Simulation of whispering-gallery-mode resonance shifts for optical miniature biosensors

Finite element analyses are made of the shifts of resonance frequencies of whispering-gallery-mode (WGM) for a fiber-microsphere coupling miniature sensor. The time-domain Maxwell's equations were adopted to describe the near-field radiation transport and solved by the in-plane TE waves application mode of the FEMLAB. The electromagnetic fields as well as the radiation energy distributions can be easily obtained by the finite element analysis. The resonance intensity spectrum curves in the frequency range from 213 to 220 THz were studied under different biosensing conditions. Emphasis was put on the analyses of resonance shift sensitivity influenced by changes of the effective size of the sensor resonator (i.e., microsphere) and/or the refractive index of the medium surrounding the resonator. It is estimated that the WGM biosensor can distinguish molecular size change to the level of 0.1 nm and refractive index change in the magnitude of ∼10-3 even with the use of a general optical spectrum analyzer of one GHz linewidth. Finally, the potential of the WGM miniature biosensor for monitoring peptide growth is investigated and a linear sensor curve is obtained.