Direct observation of self-hybridized exciton-polaritons in tungsten disulfide membranes and their valley polarization

Polaritons are quasiparticles with a hybrid nature of matter and light. Exciton-Polaritons (EPs) in transition metal dichalcogenides (TMDCs) have been extensively explored due to their large exciton binding energies. Owing to its strong excitonic behavior, EPs have been successfully demonstrated in a TMDCs monolayer integrated with various microcavities. Meanwhile, excitonic resonance in TMDCs is strong enough to realize inherent guided EPs in TMDCs membranes. However, unlike cavity EPs, guided EPs have been far less studied because of their non-radiative nature which makes it hard to be detected using far-field microscope techniques. For this reason, recent studies on guided EPs have been accomplished only by near-field scanning optical microscope (NSOM). Here, we report a direct measurement of optical dispersion relations for guided EPs in tungsten disulfide (WS₂) membrane via near-field coupling method and angle-resolved spectroscopy. Using oil-immersion microscope objective lens for the near-field coupling, we observed the anti-crossing behavior of the dispersion relation near the exciton resonance in freestanding WS₂, which is a signature of polaritonic dispersion. We show that WS₂ membranes are capable of supporting EPs at a nanometerthickness limit. We also confirmed the pumping power dependence of polaritonic dispersions in WS₂ membranes. Lastly, we first report the valley-polarized guided EPs in WS₂ layers at room temperature. This valley polarization originates from strong coupling of photons with excitons in two different K and K’ valleys. We believe that our results pave a new way for valleytronic and nanophotonic applications.

Reference

Shin, D.-J., Cho, H., Sung, J. and Gong, S.-H. (2022), Direct Observation of Self-Hybridized Exciton-Polaritons and Their Valley Polarizations in a Bare WS₂ Layer. Adv. Mater.. (https://doi.org/10.1002/adma.202207735)