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Bandpass transmission spectra of a whispering-gallery microcavity coupled to an ultrathin fiber

Tapered fibers with diameters ranging from 1 to 4 μm are widely used to excite the whispering-gallery (WG) modes of microcavities. Typically, the transmission spectrum of a WG cavity coupled to a waveguide around a resonance assumes a Lorentzian dip morphology due to resonant absorption of the light within the cavity. In this paper, we demonstrate that the transmission spectra of a WG cavity coupled with an ultrathin fiber (500–700 nm) may exhibit both Lorentzian dips and peaks, depending on the gap between the fiber and the microcavity. By considering the large scattering loss of off-resonant light from the fiber within the coupling region, this phenomenon can be attributed to partially resonant light bypassing the lossy scattering region via WG modes, allowing it to be coupled both to and from the cavity, then manifesting as Lorentzian peaks within the transmission spectra. This implies the system could be implemented within a bandpass filter framework.