Abstract

Chemical and biological sensing with Whispering Gallery Mode (WGM) resonators is a rapidly growing field [1, 2]. The evanescent field of the high Q, narrow linewidth modes interacts with the surrounding, providing high sensitivity to changes in the refractive index of the surrounding or the binding of particles and molecules. In the last years such resonators were used to detect nearly any kind of biological matter, even down to the single molecule level [3]. The most established material used in this field thus far is fused silica in form of microspheres or toroids. We investigate for the first time single crystalline, birefringent magnesium fluoride (MgF₂) resonators having a refractive index much closer to that of water than fused silica. The small index contrast with respect to water allows the evanescent field to penetrate deep into the surrounding, enhancing the interaction and therefore boosts the sensitivity. Furthermore, the birefringence amplifies the sensitivity difference between TE and TM modes significantly, which is advantageous for schemes that trace TE and TM modes simultaneously like temperature-drift compensating measurements.

© 2015 IEEE