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We put forward the complex helicity theorem. It constitutes a novel law thatrules 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 frommagnetodielectric dipolar nanoparticles. As for the former, we show that itsreactive helicity may be experimentally observed as we introduce a reactivespin momentum and a reactive orbital momentum in terms of which we express theimaginary field momentum, whose transversal component produces an optical forceon a magnetoelectric particle that, as we illustrate, may surpass and can bediscriminated from, the known force due to the so-called extraordinarymomentum. We also uncover a non-conservative force on such a magnetoelectricparticle, acting in the decay direction of the evanescent wave, and that mayalso be discriminated from the standard gradient force; thus making thereactive power of the wavefield also observable. Concerning the light scatteredby magnetoelectric nanoparticles, we establish two optical theorems that governthe accretion of reactive helicity and reactive power on extinction of incidentwave helicity and energy. Like a nule total -- internal plus external --reactive power is at the root of a resonant scattered power, we show that azero total reactive helicity underlies a resonant scattered helicity. The firstKerker condition, under which the particle becomes dual on illumination withcircularly polarized light, we demonstrate to amount to a nule overallscattered reactive helicity. and emission. We discover a discriminatoryproperty of the reactive helicity of chiral light incident on a chiralnanoparticle by excitation of the external reactive power. This for opticalnear-field enantiomeric separation, we call reactive dichroism.

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