Fig. 4: Measurement of rotational and translational dynamics in a uniform optical torque field.

Individual rotors freely diffuse and spin in a force-free optical torque field. a The ensemble-averaged mean square displacement 〈Δr²〉 is linear (diffusive) for different photonic fluxes J. A representative trajectory of a single particle for different photonic fluxes is shown in the inset. b Translational diffusion increases with J as the gravitational height h_g increases (solid line) until it approaches the bulk value (dashed line). Colors of points in (b) correspond to the legend in (a). c The effective birefringence of a particle, Δn, can be measured by monitoring the minimal ellipticity, ϕ_onset, where rotation begins for different particle sizes (inset). d Measured spin rates for different fluxes and different particle sizes (inset) as predicted by Eq. (4) (solid lines), are consistent with measured birefringence. Error bars correspond to the standard deviation.