Ultrafast light-driven optical rotation and hidden orders in bulk WSe$_2$

TL;DR

This study demonstrates ultrafast optical Kerr rotation in bulk WSe$_2$ driven by circularly polarized light, revealing hidden quantum entanglement and long-lived photo-induced states, challenging previous assumptions about bulk inversion symmetry limitations.

Contribution

It shows that bulk WSe$_2$ exhibits significant Kerr rotation and hidden quantum entanglement, expanding valleytronics applications beyond monolayers.

Findings

Sizable Kerr rotation observed in bulk WSe$_2$ across a wide energy range.

Long-lived photo-induced electron-hole densities with a lifetime of about 500 fs.

Spectral features linked to A-, B-, and C-exciton edges and their origin from Pauli blocking.

Abstract

Single-layer semiconducting transition-metal dichalcogenides, lacking point inversion symmetry, provide an efficient platform for valleytronics, where the electronic, magnetic, valley and lattice degrees of freedom can be selectively manipulated by using polarized light. This task is however thought to be limited in parent bulk compounds where the point inversion symmetry is restored. Exploiting the underlying quantum physics in bulk materials is thus one of the biggest paradigmatic challenges. Here we show that a sizable optical Kerr rotation can be efficiently generated in a wide energy range on ultrafast timescales in bulk WSe$_2$, by means of circularly-polarized light. We rationalize these findings as a result of the hidden spin/layer/valley quantum entanglement. The spectral analysis reveals clear features at the three characteristic frequencies corresponding to the A-, B- and C-exciton edges. The origin and the relative sign of all these features is shown to stem from the selective Pauli blocking of intralayer and interlayer optical transitions. The long lifetime of the broadband Kerr response ($\tau \sim 500$ fs) provides a strong indication that coupled photo-induced electron and hole densities survive in bulk compounds longer than previously expected. The present report demonstrates that a hidden quantum entanglement is operative also in bulk centrosymmetric layered materials, opening the way for an effective exploitation of bulk WSe$_2$ in optoelectronic applications.