Direct probe of spectrally narrowed emission from π-conjugated polymers: The elucidation of mechanism for spectral line narrowing

Femtosecond time-resolved emission as well as transient absorption spectroscopy of π-conjugated polymeric thin film revealed an apparent induction time for the appearance of spectrally narrowed emission (SNE) in contrast with an instantaneous buildup of normal emission. These ultrafast intensity-dependent spectroscopic studies demonstrate that the origin of SNE phenomena in π-conjugated polymer is amplified spontaneous emission (ASE), which induces prompt and efficient depopulation of singlet excitons. Furthermore, comparison of temporal profiles of SNE, normal emission and stimulated emission reveals that the photoexcitations responsible for SNE processes are singlet excitons. The temporal profiles of SNE measured by changing excitation power density, illuminated spot size, and average π-conjugation length of polymer show that a build-up time plays an important role in generating intense SNE. These findings imply that pumping geometry, sample morphology, defect sites, as well as crystallinity are strongly correlated with ASE processes.