image: Figure 1 | Conceptual scheme of the synthesis of spatiotemporal vortex bursts with time-varying dynamic properties. a, Time-varying longitudinal OAMs of helical pulse. b, Time-varying transverse OAMs of spatiotemporal vortex bursts. c, Analogy between optical frequency combs and pulse bursts. d, Principle for spatiotemporal vortex bursts generation via spatiotemporal multiplexing technique.
Credit: Qiwen Zhan et al.
An optical comb, for example optical frequency comb, consists of a series of evenly spaced, phase-stable, narrow-linewidth spectral lines, forming an orderly and regular spectrum that resembles the teeth of a comb. In the time domain, this corresponds to a train or burst of pulses. When the individual comb lines of different frequencies are combined with spatial vortex beams carrying distinct topological charges, a self-torqued beam with a helical temporal structure can be generated. Such beams exhibit time-dependent variations in their longitudinal OAM, undergoing ultrafast changes over time. These temporally varying OAM beams hold significant potential for applications in light–matter interactions, spectroscopy, and nonlinear optics. However, current research has primarily focused on manipulating the purely spatial degrees of freedom of light fields. On ultrashort timescales, spatiotemporal structured beams remain limited to single-pulse configurations, with restricted controllability, insufficient mode purity, and frequency dependence. Consequently, a key scientific and technological challenge lies in achieving independent and on-demand control of individual comb teeth at different time nodes, thereby enabling the construction of a “spatiotemporal vortex comb”.
In a recent Light: Science & Applications publication, a team led by Professors Qiwen Zhan and Yangjian Cai, together with their collaborators from the University of Shanghai for Science and Technology and the Shandong Normal University, reports a new brand concept of spatiotemporal vortex bursts—an optical-comb–like structure endowed with time-dependent photonic characteristics. This spatiotemporal vortex burst is composed of tailored spatiotemporal vortex wavepackets arranged at variable temporal locations. These combs exhibit time-varying transverse OAM on the picosecond scale, enabled by programmable arrangements of both the radial and azimuthal quantum numbers of spatiotemporal Laguerre–Gaussian wavepackets. Through this approach, they achieve precise and simultaneous control of ultrashort pulses in both spatial and temporal dimensions, advancing beyond conventional methods that rely only on static or spatial modulation. These scientists summarize the unique advantages of their spatiotemporal vortex bursts:
“We demonstrated a spatiotemporal vortex comb that enables time-resolved control of transverse OAM states with picosecond spacing, while preserving high modal purity across all comb teeth. This innovative approach realizes time-varying yet pure OAM states, opening new avenues for advanced control of spatiotemporal structured light.”
“We demonstrated ultrafast control of transverse OAM chirality, generating staggered azimuthal momentum pulse bursts—optical “vortex dipoles” analogous to spin currents (resembling fluidic Kármán vortex streets)—and enabling dynamic tailoring of spatiotemporal Laguerre–Gaussian wavepackets, offering a novel tool to probe light–matter energy transfer and advance structured light control.”
The distinctive feature of their work is the integration of burst dynamics with spatiotemporal structured light control, which produces optical fields with dynamic and reconfigurable transverse OAM properties. Unlike conventional vortex beams, the spatiotemporal vortex comb teeth in their scheme are isolated and orthogonal in both spatial and temporal coordinates. This unique property ensures that, in nonlinear interactions such as sum-frequency generation, difference-frequency generation, or harmonic generation, the pulses act independently, thereby enhancing efficiency and flexibility. Since all comb teeth share identical central frequencies, they can also be precisely synchronized, offering a means to optimize nonlinear conversion processes.
Their methodology introduces an innovative route to constructing higher-dimensional spatiotemporal optical structures with tunable dynamics. Each comb within the ultrafast burst can carry customized space–time coupling properties, which makes them promising drivers for electron acceleration, radiation source development, and laser wakefield acceleration. In addition, the high repetition rate and controllable spatiotemporal characteristics of the bursts provide a new time-dependent degree of freedom, with promising applications in micromachining, where precision is essential, and in nonlinear spectroscopy, where separated but coherent pulses can probe ultrafast dynamics with high resolution.
Article Title
Ultrafast bursts of tailored spatiotemporal vortex pulses