Coherent control of optical spin-orbit interactions.

Optical spin-orbit interactions (SOI) are an intriguing physical phenomenon where the spin (circular polarization) of light influences and governs its spatial degrees of freedom. Yet, achieving real-time and flexible control over SOI remains a formidable challenge. Here, we propose a coherent control method for the flexible manipulation of optical SOI using a thin isotropic crystal. The output horizontally and vertically polarized waves from the crystal exhibit strong and distinct dependencies on the phase delay between the input signal and control waves, leading to substantial spin-orbit beam shifts. By tuning the relative phase between the signal beam and the control beam, we observe transitions from Gaussian to first-order orbital angular momentum (OAM) vortex modes. The achievable transition speed reaches 11.5 gigahertz, now limited by the external electro-optic link in the control arm. Notably, the dual-port architecture offers coherent and reversible selection of spin-OAM states, with the output port uniquely determined by the phase delay between two counter-propagating input beams. This ultrafast coherent control method opens avenues for advanced manipulation of SOI.