Long Range Valley Pseudospin Transport via Bloch Surface Wave Polariton in Monolayer Tungsten Disulfide

Monolayer transition metal dichalcogenide (TMD) has attracted much attention due to its unique spin-valley locking property. The valley degree of freedom acts as a pseudospin and can be differentiated by opposite circularly polarized light, and the energy degeneracy can be lifted by the Zeeman effect, which makes it possible to spatially separate pseudospins during excited state transport for applications such as quantum information processing. However, the exciton diffusion length of monolayer TMDs is limited to hundreds of nanometers to a few micrometers, making it difficult for significant spatial separation of the spin states. In contrast to excitons, Bloch surface wave polaritons (BSWPs) have been found to propagate nearly 100 µm^[1]. In this work, we demonstrate spatial pseudospin (K and K’ valleys) separation under an external magnetic field by strong coupling of excitons in monolayer tungsten disulfide (WS₂) films to BSWP at ~4K. The propagation of K and K’ valley pseudospins is inseparable without the presence of an external magnetic field, while under 7T, spatial population separation up to 10 µm is observed for different valley pseudospin species. The difference in propagation between pseudospin valley exciton-polaritons is most pronounced at wavelengths close to the anti-crossing region, where the BSWP has a higher exciton content. This implies that the exciton content of the BSWPs determines the propagation behavior. Thus, the long range pseudospin transport enabled by BSWPs and the degeneracy lifted by an external magnetic field both contribute to the net spatial separation of different valley pseudospin species. These results not only contribute towards our understanding of the governing physics of BSWP transport, but also provide a pathway for realizing on-chip quantum communication and information processing. <br/><br/>[1] Liu, B.; Lynch, J.; Zhao, H.; Conran, B. R.; McAleese, C.; Jariwala, D.; Forrest, S. R. Long-Range Propagation of Exciton-Polaritons in Large-Area 2D Semiconductor Monolayers. <i>ACS Nano</i> <b>2023</b><i>, 17</i> (15), 14442– 14448, DOI: 10.1021/acsnano.3c03485