Reactive helicity and reactive power in nanoscale optics: Evanescent waves. Kerker conditions. Optical theorems and reactive dichroism

TL;DR

This paper introduces the complex helicity theorem, linking reactive helicity and power in evanescent waves and nanoparticle scattering, revealing new optical forces, resonances, and chiral light interactions in nanoscale optics.

Contribution

It presents the complex helicity theorem, new optical theorems for reactive helicity and power, and demonstrates their implications for nanoparticle scattering and chiral light interactions.

Findings

Reactive helicity can be experimentally observed via reactive spin and orbital momentum.

A non-conservative force acts on magnetoelectric particles in evanescent fields, distinguishable from known forces.

Zero total reactive helicity corresponds to resonant scattered helicity and the first Kerker condition.

Abstract

We put forward the complex helicity theorem. It constitutes a novel law that rules the build-up of the reactive helicity through its zero time-average flow. Its imaginary Poynting momentum accounts for the accretion of reactive power, are illustrated in: evanescent waves and fields scattered from magnetodielectric dipolar nanoparticles. As for the former, we show that its reactive helicity may be experimentally observed as we introduce a reactive spin momentum and a reactive orbital momentum in terms of which we express the imaginary field momentum, whose transversal component produces an optical force on a magnetoelectric particle that, as we illustrate, may surpass and can be discriminated from, the known force due to the so-called extraordinary momentum. We also uncover a non-conservative force on such a magnetoelectric particle, acting in the decay direction of the evanescent wave, and that may also be discriminated from the standard gradient force; thus making the reactive power of the wavefield also observable. Concerning the light scattered by magnetoelectric nanoparticles, we establish two optical theorems that govern the accretion of reactive helicity and reactive power on extinction of incident wave helicity and energy. Like a nule total -- internal plus external -- reactive power is at the root of a resonant scattered power, we show that a zero total reactive helicity underlies a resonant scattered helicity. The first Kerker condition, under which the particle becomes dual on illumination with circularly polarized light, we demonstrate to amount to a nule overall scattered reactive helicity. and emission. We discover a discriminatory property of the reactive helicity of chiral light incident on a chiral nanoparticle by excitation of the external reactive power. This for optical near-field enantiomeric separation, we call reactive dichroism.