Spatial dispersion in an array of metallic nanorods

The low-frequency behavior of a set of wires with a very high conductivity is studied. The effective non-local constitutive relation is derived for wires with a finite height. Some numerical examples are described.

Optics of Metallic Nanostructures

Metamaterials such as metal-dielectric multilayers and cylindrical nanowires are well known, for instance because of their hyperbolic dispersion. Here we examine in detail the mode characteristics in an array of square and rectangular metallic nanorods. In particular we propose a method to describe the dispersion via the coupling of specific elementary excitations. Apparently, these fundamental modes depend on the size and shape of the nanorods, and on the particular symmetry of the Bloch modes. Specifically, we show that arrays of relatively small square nanorods are associated with coupling of single rod modes. In contrast, large nanorod arrays correspond with a basic structure consisting of four metallic corners. In the medium size case, the nature of the elementary excitation depends on the frequency range and Bloch mode symmetry. Finally, we study rectangular nanorods, which turn out to derive from a basic geometry with two semi-infinite rods. The analysis method is thus useful for a better comprehension of many other types of metamaterials.

From a study of the polarization dependence of the surface-enhanced Raman (SERS) modes of a probe molecule (pyridine) adsorbed on a locally anisotropic, metallic surface, we show that the polarization asymmetry associated with the in-plane (ring stretching) vibrational modes is consistently correlated with the morphological anisotropy, while the out-of-plane (ring deformation) modes are almost insensitive to it. As a direct application of this “polarization selection rule” for SERS, we further show that the polarization asymmetry of the in-plane vibrational modes provides a quantitative estimate of the degree of local structural anisotropies in the system. Such mesoscale, locally ordered, anisotropic structures occur naturally in a wide variety of aggregated and self-assembled systems. As a model system for this study, we have used metal nanorod arrays, whose tips can be made to form two-dimensional clusters that exhibit anisotropy at mesoscopic length scales.

The optical waveguiding capabilities of polycyanurate thermoset nanorod (PCNs) arrays mounted on planar gold substrates were investigated using optical waveguide spectroscopy (OWS). The nanorods, with diameter of 60 and 150 nm, and length of 650 nm, are formed by thermal polymerization of cyanate ester oligomers within porous anodic aluminium oxide templates. We discuss the waveguiding properties of the PCNs arrays when the nanorod diameter is much smaller (<1/10) than the wavelength of the guided light. We monitored the changes in the waveguiding modes by exchanging the interrod filling environment with water, ethanol, and isopropanol. This study led to a sensing figure of merit as high as 196 reciprocal refractive index units, which is higher than that of other previously studied sensors based on angular modulation. Furthermore, numerical simulations were performed by applying an Effective Medium Theory (EMT) based on the Maxwell‐Garnett approximation for the prediction of the optical properties of anisotropic nanorod arrays. Simulations based on Fresnel equations were used for estimating the optimal fabrication geometry, with varying the nanorod diameter and the interrod distance. Based on these results, we introduce the applicability of the PCNs arrays platform as a concept in optical biosensors and other related applications (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Clustered copper nanorod arrays constitute single-component, two-level hierarchical structures that mimic rose petals by showing high contact angles as well as considerable contact angle hysteresis (CAH). The cluster morphology of the nanorod arrays – electrochemically grown within porous alumina templates – is determined by the template geometry and the drying conditions. The CAH (a measure of the adhesion) is a function of the nanorod length and the cluster size to inter-cluster separation ratio. The measured pinning force was 176 μN, among the highest reported. The first level of the hierarchical nano-microstructure consists of the nanorod clusters and the air trapped between them – leading to a Cassie–Baxter type hydrophobicity. The second level comprises the tips of the clustered copper nanorods which pin the water droplets due to the inherently hydrophilic nature of metals. This conjecture is supported by fluorescence imaging and surface polymerization experiments. Thus, a clustered metal nanorod array provides a simple model system for studying the coexistence and complex interplay of hydrophobicity and adhesion, while also providing some degree of control over these mutually contradictory properties.

The dispersion of surface plasmon‐polaritons and their associated fluctuation densities and electric fields are calculated for a spatially dispersive semi‐infinite electron plasma, with, parameters appropriate to the metallic state. The model is hydrodynamic and the electron density is assumed to be uniform and equal to the bulk value, except for a small region near the surface. In this surface neighbourhood the electron density is assumed to fall in a step‐function manner as in previously reported successful models of surface profile properties. The momentum range covers that which is experimentally available and includes both the retardation and electrostatic regimes. It is emphasised that the inclusion of spatial dispersion, by the introduction of a pressure gradient, is an essential feature. This is because the distance over which the surface density changes is such that the transit time of the carriers is the same order of magnitude as the inverse plasma frequency. Resonances are possible, therefore, which are entirely absent in pure retardation models which ignore spatial dispersion. Fluctuation densities, electric fields, and polarisation plots are presented. The previously reported assertion that the charge distributions can be rigorously classified into multipoles is investigated numerically.

