High-precision three-dimensional motion sensing with optical Stokes skyrmions

Optical skyrmions have been realized across various electromagnetic quantities, including Stokes parameters, photonic spin, and Poynting vector. However, their potential for Doppler-based metrology has remained unexplored. Here, leveraging the vectorial spatial structure of the skyrmion, we report, to our knowledge, the first application of an optical Stokes skyrmion beam to simultaneously probe the translational and rotational motion of an object in a proof-of-principle experiment. The orthogonal polarization components of the skyrmion carry distinct phase profiles, enabling the simultaneous excitation of linear and rotational Doppler effects. This allows us to retrieve both the translational and rotational motion speed as well as to unambiguously determine its motion direction. Notably, unlike conventional vortex or vector beam, which suffers from an on-axis intensity null, the Stokes skyrmion features a non-zero on-axis intensity, enabling high-precision detection of rotation motion with ultra-small radius. We further study the influence of noise and imperfect particle shapes on the Doppler metrology, showcasing its robustness in sensing the particle motion. These findings establish Stokes skyrmions as an advantageous platform for advanced motion metrology in three dimensions.