Picosecond measurements of photogenerated carrier transport at semiconductor nanocrystal-glass interfaces and microlens fabrication in semiconductor doped glasses.

摘要

This thesis covers two topics of study involving the same material, semiconductor nanocrystal doped glass. The majority of this work concerns photogenerated carrier dynamics and recombination at the interface between semiconductor nanocrystals and their host glass matrix. The last chapter introduces a new technique to fabricate refractive microlenses in an extremely cost-effective manner.; Studied via two-color, picosecond pump-probe spectroscopy, a close examination of photogenerated carrier dynamics in these systems reveals a highly nonexponential relaxation that is best characterized by a stretched exponential function, A Exp ({dollar}-(t/tau)spbetarbrack,{dollar} rather than a multiexponential function description, {dollar}asb1{dollar} Exp ({dollar}-ksb1trbrack{dollar} + {dollar}asb2{dollar} Exp ({dollar}-ksb2trbrack{dollar} + ..., which has been used widely in the literature to describe these systems. A stretched exponential decay is predicted by models in which the recombination of photoexcitations is mediated by continuous-time random-walk motion among a distribution of localized sites, a small fraction of which serve as instantaneous recombination centers. Examination of the fitting parameters {dollar}tau{dollar} and {dollar}beta{dollar} as functions of temperature, excitation intensity, and excitation energy combined with absorption and luminescence measurements support the view of photoexcitations which are ultimately affected by the semiconductor-glass interface. The decays of this excitation reveal the nature of the transport, indicating that at temperatures below {dollar}sim{dollar}250 K, photoexcitations move in a diffusive manner, where motion occurs with an average time scale, while at higher temperatures the data show that photoexcitations move in a dispersive manner, where motion occurs on all time scales.; In the second part of this thesis, experiments are described in which the use of cw lasers form small, near-spherically curved regions on the surface of semiconductor-nanocrystal-doped glasses. These regions have been demonstrated to act as refractive lenses with diameters observed between 3 and 170 {dollar}mu{lcub}rm m{rcub}.{dollar} A review of current refractive microlens fabrication techniques is followed by a description of this fabrication method and characterization of the resultant microlenses. Microlenses with near-diffraction-limited performance are achieved, with numerical apertures of 0.30.