Metamaterial perfect absorber with unabated size-independent absorption

Metamaterial absorbers open a new door for the design of optical harvesting devices ranging from the microwave to optical regimes. The top resonator in these structures is critical for the function of metamaterial absorbers. The resonant frequency, bandwidth, and maximum absorption mainly depend on the choice of material, shape, and size of the top resonator. The maximum absorption is generally impaired as the size of the resonator changes, due to the high sensitivity of impedance matching with the medium. In this paper, we experimentally demonstrate a metamaterial perfect absorber with unabated absorption as its resonator's size changes. The perfect absorber is based on an array of metal squares inscribed with a hollow square. The absorption maxima stay above 98% as the size changes from 600 to 1500 nm in the mid-infrared region, agreeing with simulated results yielding an absorption of ~100%. The unabated absorption properties can be interpreted by the equivalent circuit theory. Moreover, the experimental absorption remains above 91% for incident angles change up to 50°, both for TE and TM polarization. Our work offers a method for achieving stable perfect absorption in sensing, filtering, and selective thermal emission.