Browsing by Subject "Plasmonic"
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Item Large-area resonant and non-resonant optical nanostructures(2014-08) Li, Ping-Chun, active 21st century; Yu, Edward T.Manipulation of light via subwavelength nanostructures is currently a subject of intense research interest, and is enabling the development of nanostructured photonic crystal, metamaterials and metasurfaces that provide a variety of new optical and electromagnetic functionalities, or that enable existing functionalities to be realized in new and often extremely compact form factors. This dissertation will include wide-angle wavelength-selective metasurface, omnidirectional enhancement in photovoltaic performance via subwavelength gradient anti-reflection coating, and applications of birefringent nanocylinders for single-molecule spectroscopy. In wide-angle wavelength-selective metasurface, high and broad reflectance (~95%) with low absorption (<5%) are shown to be achieved with multilayer metasurface structures. These characteristics are shown to be independent of interlayer misalignment and defects within individual layers. Interactions between different metasurface layers due to Fabry-Perot resonance are also examined with analytical models and numerical simulations. Wavelength-selective focusing at optical wavelengths which is enabled by large-area nanosphere lithography on a flexible substrate is demonstrated. In omnidirectional enhancement in photovoltaic performance via subwavelength gradient anti-reflection coating, large-area "moth-eye" structure fabricated on a flexible substrate is shown to have high transmittance (>85%) at large angle of incidences (>70°) and insensitivity to polarizations. Integration of the "moth-eye" anti-reflection coating together with nanostructured gradient A1₂O₃/TiO₂ on a GaAs solar cell shows significant improvements on external quantum efficiency (EQE) and short circuit current over all angle of incidences compared with conventional thin film anti-reflection coating. Detailed design, simulation, and fabrication of these nanostructured anti-reflection coating for reducing the discontinuity in refractive index profile will also be discussed. In application of birefringent nanocylinders for single-molecule spectroscopy, the design and fabrication method for large quantity of subwavelength birefringent nanoparticle are also discussed. These birefringent nanoparticles are shown to be stably trapped in an optical torque wrench setup, and enable observation of the dynamical response of a double-stranded DNA under torsional and extensional forces.Item Micro-patterning colloidal quantum dots based light sources for cellular array imaging(2014-08) Bhave, Gauri Suresh; Zhang, John X. J.; Dunn, Andrew Kenneth, 1970-Lab-on-chip systems have been developed for various applications like point of care diagnostics and compact imaging systems. Compact, on-chip imaging systems face a challenge in the integration of multicolor light sources on-chip. This is because of the unavailability of compact, individually addressable, multicolor light sources on a single planar substrate. Colloidal Quantum Dot based Light Emitting Diodes (QDLEDs), which have found wide appeal, due to their unique properties like their tunable and narrow emission bandwidth and easy fabrication, are ideal for lab-on-chip integration. Among different types of QDLED structures implemented, inorganic QDLEDs have shown great promise. We have demonstrated designs and fabrication strategies for creating QDLEDs with enhanced performance. In particular: (I) We introduce a sandwich structure with a spin coated inorganic hole transporting layer of nickel oxide underlying the QD layer and with a spin coated zinc oxide electron transporting layer, with patterning of anode and cathode on the substrate. Compared to the use of sputtered thin films, solution processed charge transporting layers (CTLs) improve robustness of the device, as crystalline ZnO shows low CB and VB edge energy levels, efficiently suppressing hole leakage current resulting in LEDs with longer lifetimes. We also use Atomic Layer Deposition to deposit an additional hole injecting layer to protect the QDs from direct contact with the anode. With this device design, we demonstrate a working lifetime of more than 12 hours and a shelf-life of more than 240 days for the devices. Our solution based process is applicable to micro-contact printed and also spin-coated QD films. QDLEDs with spin-coated CTLs show a lifetime increase of more than three orders of magnitude compared to devices made using sputtered CTLs. (II) We implement strategies of the enhancement of light extraction from the fabricated QDLEDs. We discuss the integration of a two dimensional grating structure based on a metal-dielectric-metal plasmonic waveguide with the metal electrode of a QDLED, with the aim of enhancing the light intensity by resonant suppression of transmitted light. The grating structure reflects the light coupled with the metal electrode in the QDLED and we found an increase of 34.72% in the electroluminescence intensity from the area of the pattern and an increase of 32.63% from photoluminescence of QDs deposited on a metal surface. (III) We demonstrate the capability of our fabricated devices as a light source by measuring intensity across stained cells with QDLEDs of two different wavelengths and show the correlation as expected with the absorption profile of the fluorescent dye. We measure the absorption from the biological samples using QDLEDs fabricated with various design modifications, as a quantification of the improvements in device performance, directly affecting to our target application.Item Plasmonic properties of subwavelength structures and their applications in optical devices(2010-12) Wang, Wei, 1983 July 24-; Shi, Li, Ph. D.; Chen, Shaochen; Zhang, Xiaojing; Ferreira, Paulo