Role of complex energy and momentum in open cavity resonances

Complex power, also known as alternating current (AC) power, is a well-established concept in an electric circuit composed of resistive and reactive elements. On the other hand, the role of complex power in optics has been elusive. In this work, we reveal that the complex energy and momentum determine the resonance frequency and the decay rate of open cavity resonance, the so-called quasinormal modes (QNMs), respectively. We also demonstrate the role of the complex energy and momentum in typical open cavities analytically and numerically: the Fabry-Perot cavity, the surface plasmon polaritons (SPPs), the plasmonic nanorod, the nanosphere, and the dielectric supercavity.