# Nonlinear metasurfaces governed by bound states in the continuum

## TL;DR

This paper introduces a novel approach using bound states in the continuum to enhance nonlinear responses in metasurfaces, demonstrated through third-harmonic generation experiments with silicon meta-atoms.

## Contribution

It develops a general method to utilize sharp resonances from bound states in the continuum for engineering and boosting nonlinear optical effects in metasurfaces.

## Key findings

- Third-harmonic generation efficiency depends on asymmetry parameter.
- Critical coupling affects nonlinear conversion efficiency.
- Experimental validation with silicon metasurfaces.

## Abstract

Nonlinear nanostructured surfaces provide a paradigm shift in nonlinear optics with new ways to control and manipulate frequency conversion processes at the nanoscale, also offering novel opportunities for applications in photonics, chemistry, material science, and biosensing. Here, we develop a general approach to employ sharp resonances in metasurfaces originated from the physics of bound states in the continuum for both engineering and enhancing the nonlinear response. We study experimentally the third-harmonic generation from metasurfaces composed of symmetry-broken silicon meta-atoms and reveal that the harmonic generation intensity depends critically on the asymmetry parameter. We employ the concept of the critical coupling of light to the metasurface resonances to uncover the effect of radiative and nonradiative losses on the nonlinear conversion efficiency.