The emission dynamics in photoexcited planar conjugated polymer waveguides
is investigated at high excitation densities. Using femtosecond pump/probe
experiments and photoluminescence spectroscopy we investigate the interplay
of nonlinear radiative and nonradiative recombination processes. Amplified
spontaneous emission (ASE) leads to an ultrafast depletion of the excited
state at excitation densities above 10(18) cm(-3) in an ladder-type poly(p-
phenylene) film deposited on a glass substrate. Owing to the different wave
guide structure ASE is not observed for the same polymer deposited on an in
dium-tin-oxide (ITO)-coated substrate. Instead, we observe nonradiative bim
olecular annihilation with a coefficient gamma = 4.2 X 10(-9) cm(3) s(-1).
Our results demonstrate that even in the absence of a resonator collective
stimulated emission can be much more efficient than nonradiative recombinat
ion. A mandatory prerequisite, however, is a suitable waveguide design. The
use of ITO as a hole-injecting contact is problematic due to its high refr
active index and its relatively high losses. (C) 1999 American Institute of
Physics. [S0021-8979(99)09101-X].