Optical gain in fluorenyl-thiophene co-oligomer thin films

We report on the emission properties of thin films of a fluorenyl-thiophene co-oligomer under strong optical excitation. A clear line narrowing is observed in the emission spectra as a function of the excitation density, due to a Stimulated Emission band attributed to Amplified Spontaneous Emission (ASE). The ASE threshold of about 150 lJ cm_2 is comparable with the best values reported for other oligomeric and/or co-oligomeric films or crystals. A maximum gain value of about 8 cm_1 is obtained for an excitation density of about 940 lJ cm_2. These results propose fluorene-thiophene co-oligomers as interesting materials for high performance optically pumped laser devices.