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  • Received: Sep. 17, 2018

    Accepted: Nov. 14, 2018

    Posted: Feb. 21, 2019

    Published Online: Feb. 21, 2019

    The Author Email: Xudong Fan (

    DOI: 10.1364/PRJ.7.000050

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    Xiaoqin Wu, Yipei Wang, Qiushu Chen, Yu-Cheng Chen, Xuzhou Li, Limin Tong, Xudong Fan. High-Q, low-mode-volume microsphere-integrated Fabry–Perot cavity for optofluidic lasing applications[J]. Photonics Research, 2019, 7(1): 01000050

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Photonics Research, Vol. 7, Issue 1, 01000050 (2019)

High-Q, low-mode-volume microsphere-integrated Fabry–Perot cavity for optofluidic lasing applications

Xiaoqin Wu1,2, Yipei Wang3, Qiushu Chen1, Yu-Cheng Chen1, Xuzhou Li1, Limin Tong2, and Xudong Fan1,*

Author Affiliations

  • 1Department of Biomedical Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA
  • 2State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, China
  • 3Department of Electrical & Computer Engineering, University of Michigan, Ann Arbor, Michigan 48109, USA


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 0.35.1 μm3 while maintaining a high Q-factor up to 1×1045×104 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.

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