We propose an ultra-thin silicon metasurface supporting a high-quality leaky mode which is formed by partiallybreaking a bound-state-in-the-continuum (BIC) generated by the collective magnetic dipole (MD) resonance excitedin the subdiffractive periodic systems. Such a quasi-BIC MD state leads to a robust near-field enhancementand a significant boost of the nonlinear process, resulting in measured 500-fold enhancement of third-harmonicemission in comparison to the conventional silicon disk metasurface. We further experimentally demonstratethe highly-efficient nonlinear image tuning via polarisation and wavelength control, opening the way for variousapplications in high-performance nonlinear metadevices including tunable laser, tunable displays, nonlinearimaging.
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