1 Mechanical Engineering Department, College of Engineering, King Saud University, Riyadh, Saudi Arabia.
2 School for Engineering of Matter, Transport and Energy,
Arizona State University, Tempe, Arizona, USA.
*Email: firstname.lastname@example.org (H. Alshehri), email@example.com (L. Wang)
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.
Table of Contents
Keywords: Metamaterials; Selective absorption; Magnetic polariton.
Metamaterials, which are artificial micro- or nano-structures with electromagnetic properties that do not occur naturally, have garnered great interest in recent years. Moreover, their radiative properties can be changed by altering their geometric parameters. This versatility allows them to have many applications including: solar absorption,[1-4], imaging,[5,6] infrared spectroscopy,[7,8] radiative cooling,[9,10] invisibility cloaking,[11,12] light trapping, sensing,[14-16] selective thermal emitters,[4,17] and perfect absorbers.[1,18]
In metamaterial absorbers, radiation can be absorbed due to the excitation of resonance modes such as surface plasmon resonance (SPP)[19,20] and magnetic polariton (MP).[21,22] SPP refers to collective oscillations of free electrons at a metal-dielectric interface in response to the electric field of an incident electromagnetic wave, while MP stems from coupling between the incident waves and the artificial magnetic resonance inside a metamaterial structure. Metamaterials that exhibit these resonance modes are typically made from a metal-dielectric-metal stack which can have periodic metallic nano- or micro-structures on top, including grating,[1,23-25] nanopyramids,