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

Abstract

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.