Optical spatiotemporal vortices

Spatiotemporal vortices of light,featuring transverse orbital angular momentum(OAM)and energy circulation in the spatiotemporal domain,have received increasing attention recently.The experimental realization of the controllable generation of spatiotemporal vortices triggers a series of research in this field.This review article covers the latest developments of spatiotemporal vortices of light ranging from theoretical physics,experimental generation schemes,and characterization methods,to applications and future perspectives.This new degree of freedom in photonic OAM endowed by spatiotemporal vortices paves the way to the discovery of novel physical mechanisms and photonic applications in light science.

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.

Intense harmonic generation driven by a relativistic spatiotemporal vortex beam

Spatiotemporal optical vortex(STOV)pulses carrying purely transverse intrinsic orbital angular momentum(TOAM)are attracting increasing attention because the TOAM provides a new degree of freedom to characterize light–matter interactions.In this paper,using particle-in-cell simulations,we present spatiotemporal high-harmonic generation in the relativistic region,driven by an intense STOV beam impinging on a plasma target.It is shown that the plasma surface acts as a spatial–temporal-coupled relativistic oscillating mirror with various frequencies.The spatiotemporal features are satisfactorily transferred to the harmonics such that the TOAM scales with the harmonic order.Benefitting from the ultrahigh damage threshold of the plasma over the optical media,the intensity of the harmonics can reach the relativistic region.This study provides a new approach for generating intense spatiotemporal extreme ultraviolet vortices and investigating STOV light–matter interactions at relativistic intensities.

Engineering photonic angular momentum with structured light:a review

Structured light with inhomogeneous phase,amplitude,and polarization spatial distributions that represent an infinite-dimensional space of eigenstates for light as the ideal carrier can provide a structured combination of photonic spin and orbital angular momentum(OAM).Photonic spin angular momentum(SAM)interactions with matter have long been studied,whereas the photonic OAM has only recently been discovered,receiving attention in the past three decades.Although controlling polarization(i.e.,SAM)alone can provide useful information about the media with which the light interacts,light fields carrying both OAM and SAM may provide additional information,permitting new sensing mechanisms and light–matter interactions.We summarize recent developments in controlling photonic angular momentum(AM)using complex structured optical fields.Arbitrarily oriented photonic SAM and OAM states may be generated through careful engineering of the spatial and temporal structures of optical fields.Moreover,we discuss potential applications of specifically engineered photonic AM states in optical tweezers,directional coupling,and optical information transmission and processing.