A Narrowband Optical Perfect Absorber and Refractive Index Sensor Based on an Epsilon-Near-Zero Metamaterial Using Ag-Ge-Si Nanowires

In this paper, a narrowband optical absorber with an absorption peak of 99.9% at the telecommunication wavelength of 1310 nm is presented. It consists of nanowires composed of five periods of Ag-Ge-Si disks on a Si substrate. The design of the stacks is based on epsilon-near-zero (ENZ) metamaterials, which causes this structure to have a relatively small thickness (i.e., thickness < 5λ, where λ is the working wavelength). By adjusting the thickness of the stacks using the effective medium theory (EMT) model, we have achieve the desired ENZ wavelength. The perfect absorption feature has been obtained by the synergy of resonances of surface plasmon polaritons (SPP) and ENZ modes. The proposed structure has been simulated using the finite difference frequency domain method. The simulation results indicate that the absorption rate remains almost 90% up to an angle of 75° and is insensitive to polarization. Furthermore, by changing the arrangement of the neighboring stacks, we see an increase in the sensitivity of the absorption peak to the refractive index variations of the environment. In general, narrowband absorbers have good potentials for applications such as filtering, sensing, and detecting, which underlines the significance of this work.