  In this work, a metamaterial selective solar absorber made of nanostructured
titanium gratings deposited on an ultrathin MgF2 spacer and a tungsten ground
film is proposed and experimentally demonstrated. Normal absorptance of the
fabricated solar absorber is characterized to be higher than 90% in the UV,
visible and, near infrared (IR) regime, while the mid-IR emittance is around
20%. The high broadband absorption in the solar spectrum is realized by the
excitation of surface plasmon and magnetic polariton resonances, while the low
mid-IR emittance is due to the highly reflective nature of the metallic
components. Further directional and polarized reflectance measurements show
wide-angle and polarization-insensitive high absorption within solar spectrum.
Temperature-dependent spectroscopic characterization indicates that the optical
properties barely change at elevated temperatures up to 350{\deg}C. The
solar-to-heat conversion efficiency with the fabricated metamaterial solar
absorber is predicted to be 78% at 100{\deg}C without optical concentration or
80% at 400{\deg}C with 25 suns, and could be further improved with better
fabrication processes and geometric optimization during metamaterial design.
The strong spectral selectivity, favorable diffuse-like behavior, and excellent
thermal stability make the metamaterial selective absorber promising for
significantly enhancing solar thermal energy harvesting in various system at
mid to high temperatures.
