Gain narrowing in optically pumped thin films, both neat and undiluted, of luminescent conjugated polymers with different molecular structures was demonstrated. These results indicate that the polymers studied have large cross sections for stimulated emission, that population inversion can be achieved at low pump energies, and that the emitted photons travel distances greater than the gain length within the gain medium. The use of simple waveguide structures is sufficient to cause low gain narrowing thresholds in submicrometer-thick films.

By measuring the gain and loss in thin-film planar waveguides using a standard technique developed for inorganic laser materials, we show that the narrow-line emission from photopumped waveguides of the conjugated polymer poly[2-butyl-5-${(2\mathrm{}}^{\prime }$-ethyl-hexyl)-1,4-phenylenevinylene] results from amplification of spontaneous emission (ASE). The narrowed linewidth of the ASE spectrum is determined by gain saturation. The ASE emitted from the edge of the waveguide is polarized because the waveguide is birefringent and only allows TE modes to propogate and be amplified. We compare our results and conclusions to other reports that claim that spectral narrowing in conjugated polymers is a result of superfluorescence or excitonic interactions. The techniques presented in the paper offer a simple and useful way for evaluating conjugated polymers as materials for solid state lasers.