BIC-stimulated tunable second-harmonic generation in transition metal dichalcogenide film coupled with high-quality dielectric metasurface

Atomically thin films of transition metal dichalcogenides (TMDCs) are a promising class of novel materials that can contribute to the development of new information processing methods in optoelectronics and photonics. However, the functionalities of TMDCs monolayers for nonlinear-optical applications are limited by the atomic length of the light-matter interaction. Marrying these monolayers with resonant dielectric metasurfaces enables higher efficiencies of nonlinear processes, thereby circumventing the aforementioned drawback. This paper demonstrates the amplification of second-harmonic generation efficiency up to 3 orders of magnitude attributed to quasi-bound state in the continuum (quasi-BIC). The enhancement occurs during the gradual spectral tuning of the quasi-BIC state, excited in a metasurface made of titanium dioxide nanodisks, until its spectral position coincides with a solitary bright exciton of a molybdenum diselenide monolayer. This spectral overlap is achieved by varying the angle of incidence of incoming radiation.