Quasinormal modes in transformation media

Abstract

The quasinormal mode (QNM) is generally a mathematical tool in analysing open systems. As the concept of Parity-Time (PT) symmetry was transplanted from quantum mechanics to classical wave systems, massive intriguing phenomena have been found, especially in the vicinity of the so-called exceptional points (EPs). In this paper, with the help of transformation optics (TO), we focus on finding EPs of three-layer PT symmetric models, especially the Metal-Insulator-amplifying Metal (MIaM) ones. Theoretical and numerical methods both indicate that the EPs only exist in the complex frequency regime. Searching within the real frequency spectrum would never find EPs across which complex conjugate wavenumbers occur. Equivalently, only QNMs exist in the symmetry broken phase.