Visible transmission through metal-coated colloidal crystals

Large-scale periodically structured metal films with enhanced optical transmission in visible frequencies were fabricated by depositing silver onto colloidal crystals. The obtained transmission properties are similar to those observed through periodical hole arrays in planar metal films. We have experimentally observed two enhanced transmission pass bands in visible frequencies in these metal films due to the excitation of surface plasmon polaritons. The peak positions of the pass bands depend on the size of the colloidal spheres. The transmission spectra highly depend on the incident angle for p-polarized light but are weak for s-polarized light. Our fabrication method provides a promising approach for the fabrication of large-scale low-cost plasmonic crystals with submicrometer periodicity.     

Length-scale-dependent relaxation in colloidal gels

We use molecular dynamics computer simulations to investigate the relaxation dynamics of a simple model for a colloidal gel at a low volume fraction. We find that due to the presence of the open spanning network this dynamics shows at low temperature a nontrivial dependence on the wave vector which is very different from the one observed in dense glass-forming liquids. At high wave vectors the relaxation is due to the fast cooperative motion of the branches of the gel network, whereas at low wave vectors the overall rearrangements of the heterogeneous structure produce the relaxation process.     

Field-induced phases of an orientable charged particle in a dilute background of point charges

We report numerical simulations of a two-dimensional dynamical model comprised of a rodlike particle surrounded by a cloud of smaller particles of the same charge, in the presence of an alternating electric field inside a box. We show that this system displays a remarkable dynamical effect; at low forcing frequencies the rod tends to align perpendicularly to the external field, whereas for higher field frequencies the standard orientation (parallel to the field) prevails. Interestingly, the transition between orientations is abrupt enough to resemble a phase transition. The fact that the "anomalous" orientation (perpendicular to the field) takes place is also interesting in the light of some recent laboratory experiments on colloidal solutions, where anomalous orientation at low frequencies was observed. Our toy model suggests that future physically realistic simulations of these systems should explore whether the anomalous orientation may be due to the collective dynamics of the colloidal particles, without necessarily involving more sophisticated electro-osmotic effects.     

A unique method for reducing losses of bends in dielectric optical wave guide

The dielectric optical wave guide is finding growing attention at millimeter wave frequencies. However the dielectric optical waveguide radiates at bends and thus transmission loss increases. These radiations are in the outword direction of bends. This output radiation at output bends arises due to change in phase velocities of the propagating wave at the centre of the dielectric guide and the phase velocity at the outer surface of the dielectric guide. A unique methiod is suggested to avoid these radiation losses.Experimental results are also shown at microwave frequencies.     

Visualization of wave propagation in muffler

This paper presents a prediction technique for wave propagation in muffler using Boundary Element Method (BEM). The results of the numerical calculation are compared with the plane-wave theory and also the experimental results. It is shown that at high frequencies the plane-wave theory does not yield correct results due to the occurrence of two- and three-dimensional wave motion. On the other hand, the BEM is able to predict the acoustic wave behavior in the high frequencies where the planewave theory fails.     

Generalized Bloch's theorem for viscous metamaterials: Dispersion and effective properties based on frequencies and wavenumbers that are simultaneously complex

It is common for dispersion curves of damped periodic materials to be based on real frequencies as a function of complex wavenumbers or, conversely, real wavenumbers as a function of complex frequencies. The former condition corresponds to harmonic wave motion where a driving frequency is prescribed and where attenuation due to dissipation takes place only in space alongside spatial attenuation due to Bragg scattering. The latter condition, on the other hand, relates to free wave motion admitting attenuation due to energy loss only in time while spatial attenuation due to Bragg scattering also takes place. Here, we develop an algorithm for 1D systems that provides dispersion curves for damped free wave motion based on frequencies and wavenumbers that are permitted to be simultaneously complex. This represents a generalized application of Bloch's theorem and produces a dispersion band structure that fully describes all attenuation mechanisms, in space and in time. The algorithm is applied to a viscously damped mass-in-mass metamaterial exhibiting local resonance. A frequency-dependent effective mass for this damped infinite chain is also obtained.     

