Self-Hybridized, Polarized Polaritons in ReS2 Crystals

Strong coupling between excitons and photons in a photonic cavity provides a promising platform for exploring a wide array of physical effects. To date, there are many examples of strong coupling in group VI TMDCs, but no examples of group VII polaritons have been demonstrated. Here, we show evidence of polaritons in ReS2 flakes, where the photonic confinement arises from internal reflections within the flake itself, without requiring a separate cavity to confine the field. By examining several different samples of varying thicknesses (each thickness with a different intrinsic cavity mode), we vary the detuning and map out a polariton dispersion. The polaritons exhibit mutiple peaks in the absorption spectra, a fact that may help elucidate the observed multiple resonances in similar group VII TMDCs. As a unique feature of the anisotropic crystal structure of group VII TMDCs, these polaritons inherit the underlying polarization dependence of the two lowest order bright ReS2 excitons, displaying a polarization-tunable absorption, with maximum absorption greater than 95%.