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, λ 2 , regardless of whether they are plasmonic nanoparticles, twolevel quantum systems, or RF antennas. The maximum cross-section is an intrinsic property of the incident field : plane waves, with infinite power, can be decomposed into multipolar orders with finite powers proportional to λ 2 . In this Article, we identify λ 2 /c and λ 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.

Maximal single-frequency electromagnetic response

“…Maximizing optical forces and torques has been a topic of substantial interest [14,[29][30][31][32], and is one that our framework applies to very naturally. One can compute force and torque via surface integrals of quantities related to the Maxwell stress tensor, which is a quadratic function of the electric and magnetic fields.…”

“…We use inverse design to discover ultrathin absorber designs closely approaching the bounds. The framework developed here has immediate applicability to any linear and quadratic response functions in electromagnetic scattering problems, including those that arise in near-field radiative heat transfer (NFRHT) [26][27][28], optical force/torque [14,[29][30][31][32], and more general nanophotonic mode coupling [33].…”

Open-Source Software for Electromagnetic Scattering Simulation: The Case of Antenna Design

“…SCUFF-EM was born with the original intention of studying highly specialized phenomena in photonics, such as the Casimir forces or the radiative transfer [56][57][58][59]. However, it evolved in a complete suite for the BEM analysis of electromagnetic scattering for nanophotonics, RF/microwave engineering, electrostatics, and so on [58].…”

Photonic Inverse Design of Simple Particles with Realistic Losses in the Visible Frequency Range

“…Furthermore, the implementation of improved gradient-based shape optimization methods can lead to fabrication-tolerant ultra-highly-efficient grating couplers [4], while the adjoint method in which the gradient of the objective function with respect to all parameters is evaluated via only a couple of full-field simulations can lead to novel compact nonlinear photonic devices with record-high performance [5]. Finally, computing globally-optimal "holographic" incident beams for a fixed collection of scatterers becomes feasible via a quadratic scattering-channel framework utilizing the upper bounds to optical force and torque, which are not achievable for spherically-symmetric structures [6].…”