Controllable orbital-to-spin angular momentum conversion in tight focusing of spatiotemporal vortex wavepacket

In this paper, we investigate the tight focusing of the radially polarized spatiotemporal vortex (STV) wavepackets. We find that, by changing the initial phase of the incident polarization state, the intensity envelope of the tightly focused first-order radially polarized STV wavepacket can be well controlled, yet the intensity envelope just rotates in whole for the tightly focused high-order radially polarized STV wavepacket. Furthermore, we show that, when the initial phase of incident polarization state takes π/2, the transverse double vortex structure arises in the focal region. More interestingly, when the initial phase takes π/2, the pure longitudinal spin angular momentum and transverse orbital angular momentum can be obtained in the tight focusing of the first-order radially polarized STV wavepacket. These effects are the manifestation of the spin-orbit interaction determined by the transverse orbital angular momentum and the incident polarization state. Our works present a technique to modulate the optical angular momentum in the tight focusing of the radially polarized STOV wavepacket, have potential application in the fields of optical switches, optical capture, quantum communication and nano-manipulation.