Quasi-Bound States in the Continuum on Dislocated Bilayer Metal Gratings for Spatiotemporal Vortex Pulse Generation

Spatiotemporal vortex pulses (STVPs) with transverse orbital angular momentum (OAM) have recently stimulated great interest, influencing wide disciplines from classical to quantum optics. However, generating sophisticated STVPs in high-efficiency and compact systems remains a challenge. This work outlines an ultra-compact metasurface approach to efficiently generate STVPs through the coupling of free-space plane waves with a quasi-bound state in the continuum (quasi-BIC). The approach leverages external excitation of the quasi-BIC at the Gamma-point by slightly dislocating two sub-layer gratings to break the mirror symmetries. This operation converts the incident wave into a unidirectional surface mode and induces a Fano resonance, resulting in STVPs in the frequency-momentum domain. The method is demonstrated experimentally with a meta-grating to produce an electromagnetic (EM) STVP with a topological charge of l = -1 with high fidelity and use the set-up to unravel the hitherto unexplored diffraction and dispersive properties of near-field STVPs. The present work can be extended from the microwave to the visible light regime by simply scaling the metasurface, and allowing high-order OAM generation through cascaded elements, thereby paving a robust way to build up ultra-compact STVP multiplexing devices for future applications.