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Immense optical field enhancement was predicted to occur for the Berremanmode in ultrathin films at frequencies in the vicinity of epsilon near zero(ENZ). Here, we report the first experimental proof of this prediction in themid-infrared by probing the resonantly enhanced second harmonic generation(SHG) at the longitudinal optic phonon frequency from a deeplysubwavelength-thin aluminum nitride (AlN) film. Employing a transfer matrixformalism, we show that the field enhancement is completely localized insidethe AlN layer, revealing that the observed SHG signal of the Berreman mode issolely generated in the AlN film. Our results demonstrate that ENZ Berremanmodes in intrinsically low-loss polar dielectric crystals constitute apromising platform for nonlinear nanophotonic applications.

Second Harmonic Generation from Phononic Epsilon-Near-Zero Berreman Modes in Ultrathin Polar Crystal Films