Direct measurements of the extraordinary optical momentum and transverse spin-dependent force using a nano-cantilever

Radiation pressure is associated with the momentum of light, and it plays a crucial role in a variety of physical systems. It is usually assumed that both the optical momentum and the radiation-pressure force are naturally aligned with the propagation direction of light, given by its wavevector. Here we report the direct observation of an extraordinary optical momentum and force directed perpendicular to the wavevector, and proportional to the optical spin (degree of circular polarization). Such an optical force was recently predicted for evanescent waves and other structured fields. It can be associated with the 'spin-momentum' part of the Poynting vector, introduced by Belinfante in field theory 75 years ago. We measure this unusual transverse momentum using a femtonewton-resolution nano-cantilever immersed in an evanescent optical field above the total internal reflecting glass surface. Furthermore, the measured transverse force exhibits another polarization-dependent contribution determined by the imaginary part of the complex Poynting vector. By revealing new types of optical forces in structured fields, our findings revisit fundamental momentum properties of light and enrich optomechanics.