Stimulated emission and lasing in solid films of conjugated polymers: ultrafast photophysics and photon confinement via scattering

Abstract
Two major hurdles to producing solid–state polymer laser diodes are identified: the presence of a photoinduced absorption (PA) which inhibits the stimulated emission (SE) necessary for lasing; and the difficulty with making emitted light in an excited polymer travel farther than the gain length. The first of these difficulties is addressed utilizing femtosecond spectroscopic experiments on a new conjugated polymer, poly(2–butyl–5–(2′–1,4–phenylene vinylene) (BuEH–PPV). The 60 ps SE decay time of BuEH–PPV is nearly an order of magnitude longer than that of other conjugated polymers, and is limited only by a dynamic blue–shift of the PA. Long SE times in polymer films open the possibility for addressing the second difficulty by producing lasing using multiple scattering. Above a critical excitation threshold, addition of TiO2 nanoparticles to solutions and dilute blend films of poly(2–methoxy,5–(2′–ethyl–hexyloxy)1,4–phenylene–vinylene) (MEH-PPV) confines the emitted photons in the excited film until gain exceeds loss. With this technique, lasing is observed in a solid–state polymer for the first time. The combination of new materials with persistent SE like BuEH–PPV and lasing by photon confinement should open major new avenues in the production of polymer laser diodes.