Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: High-Q MgF$_2$ whispering gallery mode resonators for refractometric
  sensing in aqueous environment   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 $1 \times 10^8$.

5
The categories of this paper are: gr-qc
1
The categories of this paper are: cs
6
The categories of this paper are: astro-ph
36
The categories of this paper are: cmp-lg
35
The categories of this paper are: mtrl-th
34
The categories of this paper are: chem-ph
28
The categories of this paper are: alg-geom
4
The categories of this paper are: cond-mat
7
The categories of this paper are: hep-th
13
The categories of this paper are: quant-ph
16
The categories of this paper are: stat
33
The categories of this paper are: plasm-ph
37
The categories of this paper are: supr-con
23
The categories of this paper are: solv-int
25
The categories of this paper are: patt-sol
32
The categories of this paper are: atom-ph