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 mu m in liquid-filled plano-plano Fabry-Perot (FP) cavities, which can provide an extremely low effective mode volume down to 0.3-5.1 mu m3 while maintaining a high Q-factor up to 1 x 10(4)-5 x 10(4) and a finesse of similar to 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 mu m and high tolerance to mirror misalignment as large as 2 degrees. 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-modeMIFP optofluidic lasers that have a lasing threshold as low as a few microjoules per square millimeter and a lasing spot radius of only similar to 0.5 mu m. 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. (C) 2018 Chinese Laser Press