Selective Thermal Emission from Thin-Film Metasurfaces

The mid-infrared (mid-IR), as the spectral range where all finite temperature biological and mechanical objects emit thermal radiation, and where numerous molecular species have strong vibrational absorption resonances, is of significant importance for both security and sensing applications. The design of materials with engineered absorption resonances, which by Kirchoff's Law, should give strongly selective emission at the design resonance upon thermal excitation, allows for the control of the spectral character of the material's thermal emission. Designed as a thin film coating, these structures can be applied to grey-body emitters to shift the grey-body thermal emission into predetermined spectral bands, altering their appearance on a thermal imaging system. Here we demonstrate strongly selective mid-infrared absorption and thermal emission from three classes of sub-wavelength thin-film materials. First, we demonstrate selective thermal emission from patterned, commercially-available steel films, via selective out-coupling of thermally-excited surface modes. Subsequently, we show nearperfect absorption (and strongly selective thermal emission) for wavelengths between 5-9 mu m with patterned metal-dielectric-metal structures. Finally, we demonstrate strong absorption from large area, unpatterned, thinfilm high-index dielectric coatings on highly-doped Si substrates, tunable across the mid-IR (5 - 12 mu m). Our results are compared to numerical simulations, as well as analytical models, with good agreement between experiments and models.