Perfect Absorber Based on Epsilon-Near-Zero Metamaterial as a Refractive Index Sensor

We have designed a multilayer metamaterial structure which exploits the epsilon-near-zero (ENZ) mode for perfect absorption. The design procedure is based on the effective medium theory (EMT) model. The proposed multilayer structure consists of layers of Ag, InSb, SiO2. The thickness of the layers is adjusted so that the ENZ mode is at the communication wavelength of 1550 nm. Two alternations of the layered structure are placed on a metal (Ag) substrate to prevent light transmission. Placing the nano-ring cavity arrays at the top of the multilayer structure leads to excitation of the surface plasmon polaritons (SPPs) and the cavity mode. Consequently, a resonance peak with nearly perfect absorption of 99.92% for normal incidence is obtained. Since the dimensions of the nano-ring affect the resonance wavelength, it is tuned to be exactly at the wavelength of the ENZ mode, resulting in perfect absorption. Simulation results based on the finite difference frequency domain method indicate that the absorption is insensitive to polarization and the absorption efficiency remains above 90% up to a 60 degrees incident angle. The proposed absorber can be used for various optical communication applications such as filters, detectors and sensors. Finally, the structure's sensitivity to the environmental refractive index variations has been used here for refractive index sensing. A sensitivity of 200 nm/Refractive Index Unit (RIU) is obtained in this case.