"COSMOS" research group from University of Shanghai for Science and Technology (USST) proposed and studied a new type of spatiotemporal optical field. The field features multiple vortex phase structure in different space-time domains. By changing the phase structure, it is possible to achieve a vectorial control over the orbital angular momentum (OAM) carried by the wavepacket. This spatiotemporal wavepacket may bring new opportunities in applications such as developing novel optical communication, studying light-and-matter interaction, and spin-orbital-interaction (SOI) studies.
Photonic orbital angular momentum (OAM) of spatiotemporal optical vortices
Figure 1. Conventional vortex beam (left) and spatiotemporal optical vortices (STOV) wavepacket (right). The vortex beam carries a longitudinal OAM that is in parallel with the light propagation direction. STOV carries a transverse OAM that is in perpendicular with the light propagation direction.
Differed from the vortex beam (left plot in the figure), spatiotemporal optical vortices (STOV) wavepacket (right plot in the figure) has a spiral phase of exp(ilθx-t) in the spatiotemporal x-t plane. Such a spiral phase structure grants the STOV wavepacket unique photonic property so that it possesses a transverse orbital angular momentum (OAM). It is also noteworthy that the vortex beam possesses a longitudinal OAM with the direction of the OAM parallel with the light propagation direction. For the STOV wavepacket, the OAM is transverse and it is perpendicular with the light propagation direction.
STOV wavepacket with phase singularities in multiple space-time domain
Current research in STOV wavepacket focuses on studying STOV with a singular phase structured in the x-t plane. Only few research works have studied spatiotemporal optical field with multiple phase singularities in the spatiotemporal domain or with phase singularity placed in different domains. In this work, we propose and study a novel spatiotemporal optical field with phase singularities embedded in multiple space-time domains. As shown in Fig. 2, the field is designed to have two phase singularities placed in the x-t and x'-t plane. The x'-axis has an angle offset of θx-x' with respect to the x-axis. Figure 2 shows the intensity and phase distribution of the wavepacket in space-time. When θx-x' is changing, the intensity-null of the wavepacket rotates in the x-y plane, indicating the rotation of the OAM carried by the wavepacket. Figure 3 shows the numerical analysis of the OAM when θx-x' varies from -π to π. Both the magnitude and the orientation of the OAM changes with a changing θx-x'. It means we can achieve a vectorial control of the OAM by engineering the spatiotemporal wavepacket.
Optical wavepackets with multiple phase singularities embedded in different space-time domains allow a vectorial control over the OAM carried by the wavepackets. This may facilitate applications such as developing novel optical communication, studying light-and-matter interaction, and spin-orbital-interaction (SOI) studies. This work is published in Chinese Optics Letters 21(8): Liangliang Gu, Qian Cao, and Qiwen Zhan. Spatiotemporal optical vortex wavepackets with phase singularities embedded in multiple domains[J]. Chinese Optics Letters, 2023, 21(8): 080003,and is selected as the cover paper of the journal.
Figure 2. Spatiotemporal intensity and phase profile of STOV wavepackets with phase singularities embedded in x-t and x'-t domain. The angle difference between x- and x'-axis changes from 0 to π in an interval of π/4 (from left to right).
Figure 3. Vectorial control over the OAM carried by a STOV wavepacket with phase singularities in multiple space-time domain.
