Energy of eigen-modes in magnetohydrodynamic flows of ideal fluids

Analytical expression for energy of eigen-modes in magnetohydrodynamic flows of ideal fluids is obtained. It is shown that the energy of unstable modes is zero, while the energy of stable oscillatory modes (waves) can assume both positive and negative values. Negative energy waves always correspond to non-symmetric eigen-modes -- modes that have a component of wave-vector along the equilibrium velocity. These results suggest that all non-symmetric instabilities in ideal MHD systems with flows are associated with coupling of positive and negative energy waves. As an example the energy of eigen-modes is calculated for incompressible conducting fluid rotating in axial magnetic field.Comment: 10 pages, 3 figure

Status of the QCDSP project

We describe the completed 8,192-node, 0.4Tflops machine at Columbia as well as the 12,288-node, 0.6Tflops machine assembled at the RIKEN Brookhaven Research Center. Present performance as well as our experience in commissioning these large machines is presented. We outline our on-going physics program and explain how the configuration of the machine is varied to support a wide range of lattice QCD problems, requiring a variety of machine sizes. Finally a brief discussion is given of future prospects for large-scale lattice QCD machines.Comment: LATTICE98(machines), 3 pages, 1 picture, 1 figur

Influence of the crystalline structure of metal films on the performance of plasmonic biosensors

The development of plasmonic biosensors requires careful consideration of various factors associated with the deposition of thin metal films. The quality of metal films is mostly determined by their polycrystalline structure, where smaller crystallite sizes produce higher optical losses. In present work we obtained the dependence of optical losses in thin gold and copper films on their thicknesses. These experimental results are used for the analysis of the performance of plasmonic biosensors, which demonstrate an increase in the limit of detection of up to 50% and 35% for gold and copper films, respectively

Morphology and effective dielectric functions of ultra-thin gold films

In the present study, a new method for analyzing the dielectric functions in dependence with the thickness of ultra-thin gold films is proposed. The reliability of the method is carefully verified through the correlation of optical properties and structural morphology of gold films at thicknesses around the percolation threshold (from 2 to 20 nm). The optical characterization of deposited films is performed in visible and NIR regions (300-1500 nm) by a spectroscopic ellipsometry along with transmission spectra measurements at normal incidence. Fitting of the ellipsometric data allows one to calculate the effective complex dielectric function of cluster-like and continuous films of different mass-equivalent thickness. Surface morphology of the films is analysed by scanning electron microscopy

Static and Dynamic Properties of Inhomogeneous Elastic Media on Disordered Substrate

The pinning of an inhomogeneous elastic medium by a disordered substrate is studied analytically and numerically. The static and dynamic properties of a $D$-dimensional system are shown to be equivalent to those of the well known problem of a $D$-dimensional random manifold embedded in $(D+D)$-dimensions. The analogy is found to be very robust, applicable to a wide range of elastic media, including those which are amorphous or nearly-periodic, with local or nonlocal elasticity. Also demonstrated explicitly is the equivalence between the dynamic depinning transition obtained at a constant driving force, and the self-organized, near-critical behavior obtained by a (small) constant velocity drive.Comment: 20 pages, RevTeX. Related (p)reprints also available at http://matisse.ucsd.edu/~hwa/pub.htm

Hybrid graphene-nanometallic structures

The development of functional hybrid nanomaterials based on graphene and metal nanostructures is currently the subject of intense research interest. In this study we discuss different aspects relevant to growth morphology, properties (optical, electrical, and structural), as well as the nanoscale architecture of hybrid graphene-nanometallic structures based on graphene as a multifunctional platform. The peculiarities of growth kinetics of thin gold and copper films on graphene are analysed (by use of the variety of experimental methods) and compared. It is shown that the presence of graphene affects strongly the optical, electrical, and structural properties of thin metal film

Near-field characterization of ultra-thin metal films

Ultra-thin metal films will form the basis of next-generation optoelectronics. However, characterization of their performance requires consideration of nanocrystalline structure and analysis of local optical and electrical properties. In present study, we use scanning near-field optical microscopy (SNOM) for nanoscale probing of optical conductivity of ultrathin metal films. We obtained surface maps of scattered near-field signal for gold films grown on monolayer graphene and MoS2 films as well as on a pure Si/SiO2 substrate. These results clearly demonstrate the difference in generated optical responses and can be used in the development of various devices utilizing ultrathin metal films.</p

Fractal Shaped Periodic Metal Nanostructures Atop Dielectric-Metal Substrates for SERS Applications

Controlled and reliable field enhancement (FE) effects associated with the excitation of plasmons in resonant metal nanostructures constitute an essential prerequisite for the development of various sensing configurations, especially those utilizing surface-enhanced Raman scattering (SERS) spectroscopy techniques. Leveraging advantages of random nanostructures in providing strong collective resonances in a broad wavelength range with the design flexibility of individual gap plasmon resonators, we experimentally investigate fractal-shaped arrays of gap plasmon resonators and characterize the occurring FE effects by mapping SERS signals from uniformly spread Rhodamine 6G with high-resolution Raman microscopy. In such a geometry, the total FE is expected to benefit from both FE associated with gap plasmon excitation and FE due to constructive interference of the surface plasmon modes reflected and diffracted by fractal-shaped boundaries. Linear reflection imaging spectroscopy is used to verify that the fabricated nanostructures exhibit spatially distributed resonances (bright spots) close to the excitation wavelengths used for the Raman microscopy. The positions of bright spots are argued to be influenced by fractal-shaped boundaries, particle dimensions, polarization, and wavelength of the incident and scattered light. Experimentally obtained SERS images from similar fractal (gold) structures fabricated with different dielectric SiO2 spacer thicknesses (0, 20, and 40 nm) featured diffraction-limited bright spots corresponding to local SERS enhancements of up to a107 (relative to Raman signals obtained with a glass substrate) for 40 nm thick SiO2 layers. Our results indicate that the strategy of combining fractal array geometry with gap plasmon resonances is promising for the design of highly efficient SERS substrates for potential applications in surface-enhanced multichannel sensing, including single-molecule spectroscopy. </p

Optical properties of thin graphene oxide films and their biosensing applications

Graphene oxide (GO) is a promising platform to realize immobilization matrices for plasmonic biosensors. Here, we discuss various aspect of the deposition of GO thin films, their optical properties, and the influence of GO linking layers on biosensing sensitivity. Using spectroscopic ellipsometry, we obtain dielectric functions of thin GO films deposited by spray-coating in the visible and near-infrared ranges. In addition, we demonstrate that the optimization of GO linking layers may sufficiently increase the sensitivity of SPR biosensing.</p