Spatiotemporal instabilities of terahertz OAM beams from air plasma via chirping a few-cycle vortex pump field

We studied the spatiotemporal instability during the evolution of ultra-broadband (0.1–30 THz) terahertz (THz) orbital angular momentum (OAM) radiation generated from an air plasma via chirping a few-cycle vortex pump laser. Generally, the simulation results reveal three stages in the filamentation process, i.e., stable propagation zone, unstable propagation zone and collapse zone. At the first stage, THz OAM pulses are generated and propagate steadily; at the second stage, the spatiotemporal structure of THz OAM pulses is deformed near the transverse intensity peak, which is induced by spatiotemporal instabilities of pump few-cycle pulse generic to the filamentation dynamics; at the third stage, some separate hot spots occur in THz intensity patterns, and the corresponding amplitude lobes have long fork tails. More interestingly, a deep analysis of chirp parameter shows that the length of the stable propagation zone is closely related to its sign and value. Positive chirp parameter favors a longer stable propagation zone, and the length of the stable zone grows with the chirp parameter. On the contrary, the length of unstable propagation zones are insensitive to the variation of chirp parameters. The results may define the boundary of producing stable THz OAM beams and provide an efficient way of control stable length via chirping pump field, which are favorable for experimental convenience in THz OAM beam generation, detection and collection.