A hybrid Au-Ag subwavelength metallic zone plate-like structure was put forth for the purpose of preventing oxidation and sulfuration of Ag film, as well as realizing superfocusing. The Au film acts as both a protector and modulator in the structure. Focusing performance is analyzed by means of three-dimensional finite-difference and time-domain (FDTD) algorithm-based computational numerical calculation. It can be tuned by varying thicknesses of both Au and Ag thin films. Our calculation results show that thickness difference between the Au and Ag thin films plays an important role for transmission spectra. Ratio of Au to Ag film thicknesses, hAu/hAg is proportional to the relevant peak transmission intensity. In case of hAu≈hAg=50 nm, both transmission intensity and focusing performance are improved. In addition, the ratio hAu/hAg strongly influences position of peak wavelengths λAu and λAg generated from beaming through the metallic structures.
"Hybrid Au-Ag subwavelength metallic structures with variant periods for superfocusing," Journal of Nanophotonics 3(1), 033504 (1 June 2009). https://doi.org/10.1117/1.3159299