Observation of Larmor-like precession of a single birefringent particle due to spin-dependent forces in tilted optical tweezers

We observe concurrent precessional and partial orbital motion of highly birefringent liquid crystal (LC) particles trapped in a spherically aberrated optical trap, which is built employing a tilted refractive index stratified medium. For input circularly polarized light, the breaking of azimuthal symmetry induced by the tilt leads to an asymmetric intensity distribution in the radial direction near the trap focal plane, which—in combination with the spin–orbit conversion effects for input circularly polarized light—results in nonuniform canonical and spin momentum densities in those regions. In addition, while the canonical momentum remains always oriented toward the axial direction, the spin momentum reverses direction along spatial loops in the radial direction. As a consequence, the total momentum precesses around the canonical momentum vector along elliptical spatial loops—akin to a Larmor-like precession of magnetic moment (total momentum in our case) around a magnetic field (canonical momentum). We probe this precession experimentally using the single trapped LC particles—with the direction of precession and orbital motion determined by the helicity of the input light, with the precession frequency varying linearly with the laser power. Our experimental results are validated by numerical simulations of the system where we employ the Debye–Wolf theory for tight focusing in the presence of a tilted stratified media.