Detection of the transverse spin of light by twisting anisotropic particles near an optical nanofiber waveguide

In contrast to paraxial waves, strongly confined light can carry significant transverse spin angular momentum. Here we report on its direct detection in the evanescent electromagnetic field near the ultrathin waist of an optical nanofiber waveguide. We demonstrate the spin by its contribution to rotation of an anisotropic microsphere held and spun near the nanofiber waist by optical tweezers. By setting the driving spin angular momentum in the optical tweezers to be parallel or antiparallel with respect to the transverse spin near the nanofiber, we can speed up or slow down the particle's rotation by about a half of the rotation rate observed without the light in the fiber. We also explore the dependence of this optomechanical effect on the propagation direction and polarization of the guided light.