Excitonic bound states in the continuum in van der Waals heterostructure metasurfaces

We investigate the formation of excitonic bound states in the continuum in van der Waals (vdW) heterostructures composed of two-dimensional excitonic vdW layers and an optically resonant patterned vdW thin film. We show that the radiative losses of the exciton can be completely suppressed---not through conventional methods such as total internal reflection, Bragg mirrors, or metallic layers, but instead via destructive interference of exciton emission rates to distinct optical modes of the metasurface. We formulate the general conditions of excitonic BICs as a vanishing Purcell factor with nonvanishing vacuum local density of states at the exciton frequency. We propose a mechanism to achieve excitonic quasi-BICs with almost complete suppression of radiation via exciton coupling with a guided-mode resonance and multiple Fabry-P\'erot modes. We show that in unpatterned vdW slabs, the Purcell factor suppression is defined exclusively by the slab's permittivity achieved via positioning the 2D exciton layer in the minimum of the mode electric field. We confirm through numerical simulations that, in periodically patterned heterostructure metasurfaces, the Purcell factor can be suppressed by more than five orders of magnitude, and this effect is not due to vanishing local electric fields. Our results demonstrate the formation of excitonic quasi-BICs and their potential for advancing quantum optics and information processing.