Sensors based on whispering gallery mode resonators are able to detect single molecules and nanoparticles in liquid and gaseous environments and thus constitute an ultra-sensitive, on-chip platform for label-free biological, chemical and medical sensing applications. To reduce measurement noise a self-referencing detection method based on mode splitting was recently demonstrated [1]. Nanoparticles attaching to the resonator induce a mode-splitting of the originally degenerate clockwise (CW) and counter-clockwise (CCW) propagating modes of the WGM resonator. The resulting doublet allows a precise sizing of the particles. However, in practice this spectral feature is often obscured by the width of the resonance line which hides the doublet structure. This happens particularly in liquid environments that reduce the effective quality or Q factor of the resonator and cause a broadening of the resonance lines.
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