Nonlinear optical studies of the properties of nanoparticles

The purpose of this research is to investigate long-lifetime nonlinear optical processes in lead sulfide quantum dots in glass in which the carriers (electrons and holes) are considered to be strongly confined. As part of this study, a close look was taken at time-resolved heat transfer issues and the temperature dependence of the optical absorption spectrum of quantum dot material.; Lead sulfide quantum dots were grown in borosilicate glasses. The size of the crystallites is controlled by varying the time and temperature of heat treatment. Samples heat treated from 540 to 580°C for 12 to 35 hours exhibit various blue shifts of the lowest absorption peak. A four-band envelope-function theory is addressed and used to deduce quantum dot size from the absorption spectrum. Three glass samples containing quantum dots of approximately 3 nm radius were prepared for nonlinear optical measurements. The temperature-dependent spectrum of these and other samples was also measured.; Numerical calculations by applying the finite difference method was used to model the time-dependent heat transfer process of a quantum dot embedded in the glass, both during and after application of an optical pulse. The variation of temperature of the lead sulfide quantum dot under our experimental condition is found to be quite small and does not significantly affect the optical properties or the resulting deductions of measurements on electronic processes. On the other hand, it is shown that a focused intense laser pulse (1J-100ns) offers enough thermal energy to heat up 10-nm PbS and CdSe nanoparticles to hundreds of degrees—a temperature which supports the idea of laser-induced phase transformation.; Nanosecond pump-probe measurement and steady-state photomodulation were carried out to understand the microsecond scale lifetime bleaching (induced transmission) effect in lead sulfide quantum dots in glass. The saturation behavior and pump intensity-independent sub-microsecond scale rise time suggest transitions involving a saturable intermediate state(s) that lies in the band(s). The small magnitude ∼5% of the maximum of change of absorption coefficient, microsecond scale decay time, and change of profile of absorption spectrum are believed to be from indirect effect of trapped carriers in low density traps. These trapped carriers change the distribution of the wave function of band edges reduced oscillator strength and lead to a red shift of the spectrum. Two simulations were carried out to reconstruct the carrier transition during the optical process. Potential applications of the material are also discussed.