Simultaneous observation of bright and dark polariton states in subwavelength gratings made from quasi-bulk WS₂

Over the last decade, layered crystals, referred to as van der Waals (vdW) materials, have attracted tremendous interest due to their unique properties in their single and few layer forms. Their bulk counterparts, however, have only been recently explored as building blocks for nanophotonics. Indeed, bulk vdW materials offer promising properties such as high refractive indices and adherence to any type of substrate. We present here a variety of 1D grating structures composed of bulk transition metal dichalcogenide (TMD) WS₂ as a highly tunable and versatile platform for observation of a multi-level polaritonic system. The WS₂ excitons are simultaneously strongly coupled with the two grating photonic modes, including the bound state in the continuum (BIC) of the lower energy mode, giving rise to polariton-BICs. The polaritonic dispersion shapes can be varied in a straightforward fashion by choosing WS₂ films of different thicknesses and changing the gratings periods.