Microwave magnetoelectric fields: helicities and reactive power flows

摘要

The dual symmetry between the electric and magnetic fields underlies Maxwell's electrodynamics. Due to this symmetry, one can describe topological properties of an electromagnetic field in free space and obtain the conservation law of optical (electromagnetic) helicity. What kind of the field helicity one can expect to see when the electromagnetic-field symmetry is broken? The near fields originated from small ferrite particles with magnetic-dipolar-mode oscillations are the fields with the electric and magnetic components, but with broken dual (electric-magnetic) symmetry. These fields-called magnetoelectric (ME) fields-have topological properties different from such properties of electromagnetic fields. The helicity states of ME fields are topologically protected quantum-like states. In this paper, we study the helicity properties of ME fields. We analyze conservation laws of the ME-field helicity and show that the helicity density is related to an imaginary part of the complex power-flow density. We show also that the helicity of ME fields can be a complex value. The shown topological properties of the ME fields can be useful for novel near- and far-field microwave applications. The obtained results can find application for development of novel microwave metamaterials. Strongly localized ME fields, having both the real and imaginary helicity parameters, open unique perspective for sensitive microwave probing of structural characteristics of chemical and biological objects.