Nonlinear optical metasurfaces empowered by bound-states in the continuum

Optical bound-states in the continuum (BICs) have greatly enriched the field of nonlinear optics with novel ways to control and manipulate light-matter interaction at the nanoscale. This has been made possible by their unique physical properties, including effective confinement of light, non-trivial topological features, and robustness upon the propagation of the optical field both in the real and momentum space. Regarding the exploration of nonlinear optical response in various photonic nanostructures supporting BICs, particular attention has been paid to optical metasurfaces, chiefly due to their ability to control the light flow at subwavelength scale, design and fabrication flexibility, and convenient phase-matching conditions. In this review, we outline and discuss recent advances in metasurface-based frequency conversion processes utilizing the versatile physics of BICs, with a particular emphasis on the main physics background pertaining to nonlinear optical phenomena and optics of BICs, as well as state-of-the-art functionalities enabled by BIC-driven nonlinear metasurfaces. These applications include harmonic generation, harmonic chiroptical effects, generation of complex quantum states, and broadband terahertz generation. In addition, several emerging research fields and the existing challenges of photonic nanodevices relying on BICs are discussed.