Protein-based microlasers: continuous transition from whispering gallery mode to random lasing

Whispering gallery mode (WGM) and random biolasers are important for bio-integration and biosensing applications due to their biocompatibility. However, these two laser types typically require different fabrication techniques, and limited research has explored transitioning between them using the same fabrication process. In this work, we present a comprehensive investigation into the transition from WGM to random lasing in protein-based microsphere lasers. Through a dehydration method, we can fabricate dye-doped protein microspheres with diameters ranging from 20 to 150 mu m. By varying the polystyrene (PS) particle concentration within these microspheres, we observed a continuous transition from WGM to random lasing under optical pumping, driven by enhanced light scattering as PS concentration increased. Key lasing parameters, including the lasing spectrum and lasing threshold versus laser size and PS concentration, were analyzed and compared. The results indicate that microsphere random lasers (RLs) exhibit shorter lasing wavelengths. Particularly, a 63 mu m WGM laser has a central lasing wavelength at 630 nm, while a RL of similar size shows a peak wavelength at 590 nm, a 40 nm blue shift. The lasing threshold increases with smaller laser size and higher PS concentration, with WGM lasers requiring several mu J mm-2, potentially ten times lower than RLs. Our work opens the possibility of designing flexible, wavelength-tunable biological microlasers. Moreover, it provides an understanding of the distinct characteristics of WGM and random lasing, making a meaningful contribution to laser research.