Flux closure structures in cobalt rings

Measurements are reported on the magnetization reversal in submicron magnetic rings fabricated by high-resolution electron beam lithography and lift-off from cobalt thin films. For all dimensions investigated, with diameters of 300-800 nm and a thickness of 10-50 nm, the flux closure state is the stable magnetization configuration. However, with increasing diameter and decreasing film thickness a metastable near single domain state can be obtained during the reversal process in an in-plane applied field.     

Enhanced third harmonic reflection and diffraction in Silicon on Insulator photonic waveguides

Enhanced third harmonic (TH) generation from Silicon-On-Insulator (SOI) planar waveguides as well as SOI photonic crystal (PhC) slabs is studied in different angular configurations, both in the visible and infrared energy ranges. In the SOI planar waveguide, the multilayer structure causes the optical properties such as TH reflection to be different from those of bulk silicon samples. This behavior is well reproduced by calculations of TH reflectance.Measurements of third-harmonic reflection and diffraction from one-dimensional PhC slabs etched in the SOI waveguide are also reported. The angular positions of TH peaks at various diffraction orders agree well with those calculated from a nonlinear grating equation. Both reflection and diffraction processes contribute to enhanced TH generation efficiency in the PhC slabs.TH reflectance measurements performed on PhC slabs in the near infrared show a resonant interaction between the incident beam and the photonic structure, dependent on the angle of incidence. This leads to a nonlinear conversion efficiency which is strongly enhanced with respect to that of the SOI waveguide, due to the excitation of strong local fields associated with the presence of photonic modes in the PhC slab.     

High strength and high conductivity composite wires for high pulsed magnet

Magnets for high pulsed magnetic fields require a winding with high electrical conductivity, high strength, high heat capacity, and a softening temperature greater than 300 °C. Different conductors, developed on DRET contracts, are presented. The ultimate tensile stress at room temperature of macrocomposites, where the reinforcement is either a jacket or fibres, checks the rule of mixtures, whereas _uts of Cu/Nb nanocomposites with continuous Nb nanofibres exhibit a very interesting deviation from the rule of mixtures leading to _uts(77K) = 1206 MPa for 30% Nb.To be presented at the Ninth International Conference on the Mechanics of Composite Materials, 17–20 October 1995, Riga.Published in Mekhanika Kompozitnykh Materialov, Vol. 31, No. 4, pp. 518–525, July–August, 1995.     

Sub-micrometre dielectric and metallic yablonovite structures fabricated from resist templates

This paper reports the optical characterisation and numerical simulations of sub-micrometre yablonovite templates fabricated in poly-methyl methacrylate (PMMA) resist along with the pattern transfer to metals and other dielectrics. The fabrication is based on three consecutive exposures to an X-ray beam through a triangular lattice of holes. Up to seven (111) crystal periods are repeatedly obtained in thick PMMA layers. Optical characterisations of 1.3 m period templates show well-defined stop-bands in the mid-infrared, in good agreement with the numerical simulations. Two simple and low-cost methods are used to replicate the yablonovite structures in other materials. The pattern transfers to a metal (copper) and a high-refractive-index dielectric (titanium dioxide) are obtained by electrodeposition and a sol–gel technique, respectively. Very regular yablonovite-type metallic structures of 2 m lattice constant and 4.3 m height are fabricated with metallic wires of 500 nm diameter over a surface of 150 × 150 m².     

Micro-photoluminescence of GaN quantum dots embedded in 100 nm wide cylindrical AlN pillars

Individual pillars were etched from a sample embedding a single plane of GaN/AlN quantum dots, deposited by molecular beam epitaxy on a sapphire substrate. Pillars with diameters ranging from 0.1 to 5 μm were fabricated by electron-beam lithography and SiCl₄ reactive ion etching. The PL from a single pillar could be measured by using a confocal microscope, with a spatial resolution of 600 nm. We report an intense PL signal from pillar diameters as small as 0.1 μm at room temperature. By increasing the power of the excitation laser from 0.05 to 200 μW, we induced a blue-shift of the PL energy peak from 2.38 to 2.86 eV, accompanied by a substantial broadening of the PL line. This is explained by the photo-induced screening of the internal electric field, which is close to 10 MV/cm in GaN/AlN heterostructures. Finally we report and tentatively explain a photodarkening effect, i.e., the progressive decrease of the PL intensity over two orders of magnitude, after one hour of continuous laser excitation. However, this effect does not seem to be correlated to the etching process.     

Optical analogy to electronic quantum corrals

We describe full multiple-scattering calculations of localized surface photonic states set up by lithographically designed nanostructures made of a finite number of dielectric pads deposited on a planar surface. The method is based on a numerical solution of the dyadic Dyson's equation. When the pads are arranged to form a closed circle, we find field patterns that look like the electronic charge density recently observed above quantum corrals. We propose two experimental techniques that could be used to observe these electromagnetic modes in direct space.