  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. To measure the chiral
properties of a beam of given handedness at the nanoscale, we determine the
photo-induced force exerted on a sharp tip, which is illuminated first by the
beam of interest and second by an auxiliary beam of opposite handedness, in a
sequential manner. We show that the difference between the force measurements
is directly proportional to the chiral properties of the beam of interest. In
particular, the gradient force difference $\Delta\langle F_{grad, z}\rangle$ is
found to have exclusive correspondence to the time-averaged helicity density of
the incident light, whereas the differential scattering force provides
information about the spin angular momentum density of light. We further
characterize and quantify the helicity-dependent $\Delta\langle F_{grad,
z}\rangle$ using a Mie scattering formalism complemented with full wave
simulations, underlining that the magnitude of the difference force is within
an experimentally detectable range.
