A unique resonance mode observed in a prism-coupled micro-tube resonator sensor with superior index sensitivity

Silica micro-tube resonators were studied by using a prism-coupled method and act as refractive index sensing elements. Two types of refractive index sensing modes were observed experimentally by tuning the incident angles in the prism-coupled configuration. The resonance mode with sensitivity below 100nm/RIU is a result of the typical evanescent wave interaction with the fluid, while the mode with record high sensitivity of ~ 600nm/RIU is a form of non-evanescent wave with high optical field in the low index fluid region. An analysis of the field distribution of the resonance modes in the micro-tube also revealed the existence of high order modes with strong optical field inside the low index liquid core, which leads to the high index sensitivity. Theoretical calculation of the sensitivity for this specific mode obtained by the Mie scattering method is consistent with the experimental result. A ray optics picture is presented to elucidate the physical nature of this special resonance mode observed in the micro-tube resonator.