Resonant Saturable uv Absorber

Recent experiments succeeded in sub-100fs pulse generation from solid-state lasers by use of a saturable single-quantum-well absorber (SQW) yielding self-starting 34fs pulses in a Ti:sapphire laser1 as well as 90fs pulses in a diode pumped Cr:LiSAF laser2. These experimental findings can hardly be explained by the common theoretical understanding of passive mode locking in solid-state lasers which requires a fast saturable loss for stable pulse generation. The relaxation of the absorption bleaching in a GaAs quantum well at room temperature exhibits a bitemporal behavior where the fast component of about 200fs owing to thermal ionization of the exzitons is remarkably slower than the meanwhile achieved pulse width in the experiments. Recently we have proposed a novel mechanism for mode locking solid state lasers by use of a slow off-resonant saturable two-level absorber3.