Negative Group Velocity: Is It a Backward wave or Fast Light?

TL;DR

This paper explores the phenomenon of negative group velocity in photonic systems, clarifying when it corresponds to fast light or backward waves, and emphasizing the role of causality in determining the nature of wave propagation.

Contribution

It demonstrates that causality constraints determine whether negative group velocity results in fast light or backward waves, and shows that the same system can support both depending on parameters.

Findings

Causality rules out backward waves for certain conditions.

The same system can exhibit both fast light and backward wave solutions.

Parameter changes can switch between the two wave types.

Abstract

When a negative slope of the dispersion curve is encountered, the propagating light may be either 'fast light' or 'backward propagating'. We show that causality considerations select only one of which for each scenario, and demonstrate that the same photonic (plasmonic) system can support both type of solutions depending on parameters.