J.; Shvets, GennadyA metallic hole array of a rectangular converging-diverging channel (RCDC) shape exhibits extraordinary transmission for wavelengths larger than the periodicity of the holes. We use a three-dimensional (3D) finite element method to analyze the transmission characteristics of two-dimensional metallic hole arrays (2D-MHA) with RCDC. For a straight channel MHA, when the aperture size is reduced, the transmission peaks have a blue-shift. The same result is observed for a smaller gap throat for the RCDC structure. For the rectangular holes with a high length-width ratio, a similar blue-shift in the transmission peaks as well as a narrower full width at half maximum (FWHM) are observed. The asymmetry from the rectangular shape gives this structure high selectivity for light with different polarizations. Furthermore, the RCDC shape gives extra degrees of geometrical variables to 2D-MHA for tuning the location of the transmission peak and the FWHM. Tunable extraordinary transmission via changing temperature of a porous metallic layer on top of a thin layer of dielectric strontium titanate (STO) is then studied. The metallic layer has a through-hole array and each hole has a circular converging-diverging channel (CDC) shape, which induces the excitation of surface plasmon polaritons (SPPs) and then results in a controllable extraordinary optical transmission in the terahertz (THz) frequency range. We use a three-dimensional (3D) finite element method to analyze the transmission characteristics of the structure. Location and magnitude of the transmission peaks can be adjusted by the hole size, converging angle, and thicknesses of metal and STO layers. Remarkably, the suggested structure presents a strong transmission dependency on temperature, which offers a new approach to actively and externally tune the transmission. Currently, the performances of thin film solar cells are limited by poor light absorption and carrier collection. In this research, large, broadband, and polarization-insensitive light absorption enhancement is realized via integrating with unique metallic nanogratings. Through simulation, three possible mechanisms are identified to be responsible for such an enormous enhancement. A test for totaling the absorption over the solar spectrum shows an up to ~30% broadband absorption enhancement when comparing to bare thin film cells. Overall performance of a thin film solar cell is determined by the efficiency of conversing photons to electrons that include light absorption, carrier generation and carrier collection processes. Photon management via hybrid designing has been emerging as a powerful means to further boost the conversion efficiency. Here a new nanograting solar cell design, which can be universal and a new solar cell platform technology, is proposed with goals to achieve large enhancement on broadband light absorption and carrier generation, most importantly, under the much reduced usage of active and non-earth-abundant materials. A test for the short circuit current density in CuIn[subscript x]Ga([subscript 1-x])Se₂ (CIGS) thin film solar cells shows an up to ~250% enhancement when comparing to the corresponding bare thin film cells. Besides that, by placing metal strips on top of the nanograting, which act as the top electrode, this design is able to reduce the use of non-earth-abundant materials such as indium that is normally used in both active and transparent conducting materials.Item Template-assembly and spectroscopic study of colloidal quantum dot molecules(2014-05) Lei, Kin Wai; Fan, Donglei; Zhu, Xiaoyang, 1963-; Li, Xiaoqin (Elaine); Korgel, Brian; Mullins, Charles BBlock copolymer template-assembly of quantum dots and plasmonic nanostructures is developed to provide a well-controlled platform to study the electronic coupling and Förster resonance energy transfer (FRET) between quantum dots (QDs), as well as the influence of surface plasmons on energy transfer. By fine-tuning the aspect ratio of the geometric features of PS-b-PMMA copolymer template, QDs are assembled into an array of QD clusters within nanoscopic holes on the template using capillary force assembly. Coupled QD clusters, termed quantum dot molecules (QDMs), are assembled in an array to probe the local coupling within QD molecules when the native insulating ligands are exchanged with shorter ligands. From absorption measurement of 1st exciton peak position of PbSe QDMs upon ligand exchange, a larger red-shift is found for QDMs than for a close packed film of PbSe QDs with the same ligand exchange, demonstrating localized electronic coupling of these QD molecules. Template-assembly of nanoparticles is further generalized to uniformly couple QDs clusters with plasmonic nanodisks of noble metals. Using PS nanospheres as reactive ion etch mask, Au nanodisks are fabricated on CdSe/ZnS core shell QD clusters, separated by a tunable space layer of PMMA. This highly controllable surface plasmon-coupled QD system minimizes the uncertainty in interfacial homogeneity, characterized by cross sectional scanning electron microscopy (SEM). Photoluminescence (PL) peak ratio of donor to acceptor emission and donor lifetime measurements show strong evidence of surface plasmon coupled energy transfer between donor-acceptor QDs, which depends on the position of the surface plasmon peaks as well as the separation between plasmonic structure and FRET QD clusters. The result suggests that a larger overlap of surface plasmon peak with the emission peak of acceptor leads to greater decrease in PL lifetime of donor. Donor lifetime decreases dramatically in the presence of both acceptor and surface plasmon compared to just in the presence of surface plasmon. Coupling between plasmonic nanodisks and QD clusters also decreases and results in longer donor lifetime as the thickness of PMMA separation layer increases.