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

Aaron Yevick, Daniel J. Evans, David G. Grier

    Research output: Contribution to journalArticlepeer-review


    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 languageEnglish (US)
    Article number20150436
    JournalPhilosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences
    Issue number2087
    StatePublished - Feb 28 2017


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

    ASJC Scopus subject areas

    • General Mathematics
    • General Engineering
    • General Physics and Astronomy


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