Laser action of two conjugated polymers in solution and in solid matrix: The effect of aggregates on spontaneous and stimulated emission

We present an investigation of amplified spontaneous emission (ASE) and laser action of two polymers, emitting in the blue, with alternating conjugated and flexible aliphatic segments of different lengths, in tetrahydrofuran (THF) solutions and in solid poly(methyl methacrylate) (PMMA) matrices. Laser action and ASE occur simultaneously, permitting their main differences such as coherence and tunability to be distinguished and the detrimental effect of ASE on laser action to be investigated. The polymer with the longer aliphatic segment is the more efficient material in both laser and ASE. The neat films of these polymers showed no spectral narrowing; this is attributed to the presence of aggregates. An investigation of the role of aggregates on spontaneous emission and mainly on ASE was realized in solutions by increasing the aggregate concentration through a technique based on a combination of good and poor solvent. It is proved that in these solutions both aggregates and isolated chains exist. Aggregates are responsible for the spontaneous emission while the isolated chains for the ASE.