Nonlinear emission and recombination in conjugated polymer waveguides

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 waveguide structure ASE is not observed for the same polymer deposited on an indium-tin-oxide (ITO)-coated substrate. Instead, we observe nonradiative bimolecular 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 recombination. A mandatory prerequisite, however, is a suitable waveguide design. The use of ITO as a hole-injecting contact is problematic due to its high refractive index and its relatively high losses. (C) 1999 American Institute of Physics. [S0021-8979(99)09101-X].