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

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 and affects the polarization state of the detected SHG. In this work, it is shown that the temperature-induced change of the SHG intensity dependence on the excitation field polarization is a fingerprint of VPI in TMDs. In particular, pixel-by-pixel VPI mapping based on polarization-resolved raster-scanning imaging microscopy was performed inside a cryostat to generate the SHG contrast in the presence of VPI from every point of a TMD flake. The generated contrast is marked by rotation of the SHG intensity polar diagrams at low temperatures and is attributed to the VPI-induced SHG.

Optical Harmonic Generation in 2D Materials

“…The thermal changes are believed to influence valley population imbalance and spin valley polarization. [ 238 ] The above studies suggest integrated SHG and valley polarization dependent SHG for applications in thermal sensing. Furthermore, THG is expected to provide useful sensing of temperature variation.…”

“…Valley polarization is an important way to tune one photon PL emission in TMDs which is controlled by valley selection rules for one‐photon case for K(−K) valley of the Brillouin zone. [ 238,273,274 ] Similarly, valley polarization obeys selection rules for the two photon case of SHG emission. Xiao et al.…”

All-optical polarization and amplitude modulation of second-harmonic generation in atomically thin semiconductors

“…The small asymmetry of the lobes along the two AC directions (∼ 15 • /70 • and ∼ 190 • /250 • ) is attributed to polarization scrabbling effects due to the use of a dichroic mirror in reflection geometry [48]. Finally, based on the results in Fig.…”

“…The experimental results show a weak modulation also for the SH emitted along the ZZ direction, although this is not expected from theory. This could arise from weak strain (i.e., below the limit detectable by our SHG polarization measurements [39,56]), small deviations in the alignment of the detection polarizer with respect to the AC/ZZ directions or from additional terms in the χ (2) arising from the valley degree of freedom [48,57].…”

Nonlinear Optical Imaging of In‐Plane Anisotropy in Two‐Dimensional SnS

“…and WSe 2 . [31][32][33][34][35][36][37] Specifically, it has enabled direct optical imaging of the atomic edges and boundaries of a 2D material, based on the observation of electronic structure changes at the edges of monolayer MoS 2 . [31] Two-dimensional (2D) tin(II) sulfide (SnS) crystals belong to a class of orthorhombic semiconducting materials with remarkable properties, such as in-plane anisotropic optical and electronic response, and multiferroic nature.…”