Direct measurement of the extraordinary optical momentum using a nano-cantilever

Radiation pressure has been known since Kepler's observation that a comet's tail is always oriented away from the sun, and in the past centuries this phenomenon stimulated remarkable discoveries in electromagnetism, quantum physics and relativity [1-3]. In modern terms, the pressure of light is associated with the momentum of photons, which plays a crucial role in a variety of systems, from atomic [4-7] to astronomical [8,9] scales. Experience from these cases leads us to assume that the direction of the optical momentum and the radiation-pressure force are naturally aligned with the propagation of light, i.e., 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 (i.e., degree of circular polarization). This transverse spin-dependent optical force, a few orders of magnitude weaker than the usual radiation pressure, was recently predicted for evanescent waves [10] and other structured fields [11]. Fundamentally, it can be associated with the enigmatic "spin momentum," introduced by Belinfante in field theory 75 years ago [12-14]. We measure this unusual transverse momentum using a nano-cantilever with extremely low compliance (capable of femto-Newton resolution), which is immersed in an evanescent optical field directly above the total-internal-reflecting glass surface. Such sensors, perpendicular to a substrate, have already shown an extreme force resolution in various systems [15-19]. Our findings revisit fundamental momentum properties of light, while the experimental technique opens the way for precision measurements of fine optical forces in structured fields at subwavelength scales.