Observation of temporal Wood's anomaly in folded time gratings: surface-wave-enhanced transmission and the emergence of gain

Abstract

Wood's anomaly is a fundamental wave phenomenon that stems from the interplay between farfields and surface-wave (SW) resonances through structured interfaces. Recent theories have suggested temporal analogs of such SW coupling processes by employing frequency transitions via time-periodic interfaces, rather than classical wavevector (momentum) transitions via space-periodic gratings. In this paper, we observe this phenomenon experimentally by devising a folded time grating of a single, waveguide-enclosed, time-modulated element; this substantially reduces complexity and power consumption and facilitates transmissive operation. We support our findings by deriving a comprehensive Floquet-Bloch analysis that exhibits excellent agreement with measurements. Importantly, we utilize our framework and experiment to reveal a unique regime of temporal Wood's anomaly, in which coupling to negative SW frequencies manifests tunable parametric amplification unattainable via traditional spatial modulation. Beyond the fundamental contribution, our results provide an economical path for universally synthesizing intricate temporal apertures to enhance dynamic filtering and leaky-wave antennas.