High-Q, low-mode-volume microsphere-integrated Fabry–Perot cavity for optofluidic lasing applications
We develop a hybrid optofluidic microcavity by placing a microsphere with a diameter ranging from 1 to 4 μm in liquid-filled plano-plano Fabry–Perot (FP) cavities, which can provide an extremely low effective mode volume down to 0.3–5.1 μm3 while maintaining a high Q-factor up to 1×104–5×104 and a finesse of ～2000. Compared to the pure plano-plano FP cavities that are known to suffer from the lack of mode confinement, diffraction, and geometrical walk-off losses as well as being highly susceptible to mirror misalignment, our microsphere-integrated FP (MIFP) cavities show strong optical confinement in the lateral direction with a tight mode radius of only 0.4–0.9 μm and high tolerance to mirror misalignment as large as 2°. With the microsphere serving as a waveguide, the MIFP is advantageous over a fiber-sandwiched FP cavity due to the open-cavity design for analytes/liquids to interact strongly with the resonant mode, the ease of assembly, and the possibility to replace the microsphere. In this work, the main characteristics of the MIFP, including Q-factor, finesse, effective mode radius and volume, and their dependence on the surrounding medium’s refractive index, mirror spacing, microsphere position inside the FP cavity, and mirror misalignment, are systematically investigated using a finite-element method. Then, by inserting dye-doped polystyrene microspheres of various sizes into the FP cavity filled with water, we experimentally realize single-mode MIFP optofluidic lasers that have a lasing threshold as low as a few microjoules per square millimeter and a lasing spot radius of only ～0.5 μm. Our results suggest that the MIFP cavities provide a promising technology platform for novel photonic devices and biological/chemical detection with ultra-small detection volumes.
基金项目：National Science Foundation (NSF)10.13039/100000001 (DBI-1451127, ECCS-1607250); International Postdoctoral Exchange Fellowship Program (20160007).
Yipei Wang：Department of Electrical & Computer Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
Qiushu Chen：Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
Yu-Cheng Chen：Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
Xuzhou Li：Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
Limin Tong：State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, China
Xudong Fan：Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
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Xiaoqin Wu, Yipei Wang, Qiushu Chen, Yu-Cheng Chen, Xuzhou Li, Limin Tong, and Xudong Fan, "High-Q, low-mode-volume microsphere-integrated Fabry–Perot cavity for optofluidic lasing applications," Photonics Research 7(1), 50-60 (2019)