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

## TL;DR

This study reveals that coupling a whispering-gallery microcavity with an ultrathin fiber can produce both Lorentzian dips and peaks in the transmission spectrum, influenced by the fiber-cavity gap and scattering losses, enabling potential bandpass filter applications.

## Contribution

It demonstrates the occurrence of Lorentzian peaks in the transmission spectra of WG microcavities coupled with ultrathin fibers, a phenomenon not previously reported.

## Key findings

- Transmission spectra show both dips and peaks depending on fiber-cavity gap.
- Large scattering loss allows off-resonant light to bypass the cavity.
- System can function as a bandpass filter.

## Abstract

Tapered fibers with diameters ranging from 1-4 micron 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 (500-700nm) 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, thence manifesting as Lorentzian peaks within the transmission spectra, which implies the system could be implemented within a bandpass filter framework.