Optimal nanoparticle forces, torques, and illumination fields

Abstract

A universal property of resonant subwavelength scatterers is that their optical cross-sections are proportional to a square wavelength, ${\lambda }^2$, regardless of whether they are plasmonic nanoparticles, two-level quantum systems, or RF antennas. The maximum cross-section is an intrinsic property of the \emph{incident field}: plane waves, with infinite power, can be decomposed into multipolar orders with finite powers proportional to ${\lambda }^2$. In this Article, we identify ${\lambda }^2/c$ and ${\lambda }^3/c$ as analogous force and torque constants, derived within a more general quadratic scattering-channel framework for upper bounds to optical force and torque for any illumination field. This framework also solves the reverse problem: computing globally optimal "holographic" incident beams, for a fixed collection of scatterers. We analyze structures and incident fields that approach the bounds, which for wavelength-scale bodies show a rich interplay between scattering channels, and we show that spherically symmetric structures are forbidden from reaching the plane-wave force/torque bounds. This framework should enable optimal mechanical control of nanoparticles with light.

arXiv comment6 pages plus references and supplementary materials

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Cited by in corpus (3)

• Optimal Wave Fields for Micro-manipulation in Complex Scattering Environments
• Tractor beams with optimal pulling force using structured waves
• Dual-beam intracavity optical trap with all-optical independent axial and radial self-feedback schemes