Space-squeezing optics in the microwave spectral region

TL;DR

This paper demonstrates a microwave spaceplate device that emulates free-space diffraction effects with a thin metamaterial, enabling miniaturization of optical systems by compressing space in the microwave spectrum.

Contribution

First experimental demonstration of a microwave spaceplate operating in ambient air, using a tunable Fabry-Pérot cavity with metasurfaces to achieve significant space compression.

Findings

Achieved a space compression factor of approximately 6.

Operates over a fractional bandwidth of 6%.

Can reach arbitrarily high compression by limiting NA and bandwidth.

Abstract

Optical systems often consist largely of empty space, as diffraction effects that occur through free-space propagation can be crucial to their function. Contracting these voids offers a path to the miniaturisation of a wide range of optical devices. Recently, a new optical element - coined a 'spaceplate' - has been proposed, that is capable of emulating the effects of diffraction over a specified propagation distance using a thinner non-local metamaterial [Nat. Commun. 12, 3512 (2021)]. The compression factor of such an element is given by the ratio of the length of free-space that is replaced to the thickness of the spaceplate itself. In this work we test a prototype spaceplate in the microwave spectral region (20-23\,GHz) - the first such demonstration designed to operate in ambient air. Our device consists of a Fabry-P\'erot cavity formed from two reflective metasurfaces, with a compression factor that can be tuned by varying the size of perforations within each layer. Using a pair of directive horn antennas, we measure a space compression factor of up to approx. 6 over an NA of 0.34 and fractional bandwidth of 6%. We also investigate the fundamental trade-offs that exist between the compression factor, transmission efficiency, numerical aperture (NA) and bandwidth of this single resonator spaceplate design, and highlight that it can reach arbitrarily high compression factors by restricting its NA and bandwidth.