Fresnel reflection from a cavity with net roundtrip gain

TL;DR

This paper investigates how a Gaussian beam interacts with an active cavity with net gain, revealing that interference from beam side-tails ensures field convergence despite expectations of divergence.

Contribution

It introduces a new understanding of field behavior in active cavities, highlighting the role of beam side-tails in maintaining convergence.

Findings

Side-tail leakage causes interference that stabilizes the field.

The mechanism explains phenomena like amplified total internal reflection.

Maxwell's equations remain consistent with convergent solutions.

Abstract

A planewave incident on an active etalon with net roundtrip gain may be expected to diverge in field amplitude, yet Maxwell's equations admit only a convergent solution. By examining a Gaussian beam obliquely incident on such a cavity, we find that the "side-tail" of the beam leaks into the cavity and gives rise to a field that interferes with the main portion of the beam, which is ultimately responsible for the convergence of the field. This mechanism offers perspective for many phenomena, and we specifically discuss the implications for amplified total internal reflection.