Crystalline MgF2 whispering gallery mode resonators for enhanced bulk index sensitivity

We report on experiments on refractrometric sensing with crystalline Whispering Gallery Mode (WGM) resonators made of magnesium fluoride, which has a refractive index that is only slightly larger than that of water (Delta n approximate to 0.05). The resulting evanescent field of a WGM resonator placed in an aqueous environment penetrates therefore deep into the surrounding medium, which makes it a promising candidate for sensing applications. We measured a bulk index sensitivity of 1.09 nm/RIU (refractive index unit) in a resonator with a radius of R = 2.91mm and intrinsic Q-factors of more than 10(8) in aqueous environments. Furthermore, we describe the fabrication process of crystalline WGM resonators