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

Optical microresonators are finding widespread application at the frontiers of nanophotonic technology, driven by virtue of their ability to confine light to the nanoscale and enhance light-matter interactions. Recently, our group has developed a new method of photothermal absorption spectroscopy, whereby toroidal optical microresonators act as microscale thermometers to detect the thermal relaxation of optically pumped, nanoscopic analytes. However, translation of this technology to chemically dynamic systems requires a platform that is mechanically stable, amenable to solution, and visibly transparent. We here report microbubble absorption spectrometers as a new and exciting platform that meets these requirements. Microbubbles integrate a two-port microfluidic 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. Furthermore, their all-glass fabrication results in an ultra-low photothermal background at visible wavelengths. We first leverage these virtues 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, indicating new routes forward towards the controlled construction of hybrid photonic-plasmonic systems. Critically, the reported platform advances microresonator technology by permitting room-temperature, aqueous experimental conditions, ushering the possibility of time-resolved experiments on non-emissive, nanoscale analytes engaged in catalytically and biologically relevant chemical dynamics.