Cooperative emission in ∏-conjugated polymer thin films

Picosecond dynamics of exciton emission and absorption have been studied in neat thin films of a variety of poly (phenylene vinylene) derivatives. We found that the stimulated emission band of 120nm width and {approximately}1ns duration, which is observed at low exciton density n, collapses at n{gt}10{sup 17}cm{sup -3} into a much narrower band of 7nm width and lifetime {tau}{lt}10ps. Based on its excitation intensity dependence, polarization, lifetime, illuminated area, and film thickness dependencies, we assign this narrow band to superfluorescence rather than to amplified spontaneous emission. {copyright} {ital 1997} {ital The American Physical Society}