• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 1
  • 1
  • Tagged with
  • 2
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • 1
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
1

Investigating the far- and near-field thermal radiation in carbon-based nanomaterials

Zhang, Zihao 07 January 2016 (has links)
Two classes of carbon nanomaterials—carbon nanotubes and graphene—have promoted the advancement of nanoelectronics, quantum computing, chemical sensing and storage, thermal management, and optoelectronic components. Studies of the thermal radiative properties of carbon nanotube thin film arrays and simple graphene hybrid structures reveal some of the most exciting characteristic electromagnetic interactions of an unusual sort of material, called hyperbolic metamaterials. The features and results on these materials in the context of both far-field and near-field radiation are presented in this dissertation. Due to the optically dark nature of pyrolytic carbon in the wavelength range from visible to infrared, it has been suggested vertically aligned carbon nanotube (VACNT) coatings may serve as effective radiative absorbers. The spectral optical constants of VACNT are modeled using the effective medium theory (EMT), which is based on the anisotropic permittivity components of graphite. The effects of other EMT parameters such as volume filling ratio and local filament alignment factor are explored. Low reflectance and high absorptance are observed up to the far-infrared and wide range of oblique incidence angles. The radiative properties of tilt-aligned carbon nanotube (TACNT) thin films are illustrated. Energy streamlines by tracing the Poynting vectors are used to show a self-collimation effect within the TACNT thin films, meaning infrared light can be transmitted along the axes of CNT filaments. Graphene, a single layer sheet of carbon atoms, produces variable conductance in the terahertz frequency regime by tailoring the applied voltage gating or doping. Periodically embedding between dielectric spacers, the substitution of graphene provides low radiative attenuation compared to traditional metal-dielectric multilayers. The hyperbolic nature, namely negative angle of refraction, is tested on the graphene-dielectric multilayers imposed with varying levels of doping. EMT should be valid for graphene-dielectric multilayers due to the nanometers-thick layers compared to the characteristic wavelength of infrared light. For metal- or semiconductor-dielectric multilayers with thicker or lossier layers, EMT may not hold. The validity of EMT for these multilayers is better understood by comparing against the radiative properties determined by layered medium optics. When bodies of different temperatures are separated by a nanometers-size vacuum gap, thermal radiation is enhanced several-fold over that of blackbodies. This phenomenon can be used to develop more efficient thermophotovoltaic devices. Due to their hyperbolic nature, VACNT and graphite are demonstrated to further increase evanescent wave tunneling. The heat flux between these materials separated by vacuum gaps smaller than a micron is vastly improved over traditional semiconductor materials. A hybrid structure composed of VACNT substrates covered by doped graphene is analyzed and is shown to further improve the heat flux, due to the surface plasmon polariton coupling between the graphene sheets.
2

Polarization-resolved nonlinear microscopy in metallic and ferroelectric nanostructures for imaging and control in complex media / Microscopie non-linéaire polarisée dans les nanostructures métalliques et ferroélectriques pour l'imagerie et le contrôle dans les milieux complexes

Rendón Barraza, Carolina 02 December 2016 (has links)
Les signaux non linéaires provenant de nanostructures métalliques et cristallines sont connus pour être fortement dépendants vis à vis de la polarisation. Ceci est dû à leur propriété de symétrie locale, reliée à leur réponse volumique ou surfacique. Les signaux de polarisation venant de nanostructures de taille inférieure à la limite de diffraction sont généralement mesurés avec un spot limité par la diffraction (300 nm) ce qui représente la moyenne du signal. Cette technique a pour défaut de perdre l'information spatiale du signal de polarisation. Nous avons développé une nouvelle méthode de microscopie à polarisation non-linéaire qui exploite l'information en dessous de la limite de diffraction.Une analyse de Fourier d'un signal non linéaire a été faite en dessous de la limite de diffraction sur une image sur-échantillonnée et corrigée (taille du pixel=50 nm). Le gain en résolution est du à la sensibilité spatiale de la polarisation. Pour ce faire, nous avons mesuré un signal polarisé de seconde harmonique de nanostructures plasmoniques de différentes formes (150 nm). Nous avons montré que la nature vectorielle du champs local confiné peut être retrouvé avec une résolution de 40 nm en utilisant la nanoscopie polarisée non linéaire. Nous avons par ailleurs montré que nous pouvons imager l'hétérogénéité spatiale de nanoparticules ferroélétriques cristallines (BaTiO3) de taille allant de 100 nm à 500 nm. Ceci prouve l'existence d'une coque centrosymétrique dans des petites structures. Enfin, les nanocristaux de KTP nanostructures sont les candidats idéaux pour la générations de signaux non linéaires bien maîtrisée. / In this work, we develop a novel polarized nonlinear microscopy method that exploits sub-diffraction resolution information. Fourier analysis of the polarization modulated nonlinear signal is performed on over-sampled, drift-corrected images (50nm pixel size). The information gained by polarization-induced modulation signals provides a higher level of spatial selectivity that is directly related to the local optical response of the investigated system, at a scale below the diffraction limit. The gain in spatial scale is due to the additional spatial sensitivity brought by polarization. This approach is applied to polarized second harmonic generation imaging in plasmonic nanostructures (150nm size) of multi-branched shapes, in which the vectorial nature of the local field confinement can be retrieved with a resolution of 40 nm. We also demonstrate the possibility to image spatial heterogeneities within crystalline ferroelectric BaTiO3 nanoparticles of 70nm to 500nm size, emphasizing in particular the existence of a centrosymmetric shell in small size structures. These nanostructures will be used as starting models for coherent optical probes in biological media (cells, tissue slices or in vivo) with two objectives. First, the nonlinear nature of their emission will make them stable and tunable nanosources, able to report their localization with high accuracy in 3D, potentially sensing local environment changes, and actively inducing perturbations such as controlled temperature increase at the nanoscale. Second, the coherent nature of their emission will make them useful as local nanoprobes for wavefront and polarization correction through scattering media.

Page generated in 0.076 seconds