Ultrafast optical nonlinearities in II-VI compounds

We review our experimental and theoretical results on the femtosecond laser excited CdS and CdSe bulk and quantum dots. Optical Stark effect in bulk CdS is reported where the electron-hole continuum states in addition to the bound-exciton states are blue shifted. The nonlinear optical properties of CdSe microcrystallites embedded in glass matrices are investigated using femtosecond and nanosecond laser pulses. Spectral hole-buring is observed which is accompanied by an induced absorption feature on the high-energy side. A 25±10 fs dephasing time is obtained from these measurements. It is concluded that Coulomb effects are important in quantum dots. They give rise to a ground state two-pair resonance on the low energy side of the one-pair state and higher energy two-pair resonances which are spectrally located between the one-pair states. Single-beam saturation experiments show that the intensities required for bleaching are larger for smaller crystallites.