COOPERATIVE EMISSION IN PI-CONJUGATED POLYMER THIN-FILMS

Picosecond dynamics of exciton emission and absorption have been studi ed in neat thin films of a variety of poly (phenylene vinylene);deriva tives. We found that the stimulated emission band of 120 nm width and similar to 1 ns duration, which is observed at low exciton density n, collapses at n > 10(17) cm(-3) into a much narrower band of 7 nm width and lifetime tau much less than 10 ps: Based on its excitation intens ity dependence, polarization, lifetime, illuminated area, and film thi ckness dependencies, we assign this narrow band to superfluorescence r ather than to amplified spontaneous emission.