Toward Real-Time Monitoring and Control of Single Nanoparticle Properties with a Microbubble Resonator Spectrometer

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 new method for single-particle photothermal absorption spectroscopy, whereby the microresonators act as microscale thermometers to detect the heat dissipated by optically pumped, non-luminescent 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 new and versatile platform that meets these requirements. Microbubbles integrate a two-port microfluidic device within a Whispering Gallery Mode (WGM) 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 photo-activated etching of single gold nanorods by ferric chloride, providing a new 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 new route toward the construction of hybrid photonic-plasmonic systems. Critically, the reported platform advances microresonator spectrometer technology by permitting room-temperature, aqueous experimental conditions, opening a regime of time-resolved single-particle experiments on non-emissive, nanoscale analytes engaged in catalytically and biologically relevant chemical dynamics.