Monodisperse and size-tunable high-quality factor microsphere biolasers.

Microsphere biolasers have attracted a great deal of interest due to their potential for biosensing and cell tracking. Here we demonstrate a novel, to the best of our knowledge, microfluidic-based fabrication of nearly monodisperse dye-doped protein microsphere biolasers with a tunable size from 150 to 50 µm. In particular, for an 85 µm-bead, about 70% of the fabricated microspheres have the same size of 85 µm. Under optical pumping, the fabricated microspheres emit whispering gallery mode lasing emission with a lasing threshold of ${{7}}\;\unicode{x00B5} {\rm{J}}\;{{\rm{mm}}^{- 2}}$ and quality ($\!Q$) factor up to 3000. Interestingly, microspheres with the same size exhibit a similar lasing threshold and spectrum. The result indicates a high reproducibility of microsphere biolasers by the microfluidic-based fabrication technique. This Letter provides an effective method for mass production of high-$Q$ factor microsphere biolasers which is a significant step toward real biosensing and medical applications.