Optical bound states in the continuum in subwavelength gratings made of an epitaxial van der Waals material

TL;DR

This paper demonstrates the design and fabrication of ultrathin MoSe2 subwavelength gratings that host bound states in the continuum, significantly enhancing nonlinear optical processes and enabling compact photonic devices.

Contribution

It introduces a novel method to create MoSe2-based subwavelength gratings with bound states in the continuum, combining theoretical design and simple fabrication techniques.

Findings

Confirmation of bound states in the continuum in MoSe2 gratings

Over three orders of magnitude enhancement in third harmonic generation

Potential for ultra-compact photonic devices

Abstract

High refractive index (4.4 at 1100 nm), negligibly small absorption in near-infrared spectral range, and ease of processing make MoSe$_2$ a perfect material for applications in near-infrared photonics. So far, implementation of MoSe$_2$-based photonic structures has been hindered by the lack of large surface MoSe$_2$ substrates. The use of molecular beam epitaxy allows the production of homogeneous layers of MoSe$_2$ 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 42-nm thick, epitaxially-grown MoSe$_2$ 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 three orders of magnitude as compared to the unstructured MoSe$_2$ layer. The presented results are promising for the realization of flat, ultra-compact devices for lasing, wavefront control, and higher-order topological states of the light.