A Dual-Band Spaceplate: Contracting the Volume of Quasi-Optical Systems

A "spaceplate" approximates the angular response of free space with a much thinner nonlocal metamaterial. They have the potential to significantly shrink the volume of optical and quasi-optical systems, by allowing elements such as lenses to be moved closer together. However, spaceplates exhibit a tradeoff between their operational angular and spectral bandwidths. In this work, we present a new space-compression concept: a dual-band spaceplate capable of operating in two distinct frequency bands simultaneously. This allows the limited spectral bandwidth to be targeted to application-specific parts of the spectrum. Our design is composed of a multilayer stack of semitransparent mirrors separated by free-space voids. These layers act as a system of coupled Fabry-Perot (FP) cavities-the guided-mode resonances of which emulate the effect of free-space propagation. The stack is engineered to exhibit two resonant subbands, with the frequency separation a tunable parameter in the design. We numerically and experimentally demonstrate a dual-band spaceplate exhibiting space compression at two distinct frequency subbands centered about 21.4 and 23.7 GHz.