  Spin-orbital coupling and interaction as intrinsic light fields
characteristics have been extensively studied. Previous studies involve the
spin angular momentum (SAM) carried by circular polarization and orbital
angular momentum (OAM) associated with a spiral phase wavefront within the beam
cross section, where both the SAM and OAM are in parallel with the propagation
direction. In this work, we study a new type of spin-orbital coupling between
the longitudinal SAM and the transverse OAM carried by a spatiotemporal optical
vortex (STOV) wavepacket under tight focusing condition. Intricate
spatiotemporal phase singularity structures are formed when a circularly
polarized STOV wavepacket is tightly focused by a high numerical aperture
objective lens. For the transversely polarized components, phase singularity
orientation can be significantly tilted away from the transverse direction
towards the optical axis due to the coupling between longitudinal SAM and
transverse OAM. The connection between the amount of rotation and the temporal
width of the wavepacket is revealed. More interestingly, spatiotemporal phase
singularity structure with a continuous evolution from longitudinal to
transverse orientation through the wavepacket is observed for the
longitudinally polarized component. These exotic spin-orbit coupling phenomena
are expected to render new effects and functions when they are exploited in
light matter interactions.
