Preparation and non-linear optical properties of CdS quantum dots in Na₂O-B₂O₃-SiO₂ glasses by the sol-gel technique

Non-linear optical properties of CdS quantum dots embedded in the sol-gel derived Na₂O-B₂O₃-SiO₂ glass matrix have been studied using nanosecond degenerate four-wave mixing (DFWM), pump-probe experiments, and time-resolved subpicosecond DFWM measurements. The concentration of CdS microcrystallites was varied from 1.4 to 10.2 wt% while the particle size was controlled to be in the range of 36 nm in diameter so that the confinement effects can be realized. The third-order susceptibility, χ⁽³⁾, was determined to be of the order of 10^-7-10^-6 e.s.u. near the resonant wavelength between 450 and 470 nm by the nanosecond DFWM and pump-probe experiments, and of the order of 10^-11-10^-10 e.s.u. at the off-resonant wavelength, 580 nm, by a time-resolved DFWM measurement with 400 fs laser pulse. The decay time changed from 0.5 to 50 picoseconds as a function of the size, size distribution and number density of CdS microcrystallites in the glasses. Photodarkerting reduced the optical non-linearity of the melt-quenched samples by a factor of ≈ 20, while it had no appreciable effect on that of the sol-gel derived samples.