Ultrafast intensity-dependent stimulated emission in conjugated polymers: The mechanism for line-narrowing

Intensity-dependent, time-resolved stimulated emission experiments are performed simultaneously with emission line width measurements on the conjugated polymer BCHA–PPV. The results show a steady decrease in emission lifetime with increasing excitation intensity, consistent with a model utilizing only stimulated emission. No evidence is found for condensed excitonic species or coherent emission processes such as superfluorescence. The spatially anisotropic nature of the gain can also explain the apparent drop in fluorescence quantum yield near the line-narrowing threshold. The existence of line-narrowing in heavily oxidized samples suggests that stimulated emission might be used in electroluminescent devices to overcome quenching by defects.