Large metasurface-based optical concentrators for infrared photodetectors

We demonstrate large metasurface-based optical concentrators utilizing a modular design. In this approach each metasurface module comprising the optical concentrator is designed individually, thus reducing the required computational resources and improving the design versatility. Each module has an off-axis focal point, and this point is located in between the module center and the intended detector center. This reduces the necessary deflection angle, thus improving the concentrator design. We designed, fabricated and tested 300-$\mu$m-diameter metasurface-based optical concentrators operating in the 3-5 $\mu$m mid-wavelength infrared range. These optical concentrators are fabricated on a gallium antimonide substrate, which can be used for epitaxial growth of infrared detectors. This allows future monolithic integration of these concentrators with detectors epitaxially grown on the front side of the substrate, and concentrator fabricated on the backside. The optical concentrators enhanced the measured optical intensity at the intended detector position up to a factor of $6.4$, in the future this will improve the signal-to-noise ratio of detectors and increase their operating temperature.