Equivalence of Optical Theorems

TL;DR

This paper shows that the optical theorem for electromagnetic scattering can be expressed in multiple equivalent forms using different measurement approaches, highlighting the nonuniqueness and interpretability of optical detectors.

Contribution

It introduces multiple equivalent formulations of the optical theorem in the full vector electromagnetic context, linking them to various measurement and detector configurations.

Findings

Multiple equivalent forms of the optical theorem are derived.

The nonuniqueness of optical detectors is explained via inverse source problems.

Applications include near-field sensors, backpropagation imaging, and aperture-based detectors.

Abstract

We demonstrate, in the full vector formulation of electromagnetic fields, that the well-known optical theorem pertinent for the characterization of a scatterer's extinction power and associated cross section can be expressed in a multitude of alternative equivalent forms. These alternatives involve different forms of projective field measurements or detectors. The inherent nonuniqueness of such optical-theorem-based detectors stems from the nonuniqueness of an associated inverse source problem, and can be interpreted via well-known equivalence principles. Some of the multiple ways in which the extinction of power due to the interaction of a scattering body with a probing field can be measured remotely are derived and interpreted for a number of canonical frameworks. This includes detectors and their corresponding optical theorems synthesized in the contexts of surface-confined sensors for near-field sensing, surface sensors based on backpropagation-based imaging, a number of planar aperture realizations dealt with through classical diffraction theory, as well as detectors based on multipole representations. General aspects of the derived optical theorems are discussed in the context of envisioned practical applications.