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The challenges to realizing diode lasers based on thin films of organic semiconductors are primarily related to low charge carrier mobility in these materials. This not only limits the thickness of organic films to /spl les/100 nm in electrically pumped devices, but it also leads to changes in the optical properties of organic films induced by the large number of carriers trapped in the materials subjected to an intense electrical excitation. We describe organic waveguide laser structures composed of thin organic films and transparent indium-tin-oxide electrodes. These waveguides allow for efficient injection of an electrical current into the organic layers and provide for low optical losses required in a laser. The changes in the optical properties of organic thin films induced by electrical excitation are studied using electroluminescence and pump and probe spectroscopy. Induced transparency and absorption observed in the organic materials may be related to triplet excitons or trapped charge carriers. Pump-induced absorption is also observed in organic films under quasi-CW optical excitation. These effects must be taken into account both in the design of organic diode laser structures and in the selection of charge transporting materials.