In this paper we show how to systematically design anti-reflective metasurfaces for the mid-infrared wavelength range. To do so, we have utilized a multilayer arrangement involving a judiciously nano-perforated surface, having air holes, arranged in a hexagonal fashion. By exploiting an effective medium approach, we optimized the dimensions of the surface features in our design. Here, we report a broadband reflectivity 3.5 − 5.5 μm that is below 10% over a broad range of incident angles 00 ≤ θ𝑖 ≤ 700 , irrespective of the incident polarization (TE, TM). Our experimental results are in excellent agreement with full-wave finite element simulations. This systematic approach can be used to design a wide variety of patterned metasurfaces, capable of controlling the phase of the incident optical field.
Parity-time (PT) symmetric complex structures can exhibit peculiar properties which are otherwise unattainable in traditional Hermitian systems. This is achieved by judiciously involving balanced regions of gain and loss. Here we investigate the scattering properties of PT-symmetric diffraction gratings. The presence of the imaginary potential can modify the light transport properties in their far field. This is an outcome of a local power flow taking place between the gain and loss regions in the near field. We show that for a certain gain/loss contrast, all the negative diffraction orders can be eliminated while the positive diffraction orders can be amplified.