Localized Surface Plasmon‐Enhanced Ultrathin Film Broadband Nanoporous Absorbers

Ultrathin lossy films have attracted much attention due to their strong interference persisting inside the lossy dielectric film on a reflective substrate. Here, a plasmon‐enhanced ultrathin film broadband absorber is proposed by combining the ultrathin film absorber with localized surface plasmon resonances. This concept can be realized by patterning nanoholes on an absorber comprised of an absorptive ultrathin Ge film and a reflective Au layer, where the localized surface plasmon mode is activated by metallic pore‐shaped holes. The plasmonic enhancement is resulting from the pore‐shape localized resonance mode, which increases the optical path length through scattering and concentrates the incident light field near the interface of Ge/Au. The experimental characterization results of a nanoporous ultrathin film absorber, which is fabricated with a scalable laser interference lithography approach, demonstrate its superior light absorption performance. Several materials, such as Ag, Al, and Cu, are proposed as an alternative to Au, and they can also provide plasmonic enhancement to ultrathin films. Furthermore, through an efficient way to optimize the structural dimensions of the nanoporous ultrathin film absorber, a trilayer system of TiO₂/Ge/Au achieves the total solar absorptance over 89.3% with a wavelength range of 400–1100 nm.