Dynamic Control of Nonlinear Emission by Exciton-Photon Coupling in WS2 Metasurfaces

Transition metal dichalcogenides (TMDCs) have demonstrated significant potential as versatile quantum materials for light absorption and emission. Their unique properties are primarily governed by exciton-photon interactions, which can be substantially enhanced through coupling with resonant photonic structures. For example, nonlinear light emission, such as second harmonic generation (SHG) is doubly enhanced when the incident wave is resonant simultaneously with the excitonic and photonic resonance. However, the excitonic absorption of incident waves can significantly dump the SHG emission. Here, we propose and demonstrate a tunable enhancement of SHG by leveraging virtual coupling effects between quasi-bound states in the continuum (qBIC) optical resonances and tunable excitons in arrays of high-index WS2 crescent metaatoms. These crescent metaatoms excites a pure magnetic type qBIC resonance, enabling dynamic control and enhancement of nonlinear optical processes in visible spectrum. Our findings demonstrate that an array of WS2 crescent metaatoms, exhibiting qBIC resonance at half the exciton energy, enhances SHG efficiency by more than 98-fold compared to monolayer WS2 (1L-WS2) and four orders of magnitude relative to unpatterned WS2 film. This substantial SHG enhancement is tunable as a function of temperature and polarization angle of incident light, allowing us to obtain control of the virtual coupling and SHG efficiency in the visible spectrum (600-650 nm). Our work opens new avenues toward next-generation reconfigurable meta-optics devices.