Broadband Solar Thermal Absorber Based on Optical Metamaterials for High-Temperature Applications

In this work, a tandem grating solar absorber is proposed, which can be easily fabricated on a wafer scale and is thermally stable up to 800 K. The base of the solar thermal absorber consists of a tungsten substrate, SiO₂ spacer, and 2D tungsten nanohole array filled with SiO₂. On top of the base structure, a 2D tungsten nanodisc array is coated with an additional SiO₂ spacer, forming the tandem grating structure. The outside area of the nanodisc array is also filled with SiO₂; thus, the proposed solar absorber is geometrically flat. In the solar spectrum from 0.3 to 2.0 μm, the total absorptance of the fabricated sample is measured to be 90%. On the other hand, in the mid-infrared region from 3 to 15 μm, the total emittance is measured to be 23% at 800 K, which greatly suppresses the thermal emission loss. Finite-difference time-domain simulation suggests that the magnetic resonance and Rayleigh anomaly are responsible for the enhanced absorption of solar radiation. It is also shown that the spectral absorptance of the proposed solar absorber is nearly insensitive to the polarization angle as well as to the incidence angle in the spectrum of solar radiation.