Polarization-induced torque in optical traps

SH Simpson, DC Benito, S Hanna

Research output: Contribution to journalArticle (Academic Journal)peer-review

42 Citations (Scopus)


In the field of optical trapping and micromanipulation it is well known that linearly polarized Gaussian beams, which possess no inherent angular momentum, can exert an orienting torque on optically or geometrically anisotropic particles. Conservation of angular momentum requires that the application of such a torque be compensated for by an equivalent, and opposite, angular momentum flux in the beam. In the following paper we analyze this effect in terms of both the scattered field, and the mechanical torque experienced by the particle. It is demonstrated that, in general, the scattered field has a complicated form, carrying both spin and orbital angular momentum. However, we show that the variation of torque with rotation angle is identically equal to A+B sin(2 alpha+beta) for arbitrarily shaped particles, where A, B, and beta are constants and alpha is the angular displacement of the major axis of the particle from the polarization direction. The scattered field, and the mechanical torque, are seen to reduce to qualitatively distinct forms that depend on the symmetry group of the scattering particle. Our findings are verified and illustrated by a series of numerical calculations of the forces and torques experienced by arbitrarily shaped particles trapped in linearly polarized Gaussian beams.
Translated title of the contributionPolarization-induced torque in optical traps
Original languageEnglish
Pages (from-to)043408-1 - 043408-14
Number of pages14
JournalPhysical Review A: Atomic, Molecular and Optical Physics
Publication statusPublished - Oct 2007

Bibliographical note

Publisher: American Physical Society


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