Precise in situ radius measurement of individual optically trapped microspheres using negative optical torque exerted by focused vortex beams

Abstract

We demonstrate a new method for determining the radius of micron-sized particles trapped by a vortex laser beam. The technique is based on measuring the rotation experienced by the center of mass of a microsphere that is laterally displaced by a Stokes drag force to an off-axis equilibrium position. The rotation results from an optical torque pointing along the direction opposite to the vortex beam angular momentum. We fit the rotation angle data for different Laguerre-Gaussian modes taking the radius as a fitting parameter in the Mie-Debye theory of optical tweezers. We also discuss how micron-sized beads can be used as probes for optical aberrations introduced by the experimental setup.

arXiv comment11 pages, 5 figures

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• Array Programming with NumPy
• Angular momentum of focused beams: beyond the paraxial approximation
• Theory of Optical Tweezers
• Macroscopic direct observation of optical spin-dependent lateral forces and left-handed torques
• Negative optical torque on a microsphere in optical tweezers
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Cited by in corpus (1)

• Probing the chirality of a single microsphere trapped by a focused vortex beam through their orbital period