Bidirectional tuning of whispering gallery modes in a silica microbubble infiltrated with magnetic fluids.

A bidirectional tuning mechanism of whispering gallery modes (WGMs) in a capillary-based microbubble microresonator infiltrated with magnetic fluids (MFs) is investigated. Owing to distinct RI responses of MFs dependent on the applied magnetic field direction with respect to the capillary axis, the RI of MFs shows different variation trends when an external magnetic field is parallel or perpendicular to the capillary axis. Experimental results indicate that WGM resonance dips exhibit wavelength shift in inverse directions for the above two cases, which is in accordance with our theoretical analysis on different refractive variation behaviors of MFs. As the applied magnetic field is perpendicular or parallel to the capillary axis, the WGM resonance wavelength tuning sensitivities tend to be $ - {15.01}\;{\rm pm/mT}$-15.01pm/mT and 6.3 pm/mT, respectively. Our proposed WGM tuning scheme has several desirable advantages, including bidirectional tunability, high Q-factor, ease of fabrication, and good compatibility with functional materials, making it a promising candidate in the field of magnetic field vector sensing and magnetically manipulated micro-optic devices.