Active control of light-matter interactions using nanophotonic structures is critical for new modalities for solar energy production, cavity quantum electrodynamics (QED), and sensing, particularly at the single-particle level, where it underpins the creation of tunable nanophotonic networks. Coupled plasmonic-photonic systems show great promise toward these goals because of their subwavelength spatial confinement and ultrahigh-quality factors inherited from their respective components. Here, we present a microfluidic approach using microbubble whispering-gallery mode cavities to actively control plasmonic-photonic interactions at the single-particle level. By changing the solvent in the interior of the microbubble, control can be exerted on the interior dielectric constant and, thus, on the spatial overlap between the photonic and plasmonic modes. Qualitative agreement between experiment and simulation reveals the competing roles mode overlap and mode volume play in altering coupling strengths.
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