Nanoscale mapping of valley polarization via chiral tip-enhanced photoluminescence

One of the promising candidates for valleytronic devices is transition metal dichalcogenide (TMD) monolayers, due to their strong spin–orbit coupling and broken inversion symmetry. Since it was demonstrated that the valley degree of freedom in TMD materials is optically controllable by using circularly polarized light (CPL), this has become a key concept in valleytronics. However, valley-selective optical pumping with a diffraction-limited light source is limited to distinguishing the change in degree of valley polarization (DOP) arising from nanoscale heterogeneities, which remains unexplored. In this work, we implement chiral tip-enhanced photoluminescence (cTEPL), which uses circularly polarized light as an excitation source, and acquire a nanoscale DOP image beyond the optical diffraction limit. We confirm that far-field CPL can be successfully delivered into the near-field with an achiral plasmonic Au tip by FDTD simulation. It is experimentally validated by observing the enhancement of not only photoluminescence signals but also the DOP using the circularly polarized near-field. And the DOP enhancements are not observed when linearly polarized light is used. Moreover, we elucidate the radiative properties of DOP at nanoscale features in 2D semiconductors by imaging DOP with a spatial resolution smaller than 10 nm.