Enhancing the thermo-optic tuning performance of whispering gallery modes in a microcapillary resonator filled with nematic liquid crystal

We investigated both theoretically and experimentally, the thermo-optic tuning of whispering-gallery modes (WGMs) in a microcapillary resonator filled with nematic liquid-crystal (LC). The tuning of WGMs was realized due to the photo-thermal effect of magnetic nanoparticles (MNPs) on the surface of a fiber half-taper connected to a pump laser source, resulting in temperature-induced refractive index (RI) changes of the LC material. Based on perturbation theory, we analyzed the influence the RI and the wall thickness on the sensitivity of the proposed tuning scheme. Furthermore, we experimentally demonstrated that increasing the thickness of the MNPs coating on the fiber taper surface leads to a stronger photo-thermal effect and to a larger RI change of the LC material within the microcapillary core. Thermo-optic tuning of WGM resonances with a sensitivity of 256.63 +/- 5.67 pm/mW to the laser pump power and tuning range of 10.43 nm has been achieved. The developed thermo-optic tuning scheme has many potential applications as tunable devices, optical filters and sensors.