Photonic crystals are attracting current interest for a variety of reasons, such as their ability to inhibit the spontaneous emission of light. This and related properties arise from the formation of photonic bandgaps, whereby multiple scattering of photons by lattices of periodically varying refractive indices acts to prevent the propagation of electromagnetic waves having certain wavelengths. One route to forming photonic crystals is to etch two-dimensional periodic lattices of vertical air holes into dielectric slab waveguides. Such structures can show complete photonic bandgaps, but only for large-diameter air holes in materials of high refractive index (such as gallium arsenide, n = 3.69), which unfortunately leads to significantly reduced optical transmission when combined with optical fibres of low refractive index. It has been suggested that quasicrystalline (rather than periodic) lattices can also possess photonic bandgaps. Here we demonstrate this concept experimentally and show that it enables complete photonic bandgaps--non-directional and for any polarization--to be realized with small air holes in silicon nitride (n = 2.02), and even glass (n = 1.45). These properties make photonic quasicrystals promising for application in a range of optical devices.

Here we report a simple and versatile technique for the preparation of novel macroporous three-dimensional gold and platinum films with regular submicron spherical holes arranged in a close-packed structure. Gold and platinum films were prepared by electrochemical reduction of gold or platinum complex ions dissolved in aqueous solution within the interstitial spaces between polystyrene latex spheres (500 or 750 nm in diameter) assembled on gold surfaces. The latex sphere templates were subsequently removed by dissolving in toluene to leave the structured metal films. Scanning electron microscopy of the gold and platinum films shows a well-formed regular three-dimensional, porous structure consisting of spherical voids arranged in a highly ordered face-centered cubic (fcc) structure. The spherical voids have the same diameter as the latex spheres used to form the template. Within the metal film the spherical voids are interconnected through a series of smaller pores. The metallic framework is dense, self-supporting, and free from defects. X-ray studies show the metal to be polycrystalline with a grain size smaller than 100 nm. The optical reflectivity of the macroporous gold and platinum films shows strong diffractive optical properties, which are potentially useful in many existing and emerging applications.

We investigate the properties of gold surfaces patterned using a nanoscale "lost wax" technique by electrochemical deposition through a self-assembled latex template. Near-spherical gold nanocavities within the resulting porous films support localized surface plasmons which couple strongly to incident light, appearing as sharp spectral features in reflectivity measurements. The energy of the resonances is easily tunable from ultraviolet to near infrared by controlling the diameter and height of the nanocavities. The energies of these features agree well with the Mie resonances of a perfect spherical void.

Mesostructured metallic substrates composed of square pyramidal pits are shown to confine localized plasmons. Plasmon frequency tuning is demonstrated using white light reflection spectroscopy with a wide range of structure dimensions from 400 to 3000 nm. Using a simple plasmon cavity model, we demonstrate how the individual pit morphology and not their periodicity controls the resonance frequencies. By measuring the surface-enhanced Raman scattering ͑SERS͒ signals from monolayers of benzenethiol on the same range of mesostructures, we extract a quantitative connection between absorption, field enhancement, and SERS signals. The match between theory and experiment enables effective design of plasmon devices tailored for particular applications, such as optimizing SERS substrates. DOI: 10.1103/PhysRevB.76.035426 PACS number͑s͒: 73.20.Mf, 42.70.Qs, 72.15.Rn, 81.07.Ϫb Raman scattering is a crucial spectroscopic technique for identifying molecules through their vibrational resonances and has increasingly important applications in monitoring low concentrations of impurities or trace biomolecules.1-3 It has also been suggested for direct monitoring of coupling of molecular distortions and electronic transport in molecular electronics. [4][5][6] However, the terribly weak Raman cross section has always made such application problematic. The enormous enhancement in cross section when the molecules are held close to a metal surface with nanoscale roughness 7,8 has driven the hope that surface-enhanced Raman scattering ͑SERS͒ will become a viable and reproducible diagnostic. While improvements have been made in terms of enhancement factor, reproducibility, and in understanding that plasmons underpin such enhancements, 9 it is unclear how to precisely design nanostructures to optimize the Raman signatures. Over the last five years, we have shown that mesostructured metal films comprised of arrays of voids form excellent surfaces for localizing plasmons while retaining strong coupling to external light.10-13 Recently, we showed, using angularly resolved SERS on such void substrates, that incident photons are transducted both into and out of molecules via plasmons, 14 giving hope that reproducible substrates can be designed for specific applications. While most SERS research has focused on using the nanoscale junctions between metallic particles such as colloids 15 or lithographic arrays 16 that have broad plasmon resonances which can only be tuned through control of shape anisotropy or gap dimensions, the voids show strong sharp tunable plasmon resonances. Previous work has shown that optimized samples possess plasmon absorption that lies between the laser wavelength and outscattered Raman emission; 17-19 however, they are unable to make clear the quantitative link between plasmon resonant absorption and SERS emission.In this paper, we present calibrated spectroscopic measurements on systematically engineered plasmonic mesostructured metal surfaces. We show the quantitative connection between the resonance arising from ...

Self-phase modulation has been observed for ultrashort pulses of wavelength 800nm propagating through a 1 cm-long Ta2O5 rib waveguide. The associated nonlinear refractive index n2 was estimated to be 7.23x10-19 m2/W, which is higher than silica glass by more than one order of magnitude. Femtosecond time of flight measurements based on a Kerr shutter configuration show that the group velocity dispersion is small at a wavelength of 800 nm, confirming that dispersion may be neglected in the estimation of n2 so that a simplified theory can be used with good accuracy.