Spectra super-narrowing and random lasing dynamics in neat and TiO2-doped polymer films

Abstract

We investigated the dynamics of random lasing in 2,5-dioctyloxy poly(p-phenylene-vinylene) (DOO-PPV) neat and TiO2–doped films. We observed laser-like sharp emission (i.e., spectral width less than 1nm) in the films under certain threshold excitation condition. Fig.1 shows the random lasing time-integrated spectra in neat DOO-PPV film at various excitation intensities using excitation wavelength of 532nm (35ps,10Hz). Near the threshold intensity Io,ceven single mode emission can be achieved. At excitation below the threshold (i.e., I=0.5Io), only the broad photoluminescence (PL) background can be seen, although here the intensity scale is too large to see this PL clearly. At threshold intensity Io and just above it, three super-narrow emission peaks emerge. Increasing excitation intensity will increase number of super-narrow emission peaks. At very high excitation (i.e., 5 to 6 time above Io), all the peaks merge together into a single stimulated emission peak (with spectral width 5-10nm) commonly seen in lasing action experiment with an emission lifetime of a few picoseconds [1]. Fig 2 shows the time-resolved emission under the excitation condition where only a single super-narrow peak emerges. This time-resolved PL shows laser-like sharp emission lifetime at 622 nm was much less than the broad spontaneous emission lifetime and only limited by the time-resolution (~20ps) of our streak camera system. Similar random lasing dynamics and spectra super-narrowing were also measured in TiO2-doped (1×109cm−3) DOO-PPV polymer films. This ultrafast emission decay strongly indicates the super-narrow spectral peak is due to laser action.