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Rotation and Negative Torque in Electrodynamically Bound Nanoparticle Dimers.

Nishant Sule1, Yuval Yifat1, Stephen K Gray2

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Area of Science:

  • Nanophotonics and optical trapping.
  • Plasmonics and nanoparticle interactions.
  • Computational physics and simulation.

Background:

  • Focused laser beams can trap and manipulate nanoparticles.
  • Circularly polarized light can induce rotation in optical systems.
  • Understanding nanoparticle interactions is key to optical matter control.

Purpose of the Study:

  • To investigate the rotational dynamics of electrodynamically bound dimers (EBDs) of silver nanoparticles.
  • To explore the influence of incident beam power and particle separation on EBD rotation.
  • To elucidate the role of negative torque in opposite-handed rotational dynamics.

Main Methods:

  • Experimental trapping of 150 nm silver nanoparticles in circularly polarized Gaussian beams.
  • Theoretical modeling using coupled-dipole/effective polarizability approximations.
  • Electrodynamics-Langevin dynamics simulations to analyze rotational behavior.
  • Statistical analysis of experimental EBD trajectories.

Main Results:

  • EBD rotation frequency scales linearly with incident beam power (up to ~4 kHz at 14 mW).
  • Negative torque, arising from field retardation and interparticle interactions, induces opposite-handed rotation.
  • Simulation and experimental results confirm opposite-handed rotation dependent on particle separation.
  • Retardation effects are crucial in determining the direction of nanoparticle dimer rotation.

Conclusions:

  • Novel rotational dynamics of nanoparticles in optical matter are demonstrated using circular polarization.
  • Negative torque provides a mechanism for controlling orientational dynamics.
  • Interparticle separation offers a tunable parameter to influence nanoparticle rotation.
  • This work opens new avenues for precise control of nanoparticle assemblies.