Enhanced high Q whispering gallery resonator sensing




Yu, Wenyan

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This thesis presents a novel method to fabricate metallic nanostructures on whispering gallery mode (WGM) cavity surfaces. The unique properties of WGM cavities have shown their promising future in both fundamental research and engineering applications. High sensitivity biosensors are one of the most important applications. Thanks to their ultra high quality factor (Q) and small optical mode volume, the resonant frequency shift of a single nanoparticle binding becomes detectable. The basic principles of a WGM cavity and its coupling mechanism with an optical coupler are discussed in detail. From the WGM sensing principle, people have demonstrated the positive contributions of the surface plasmon to the sensitivity. Furthermore, we implement the localized surface plasmon resonance (LSPR) on the cavity surface by depositing metallic dots. We use the focused ion beam (FIB) to directly deposit metallic nanodots on the spherical cavity surface for the first time. The quality factor of the cavity with metallic dots is above 10^7 in both air and water, which is more than one order larger than other published results. Also, the new method is much more controllable and repeatable than previous methods. It reveals a new fabrication method for potential ultra sensitive sensors based on WGM cavities. In addition, we offer a new mode solver for the toroidal WGM cavity. The microtoroid is a better platform for further investigation of WGM sensing than the microsphere. By expanding cavity modes to a set of normal fiber modes, we formulate the new mode solver based on simple physical principles. The simulation results of the radiative quality factor based on the new mode solver are presented as well.



optical cavity, sensing, whispering gallery mode