### 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) |
---|---|

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 |

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### Keywords

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

### ASJC Scopus subject areas

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

### 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

**Photokinetic analysis of the forces and torques exerted by optical tweezers carrying angular momentum.** / Yevick, Aaron; Evans, Daniel J.; Grier, David G.

Research output: Contribution to journal › Article

*Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences*, vol. 375, no. 2087, 20150436. https://doi.org/10.1098/rsta.2015.0432

}

TY - JOUR

T1 - Photokinetic analysis of the forces and torques exerted by optical tweezers carrying angular momentum

AU - Yevick, Aaron

AU - Evans, Daniel J.

AU - Grier, David G.

PY - 2017/2/28

Y1 - 2017/2/28

N2 - 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.

AB - 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.

KW - Angular momentum

KW - Brownian vortex

KW - Optical trapping

KW - Spin-curl force

UR - http://www.scopus.com/inward/record.url?scp=85011044819&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=85011044819&partnerID=8YFLogxK

U2 - 10.1098/rsta.2015.0432

DO - 10.1098/rsta.2015.0432

M3 - Article

VL - 375

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

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

SN - 0962-8428

IS - 2087

M1 - 20150436

ER -