Triple-layer-coated microspheres for refractive index sensor with internally referenced self-compensated thermal effect

Abstract

Optical microcavity has an important and promising application in high sensitivity sensing, but thermal drift hinders its practical use. In this study, we propose a triple-layer-coated microsphere resonator, which has a high sensitivity in refractive index sensing with low thermal drift. The refractive indexes of the three layers from the inside to the outside are high, low, and high, respectively. The two high refractive index layers can support their own whispering-gallery modes, called the inner mode (IM) and the outer mode (OM). We study the performance of IM and OM with waveguide coupling in refractive index sensing and temperature sensing. The results show that when the thickness of the middle layer is 550 nm, the refractive index sensitivity of IM and OM will be 0.0168 nm/RIU, 102.56 nm/RIU, and the temperature sensitivity will be –19.57 pm/K and –28.98 pm/K, respectively. The sensing is carried out by monitoring the difference in resonant wavelength between IM and OM and the sensing characteristics are optimized by adjusting the thickness of the middle layer. Further, when <inline-formula><tex-math id="M3">\begin{document}${t_B}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191265_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="1-20191265_M3.png"/></alternatives></inline-formula> = 400 nm, the refractive index sensitivity can arrive at 75.219 nm/RIU, the detection limit can reach 2.2 × 10<sup>–4</sup> RIU, and the thermal drift is reduced to 3.17 pm/K, thereby eliminating the effect of thermal drift to a great degree. This study provides the guidance for designing and improving the microsphere refractive index sensors.