Morphological design of optical cavities for frequency-selective black absorbers

We study various morphological effects due to optical cavities that are formed into metal substrates for the implementation of frequency-selective black absorbers. The absorption spectra (λ = 500-3000 nm) of patterned metal substrates are investigated by conducting full-vectorial electromagnetic simulations. The diameter of optical cavities determines a cut-off wavelength at which absorption begins to drop off exponentially. The cut-off wavelength is gradually redshifted by increasing the diameter of the optical cavities, which is associated with the tuning of the fundamental transverse mode. The height of optical cavities determines the number and amplitude of absorption peaks, which originate from Fabry-Perot modes with different longitudinal orders. Also, the absorption features depend strongly on the refractive index of the material within optical cavities; optical cavities filled with a dielectric yield improved absorption, even with a relatively shallow height. With an integration of patterned tantalum (Ta) and tungsten (W) thermal emitters, the power conversion efficiencies of thermophotovoltaics are predicted, accounting a body temperature of 1300 K and the quantum efficiency of a typical infrared photovoltaic cell. Tailored optical cavities lead to a dramatic enhancement in the power conversion efficiency up to 11.6 and 2.1 fold compared to planar structures, for Ta and W thermal emitters, respectively. These numerical findings and underlying physics will provide valuable design strategies to thermal radiation engineered applications such as solar absorbers, radiative coolers, as well as thermophotovoltaics.