Selective Color Metamaterial Absorber Made of Aluminum Nanodisk Arrays by Excitation of Magnetic Polaritons

We report the fabrication and optical characterization of aluminum nanodisk (AlNDs) metamaterials as selective color absorbers by exciting magnetic polaritons (MP). Anodized aluminum oxide templates are transferred onto aluminum-coated silicon wafer, followed by e-beam evaporation of aluminum and template removal leaving behind fabricated AlNDs. Scanning electron microscopy reveals AlNDs are successfully molded with disk diameter of ~390 nm and periodicity of 450 nm, however, morphology variations in disk geometry can be seen. A home-built microscale optical reflectance and transmittance microscope is developed to characterize the optical reflectance of AlNDs. A reflectance dip is experimentally observed with resonance wavelengths varying with AlND thickness, indicating selective absorption within visible spectrum. Moreover, AlNDs with different thicknesses exhibit purple, red, and green coloration as a result of tunable reflectance spectra. Simulations confirm measured spectra and elucidate mechanisms of selective absorption as excitation of MP and surface plasmon polariton. Discrepancy between measured and simulated reflectance spectra is found to be associated with morphology variation and wave diffraction. With a silica spacer, AlND absorber shows dual-band selective absorption by exciting multiple MPs illustrated by simulated electromagnetic field distributions. The results could facilitate low-cost development of selective metamaterial absorbers for solar thermal, radiative cooling, and sensing applications.