Generation of Spatiotemporal Microwave Vortices With Transverse Orbital Angular Momentum via Momentum--Frequency Spectral Singularity Engineering

Abstract

Spatiotemporal vortices (STVs) carrying transverse orbital angular momentum (T-OAM) provide a distinct degree of freedom for wave manipulation in coupled space and time. However, their implementation in the microwave regime is challenging because optical pulse-shaping techniques cannot be directly transferred to microwaves, while array-based spatiotemporal synthesis generally requires multiple synchronized radio frequency channels. Here, we propose a metasurface-based approach for generating spatiotemporal microwave vortices (STMVs) by engineering an isolated transmission singularity in the joint momentum–frequency $(k_{x}-\omega)$ domain. Temporal coupled-mode theory (TCMT) and full-wave decomposition show that the symmetry-breaking metasurface unit converts a transmission nodal line into an isolated zero, where destructive interference between background transmission and resonant radiation produces a singularity in the $(k_{x}-\omega)$ domain. Fourier synthesis maps this spectral singularity into an edge-dislocation vortex in the longitudinal $xt$ plane, with T-OAM oriented perpendicular to the propagation direction. This vortex field differs from conventional spatial orbital angular momentum (OAM) vortices defined in the transverse $xy$ plane with longitudinal OAM. A standard horn antenna provides the required transverse-wavevector spectrum and excites the designed metasurface without phased-array feeding or synchronized temporal modulation. Full-wave simulations demonstrate the generation of STMV with a winding number $q=1$ , an average T-OAM value of 1.178, and a maximum $l = 1$ T-OAM mode purity of 0.830. Experimentally reconstructed fields retain $q=1$ , with a T-OAM value of 1.43 and a maximum $l=1$ mode purity of 0.737. These results demonstrate a compact passive route for generating and characterizing STMV carrying T-OAM, which is important for potential microwave applications of STMVs.