Mode Selection Method in Spherical Optical Cavities with Thin Metal Film

Abstract

A new method for the selection of the radial modes of a spherical microresonator by the quality factor, based on the effect of absorption in a thin metal film deposited on a sphere together with a dielectric layer, is proposed. The method was proved by numerical simulation on the example of the second and third radial modes TEl,l,q, l = 455, q = 2, 3 selection in 80 μm radius quartz microsphere with 5 nm thick nickel film. The structure of the metal-dielectric coating was determined using analytical formulas in such a way that the metal film was placed at the node of the selected mode E-field distribution. The finite element method was used to refine the calculation, optimizing the position of the film in terms of obtaining the maximum quality factor. As a result, it was shown that the selected mode could have a relatively high quality factor (Q~107), while the other radial modes are significantly suppressed (Q~103). In addition, it is shown that selection is carried out by the mode index number (l) simultaneously with selection by the radial index.