Nanolayer characterization through wavelength multiplexing of a microsphere resonator

We optically characterize nanolayer ($<150 \mathrm{nm}$) formation in situ on a silica microsphere in an aqueous environment by simultaneously following the shifts of whispering-gallery modes at two wavelengths. This approach was inspired by layer perturbation theory, which indicates that these two measurements can be used to determine independently both the thickness and the optical dielectric constant. The theory is verified for extreme cases and used to characterize a biophysically relevant hydrogel nanolayer with an extremely small excess refractive index of 0.0012.