Organic random lasers in the weak-scattering regime

We used the ensemble-averaged power Fourier transform (PFT) of random laser emission spectra over the illuminated area to study random lasers with coherent feedback in four different disordered organic gain media in the weak scattering regime, where the light mean free path, $\ensuremath{\ell}*$ is much larger than the emission wavelength. The disordered gain media include a $\ensuremath{\pi}$-conjugated polymer film, an opal photonic crystal infiltrated with a laser dye (rhodamine 6G; R6G) having optical gain in the visible spectral range, a suspension of titania balls in R6G solution, and biological tissues such as chicken breast infiltrated with R6G. We show the existence of universality among the random resonators in each gain medium that we tested, in which at the same excitation intensity a dominant random cavity is excited in different parts of the sample. We show a second universality when scaling the average PFT of the four different media by $\ensuremath{\ell}*$; we found that the dominant cavity in each disordered gain medium scales with $\ensuremath{\ell}*$. The excellent agreement obtained with computer simulations using a distribution of random microdisks, each contributing a number of longitudinal whispering gallery modes within the gain spectrum, unambiguously shows that random lasers in the weak scattering regime cannot be described by gain amplification of localized photon states.