Force Detection of Electromagnetic Beam Chirality at the Nanoscale

Abstract

Many nanophotonic applications require precise control and characterization of electromagnetic field properties at the nanoscale. The chiral properties of the field are among its key characteristics, yet measurement of optical chirality at dimensions beyond the diffraction limit has proven difficult.Here we theoretically show that the chiral properties of light can be characterized down to the nanometer scale by means of force detection. We demonstrate that the photo-induced force exerted on a sharp chiral tip, subjected to sequential illumination by two circularly polarized beams of opposite handedness, provides a useful probe of the chirality of the electromagnetic field. The gradient force difference ∆ F grad,z is found to have exclusive correspondence to the time-averaged helicity density, whereas the differential scattering force provides information about the spin angular momentum density of light. We further characterize and quantify the helicity-dependent ∆ F grad,z using a Mie scattering formalism complemented with full wave simulations, underlining that the magnitude of the difference force is within an experimentally detectable range.