Total helicity of electromagnetic fields and matter

The electromagnetic helicity of the free electromagnetic field and the static magnetic helicity are shown to be two different embodiments of the same physical quantity, the total helicity. The total helicity is the sum of two terms: a term that measures the difference between the number of left-handed and right-handed photons of the free field, and another term that measures the screwiness of the static magnetization density in matter. Each term is the manifestation of the total helicity in different frequency regimes: omega > 0 and omega = 0, respectively. This unification establishes the theoretical basis for studying the conversion between the two embodiments of total helicity upon light-matter interaction.

Automated summary

The electromagnetic helicity of the free electromagnetic field and the static magnetic helicity are revealed to be two different manifestations of the same physical quantity, the total helicity. The total helicity is a combination of two terms measuring different aspects of helicity in different frequency regimes, establishing a theoretical basis for studying the interconversion between these two forms of total helicity in light-matter interactions.