Large-Scale, Bandwidth-Adjustable, Visible Absorbers by Evaporation and Annealing Process.

Optical absorbers have received a significant amount of attention due to their wide range of applications in biomedical sensing, solar cell, photon detection, and surface-enhanced Raman spectroscopy. However, most of the optical absorbers are fabricated with high-cost sophisticated nanofabrication techniques, which limit their practical applications. Here, we introduce a cost-effective method to fabricate an optical absorber by using a simple evaporation technique. The absorbers are composed of evaporated nanoparticles above a silver (Ag) mirror separated by a silicon oxide layer. Experimental results show over 77% absorption in the wavelength range from 470 to 1000 nm for the absorber with isolated Ag nanoparticles on the top. The performance of the absorber is adjustable with the morphology and composition of the top-layer nanoparticles. When the top layer was hybrid silver-copper (Ag-Cu) nanoparticles (NPs), the absorption exceeding 90% of the range of 495-562 nm (bandwidth of 67 nm) was obtained. In addition, the bandwidth for over 90% absorption of the Ag-Cu NP absorber was broadened to about 500 nm (506-1000 nm) when it annealed at certain temperatures. Our work provides a simple way to make a highly efficient absorber of a large area for the visible light, and to transit absorption from a narrow band to broadband only by temperature treatment.