Random lasing from weakly scattering media; spectrum universality in DOO–PPV polymer films

When films of a π-conjugated polymer are optically excited above a certain threshold intensity, then the emission spectrum acquires a multimode finely structured shape, which depends on the position of the excitation spot. We demonstrate that the power Fourier transform (PFT) of the emission spectrum exhibits a certain peak-like structure, which also depends on the excitation spot. Our intriguing observation is that averaging of the individual PFTs does not lead to a structureless curve, but rather yields a series of distinct transform peaks. We develop a simple theory that explains the presence of peaks in the average PFT and predicts their shape.