Wide-Angle Spectrally Selective Absorbers and Thermal Emitters Based on Inverse Opals

Engineered optical absorbers are of substantial interest for applications ranging from stray light reduction to energy conversion. We demonstrate a large-area (centimeter-scale) metamaterial that features near-unity frequency-selective absorption in the mid-infrared wavelength range. The metamaterial comprises a self-assembled porous structure known as an inverse opal, here made of silica. The structure's large volume fraction of voids, together with the vibrational resonances of silica in the mid-infrared spectral range, reduce the metamaterial's refractive index to close to that of air and introduce considerable optical absorption. As a result, the frequency-selective structure efficiently absorbs incident light of both polarizations even at very oblique incidence angles. The absorber remains stable at high temperatures (measured up to similar to 900 degrees C), enabling its operation as a frequency-selective thermal emitter. The excellent performance of this absorber/emitter and ease of fabrication make it a promising surface coating for passive radiative cooling, laser safety, and other large-area applications.