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

## TL;DR

Researchers created a new photonic structure using MoSe2 that supports unique optical modes and boosts harmonic generation efficiency.

## Contribution

The study demonstrates the first epitaxial MoSe2 subwavelength grating supporting optical bound states in the continuum.

## Key findings

- MoSe2 subwavelength gratings host bound states in the continuum confirmed by reflectivity measurements.
- Third-harmonic generation efficiency is enhanced by over 1,000 times compared to unstructured MoSe2.
- The structures are promising for compact photonic devices like lasers and wavefront controllers.

## 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.