Calculations of emf induced in a vertical axis coil by a current step in a straight wire segment of finite length were carried out using an equivalent dipole placed along the source wire. An approximation is valid for homogeneous and layered earth models. The location of the equivalent dipole is calculated by forcing the early‐ and late‐time asymptotes of the transient sounding curve of the equivalent dipole to match those of the finite wire. This approximation works because the early‐time asymptote of the emf depends on the component of the receiver position which is parallel to the wire while the late‐time asymptote does not. Analytical integration of the early‐time asymptote yields an expression for the equivalent dipole location in closed form.The coil can be placed anywhere in the vicinity of the finite wire. Square or rectangular loop sources can be simulated by one or more finite wire segments depending on the source‐receiver geometry. The equivalent radius calculated for central loop soundings agrees well with the value derived using a circular loop with the same area as the square loop.Results show that acceptable sounding curves can be generated by the equivalent dipole for coils placed as close as 0.5 source lengths from the finite wire segment. Higher accuracy can be obtained by splitting the finite wire into two or more subsegments. Results for layered models are slightly better than homogeneous earth models when the resistivity increases with depth and slightly worse for models with resistivity decreasing with depth. Approximate calculations are about 10 times less expensive than exact calculations depending on the method used for the numerical integration.

Hexagonal close-packed tilted Ag nanorod arrays that exhibit excellent uniformity and reproducibility were prepared. The tilt angle was easily controlled by regulating the sputtering angle, accompanied by a reduction and constancy in the gap size of adjacent nanorods, which is 30° and 90° relative to the sputtering direction. The surface enhanced Raman spectroscopy (SERS) technique was used to characterize the interaction of tilted Ag nanorod arrays with polarized laser excitation. Interestingly, the SERS polarization-dependence increased with increasing tilt angle of the Ag nanorods. To elucidate the essential factors responsible for this SERS result, three-dimensional (3D) electromagnetic enhancement distribution for the proposed system was numerically simulated based on p- and s-polarization excitation. Most importantly, the fundamental reasons for the polarization dependence of SERS were obtained by a quantitative 3D numerical simulation of hotspot distribution for adjacent nanorods.

We study the Casimir torque arising from the quantum electromagnetic fluctuations due to the interaction of two interfaces in a system formed by a dense array of metallic nanorods embedded in dielectric fluids. It is demonstrated that as a consequence of the ultrahigh density of photonic states in the nanowire array it is possible to channel the quantum fluctuations, and thereby boost the Casimir torque by several orders of magnitude as compared to other known systems (e.g., birefringent parallel plates).

We present indications of thermalization and cooling of quasiparticles, a precursor for quantum condensation, in a plasmonic nanoparticle array. We investigate a periodic array of metallic nanorods covered by a polymer layer doped with an organic dye at room temperature. Surface lattice resonances of the array--hybridized plasmonic-photonic modes--couple strongly to excitons in the dye, and bosonic quasiparticles which we call plasmon-exciton polaritons (PEPs) are formed. By increasing the PEP density through optical pumping, we observe thermalization and cooling of the strongly coupled PEP band in the light emission dispersion diagram. For increased pumping, we observe saturation of the strong coupling and emission in a new weakly coupled band, which again shows signatures of thermalization and cooling.

Design of multiple resonant cavities of surface plasmas using multilayered sliver nanorods

This paper reviews recent advances in modeling of water flow in the unsaturated zone. The Richards model remains the most widely accepted and fertile framework for water flow analyses. More general formulations are reserved for the analysis of problems involving macroporosity, thermal effects, and air pressure effects. Many exact and approximate solutions have been derived for particular boundary value problems of homogeneous soils using methods such as quasi-linear analysis, Green-Ampt analysis, perturbation, and the kinematic wave approximation. Numerical simulators have become bigger and more accurate due to improvements in the areas of nonlinear solution procedures, mass conservation, computational efficiency, and computer hardware. Problems of natural heterogeneity have been addressed primarily through application of various stochastic methods to the Richards model. The stochastic formulations generally refute the concept of simple “equivalent” homogeneous properties, but do themselves offer a certain limited potential for a predictive capability.

Analysis of flow behavior in fractured lithophysal reservoirs

In order to study emulsification phenomena, devices generating well‐defined flow conditions are essential. Thus, emulsification of drop collectives under laminar shear flow is commonly performed in cylindrical Couette or Searle devices. In these devices, the flow conditions in the shear gap and in the volume underneath the rotor are often different, which can lead to inhomogeneous product properties and may complicate sample taking. Here, a novel cone‐cone shear cell is presented to study emulsification processes. The flow inside the device is examined using numerical simulations. The numerical simulations indicate that simple shear flow is realized all over the sample volume in the cone‐cone shear cell. The experimental results show that the drop breakup in the cone‐cone shear cell is equivalent to the breakup under simple shear realized in the shear gap of a conventional device, i.e., the Searle device. Critical capillary numbers are calculated from the experimental data and show breakup behavior as predicted by single‐drop experiments. Thus, the cone‐cone shear cell proved to be suitable to study emulsification mechanisms in simple shear flow.