Positive and negative extinction of active particles

TL;DR

This paper investigates how active particles with gain properties interact with electromagnetic waves, revealing unique scattering behaviors and classifying their responses based on shape and material properties.

Contribution

It introduces a five-class categorization of active and passive dielectric particles and analyzes the effects of particle shape on electromagnetic response.

Findings

Active particles can exhibit negative absorption cross-section indicating gain.

Extinction can be positive, negative, or zero depending on particle properties.

Non-sphericity influences scattering and extinction behaviors.

Abstract

Using analytical Lorenz--Mie scattering formalism and numerical methods, we analyze the response of active particles to electromagnetic waves. The particles are composed of homogeneous, non-magnetic, and dielectrically isotropic medium. Spherical scatterers and sharp and rounded cubes are treated. The absorption cross-section of active particles is negative, thus showing gain in their electromagnetic response. Since the scattering cross-section is always positive, their extinction can be either positive, negative, or zero. We construct a five-class categorization of active and passive dielectric particles. We point out the enhanced backscattering phenomenon that active scatterers display, and also discuss extinction paradox and optical theorem. Finally, using COMSOL Multiphysics and an in-house Method-of-Moments code, the effects of the non-sphericity of active scatterers on their electromagnetic response are illustrated.