Reflectivity kinetics of saturable absorbers for laser mirrors

The reflectivity kinetics of saturable absorbers based on semiconductor quantum wells was investigated with differential reflectivity Delta R/R = 10-5 accuracy by the pump probe technique for energy fluxes (0,27-5.40).1014 photons cm 2 using a femtosecond Yb3+:KY(WO4)(2) laser, Three kinds of saturable absorbers were investigated: absorbers based on multiple quantum wells with nano -structured barriers grown over a mirror; the same absorber grown over the substrate; absorbers on multiple quantum wells separated by thick barriers grown over the substrate, The substrate contribution gives a stationary background and a two -photon absorption peak for small probe delays, which complicates the kinetics of the samples grown over the substrate. For absorbers grown over the mirror two exponential decays with a short relaxation time tau(1)= 0.2-0.4 ps and with a long relaxation time tau(2) = 7.8 ps were obtained, The last relaxation time is remarkably shorter than in quantum wells with thick barriers.