Agile Free-Form Signal Filtering and Routing with a Chaotic-Cavity-Backed Non-Local Programmable Metasurface.

Filter synthesis is an inverse problem that is traditionally approached rationally by engineering the coupling between selected pairs of lumped resonators. The implicit restriction to spatially disjoint resonators strongly limits the design space, making it challenging to build extremely tunable filters. Here, agile free-form signal filtering and routing are demonstrated with an alternative purely-optimization-based approach leveraging a multi-parameter programmable system with many spatially overlapping modes. The approach is largely insensitive to system details other than the programmable system configuration. In the fabricated prototype, all ports and tunable meta-elements are strongly coupled via a quasi-2D chaotic cavity such that the meta-elements' configuration efficiently controls the transfer function between the ports. The all-metallic device enables low-loss and ultra-wideband (UWB) tunability (7.5-13.5 GHz) and guarantees signal-strength-independent linearity. First, theoretical predictions about reflectionless and transmissionless scattering modes (including transmissionless exceptional points) are experimentally confirmed. Second, these transfer function zeros are imposed at desired frequencies within an UWB range. Third, low-loss reflectionless programmable signal routing is achieved. Fourth, the trade-off between routing fidelity and bandwidth is investigated, achieving 20 dB discrimination over 10 MHz bandwidth. Fifth, UWB-tunable multi-band filtering is demonstrated that rejects (< -24 dB) or passes (≥ -1 dB) signals in specified bands whose centers, widths and number are reprogrammable.