and heterostructures.

Contribution

It introduces a polarization-dependent, time-resolved SHG imaging technique for analyzing the structure and dynamics of TMDC monolayers and heterostructures over large areas without scanning.

Findings

SHG imaging can evaluate crystalline orientation and stacking angles across large fields.

Pump-probe SHG correlates transient changes with structural features.

Time resolution is limited only by laser pulse duration.

Abstract

Two-dimensional transition metal dichalcogenides (TMDC) have shown promise for various applications in optoelectronics and so-called valleytronics. Their operation and performance strongly depend on the stacking of individual layers. Here, optical second-harmonic generation (SHG) in imaging mode is shown to be a versatile tool for systematic time-resolved investigations of TMDC monolayers and heterostructures in consideration of the material's structure. Large sample areas can be probed without the need of any mapping or scanning. By means of polarization dependent measurements, the crystalline orientation of monolayers or the stacking angles of heterostructures can be evaluated for the whole field of view. Pump-probe experiments then allow to correlate observed transient changes of the second-harmonic response with the underlying structure. The corresponding time-resolution is virtually limited by the pulse duration of the used laser. As an example, polarization dependent and time-resolved measurements on mono- and multilayer MoS$_2$ flakes grown on a SiO$_2$/Si(001) substrate are presented.