Abstract
<title>Abstract</title> <p>Spacetime interfaces provide a powerful route to wave control beyond static media. While scattering at subluminal and superluminal interfaces has been recently observed, the interluminal regime—where the interface velocity lies between the wave velocities of the two adjacent media—has remained largely unexplored. In homogeneous non-dispersive media, interluminal interfaces are predicted to produce one- or three-wave responses due to over- or under-determined boundary conditions, yet these effects have not been experimentally observed. Here, we realize tunable interluminal interfaces in a dispersive Floquet synthetic temporal lattice formed by two coupled fiber loops. We observe the classical interluminal behaviors expected in homogeneous media—one reflection with multiple transmissions, total reflection, and complete transmission—while the Floquet band structure enriches the process, enabling a single incident wave to access multiple outgoing channels via band-matching and producing characteristic spatiotemporal interference from coherent channel superposition. By cascading two co-moving interfaces into an interluminal slab, we demonstrate a generalized Floquet Fabry-Pérot resonator exhibiting order-selective transmission controlled by the slab width. Our work establishes a channel-resolved framework for interluminal scattering and paves the way for programmable spacetime photonic devices.</p>