Selective excitation of whispering-gallery modes and Fano resonance in a high-Q micro-capillary resonator with cleaned-up spectrum

We demonstrate, experimentally and theoretically, a micro-capillary resonator with surface nanoscale axial photonics for clean and nearly equidistant spectra, which belongs to a category of bottle-shaped optofluidic ring resonators. Selective excitation for the lowest five axial modes is achieved by changing the gap between the resonator and the taper and the coupling position along the micro-capillary. By scanning tapered fiber diameters carefully, the resonance state for converting a Lorentzian resonance to a Fano resonance and an electromagnetically induced transparency resonance is exhibited. The results obtained pave the way for an enhanced approach in sensing and, in particular, in microfluidic sensing.