  Whispering gallery mode (WGM) microbubble cavities are a versatile
optofluidic sensing platform owing to their hollow core geometry. To increase
the light-matter interaction and, thereby, achieve a higher sensitivity,
thin-walled microbubbles are desirable. However, a lack of knowledge about the
precise geometry of hollow microbubbles prevents us from having an accurate
theoretical model to describe the WGMs and their response to external stimuli.
In this work, we provide a complete characterization of the wall structure of a
microbubble and propose a theoretical model for the WGMs in this thin-walled
microcavity based on the optical waveguide approach. Structural
characterization of the wavelength-scale wall is enabled by focused ion beam
milling and scanning electron microscopy imaging. The proposed theoretical
model is verified by finite element method simulations. Our approach can
readily be extended to other low-dimensional micro-/nanophotonic structures.
