Cooperative Emission in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi mathvariant="italic">π</mml:mi></mml:math>-Conjugated Polymer Thin Films

Abstract

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 120 nm width and $\ensuremath{\sim}1\mathrm{ns}$ duration, which is observed at low exciton density $n$, collapses at $n&gt;{10}^{17}{\mathrm{cm}}^{\ensuremath{-}3}$ into a much narrower band of 7 nm width and lifetime $\ensuremath{\tau}\ensuremath{\ll}10\mathrm{ps}$. 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.