Date of Award
Doctor of Philosophy (PhD)
Due to their ultra-high quality factor and small mode volume, whispering gallery mode (WGM) microresonators have proven to have exceptional sensing capabilities, with single particle level sensitivity to virions, proteins, and nucleic acids. Current sensing mechanisms rely on measuring the changes in the transmission spectrum of the resonator upon adsorption of the analyte on the surface of the resonator, appearing as either shift, splitting, or broadening of the resonance mode, all of which measure the polarizability of adsorbed analytes. In this dissertation, we present two new sensing mechanisms for WGM microresonators: the measurement of a dynamic chemical reaction around the resonator, exemplified by the polymerization of hydrogel, and the Raman spectroscopy of molecules on the surface of WGM microresonator through WGM-based surface-enhanced Raman scattering. Further, an on-going work on sensing using mesoporous silica micro-bottle resonator is described in the last chapter.
Our work on the measurement of gelation of polyacrylamide hydrogel using WGM resonators is the first report of using WGM resonators to continuously monitor a chemical reaction (i.e. gelation) in situ. The results from WGM sensing is compared with rheology, a well-established technique for hydrogel characterization. From the similarities and differences in the measured results from WGM and rheology, we suggest that whereas rheology measures the viscoelastic properties of the hydrogel, WGM resonators measure the hydrogel density indirectly through its refractive index. The two techniques provide data that complement each other, which can be used to study the gelation reaction in more details.
Raman spectroscopy is a powerful technique for molecular fingerprinting, but the weak Raman signal often requires enhancement from techniques such as surface-enhanced Raman scattering (SERS). Conventionally, metallic nanostructures are used for SERS, but recently there has been increasing interest in the enhancement of Raman scattering from dielectric substrates due to their improved stability and biocompatibility compared with metallic substrates. The combination of WGM resonator and Raman spectroscopy can be a promising sensing platform with both high sensitivity and specificity. Here, we demonstrate the enhanced Raman scattering from rhodamine 6G molecules coated on silica microspheres, excited through WGMs. A total Raman enhancement factor of 1.4 × 104 is observed.
Shantanu Chakrabartty, Rajan Chakrabarty, Matthew Lew, Ulugbek Kamilov,