Ultrafast Hidden Spin Polarization Dynamics of Bright and Dark Excitons in 2H-WSe$_2$

TL;DR

This study uses advanced photoemission spectroscopy to investigate the ultrafast spin polarization dynamics of bright and dark excitons in 2H-WSe2, revealing control mechanisms and potential for spin injection in layered materials.

Contribution

It provides the first direct measurement of hidden spin polarization in excitons of 2H-WSe2 using XUV photoemission spectroscopy, highlighting ultrafast dynamics and control possibilities.

Findings

Efficient chiroptical control of bright excitons' hidden spin polarization.

Ultrafast decay of bright excitons' spin polarization due to intervalley scattering.

Dark excitons act as a local spin polarization reservoir.

Abstract

We performed spin-, time- and angle-resolved extreme ultraviolet photoemission spectroscopy (STARPES) of excitons prepared by photoexcitation of inversion-symmetric 2H-WSe$_2$ with circularly polarized light. The very short probing depth of XUV photoemission permits selective measurement of photoelectrons originating from the top-most WSe$_2$ layer, allowing for direct measurement of hidden spin polarization of bright and momentum-forbidden dark excitons. Our results reveal efficient chiroptical control of bright excitons' hidden spin polarization. Following optical photoexcitation, intervalley scattering between nonequivalent K-K' valleys leads to a decay of bright excitons' hidden spin polarization. Conversely, the ultrafast formation of momentum-forbidden dark excitons acts as a local spin polarization reservoir, which could be used for spin injection in van der Waals heterostructures involving multilayer transition metal dichalcogenides.