Scattering forces in the focal volume of high numerical aperture microscope objectives

Radiation pressure is not the only source of scattering force in the focal region of a microscope objective. Depending on the numerical aperture and the polarization characteristics of the light at the entrance pupil, the force emerging from the spin density may represent a fundamental contribution to the total force experienced by small (Rayleigh) particles when using high resolution objectives. The predicted and experimentally observed strong asymmetry of the trapping potential for linear polarization is shown to be related to non-conservative spin curl forces. We demonstrate the characteristics of this force field when using linear polarized light with circular and annular pupils and in the radial and azimuthal polarization structures.

Research Highlights
► High NA objectives induce radiation pressure and curl force as scattering forces.
► The curl force cause trapping potential asymmetrization for linear polarization.
► The curl force is fundamental for radial polarization and absent for azimuthal.
► The total intensity can approximate the scattering force amplitude in some cases.