Second‐Harmonic Generation Enhancement in Polar Dielectric Films via Nonlocal Longitudinal Modes and Berreman Modes

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Polar dielectrics offer considerable promise for mid‐infrared nanophotonics and nonlinear optics because of their low optical losses and spectral bands of negative permittivity. However, previous studies have generally shown that nonlocal effects in polar dielectrics lead to suppression effects. This work reveals a different phenomenon, whereby we utilize the nonlocal effect to achieve second‐harmonic generation (SHG) enhancement in polar dielectric films. Specifically, the longitudinal‐mode resonances in polar dielectrics introduce multiple longitudinal resonances. We combine the nonlocal longitudinal mode at the fundamental frequency with the Berreman mode at the second‐harmonic frequency, achieving simultaneous resonance of the fundamental and second‐harmonic waves, thereby significantly enhancing the intensity of SHG. Numerical results show that the second‐harmonic intensity in the nonlocal case can be hundreds of times higher than that in the local case. Our study indicates that nonlocality‐enhanced SHG provides a promising platform for the application of polar dielectric crystals in nonlinear nanophotonics.