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

作　　者：XIAOQIN WU YIPEI WANG QIUSHU CHEN YU-CHENG CHEN XUZHOU LI LIMIN TONG XUDONG FAN 

机构地区：[1]Department of Biomedical Engineering, University of Michigan [2]State Key Laboratory of Modern Optical Instrumentation, College of Optical Science and Engineering, Zhejiang University [3]Department of Electrical & Computer Engineering, University of Michigan

出　　处：《Photonics Research》2019年第1期50-60,共11页光子学研究（英文版）

基　　金：National Science Foundation(NSF)(DBI-1451127,ECCS-1607250);International Postdoctoral Exchange Fellowship Program(20160007)

摘　　要：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 μm^3 while maintaining a high Q-factor up to 1 × 10~4–5 × 10~4 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.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 μm^3 while maintaining a high Q-factor up to 1 × 10~4–5 × 10~4 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.

关 键 词：mirror radius diameter optical devices down EASE spot 

分 类 号：TN248[电子电信—物理电子学]