Resonant electromagnetic scattering in anisotropic layered media

TL;DR

This paper analyzes resonant electromagnetic scattering in layered anisotropic media, deriving exact expressions for scattering phenomena and demonstrating how to control resonance properties via incident wave angles.

Contribution

It extends previous results to evanescent sources and 2D wavevectors, proving key nondegeneracy conditions of the dispersion relation for layered media.

Findings

Resonance width and frequency can be independently controlled by incident angle.

Exact expressions for scattering phenomena are derived.

Nondegeneracy conditions for the dispersion relation are proved.

Abstract

The resonant excitation of an electromagnetic guided mode of a slab structure by exterior radiation results in anomalous scattering behavior, including sharp energy-transmission anomalies and field amplification around the frequency of the slab mode. In the case of a periodically layered ambient medium, anisotropy serves to couple the slab mode to radiation. Exact expressions for scattering phenomena are proved by analyzing a pole of the full scattering matrix as it moves off the real frequency axis into the lower half complex plane under a detuning of the wavevector parallel to the slab. The real pole is the frequency of a perfect (infinite Q) guided mode, which becomes lossy as the frequency gains an imaginary part. This work extends results of Shipman and Venakides to evanescent source fields and two-dimensional parallel wavevector and demonstrates by example how the latter allows one to control independently the width and central frequency of a resonance by varying the angle of incidence of the source field. The analysis relies on two nondegeneracy conditions of the complex dispersion relation for slab modes (relating poles of the scattering matrix to wavevector), which were assumed in previous works and are proved in this work for layered media. One of them asserts that the dispersion relation near the wavevector and frequency of a perfect guided mode is the zero set of a simple eigenvalue, and the other relates the frequency derivative of the eigenvalue to the total energy of the mode, thereby implying that this derivative is nonzero.