Relaxation oscillations in long-pulsed random lasers

We have measured the evolution of the intensity emitted by a random laser during a pump pulse that is comparable in duration to the spontaneous emission decay time. The time traces of our random laser, consisting of titanium dioxide particles and sulforhodamine B dye, show clear relaxation oscillations. We compare our experimental results to an analytic, conventional model based on the four-level rate equations for a singlemode laser. The measured frequency of well-resolved fluctuation oscillations suggests an effective mode lifetime of 5 ps.