Naturally occurring resonators in random lasing of pi-conjugated polymer films

Random lasing consists of a sequence of narrow, coherent spectral lines formed when stimulated emission in a disordered gain medium is excited above threshold excitation intensity, I-th. We studied the I-th distribution function in pi-conjugated polymer films, and its dependence on the excitation area size and polymer mixture with TiO2 nanoparticles; optical pictures of the excited film area support the statistical measurements. At I-th we found that a single dominant random lasing resonator appears in the picture, with good agreement between its diameter and the cavity size extracted from the power Fourier transform analysis of the emission spectrum. This was directly confirmed by a new technique of space/spectrum cross-correlation of the laser emission lines from the excited area with 10 x 10 mu m(2) spatial resolution. The statistical results coupled with the imaging data provide strong evidence for the model of random resonators in the gain medium for explaining random lasing in pi-conjugated polymer films.