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

TL;DR

This paper demonstrates ultrahigh-Q magnesium fluoride whispering gallery mode resonators for sensitive refractometric sensing in water, showing enhanced evanescent fields and sensitivity compared to fused silica, with stable Q factors above 10^8.

Contribution

The study introduces MgF$_2$ whispering gallery mode resonators with higher sensitivity and stability in aqueous environments, expanding capabilities for optical sensing applications.

Findings

Over 50% longer evanescent fields than fused silica

4.25 times larger sensitivity in water

Q factors exceeding 10^8 in water

Abstract

We present our experiments on refractometric sensing with ultrahigh-Q, crystalline, birefringent magnesium fluoride (MgF$_2$) whispering gallery mode resonators. The difference to fused silica which is most commonly used for sensing experiments is the small refractive index of MgF$_2$ which is very close 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 the birefringence amplifies the sensitivity difference between TM and TE type modes which will enhance sensing experiments based on difference frequency measurements. We estimate the performance of our resonators and compare them with fused silica theoretically and present experimental data showing the interferometrically measured evanescent decay and the sensitivity of mm-sized MgF$_2$ whispering gallery mode resonators immersed in water. They show reasonable agreement with the developed theory. Furthermore, we observe stable Q factors in water well above $1 \times 10^8$.