### Abstract

The theory of photokinetic effects expresses the forces and torques exerted by a beam of light in terms of experimentally accessible amplitude and phase profiles. We use this formalism to develop an intuitive explanation for the performance of optical tweezers operating in the Rayleigh regime, including effects arising from the influence of light's angular momentum. First-order dipole contributions reveal how a focused beam can trap small objects, and what features limit the trap's stability. The firstorder force separates naturally into a conservative intensity-gradient term that forms a trap and a non-conservative solenoidal term that drives the system out of thermodynamic equilibrium. Neither term depends on the light's polarization; light's spin angular momentum plays no role at dipole order. Polarization-dependent effects, such as trap-strength anisotropy and spin-curl forces, are captured by the second-order dipole-interference contribution to the photokinetic force. The photokinetic expansion thus illuminates how light's angular momentum can be harnessed for optical micromanipulation, even in the most basic optical traps.

Original language | English (US) |
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Article number | 20150436 |

Journal | Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |

Volume | 375 |

Issue number | 2087 |

DOIs | |

State | Published - Feb 28 2017 |

### Keywords

- Angular momentum
- Brownian vortex
- Optical trapping
- Spin-curl force

### ASJC Scopus subject areas

- Mathematics(all)
- Engineering(all)
- Physics and Astronomy(all)

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## Cite this

*Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences*,

*375*(2087), [20150436]. https://doi.org/10.1098/rsta.2015.0432