Effect of Molecular Weight on Amplified Spontaneous Emission in pi-conjugated Polymer Solutions

pi-conjugated polymers (PCPs) are suitable gain materials for laser applications due to their high photoluminescence (PL) quantum yield. However, the effect of molecular weight on amplified spontaneous emission (ASE) of PCPs is unknown. In this work, we investigated ASE in poly [2-methoxy-5-(2'ethylhexyloxy)-1, 4-phenylethylene] (MEH-PPV) with different molecular weight in 0.5 mg/mL, 1 mg/mL and 2 mg/mL solutions by using a pulsed laser with pulse width of 10 ns. The molecular weights are 40000-70000, 70000-100000, and 100000-150000, respectively. We found that the threshold decreases as the molecular weight increases, which is due to higher emission intensity at larger molecular weight. In addition, the threshold decreases with concentration increasing, this is because slightly aggregation is helpful for ASE. Our results have shown that molecular weight as well as concentration play an important role in the ASE threshold in MEH-PPV solutions, as well as PCP lasers.