Symmetry constraints for vector scattering and transfer matrices containing evanescent components: energy conservation, reciprocity and time reversal

TL;DR

This paper develops a comprehensive framework for understanding the constraints on vector scattering and transfer matrices, including energy conservation, reciprocity, and time reversal, especially accounting for evanescent components in three-dimensional fields.

Contribution

It introduces a general formalism for vector scattering matrices that incorporates evanescent waves and applies to both continuous and discrete spectra, extending previous unitarity-based constraints.

Findings

Derived new symmetry constraints for vector scattering matrices.

Showed that previous unitarity results are special cases of their framework.

Numerically demonstrated the formalism with a wave at a glass-air interface.

Abstract

In this work we study the scattering and transfer matrices for electric fields defined with respect to an angular spectrum of plane waves. For these matrices, we derive the constraints that are enforced by conservation of energy, reciprocity and time reversal symmetry. Notably, we examine the general case of vector fields in three dimensions and allow for evanescent field components. Moreover, we consider fields described by both continuous and discrete angular spectra, the latter being more relevant to practical applications, such as optical scattering experiments. We compare our results to better-known constraints, such as the unitarity of the scattering matrix for far-field modes, and show that previous results follow from our framework as special cases. Finally, we demonstrate our results numerically with a simple example of wave propagation at a planar glass-air interface, including the effects of total internal reflection. Our formalism makes minimal assumptions about the nature of the scattering medium and is thus applicable to a wide range of scattering problems.