Observation of space-time nonseparable helical pulses

Abstract: Manipulating optical vortices at ultrafast spatiotemporal coupled domain is still a great challenge in photonics. Especially, the single- or few-cycle level short pulses carrying stable vortex topological charge, called helical pulses, have never been experimentally realized. Here, we introduce two complementary methods for experimentally generating such space-time nonseparable helical pulses (SNHPs) across optical and microwave spectral regimes. We achieve few-cycle quasi-linearly polarized SNHPs through the polarization decomposition of optical toroidal pulses. We also generated exactly single-cycle nontransverse SNHPs directly from a microwave ultrawideband spiral emitter. These approaches not only enable the experimental realization of SNHPs but also provide a platform for further investigation into their properties and applications, such as nontrivial light-matter interactions and optical communications, marking a significant step forward in the field of structured light.