The role of Berry's geometric phase in the mode spectrum of a Fabry-Pérot resonator

Berry's geometric phase [1] is a universal phenomenon which appears naturally when dealing with three-dimensional rotations. Here we will demonstrate its unexpected role in the high-resolution transmission spectrum of a two-mirror Fabry-Perot resonator. Using analytical three-dimensional vector solutions to Maxwell's equations in spheroidal coordinates [2], we have calculated the first-order corrections to eigenmodes and eigenfrequencies in a Fabry-Perot resonator in the short-wavelength limit [3]. Analogous to atomic spectra, these analytical expressions show the “fine structure” of the spectrum of an optical resonator. Especially for high-order transverse modes with large orbital angular momentum as the one shown in the figure, the consequences are striking, leading to series of intricate mode patterns with Laguerre-Gaussian character split by their total angular momentum.