Simultaneous observation of bright and dark polariton states in subwavelength gratings made from quasi-bulk WS$_2$

Abstract: Over the last decade, layered crystals, dubbed van der Waals (vdW) materials, have attracted tremendous interest due to their unique properties in their single and few layer regimes. Their bulk counterparts, however, have only been recently explored as building blocks for nanophotonics as they offer promising properties such as high refractive indices and adherence to any type of substrates. We present here a variety of 1D grating structures composed of bulk transition metal dichalcogenide (TMD) WS$_2$ as a highly tunable and versatile platform for observation of multi-level polaritonic system. The WS$_2$ excitons are simultaneously strongly coupled with the two grating photonic modes including the Bound State in the Continuum (BIC) of the lower energetic mode giving rise to polariton-BICs (pol-BICs). The polaritonic dispersion shapes can be varied in a straightforward fashion by choosing WS$_2$ films of different thicknesses and by changing the period of the grating.