Abstract
We show that the optical force field in optical tweezers with elliptically polarized beams has the opposite handedness for a wide range of particle sizes and for the most common configurations. Our method is based on the direct observation of the particle equilibrium position under the effect of a transverse Stokes drag force, and its rotation around the optical axis by the mechanical effect of the optical torque. We find overall agreement with theory, with no fitting, provided that astigmatism, which is characterized separately, is included in the theoretical description. Our work opens the way for characterization of the trapping parameters, such as the microsphere complex refractive index and the astigmatism of the optical system, from measurements of the microsphere rotation angle.
Highlights
Negative optical forces are at the origin of single-beam optical traps [1], known as optical tweezers, which have become extremely important tools in several fields of physics [2] and cell biology [3]
We start with the theoretical results derived within Mie-Debye spherical aberration (MDSA) and MDSA+ for the parameters and conditions corresponding to our experiment
Note that the orbital optical torque (OT) vanishes when the sphere center is aligned along the beam symmetry axis (ρ = 0)
Summary
Negative optical forces are at the origin of single-beam optical traps [1], known as optical tweezers, which have become extremely important tools in several fields of physics [2] and cell biology [3]. The optical torque (OT) on a transparent and isotropic microsphere at its equilibrium position vanishes [4, 5], since Mie scattering conserves optical angular momentum (AM) when the microsphere is aligned along the beam symmetry axis [6]. We show, both theoretically and experimentally, that the resulting orbital optical torque (OT) points along the direction opposite to the incident field AM in most situations involving practical applications of optical tweezers with circularly or elliptically polarized Gaussian trapping beams. An oblate spheroid was predicted to spin around the axis of a circularly polarized Gaussian beam with the opposite handedness of the incident optical AM [8]. A negative OT was theoretically predicted for a small isotropic particle illuminated by a vortex beam in the Rayleigh scattering approximation [12]. Negative OT was demonstrated for a macroscopic inhomogenous and anisotropic disk by measuring the rotationally Doppler-shifted reflected light [14, 15] and more recently by the direct observation of the disk rotation [16]
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