Comprehensive optical and thermal investigation of optimal near-infrared absorption enhancement of nano-patterned aluminum

Compelling electromagnetic (EM) properties, such as tight EM-confinement and fabrication-compatible architectures, propel the importance of metal-based nanophotonics in small-footprint devices, such as highly efficient optical absorbers. Here, we focus on boosting the absorption of optically thick bulk Aluminum (Al), the most abundant metal, solely by corrugating its surface periodically. Application-oriented, we utilize the simplest one-dimensional (1D) rectangular-grating pattern that can be remotely engraved on a large scale, e.g., using femtosecond pulsed lasers. We devise four designs for various utilizations optimized for perfect absorption around the application-abundant 1.06μm wavelength under transverse electric (TE), transverse magnetic (TM), and unpolarized (denoted TEM) irradiations. Lacking attractive plasmonic confinement properties, the s-polarization is considered electromagnetically “degenerate” to the p-polarization case, making it rarely explored in the literature of 1D-grating architectures. Nevertheless, we reveal that perfect s-polarized absorption is also attainable in such designs without adding extra layers. The designs’ performance, properties, limitations, and the propelling electromagnetic physics are theoretically and numerically discussed in-depth. A large-area sample of the TEM-optimized design was fabricated, facilitating uniquely extensive optical and radiative heating experimental investigation with realistic near-to-normal Gaussian-beam incidence. The absorption enhancement derived from wideband and single-frequency optical and radiative heating measurements agree well, also with the numerical simulations. Despite fabrication-derived thickness deviation, the design experimentally showed a 9.6-(TE), 6-(TM), and 7.8-(TEM) fold absorption enhancement over bulk Al. Considering the one-dimensional-patterning simplicity and its large-area fabrication compatibility, it is noteworthy and pioneering, particularly for the challenging s-polarization case. We also study a polarization-sensitive diffused reflection scattering pattern, evidently postulated to result from resonated surface roughness, highlighting the importance of radiative heating measurements. The elaborated original findings and methods are highly applicable alongside the prevalent material and spectrum.