Numerical analysis of ultrasonic transmission and absorption of oblique plane waves through the human skull.

Ultrasonic transmission and absorption of oblique plane waves through the human skull are analyzed numerically for frequencies ranging from 1/2 to 1 MHz. These frequencies are optimum for noninvasive ultrasound therapy of brain disorders where numerical predictions of skull transmission are used to set the phase and amplitude of source elements in the phased array focusing system. The idealized model of the skull is a three-layer solid with ivory outer and inner layers and a middle marrow layer. Each layer is modeled as a flat, homogeneous, isotropic, linear solid with effective complex wave speeds to account for focused energy losses due to material damping and scattering. The model is used to predict the amplitude and phase of the transmitted wave and volumetric absorption. Results are reported for three different skull thicknesses: 3 mm, 6 mm, and 9 mm. Thickness resonances are observed in the transmitted wave for 3 mm skulls at all frequencies and for the 6 mm skulls below 0.75 MHz. Otherwise, the transmission is dominated by the direct wave. Skull phase errors due to shear waves are shown to minimally degrade the power at the focus for angles of incidence up to 20 degrees from normal even for low material damping. The location of the peak volumetric absorption occurs either in the outer ivory or middle marrow layer and shown to vary due to wave interference.     

Evanescent Plane Waves in Multilayer Structures

We have considered evanescent plane waves in structures with a layer of a substance with ε, μ < 0 and with a layer of a well-reflecting metal, ε < 0, μ ≥ 1. Waves with increased amplitude as compared with the initial wave have been found to occur, due to which evanescent waves with wave number as in the initial wave but with increased amplitude arise behind these layers. A composite material with ε, μ < 0 at optical frequencies are proposed. Surface waves on a metal layer are considered in detail. It is shown that surface waves with a sufficiently arbitrary wave number can be excited. It is also shown that, on very thin layers, surface waves with wave number exceeding ten times that of a homogeneous plane wave in vacuum can be excited. Propagation losses are calculated. For a silver layer, the wave path can be from 30 up to 100 wavelengths. Practical use in developing techniques for optical transformations of short-wave surface waves in 2D space, similar to those in 3D space, are pointed out.     

Revision of estimates of acoustic energy reflectance at the human eardrum

An improved analysis procedure has been applied to standing wave patterns measured previously [B. W. Lawton and M. R. Stinson, J. Acoust. Soc. Am. 79, 1003-1009 (1986)] in human ear canals. Revised acoustic energy reflection coefficients, at the eardrum, are obtained for 20 ears for frequencies between 3 and 13 kHz. The new analysis addresses anomalous features of the standing wave patterns, apparent at frequencies above 8 kHz, due primarily to the curvature of the ear canal. Much better agreement is now found, at these higher frequencies, between the theoretical form assumed for the standing wave patterns and the experimental data. The revised values of eardrum reflectance are somewhat smaller, especially for frequencies above 11 kHz. The reflectance rises from about 0.25 at 4 kHz up to 0.7 at 8 kHz, falls to a minimum of 0.5 at 11 kHz, then rises to 0.6 at 13 kHz. Considerable intersubject variability in the results is noted.     

Nonlinear interactions of surface waves with incident radiation at a narrow inhomogeneous plasma layer

The amplitudes of the waves radiating at combination frequencies from the plasma boundary due to nonlinear interaction of surface waves with radiation incident on a narrow inhomogeneous plasma layer, are determined. The method used allows for the discontinuity at the plasma boundary of the tangential components of the electric field of the wave at the combination frequencies.     

