Mode Selection Method in Spherical Optical Cavities with Thin Metal Film

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 mu m 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 similar to 10(7)), while the other radial modes are significantly suppressed (Q similar to 10(3)). In addition, it is shown that selection is carried out by the mode index number (l) simultaneously with selection by the radial index.