  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.
