Recently, a research team from Shanghai Institute of Optics and Fine Mechanics (SIOM) of the Chinese Academy of Sciences (CAS) reported the diffraction properties of spatiotemporal optical vortices (STOVs) and propose a fast STOV recognition method. The results were published in Optica on April 22, 2022.
Unlike conventional spatial vortex beams whose orbital angular momentum direction is parallel to the beam propagation direction (longitudinal orbital angular momentum), the phase singularity of spatiotemporal vortex beam exists in the space-time domain and its orbital angular momentum direction is orthogonal to the beam propagation direction (transverse orbital angular momentum). The STOV beam opens new freedom of light and help us understand the properties of light, and it will have special applications in wide fields.
Diffraction is a fundamental wave phenomenon that is well known for conventional light. However, studies on the diffraction of light with transverse OAM are limited. Furthermore, methods that enable the fast detection of STOVs are lacking.
They theoretically and experimentally investigate the diffraction properties of STOVs. Firstly, theoretical analysis shows that the diffraction pattern of a STOV has a multilobe structure, and the number of gaps corresponds to the topological charge. For example, for a STOV with topological charge l, the diffraction pattern has l gaps. The experimental results match with the theoretical calculations.
Using this special diffraction property of STOV beams, a fast detection method for STOVs with different topological charges is proposed, and this simple and fast method has potential applications where fast recognition of STOVs is needed, such as STOV-based optical communication.
This research contributes to the understanding of the physical properties of STOV beams and is expected to accelerate the application of STOVs.

Fig. 1. Experimental setup for the generation of STOV pulses (top part) and for the detection of their diffraction patterns (bottom part). (Image by SIOM)

Fig. 2. Intensity profiles and diffraction patterns of STOVs (l = ±1, ±4) in the focal plane of the spherical lens and in the observed plane. (Image by SIOM)