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,*
- 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 while maintaining a high -factor up to and a finesse of . 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 -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 . 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.