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 GHz, currently limited by the phase
modulator 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.