Recently, it has been reported that when PPV thin films are optically pumped under certain conditions with fast laser pulses, causing a dramatic narrowing of the emission linewidth occurs. This narrowing phenomenon is well documented but there is no unique interpretation has been suggested up to now. So far, four possible processes can explain such phenomenon: (i) Amplified Spontaneous Emission (ASE), (ii) Lasing, (iii) Superradiance (SR) and (iv) Superfluorescence (SF)....[ Read more ]

Recently, it has been reported that when PPV thin films are optically pumped under certain conditions with fast laser pulses, causing a dramatic narrowing of the emission linewidth occurs. This narrowing phenomenon is well documented but there is no unique interpretation has been suggested up to now. So far, four possible processes can explain such phenomenon: (i) Amplified Spontaneous Emission (ASE), (ii) Lasing, (iii) Superradiance (SR) and (iv) Superfluorescence (SF).

In this experiment, 2,5-dioctyloxy poly(p-phenylene vinylene) (DOO-PPV) films were fabricated on quartz, glass, and sapphire substrates by spin coating from polymer solutions in chloroform. Using femtosecond time-resolved up-conversion laser pumping techniques, the time-integrated and time-resolved spectra of the luminescence have been studied. Results show that the spectral narrowing in PPV is due to the ASE process.

Only the PPV polymer film on waveguiding substrates can produce a spectral narrowing peak which indicates that the ASE peak is enhanced by the waveguiding effect. Moreover, the existence of a definite threshold excitation intensity and the emission intensity increases linearly with pumping power are the characteristics of ASE model (i.e. I_ASE[varies as]I_pump). From the time-integrated spectra, it is clearly shown that the threshold intensity decreases as stripe length increase. While for time-resolved emission measurements, the decay spectra shows a dramatic reduction in emissron lifetime at the 630nm when pump above the spectral narrowing threshold. Also, the delayed emission at the ASE peak is longer than the spontaneous emission due to the 'propagation effect'. We also observed that the delay time increase with excitation stripe length, this emission delay time is directly proportional to the excitation stripe length in the ASE process (i.e. τD[varies as]L). All the above results give strong evidences to support the assignment of ASE process and show the importance of propagation effect in spectral narrowing phenomenon in conjugated polymer.