Theoretical analysis of Fresnel reflection and transmission in the presence of gain media

TL;DR

This paper provides a theoretical analysis of Fresnel reflection and transmission at interfaces involving gain media, focusing on the sign ambiguity of the transmitted wavevector and the effects on wavepacket behavior.

Contribution

It introduces a general strategy for determining the sign of transmitted wavevectors in gain media based on causality constraints, extending previous large-angle incidence analyses.

Findings

Sign ambiguity affects wave growth or decay in gain media.

A causality-based method determines wavevector signs for finite wavepackets.

Analysis includes finite-thickness slabs and semi-infinite gain media.

Abstract

When a monochromatic electromagnetic plane-wave arrives at the flat interface between its transparent host (i.e., the incidence medium) and an amplifying (or gainy) second medium, the incident beam splits into a reflected wave and a transmitted wave. In general, there is a sign ambiguity in connection with the k-vector of the transmitted beam, which requires at the outset that one decide whether the transmitted beam should grow or decay as it recedes from the interface. The question has been posed and addressed most prominently in the context of incidence at large angles from a dielectric medium of high refractive index onto a gain medium of lower refractive index. Here, the relevant sign of the transmitted k-vector determines whether the evanescent-like waves within the gain medium exponentially grow or decay away from the interface. We examine this and related problems in a more general setting, where the incident beam is taken to be a finite-duration wavepacket whose footprint in the interfacial plane has a finite width. Cases of reflection from and transmission through a gainy slab of finite-thickness as well as those associated with a semi-infinite gain medium will be considered. The broadness of the spatiotemporal spectrum of our incident wavepacket demands that we develop a general strategy for deciding the signs of all the k-vectors that enter the gain medium. Such a strategy emerges from a consideration of the causality constraint that is naturally imposed on both the reflected and transmitted wavepackets.