Longitudinal pumping of polymer microring lasers

Pulsed, longitudinal photopumped multi-mode laser emission is demonstrated in the visible spectral range from cylindrical microcavities formed by luminescent polymer thin films coated around optical fibers. Thresholds for laser oscillation for picosecond excitation are 80 pJ/mm for longitudinal pumping as compared to 4 nJ/mm (5 μJ/cm2 or 0.05 MW/cm2) for transversal pumping. An important advantage of the longitudinal pumping configuration lies in a significantly lower excitation density at threshold in comparison with the transverse pumping configuration. This improvement is ascribed to a larger overlap factor between the optical pump and the gain medium in the longitudinal geometry. The emission spectra of plastic microrings under these different excitation configurations are reported and analyzed. We note that lowering the lasing threshold is an important prerequisite criterion towards the development of electrically pumped plastic lasers.