Studies of random laser action in π-conjugated polymers

Laser action usually occurs within carefully configured resonant cavities. However, stimulated emission in disordered systems may also lead to coherent laser action above a certain threshold excitation intensity, Ith, which was dubbed as 'random lasing'. In random lasing a sequence of spectrally narrow emission lines, which are characteristic of laser modes, is superimposed on the amplified spontaneous emission band. We studied the statistical threshold distribution function, F(Ith) of Ith, of random lasers in π-conjugated polymer films and its evolution with respect to the excitation area. The measurements were supported by pictures of the excited area taken while lasing occurs. We found that random lasing is accompanied by the appearance of lasing random cavities in the pictures, indicating that the sharp lines in the emission spectrum are, in fact, laser modes related to the random cavities formed in the disordered gain medium. Moreover a correlation exists between the cavity size extracted from the Fourier transform of the random lasing emission spectra and the size of the random resonators in the pictures obtained. In addition we also found that the measured F(Ith) has a marked asymmetry with respect to the average threshold, (Ith)ave, which decreases with the excitation area. An existing theoretical model based on random resonators in the polymer film is found to be in good agreement with the obtained F(Ith) function