Spectroscopy of single nanowires: Optical properties and carrier dynamics

Quasi-one-dimensional nanostructures, with tunable electronic and optical properties, hold the promise of becoming the basic building blocks for novel photonic/optoelectronic devices. The fundamental optics and electronics that govern properties of individual semiconductor wires with sizes less than visible wavelengths have heretofore not been fully explored, either experimentally or theoretically. In this thesis work, initial experiments to determine the optical characteristics of nanowire waveguides, lasers, and array devices were performed via the combination of high-resolution optical microscopy with various spectroscopic techniques. For the first time, ultrafast optical pumping has been shown to produce waveguided photoluminescence (PL) and stimulated emission in single ZnO and GaN nanowires; analysis of the lasing spectrum, polarization, power dependence, and beam properties has thoroughly characterized the cavity resonances and the material behavior during lasing. Similar experiments with quasi-two-dimensional nanoribbons have provided evidence for optical modes that follow axial waveguide theory and others that have the properties of whispering gallery modes. Modification of nanowires to form core/sheath heterostructures produced evidence for the first single "quantum-wire" UV laser, as it contains a carrier-confined core and a sheath that supports transverse modes.; Second- and third-harmonic generation (SHG, THG) experiments have been explored both for applications in frequency conversion and as a probe of subtle symmetry changes in single wires. Time-resolved SHG (TRSHG) has been performed for the first time on single nanowires and ribbons, and the transient signal has been shown to characterize both radiative and nonradiative carrier relaxation channels. Comparison with time-resolved PL measurements has facilitated investigation of the carrier dynamics associated with the lasing process. In addition, a dense ZnO nanowire array has been demonstrated as an electron acceptor medium in dye-sensitized solar cell (DSC) prototypes using ultrafast mid-infrared transient absorption (TA) spectroscopy. Ultrafast measurements have elucidated distinct electron-injection behavior between nanowire and nanoparticle DSC's. New insights into dye/semiconductor interfacial properties that lead to efficient photon-to-current conversion in ZnO nanoparticle and nanowire solar cells are possible with these novel TA and TRSHG experiments, including the prospect of improving efficiencies by controlling charge dynamics in rationally-synthesized, engineered nanostructures.