Direct observation of the effects of spin dependent momentum of light in optical tweezers

We demonstrate that tight focusing of a circularly polarized Gaussian beam in optical tweezers leads to spin-momentum locking - with the transverse spin angular momentum density being independent of helicity, while the transverse momentum (Poynting vector) becomes helicity dependent. Our theoretical calculations, numerical simulations, and experiments reveal that the presence of a stratified medium in the path of the trapping beam significantly enhances the magnitude of transverse momentum in the radial direction with respect to the beam axis, and likewise, also leads to high off-axial intensity. This overlap allows us to experimentally observe the circular motion of a birefringent particle, trapped off-axis, in response to an input circularly polarized fundamental Gaussian beam carrying no intrinsic orbital angular momentum. The circular motion is dependent on the helicity of the input beam, so that we can identify it to be the signature of the elusive Belinfante spin in propagating light beams obtained in our optical tweezers setup. Our work can be extended to higher-order beams carrying intrinsic orbital angular momentum leading to simple routes of achieving complex particle manipulation using optical tweezers.