Spatiotemporal optical vortices generation in the green and ultraviolet via frequency upconversion[Invited]

As a newly discovered type of structured light,a spatiotemporal optical vortex(STOV),which is remarkable for its space–time spiral phase and transverse orbital angular momentum(OAM),has garnered substantial interest.Most previous studies have focused on the generation,characterization,and propagation of STOVs,but their nonlinear frequency conversion remains largely unexplored.Here,we experimentally demonstrate the generation of green and ultraviolet(UV)STOVs by frequency upconversion of a STOV carried near-infrared(NIR)pulse emitted by a high repetition rate Yb-doped fiber laser amplifier system.First,we verify that the topological charge of spatiotemporal OAM(ST-OAM)is doubled along with the optical frequency in the second-harmonic generation(SHG)process,which is visualized by the diffraction patterns of the STOVs in the fundamental and second-harmonic field.Second,the space–time characteristic of NIR STOV is successfully mapped to UV STOV by sum-frequency mixing STOV at 1037 nm and Gaussian beams in the green band.Furthermore,we observe the topological charges of the ST-OAM could be degraded owing to strong space–time coupling and complex spatiotemporal astigmatism of such beams.Our results not only deepen our understanding of nonlinear manipulation of STOAM spectra and the generation of STOVs at a new shorter wavelength,but also may promote new applications in both classical and quantum optics.