Asymptotics for 2D whispering gallery modes in optical micro-disks with radially varying index

Abstract

Whispering gallery modes [WGM] are resonant modes displaying special features: They concentrate along the boundary of the optical cavity at high polar frequencies and they are associated with complex scattering resonances very close to the real axis. As a classical simplification of the full Maxwell system, we consider two-dimensional Helmholtz equations governing transverse electric [TE] or magnetic [TM] modes. Even in this 2D framework, very few results provide asymptotic expansion of WGM resonances at high polar frequency $m\to\infty$ for cavities with radially varying optical index. In this work, using a direct Schr\"odinger analogy we highlight three typical behaviors in such optical micro-disks, depending on the sign of an effective curvature that takes into account the radius of the disk and the values of the optical index and its derivative. Accordingly, this corresponds to abruptly varying effective potentials (step linear or step harmonic) or more classical harmonic potentials, leading to three distinct asymptotic expansions for ground state energies. Using multiscale expansions, we design a unified procedure to construct families of quasi-resonances and associate quasi-modes that have the WGM structure and satisfy eigenequations modulo a super-algebraically small residual $O(m^{-\infty})$. We show using the black box scattering approach that quasi-resonances are $O(m^{-\infty})$ close to true resonances.