Fundamental limitations to gain enhancement in periodic media and waveguides

TL;DR

This paper reveals that adding gain to optical nanostructures to enhance performance is fundamentally limited because gain alters dispersion properties, potentially degrading slow-light effects crucial for device operation.

Contribution

It demonstrates that gain inclusion modifies dispersion laws in various optical systems, challenging the assumption that slow-light effects can be straightforwardly exploited for gain enhancement.

Findings

Gain modifies the dispersion law in optical systems.

Slow-light properties can be degraded by added gain.

Small gain amounts may still be beneficial.

Abstract

A common strategy to compensate for losses in optical nanostructures is to add gain material in the system. By exploiting slow-light effects it is expected that the gain may be enhanced beyond its bulk value. Here we show that this route cannot be followed uncritically: inclusion of gain inevitably modifies the underlying dispersion law, and thereby may degrade the slow-light properties underlying the device operation and the anticipated gain enhancement itself. This degradation is generic; we demonstrate it for three different systems of current interest (coupled resonator optical waveguides, Bragg stacks, and photonic crystal waveguides). Nevertheless, a small amount of added gain may be beneficial.