Force Detection of Electromagnetic Chirality of Tightly Focused Laser Beams

We theoretically show that the optical chiral properties of tightly focused laser beams can be characterized by means of force detection. To measure the chiral properties of a beam of given handedness in the microscopic focal volume, we determine the photoinduced 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(Fgrad ,z & rang; 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 (Fgrad ,z & rang; using a Mie scattering formalism complemented with full wave simulations, underlining that the magnitude of the difference force is within an experimentally detectable range.

Automated summary

The optical chiral properties of tightly focused laser beams can be characterized using force detection. By measuring the chiral properties of a beam in the microscopic focal volume, the difference in force measurements can provide information about the helicity density and the spin angular momentum density of light.