New aspects of the use of Optical Theorem in the analysis of microcavity lasers

The optical theorem (OT) is a classical identity, which links the extinction cross-section and the forward-direction far field for a particle illuminated with a monochromatic plane wave. We discuss the application of the similar identity to the lasing modes of an open dielectric microcavity containing an active region, i.e. a laser. This yields clear insight into the lasing thresholds of individual modes, reveals the role of matching between the active region and the modal electric-field pattern, and generally bridges a gap between the electromagnetic characterization of passive and active open cavities.