Refractometric sensors based on microsphere resonators

We have developed a highly sensitive refractometric sensor based on fused silica microsphere resonators. The spectral position of the whispering gallery mode (WGM) of a sphere shifts in response to the refractive index change in the surrounding medium. The strong light-matter interaction due to the extremely high $Q$ factor associated with the WGM results in a sensitivity of approximately $30\mathrm{nm}∕\mathrm{RIU}$ (refractive index units). This, together with the high spectral resolution of our sensor system $(\sim 0.01\mathrm{pm})$ , yields a detection limit of refractive index change on the order of ${10}^{-7}\mathrm{RIU}$ . Theoretical calculation is also performed and agrees well with the experimental data.