Probing valley population imbalance in transition metal dichalcogenides via temperature-dependent second harmonic generation imaging

TL;DR

This paper introduces a temperature-dependent second harmonic generation imaging technique to detect and quantify valley population imbalance in transition metal dichalcogenides, offering a new, wavelength-insensitive method for valley polarization analysis.

Contribution

It presents a novel polarization-resolved SHG method for monitoring valley population imbalance in TMDs, which is insensitive to excitation wavelength and suitable for large-area samples.

Findings

Temperature-induced SHG changes reveal valley population imbalance.

Method enables calculation of valley-induced to intrinsic susceptibilities ratio.

SHG technique is more versatile than photoluminescence for VPI detection.

Abstract

Degenerate minima in momentum space - valleys - provide an additional degree of freedom that can be used for information transport and storage. Notably, such minima naturally exist in the band structure of transition metal dichalcogenides (TMDs). When these atomically thin crystals interact with intense laser light, the second harmonic generated (SHG) field inherits special characteristics that reflect not only the broken inversion symmetry in real space, but also the valley anisotropy in reciprocal space. The latter is present whenever there exists a valley population imbalance (VPI) between the two valleys. In this work, it is shown that the temperature-induced changes of the SHG intensity dependence on the excitation fieldpolarization, is a unique fingerprint of VPI in TMDs. Analysis of such changes, in particular, enables the calculation of the valley-induced to intrinsic second order susceptibilities ratio. Unlike temperature-dependent photoluminescence (PL) measurements of valley polarization and coherence, the proposed polarization resolved SHG (PSHG) methodology is insensitive to the excitation field wavelength, an advantage that renders it ideal for monitoring VPI in large crystalline or stacked areas comprising different TMDs.