Optical Bound States in the Continuum in Subwavelength Gratings Made of an Epitaxial van der Waals Material.

High refractive index (4.4 at 1100 nm), negligibly small absorption in the near-infrared spectral range, and ease of processing make MoSe2 the perfect material for applications in near-infrared photonics. So far, implementation of MoSe2-based photonic structures has been hindered by the lack of large-surface MoSe2 substrates. The use of molecular beam epitaxy allows the production of homogeneous layers of MoSe2 with a few-inch surface and a thickness controlled at the sub-nm level. In the present work, we design by theoretical calculations and fabricate by a simple lithography process an ultrathin subwavelength grating out of a 42 nm thick, epitaxially grown MoSe2 layer. Our polarization-resolved reflectivity measurements confirm that the gratings host a peculiar type of a confined optical mode that is a bound state in the continuum. Moreover, the fabricated structures enhance the efficiency of the third-harmonic generation by over 3 orders of magnitude as compared to the unstructured MoSe2 layer. The presented results are promising for the realization of flat, ultracompact devices for lasing, wavefront control, and higher-order topological states of the light.