The radiated fields of the fundamental mode of photonic crystal fibers

The six-fold rotational symmetry of photonic crystal fibers has important manifestations in the radiated fields in terms of i) a focusing phenomena at a finite distance from the end-facet and ii) the formation of low-intensity satellite peaks in the asymptotic far field. For our study, we employ a surface equivalence principle which allows us to rigorously calculate radiated fields starting from fully-vectorial simulations of the near field. Our simulations show that the focusing is maximal at a characteristic distance from the end-facet. For large-mode area fibers the typical distance is of the order 10 Lambda with Lambda being the pitch of the triangular air-hole lattice of the photonic crystal fiber.Comment: 6 pages including 4 figures. Accepted for Opt. Expres

Mass and controlled fabrication of aligned PVP fibers for matrix type antibiotic drug delivery systems

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Combinatorial High-Vacuum Chemical Vapor Deposition of Lithium Niobate Thin Films

Lithium niobate is a key material for photonics with applications in optical modulators and light frequency converters. Present lithium niobate optical devices are realized in bulk lithium niobate single crystals while interests in higher integration and also better performance derives efforts towards high quality epitaxial lithium niobate thin films. In the past, epitaxial lithium niobate thin films obtained using chemical vapor deposition (CVD), pulsed-laser deposition and crystal-ion slicing. Among these methods CVD is the one that can provide large area growth. Optimizing films properties stays an issue in CVD because in CVD of multi-element oxides using multiple precursors the stoichiometry of the film does not reflect the precursor ratios and additionally it varies with precursors total flux values. In this thesis optimization and discovery of multi-element oxide thin films were performed in a high-vacuum CVD reactor that is capable of performing combinatorial experiments. Using this reactor epitaxial lithium niobate thin films were obtained on sapphire {001g} and lithium tantalate {001} substrates with rocking curve full-width at half-maximum (FWHM) values of 0.03° and 0.024° respectively. The epitaxial growth of lithium niobate films was confirmed from X-ray diffraction (XRD) measurements and transmission-electron microscope (TEM) observations. Niobium tetra-ethoxy-di-methy-amino-ethoxide (Nb(OEt)4(dmae)) and lithium tert-butoxide (Li(OBut)) were used as precursors. Optically induced inhomogeneities, also known as optical damage, in lithium niobate limit its usage for high laser intensity applications. Lithium niobate single crystals doped with hafnium showed improved optical damage thresholds. Therefore combinatorial HVCVD experiments were conducted and textured {001} hafnium-doped lithium niobate films, with about 6 [mol%] hafnium, were obtained on sapphire f001g. Although the crystalline quality of the film is not high (rocking curve FWHM 0.8°) its Raman spectrum is still quite close to lithium niobate single crystal and epitaxial lithium niobate films on sapphire. Hafnium tert-butoxide (Hf(OBut)4) was used as precursor for hafnium. In the HVCVD reactor, a precursor transport system was applied such that spatial control of the precursor impinging rate on the substrate can be tailored. The precursor flux distribution that was mostly used in this thesis was a linear gradient with ratio of three across a 150 mm diameter circular area. In fact combinatorial experiments were performed by producing a linear gradient for individual precursors over the substrate with a certain angle between the gradient directions. The linear gradient feature was used to perform efficiently the systematic study of single precursors providing the benefit that one deposition at certain substrate temperature gives information about deposition kinetics for a vast range of precursor flux values. A systematic study of the Nb(OEt)4(dmae) precursor was performed using this feature and chemical-reaction-limited, mass-transport-limited, and desorption-limited regimes of CVD process were identified. The possibility of screening a large range of precursor flux values led us to discovering a CVD regime in which deposition rate is decreased by raising the precursor flux. A model was proposed for interaction of gas with surfaces that can explain all the different CVD regimes observed. The precursor molecules in HVCVD reactor reach the substrate with nearly no gasphase collision. Based on this fact a lift-off process was proposed for large-area in-situ patterning of multi-element oxide thin films

Extreme light absorption in a necking-free monolayer of resonant-size nanoparticles for photoelectrochemical cells

Semiconductor photoelectrodes for water oxidation that absorb visible light usually have poor electronic transport properties and small optical absorption coefficients near their absorption edge. Therefore, innovative designs that lead to significant optical absorption in relatively thin layers of these compounds are highly desirable. Here, using full-field electromagnetic optical simulations, we demonstrate that a monolayer of resonant-size BiVO4 spheres can provide enhancement up to a factor of two in solar light absorption relative to dense planar layers. In this monolayer, BiVO4 spheres do not need to be interconnected; therefore, such monolayers are flexible and their fabrication process does not require the complicated necking steps to establish electrical contact among the nanoparticles. These resonant-size spheres support Mie resonance modes that efficiently trap light and hence significantly increase effective optical path length. Under air mass 1.5 global (AM1.5G) irradiation, the maximum achievable photocurrent density (MAPD) in a monolayer of 250 nm diameter BiVO4 spheres reaches 4.9 mA. cm(-2). This is about twofold improvement over the MAPD for a 250 nm thick dense planar layer and well above the 3.8 mA. cm(-2) MAPD for a 1 mu m thick dense planar layer. In addition, it is shown that lower-order resonance modes of the spheres are superior to higher-order modes for broadband optical absorption. The insight provided in this work can also be applied to nitride and oxynitride photoanode materials

Theoretical Study of Light Trapping in Nanostructured Thin Film Solar Cells Using Wavelength-Scale Silver Particles

We propose and theoretically evaluate a plasmonic light trapping solution for thin film photovoltaic devices that comprises a monolayer or a submonolayer of wavelength-scale silver particles. We systematically study the effect of silver particle size using full-wave electromagnetic simulations. We find that light trapping is significantly enhanced when wavelength-scale silver particles rather than the ones with subwavelength dimensions are used. We demonstrate that a densely packed monolayer of spherical 700 nm silver particles enhances integrated optical absorption under standard air mass 1.5 global (AM1.5G) in a 7 mu m-thick N719-sensitized solar cell by 40% whereas enhancement is smaller than 2% when 100 nm ones are used. Superior performance of wavelength-scale silver particles is attributed to high-order whispering gallery modes that they support. These modes scatter the light over a wider angular range, hence increasing the density of both waveguide and resonance modes within the dye-sensitized layer

Photonic design of embedded dielectric scatterers for dye sensitized solar cells

Embedded dielectric scatterers comprise an important approach for light trapping in dye-sensitized solar cells (DSCs) due to their simple fabrication process. The challenge in applying these scatterers lies in finding the optimal dimensions and concentration of the scatterers. This requires many experiments and it is often quite difficult to have a starting point for optimizing the concentration. Based on theories of light propagation in random media, we propose a simple model for DSCs with embedded silica spherical particles. Then, by full-wave optical calculations, we determine a narrow range for the concentration of silica particles that leads to the largest optical absorption in the cell. The simulation results were verified by realizing DSCs with different concentrations of silica particles. A power conversion efficiency of 8.08% in an 11 mu m-thick N719-sensitized DSC was achieved with 6 vol% embedded silica, which further increased to 9.30% by applying a white scattering layer on the rear-side of the counter electrode. The design approach, presented here, is a general approach that can be applied for other types of solar light harvesting structures with low optical absorption coefficient