Tunable Unidirectional Nonlinear Emission from\n Transition-Metal-Dichalcogenide Metasurfaces

Abstract

Nonlinear light sources are central to a myriad of applications, driving a\nquest for their miniaturisation down to the nanoscale. In this quest, nonlinear\nmetasurfaces hold a great promise, as they enhance nonlinear effects through\ntheir resonant photonic environment and high refractive index, such as in\nhigh-index dielectric metasurfaces. However, despite the sub-diffractive\noperation of dielectric metasurfaces at the fundamental wave, this condition is\nnot fulfilled for the nonlinearly generated harmonic waves, thereby all\nnonlinear metasurfaces to date emit multiple diffractive beams. Here, we\ndemonstrate the enhanced single-beam second- and third-harmonic generation in a\nmetasurface of crystalline transition-metal-dichalcogenide material, offering\nthe highest refractive index. We show that the interplay between the resonances\nof the metasurface allows for tuning of the unidirectional second-harmonic\nradiation in forward or backward direction, not possible in any bulk nonlinear\ncrystal. Our results open new opportunities for metasurface-based nonlinear\nlight-sources, including nonlinear mirrors and entangled-photon generation.\n