High-Q MgF$_2$ whispering gallery mode resonators for refractometric sensing in aqueous environment

摘要

We present our experiments on refractometric sensing with ultrahigh-Q,crystalline, birefringent magnesium fluoride (MgF$_2$) whispering gallery moderesonators. The difference to fused silica which is most commonly used forsensing experiments is the small refractive index of MgF$_2$ which is veryclose to that of water. Compared to fused silica this leads to more than 50%longer evanescent fields and a 4.25 times larger sensitivity. Moreover thebirefringence amplifies the sensitivity difference between TM and TE type modeswhich will enhance sensing experiments based on difference frequencymeasurements. We estimate the performance of our resonators and compare themwith fused silica theoretically and present experimental data showing theinterferometrically measured evanescent decay and the sensitivity of mm-sizedMgF$_2$ whispering gallery mode resonators immersed in water. They showreasonable agreement with the developed theory. Furthermore, we observe stableQ factors in water well above $1 \times 10^8$.