Interplay between Stimulated Emission and Singlet Singlet annihilation in oligothiophene dioxide thin films

We have studied the optical properties of different thienyl-S,S-dioxide oligothiophenes under strong excitation. No stimulated emission is observed in neat films due to singlet-singlet bimolecular annihilation. The bimolecular recombination constant is quantified from a rate equation model in the limit of constant annihilation rate. We demonstrate that tunable stimulated emission in the range 490–660 nm, due to optical gain with an estimated cross section of the order of sigma_g=10−17 cm2, can be obtained by blending the active molecule with small quantities of inert polycarbonate poly(bisphenol-A-carbonate) (PC). The presence of amplified spontaneous emission (ASE) for a PC:active molecule relative concentration as small as 1:500 suggests that the polycarbonate role is to reduce the intermolecular diffusion rate rather than to isolate the active molecules. Moreover, for higher PC content, a continuous decrease of the bimolecular quenching role is observed. These results demonstrate that the absence of stimulated emission in neat films is not necessarily due to intrinsic molecular properties, as strong ASE can be obtained by slightly modifying the molecule interaction during the deposition process. This approach could allow a considerable extension of the number of molecules showing stimulated emission for organic laser applications