Optical microresonators
have widespread application at the frontiers
of nanophotonic technology, driven by their ability to confine light
to the nanoscale and enhance light–matter interactions. Microresonators
form the heart of a recently developed method for single-particle
photothermal absorption spectroscopy, whereby the microresonators
act as microscale thermometers to detect the heat dissipated by optically
pumped, nonluminescent nanoscopic targets. However, translation of
this technology to chemically dynamic systems requires a platform
that is mechanically stable, solution compatible, and visibly transparent.
We report microbubble absorption spectrometers as a versatile platform
that meets these requirements. Microbubbles integrate a two-port microfluidic
device within a whispering gallery mode microresonator, allowing for
the facile exchange of chemical reagents within the resonator’s
interior while maintaining a solution-free environment on its exterior.
We first leverage these qualities to investigate the photoactivated
etching of single gold nanorods by ferric chloride, providing a method
for rapid acquisition of spatial and morphological information about
nanoparticles as they undergo chemical reactions. We then demonstrate
the ability to control nanorod orientation within a microbubble through
optically exerted torque, a promising route toward the construction
of hybrid photonic-plasmonic systems. Critically, the reported platform
advances microresonator spectrometer technology by permitting room-temperature,
aqueous experimental conditions, which may be used for time-resolved
single-particle experiments on non-emissive, nanoscale analytes engaged
in catalytically and biologically relevant chemical dynamics.