A graphene-based IR Fresnel lens formed on a multiple-internal-reflection substrate

We report on the adjustable lensing effect and switchable mode performance of a graphene-based Infrared (IR) Fresnel zone plate (FZP) formed on a double-side polished Si substrate. The focusing characteristics of the graphene FZP in the IR wavelength range have been systematically investigated as a function of the Fermi energy (E_F) of the multilayer graphene rings and the thickness of the Si substrate using the Finite-Difference Time-Domain method. This graphene FZP lens enhances the focal intensity of incident light at wavelengths where the multiple internal reflections in the Si substrate is periodically maximized. Also, the contrast ratios of reflectance and transmittance between the regions with and without graphene rings, which affects the focusing performance, are effectively controlled through intraband transitions depending on the E_F of graphene FZP pattern. Specifically, the 8-layer graphene FZP lens transmits the incident light with a wavelength of 8 μm when the E_F of the graphene is lower than 0.2 eV, while it operates as both transmissive and reflective Fresnel lens when the E_F exceeds 0.2 eV. By directly depositing Ag nanoparticles on this graphene FZP to enable E_f control of graphene rings without the need for electrodes, we have experimentally confirmed the focusing performance and mode conversion properties.