Browsing by Subject "Photonic Crystal"

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    Metallis photonic crystals for proposed applications as thermal emission devices.
    (2009-08) Denny, Nicholas Ryan
    Incandescent lighting is highly inefficient. One possible solution is to replace the traditional filament with an ordered photonic crystal filament that will increase the efficiency of the lamp. This work details steps towards the fabrication of such a filament with the fabrication of monolithic three-dimensionally ordered macroporous (3DOM) metallic photonic crystals. The 3DOM metallic materials produced in this work were comprised of W, Mo and alloys of those materials. The 3DOM W materials were produced from the precursors tungsten(VI) chloride, tungsten(V) ethoxide, tungstic acid, peroxotungstic acid, ammonium metatungstate (AMT) and an acetylated peroxotungstic acid (APTA). 3DOM Mo was produced from the precursors ammonium molybdate (AMo) and an acetylated peroxomolybdic acid (APMoA). To fabricate 3DOM W/Mo materials combinations of precursors of AMT and AMo were utilized or a combination of the syntheses APTA and APMoA to create APTA/APMoA was employed. A variety of synthetic conditions were optimized to produce large monolithic pieces of 3DOM W and 3DOM W/Mo with dimensions of up to 1×1×0.3 cm3. These conditions included varying the solvent mixture, precursor concentrations, reduction conditions and precursor infiltration technique. The 3DOM metallic monoliths were tested for thermal stability using both joule heating and radiant heating techniques in N2 atmospheres. Joule heating at 40 W for 15 min destroyed the nanostructure of the material. Radiant heating was employed to study the grain coarsening. At 800 ºC the 3DOM W monolith exhibited grain coarsening and needle formation caused by H2O in the system. Needle formation could be eliminated by rigorous evacuation or by the incorporation of Mo as an alloy. 3DOM W/Mo alloys at 95:5 wt% maintained their nanostructure and relative grain size at 800 ºC but were coarsened at 1000 ºC. At both temperatures the material did not produce needles even in the presence of minute amounts of water. The effect of Mo on the nanostructure was studied by in situ TEM heating to 1000 ºC. In 3DOM W/Mo alloys it is postulated that the Mo has a pinning effect on the dislocations in the structure. These methods provide a route towards fabricating 3DOM metallic photonic crystals for thermal emission.
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    Thermal emission control with periodic microstructures.
    (2009-02) Han, Sang Eon
    In this thesis, the control of thermal emission with periodic microstructures is investigated. An important class of these structures, known as photonic crystals, are considered and Kirchhoff's law for photonic crystal films is discussed. Using fluctuational electrodynamics and a Green's function formalism, it is proved that Kirchhoff's law is obeyed for any photonic crystal films. This formalism allows the calculation of optical coherence for periodic structures. Moreover, a generalized form of Kirchhoff's law is derived for non-uniform temperatures. Using this, the control of thermal emission by selective heating of periodic structures is explored. It is found that local periodic heating allows control over which peaks appear in the thermal emission spectrum. The modification of thermal emission using a self-assembled metallic photonic crystal called inverse opal is also discussed. Despite its simplicity in fabrication, strong absorption in inverse opals prevents any influence of the periodicity. The origin of this effect is considered and it is shown how to tailor both the absorption and the surface coupling in experimentally realizable metallic inverse opals. The results show that the optical properties of tailored tungsten inverse opals can be similar to the tungsten woodpile, where modified thermal emission is already seen. In addition to these structures, structured metal surfaces, which are even easier to fabricate, are discussed. In particular, thermal emission from the surfaces of metal films that are patterned with a series of circular concentric grooves (a bull's eye pattern) is examined. Due to thermal excitation of surface polaritons, theory predicts that a single beam of light can be emitted from these films in the normal direction that is amazingly narrow, both in terms of its spectrum and its angular divergence. Experiments of tungsten bull's eyes verify this effect in the infrared. This shows that metallic films can generate laser-like beams of infrared light by a simple thermal process.