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

Considering time-harmonic optical fields, we put forward the complex helicity and its alternating flow, together with their conservation equation: theorem. Its imaginary part constitutes a novel law that rules build-up of what establish as reactive through zero time-average flow. associated flow Poynting momentum accounts for accretion power are illustrated in two paradigmatic systems: evanescent waves fields scattered from magnetodielectric dipolar nanoparticles. As former, show may be experimentally observed introduce spin orbital terms which express field momentum, whose transversal component produces an force on magnetoelectric particle that, illustrate, surpass can discriminated known due to so-called extraordinary momentum. We also uncover nonconservative such particle, acting decay direction wave, standard gradient force; thus making wavefield observable. Concerning light by nanoparticles, theorems govern extinction incident wave energy. Like nule total, i.e., internal plus external, is at root resonant power, total underlies helicity. These quantities shown yield interpretation Kerker conditions, demonstrate linked absence, or minimum, overall Further, first condition, under becomes dual illumination circularly polarized light, amounts Therefore these underly directivity scattering emission. In addition, discover discriminatory property chiral nanoparticle excitation external power. This should useful near enantiomeric separation, effect call dichroism.