Highly-efficient nonlinear image tuning through magnetic dipole quasi-BIC ultra-thin resonators

We propose an ultra-thin silicon metasurface supporting a high-quality quasi-bound-state-in-the-continuum (BIC) generated by the collective magnetic dipole (MD) resonance excited in the subdiffractive periodic systems. Such quasi-BIC MD state leads to a robust near-field enhancement and a significant boost of the nonlinear process, resulting in measured 500-fold enhancement of third-harmonic emission in comparison to the conventional silicon disk metasurface. We further demonstrate the highly-efficient dynamical switching experimentally over nonlinear images via polarisation and wavelength control, opening the way for various applications in highly-efficient nonlinear metadevices including tunable laser, tunable displays, nonlinear imaging.