Enhanced the Tunable Nonlinearity of Epsilon‐Near‐Zero Nonlocal Metasurface by Quasi‐Guided Mode

Abstract Quasi Bound states in the continuum(Q‐BIC) with high Q‐factors provide a powerful way to enhance the nonlinearity of metasurfaces based on the epsilon‐near‐zero (ENZ). However, Q‐BIC has high Q‐factors only at specific wavevectors and specific wavelengths for given structures and is very sensitive to structural parameters. Here, a strong nonlinearity nonlocal metasurface, consisting of dielectric silicon disks on the ENZ thin film, with wavelength‐tunable in wide‐wavevectors is proposed through the strong coupling of Brillouin zone folding‐induced quasi‐guided mode (Q‐GM) and ENZ modes. All guided modes below the light‐line are folded by periodic perturbations to form Q‐GM with high Q‐factors in wide wavevectors. Benefiting from the high Q‐factors of the Q‐GM and the sufficient field overlap with ENZ modes, the localized field enhancement and an anticrossing splitting of 260 meV are achieved. The measured nonlinear refractive index coefficient of the nonlocal metasurface is 3.8 × 10 −13 m 2 W −1 at normal incidence, which is three orders of magnitude larger than that of the indium tin oxide (ITO)‐ film. Furthermore, since the Q‐GM has high Q‐factors at wide‐wavevectors, the experimentally observed nonlinear coefficient is robustness with increasing incident angles, realizing broadband tunable strong nonlinearity. The proposed nonlocal metasurface provides a promising platform for enhancing nonlinearity at wide‐angles and multi‐wavelengths.