Topological Space-Time Photonic Transitions in Angular-Momentum-Biased Metasurfaces

In this paper, topological space-time photonic transition of light in angular-momentum-biased metasurfaces is established, which yields a superposition of orbital-angular-momentum (OAM)-carrying beams at distinct frequency harmonics upon scattering whose topological charges and frequency shifts are correlated. A reflective dielectric metasurface is considered that consists of silicon nanodisk heterostructures integrated with indium-tin-oxide and gate dielectric layers placed on a back mirror forming a dual-gated field effect modulator. The metasurface is divided into several azimuthal sections wherein the nanodisk heterostructures are interconnected via biasing lines. Addressing each azimuthal section with radio-frequency biasing signals separately allows for flexible implementation of different spatiotemporal modulation profiles. Active tuning of OAM states with high mode-purity is demonstrated, which yields minimal cross-talk between OAM channels in a mode-multiplexed communication system. Moreover, the role of spatiotemporal modulation profile on the spectral and spatial diversity of OAM states is explored. It is shown that angular-momentum-biased metasurfaces allow opportunities for hybridized mode-division and wavelength-division multiple access through multiplexing and multicasting across distinct OAM states and wavelengths. The nonreciprocity of topological space-time photonic transitions across the temporal frequency domain and Hilbert space of OAM states is also investigated giving rise to distinct twisted light channels in up- and down-links.