Mid-infrared epsilon-near-zero modes in ultra-thin phononic films

Abstract

We demonstrate strong, narrow-band selective absorption and subsequent selective thermal emission from ultra-thin planar films of polar materials at mid-infrared wavelengths. Our structures consist of AlN layers of varying thicknesses deposited upon molybdenum ground planes. We demonstrate coupling to the Berreman mode at frequencies at, or near, the longitudinal optical phonon energy of AlN. Samples are characterized experimentally by temperature-, angle-, and polarization-dependent Fourier transform infrared reflection and emission spectroscopy and modeled using a transfer matrix method approach. Strong, spectrally selective thermal emission, with near angle-independent spectral position, is demonstrated from an AlN layer with thickness t<λo/100.