Hypergrating for sub-diffractional focusing enhancement in air in the mid-IR wavelength range

Hyperbolic metamaterial (HMM) has paved the way for sub-diffraction focusing inside the HMM due to the propagation of large momentum wave vectors in the HMM. However, these high momentum K modes exponentially decay outside the HMM which results in decaying of the focusing resolution in the near field of the HMM. In this work, we introduce both a HMM and a hypergrating structures for sub-wavelength focusing in air. Hypergrating is a structure that combines a HMM with a grating surface. The proposed structure consists of upper metallic slit integrated on HMM based multilayer of doped/intrinsic InAs with lower intrinsic InAs grating surface. HMM based multilayer of doped/intrinsic InAs has the advantage of tuning the focusing wavelength in the mid-IR range. The proposed structure has reported sub-wavelength focusing in air with value reaching 0.08 λ. Hypergrating structure shows focusing resolution enhancement of 0.08λ as compared to 0.15λ for a HMM without lower grating, both at wavelength of 7.3μm. The focusing resolution outside the hypergrating structure is much higher than that is observed in the HMM only due to the introduced lower grating. This structure demonstrates a good candidate for sub -wavelength IR imaging application in air.