Spatiotemporal Optical Vortices From All-Dielectric Bilayer Metagratings

TL;DR

This paper demonstrates a novel method to generate stable spatiotemporal optical vortices using all-dielectric bilayer metagratings exploiting bound states in the continuum, avoiding lossy metallic structures and complex pulse-shaping.

Contribution

The authors introduce a simple lateral shift technique in dielectric metagratings to convert BICs into quasi-BICs for efficient STOV generation, a scalable and low-loss approach.

Findings

Experimental transmission zero and phase singularity confirmed

Agreement between theoretical analysis and measurements

Potential for quantum light manipulation and spatiotemporal shaping

Abstract

Spatiotemporal optical vortices (STOVs) carry transverse orbital angular momentum within the space-time domain, rendering them powerful tools for constructing high-dimensional and quantum optical fields. However, most existing approaches rely on highly lossy metallic structures or complex pulse-shaping systems. Here, we propose and experimentally demonstrate an STOV generation scheme based on a bound state in the continuum (BIC) in an all-dielectric bilayer metagrating. By simply introducing a lateral shift between the upper and lower layers of the vertical slots on the dielectric metagrating, the {\Gamma}-point BIC transforms into a quasi-BIC (qBIC) with directional radiation and asymmetric coupling. This qBIC further leads to an isolated zero-transmission dip associated with a clear phase singularity and branch cut in the frequency-momentum response, enabling a stable STOV generation under the excitation by a spatiotemporal Gaussian pulse. The multipole analysis of the STOV generation reveals the key role of the asymmetric magnetic dipole of the qBIC. Experimentally, free-space transmission measurements reveal transmission zero and branch cut that agree excellently with theoretical analysis. Therefore, our work provides a scalable new route for manipulating spatiotemporal optical fields on low-loss all-dielectric metasurfaces via only gliding offsets, with potential applications in directional coupling of quantum light sources and spatiotemporal shaping of single-photon wave packets.