Acoustic attenuation of hybrid silencers

The acoustic attenuation of a single-pass, perforated concentric silencer filled with continuous strand fibers is investigated first theoretically and experimentally. The study is then extended to a specific type of hybrid silencer that consists of two single-pass perforated filling chambers combined with a Helmholtz resonator. One-dimensional analytical and three-dimensional boundary element methods (BEM) are employed for the predictions of the acoustic attenuation in the absence of mean flow. To account for the wave propagation in absorbing fiber, the complex-valued characteristic impedance and wave number are measured. The perforation impedance facing the fiber is also presented in terms of complex-valued characteristic impedance and wave number. The effects of outer chamber diameter and the fiber density are examined. Comparisons of predictions with the experiments illustrate the need for multi-dimensional analysis at higher frequencies, while the one-dimensional treatment provides a reasonable accuracy at lower frequencies, as expected. The study also shows a significant improvement in the acoustic attenuation of the silencer due to fiber absorption. Multi-dimensional BEM predictions of a hybrid silencer demonstrate that a reactive component such as a Helmholtz resonator can improve transmission loss at low frequencies and a higher duct porosity may be effective at higher frequencies.     

Filamentary coagulation of particles from water-heterogeneous systems in the field of an acoustic resonator

The coagulation of particles from water-heterogeneous systems in the field of a confocal ultrasonic resonator is studied. It is found that, at frequencies of several megahertz, when acoustic power of about 1 W is applied to the resonator, long stable filaments consisting of the material of the heterogeneous system are formed in the vicinity of the resonator axis. The filaments consist of thin disks formed by coalescent particles spaced at intervals strictly equal to half of the sound wavelength. The features of this coagulation are determined for suspensions of various nature (metal and dielectric particles, colloidal solutions, and oil emulsions). It is established that the coagulation in a standing acoustic wave occurs faster than under natural conditions (under the influence of gravity). The possibility of using this effect for cleaning liquids from impurities and separating hyperfine particles without employing filter materials is discussed.     

Optical edge modes in photonic liquid crystals

An analytic theory of localized edge modes in chiral liquid crystals (CLCs) is developed. Equations determining the edge-mode frequencies are found and analytically solved in the case of low decaying modes and are solved numerically for the problem parameter values typical for the experiment. The discrete edge-mode frequencies specified by the integer numbers n are located close to the stop-band edge frequencies outside the band. The expressions for the spatial distribution of the n’s mode field in a CLC layer and for its temporal decay are presented. The possibilities of a reduction of the lasing threshold due to the anomalously strong absorption effect are theoretically investigated for a distributed feedback lasing in CLCs. It is shown that a minimum of the threshold pumping wave intensity may be reached, generally, for the pumping wave propagating at an angle to the helical axis. However, for lucky values of the related parameters, it may be reached for the pumping wave propagating along the helical axis. The lowest threshold pumping wave intensity occurs for the lasing at the first low-frequency band-edge lasing mode and the pumping wave propagating at an angle to the spiral axis corresponding to the first angular absorption maximum of the anomalously strong absorption effect at the high-frequency edge of the stop band. The study is performed in the case of the average dielectric constant of the liquid crystal coinciding with the dielectric constant of the ambient material. Numerical calculations of the distributed feedback lasing threshold at the edge-mode frequencies are performed for typical values of the relevant parameters.     

Acousto-electric wave instability in ion-implanted semiconductor plasmas

A comprehensive investigation of excitation of acousto-electric modes and novel properties introduced due to presence of negatively charged colloids in magnetized compensated piezoelectric semiconductor plasma is presented analytically. We derive a compact dispersion relation for the acousto-electric waves in colloids laden semiconductor plasma by employing multi-fluid balance equation. It is found that the presence of charged colloids not only modifies the wave spectrum but also alters the amplification characteristics even though colloidal particles, on account of their heavy masses, do not participate in wave propagation. The results of this investigation should be useful in understanding the characteristics of longitudinal acousto-electric wave in ion-implanted piezoelectric semiconductor plasmas whose main constituents are electrons, holes and negatively charged colloids, which are stationary in the background.     

