Spectral analysis of near threshold random lasers

Emission spectra from random lasing systems typically have numerous narrow resonant lines. When excited very near to the laser threshold there are fewer resonant lines which clarify the emission spectrum analysis. We studied three different random lasing systems including π-conjugated polymer films, zinc oxide and TiO2 scatterers in dye solution. Fourier transform analysis of the laser emission spectra near threshold of each system shows that all the sharp lines are highly correlated, indicating that they originate from a single high symmetry resonant structure. The naturally formed microresonators have a circular geometry in the two-dimensional films, and transient spherical geometry in the scatterers/dye suspension.