Saturable absorbers with concentration-dependent absorption recovery time

The ground-state bleaching of highly concentrated rhodamine 6G solutions in methanol is studied with intense picosecond light pulses. The ground-state recovery time changes with concentration from about 3.9 ns at low concentration (10^–5 mol/dm³) to about 1 ps at high concentration (0.6 mol/dm³). The shortening of the absorption recovery time is determined by the concentration dependent quenching of theS ₁-state population due to unbound dimers and by the intensity dependent longitudinal and transverse amplified spontaneous emission.