Polarized light scattering from individual semiconductor nanowires

This thesis addresses the light scattering, particularly Raman and Rayleigh scattering from quasi one dimensional semiconductor nanowires, such as Zn1-xMnxS and GaP nanowires. Many of the results stem from measurements of individual wires.;Four original works are presented in the thesis: (1) The growth of diluted magnetic semiconductor (DMS) Zn1-xMnxS (0≤x<0.6) nanowires using a three-zone furnace and two solid sources is reported (Chapter 2.4). The vibrational properties of the DMS nanowires with different Zn/Mn ratios were studied by correlating their Raman scattering spectra with the composition and structure measured by x-Ray energy dispersive spectroscopy (XEDS) and selected area electron diffraction (SAD). We find that the transverse optical (TO) phonon band disappears at the lowest Mn concentrations, while the longitudinal optical (LO) phonon band position was found insensitive to x. Three additional Raman bands were observed between the ZnS q=0 TO and LO phonons when Mn atoms were present in the nanowires (Chapter 5); (2) Polarized Raman scattering on individual crystalline GaP nanowires with diameters 40 individual crystalline GaP nanowires with diameters 40<d<600 nm are systematically investigated. At small diameters, d<70 nm, the nanowires are found to act like a nearly perfect dipole antenna and the bulk Raman selection rules are masked leading to a polarized scattering intensity function I(theta) ∼ cos4theta where theta is the angle between nanowire axis and the incident laser polarization. For larger diameter (70<d<600nm) nanowires, a model based on the interplay between photon confinement and bulk Raman scattering are proposed to explain the experimental data. This work realizes a fundamental understanding of Raman scattering in semiconductor nanowires and furthermore, the antenna effects are essential to the analysis of all electro-optic effects in small diameter filaments (Chapter 7); (3) Results of polarized Rayleigh back-scattering studies are reported the first time for individual ∼10 mum long crystalline GaP nanowires using 514.5 nm excitation. The Rayleigh back-scattering intensity polar pattern I(theta) was measured at room temperature, where theta is the angle between the incident electric field and the nanowire axis. The collected radiation was polarized parallel to the incident electric field. For small nanowire diameter (d∼70 nm), we observed ∼cos4theta polar patterns. With increasing nanowire diameter above 100 nm, the polar scattering patterns rotate by 90° with respect to those seen in small diameter nanowires and then they rotate back again to cos4theta patterns and finally broaden to a circle. Our experimental data are compared to the Rayleigh back-scattering efficiency calculated via the discrete dipole approximation (DDA). Our DDA calculations show that the polar patterns are sensitive to both the diameter and the nanowire length. Although the calculated polar patterns qualitatively support our data, improvement in the modeling is still needed (Chapter 8); (4) Giant nonlinear Raman scattering of both Stokes and anti-Stokes TO and LO phonons from several GaP nanowire segments with length L<1.2 mum formed by cutting a long 210 nm diameter GaP nanowire is reported for the first time. The nonlinear Raman effect has been demonstrated to be stronger when the lengths of nanowire segments become shorter. We also observed that there exist threshold pump laser powers which separate the linear and superlinear region of the Raman spectra. The threshold laser powers which are indentified ∼0.7-1.5 mW are two orders of magnitude smaller than reported threshold laser power for the typical Raman crystals, such as LiIO3 and Ba(NO3) 2, one order smaller than the fabricated silicon waveguide and is comparable with that from SWNTs suspended between two SiO2 pillars. The threshold laser powers for short GaP nanowires are found to decrease linearly with decreasing length of nanowire segments. We contribute this giant nonlinear Raman effect to stimulated Raman scattering (SRS) in cavities (Chapter 9). Indeed many of the novel optical properties discussed in this thesis stem from cavity enhanced electric fields inside the nanowire and cavity enhanced emission of the scattered field.