Whispering gallery mode bio-sensor for label-free detection of single molecules: thermo-optic vs. reactive mechanism.

Thermo-optic and reactive mechanisms for label-free sensing of bio-particles are compared theoretically for Whispering Gallery Mode (WGM) resonators (sphere, toroid) formed from silica and stimulated into a first order equatorial mode. Although it has been expected that a thermo-optic mechanism should "greatly enhance" wavelength shift signals [A.M. Armani et al, Science 317, 783-787 (2007)] accompanying protein binding on a silica WGM cavity having high Q (10(8)), for a combination of wavelength (680 nm), drive power (1 mW), and cavity size (43 microm radius), our calculations find no such enhancement. The possible reasons for this disparity are discussed.