Mode-splitting for refractive index sensing in fluorescent whispering gallery mode resonators with broken symmetry

Abstract

Whispering Gallery Mode (WGM) biosensors have been widely exploited over the past decade, owing to their unprecedented detection limits and label free capability. WGM based sensing mechanisms, such as resonance frequency shift, linewidth broadening, and splitting of the two counter-propagating WGMs, have been extensively researched and applied for bio-chemical sensing. However, the mode-splitting of the originally degenerate WGMs from different equatorial planes on a fluorescent microsphere has not been fully investigated. In this work, we break the symmetry of the surrounding environment outside the microsphere by partially embedding the sphere into a high-refractive-index medium (i.e. glue), to lift the degeneracy of the modes from different WGM planes. The split-modes from multiple planes of the fluorescent microsphere are indiscriminately collected. It is found that the effective quality factor Q of the WGMs increases non-conventionally as the Refractive Index (RI) of the probing liquid increases up to the point where it is equal to that of the glue. This presents a new methodology for quantifying changes in the probing environment based on the Q spoiling of the resonances as determined by the RI difference between the environment and that of the reference glue. Furthermore, we find that this sensing platform opens the door to simple self-referenced sensing techniques based on the analysis of the spectral positions of subsets of the split modes.