A Thickness Dependent Enhancement of Optical Resolution in the Vicinity of an Epsilon-near-zero Slab

Recent studies reports that an epsilon-near-zero (ENZ) thin slab between a specimen and a substrate contributes in enhancing the spatial resolution of the optical system. Here, we investigate the ENZ thickness dependence of the resolution enhancement. By employing the edge response function, the resolution of the optical system is directly measured when imaging a sharp edge of a metal film. We found that the optimum ENZ slab thickness was 700 nm and the achieved resolution was 11 {\mu}m at the wavelength of 8 {\mu}m. Owing to the enhanced resolution by ENZ slab, we successfully imaged the subwavelength slit arrays.