Strong Coupling in a Photonic Molecule Formed by Trapping a Microsphere in a Microtube Cavity

A photonic molecule formed by trapping a microsphere cavity into a hollow microtube cavity is demonstrated, which provides a novel design over conventional photonic molecules comprised of solid-core whispering gallery mode microcavities with externally tangent configuration. Periodic spectral modulations of mode intensity, resonant mode shift, and quality factor are observed owing to the largely mismatched cavity sizes. The intercavity coupling strength can be tuned by shifting the excitation position off the tangent point of the microsphere-tube system along the tube axis, rather than the conventional strategy of changing the spacing between coupled cavities. In particular, anticrossing feature of coupled modes is revealed to verify the existence of optical strong coupling in the microsphere-tube system. Numerical simulation results show an excellent agreement with the experimental observations. The present work provides a flexible strategy for designing photonic molecules and tuning the coupling behavior of resonant modes, which is of high interest for both fundamental and applied studies.