On-chip $Si/SiOx$ microtube refractometer

The authors fabricate rolled up microtubes consisting of $Si/SiOx$ on Si substrate and analyze the possibility to use them as a refractometric sensor. An aqueous sugar solution is inserted into the microtube, which leads to a change in refractive index and, as a result, to a detectable spectral shift of the whispering gallery modes. Experimental results can fit well with finite-difference time-domain simulations, which are used to determine the sensitivity of this tube refractometer. The ratio of spectral sensitivity to channel cross-sectional area of the refractometer is particularly striking and allows analysis of fluid volumes in the range of femtoliters. A comparative discussion with other existing refractometer schemes concludes this work.