Investigation of four wave mixing effect with different number of input channels at various channel spacing

In this paper the design, implementation and performance analysis of four wave mixing (FWM) in optical communication system for different number of input channels is presented using various values of channel spacing. Here, all the input channels have been spaced evenly at various values like 6.25 GHz, 12.5 GHz, 25 GHz, 40 GHz, 50 GHz with the different number of channels at the input i.e. with 2, 4, 6, 8, 12 input channels. The simulation results reveal that the four wave mixing is minimum when the channel spacing is maximum i.e. 50 GHz with minimum number of channels i.e. 2 input channels. It is observed that on increasing the channel spacing, the interference between the input frequencies decreases and hence the four wave mixing also decreases. Also, on increasing the number of input channels/users, the interference between the input frequencies increases and thus, the four wave mixing also increases.     

Electrically guided assembly of planar superlattices in binary colloidal suspensions

Binary colloidal suspensions are assembled into planar superlattices using ac electric fields. Either triangular or square-packed arrays form, depending on the frequency and relative particle concentrations. The frequency dependence is striking since superlattices develop at low and high frequencies but not at intermediate frequencies. We explain the low frequency behavior (<3 kHz) in terms of induced-dipole repulsion balanced by attraction resulting from electrohydrodynamic (EHD) flow. At high frequencies (20-200 kHz), EHD flow is negligible but aggregation occurs since dipole-dipole interactions become attractive.     

Second-order random wave solutions for interfacial internal waves in N-layer density-stratified fluid

This paper studies the random internal wave equations describing the density interface displacements and the velocity potentials of N-layer stratified fluid contained between two rigid walls at the top and bottom. The density interface displacements and the velocity potentials were solved to the second-order by an expansion approach used by Longuet-Higgins （1963） and Dean （1979） in the study of random surface waves and by Song （2004） in the study of second- order random wave solutions for internal waves in a two-layer fluid. The obtained results indicate that the first-order solutions are a linear superposition of many wave components with different amplitudes, wave numbers and frequencies, and that the amplitudes of first-order wave components with the same wave numbers and frequencies between the adjacent density interfaces are modulated by each other. They also show that the second-order solutions consist of two parts： the first one is the first-order solutions, and the second one is the solutions of the second-order asymptotic equations, which describe the second-order nonlinear modification and the second-order wave-wave interactions not only among the wave components on same density interfaces but also among the wave components between the adjacent density interfaces. Both the first-order and second-order solutions depend on the density and depth of each layer. It is also deduced that the results of the present work include those derived by Song （2004） for second-order random wave solutions for internal waves in a two-layer fluid as a particular case.     

Cross-frequency correlation of intensity fluctuations for a deeply modulated phase screen

We consider the intensity fluctuation patterns produced beyond a deeply modulated phase changing screen when two plane waves, each with a different frequency, are incident on the same screen. The spatial frequency spectrum corresponding to the cross-correlation between these two different intensity fluctuation patterns is derived. This spectrum has two distinct regions, one of low spatial frequencies which is independent of the incident wave frequencies, and another at high spatial frequencies which is wave frequency dependent. As the difference between the incident wave frequencies increases the upper cut-off spatial frequency of the spectrum decreases. The corresponding cross-scintillation index is also considered and it is shown that this is independent of the incident wave frequencies close to the phase screen and including the focal region where there is a peak in the index. In the far field the scintillation index approaches unity as the distance from the screen increases in the monochromatic case. However, for different incident wave frequencies the far-field scintillation index falls off both with distance from screen and with increasing wave frequency difference.     

Effect of ultrasonic wave on the syntheses of Au and ZnO nanoparticles by laser ablation in water

We synthesized Au and ZnO nanoparticles by laser ablation in distilled water with the superposition of an ultrasonic wave. The effect of the ultrasonic wave was examined on the optical absorbance of colloidal solution and the crystallinity of synthesized nanoparticles. The absorbance of colloidal solution was enhanced by the ultrasonic wave, indicating more efficient production rate of nanoparticles. In addition, the ultrasonic wave enhanced the crystallinity of synthesized nanoparticles. These enhancements are attributed to the fact that the ultrasonic wave drives the repetitive formations and collapses of cavitation bubbles.     

On transmittance and localization of the electromagnetic wave in two-dimensional graphene-based photonic crystals

Two-dimensional graphene-based photonic crystal (GPC) formed by a periodic array of the homogeneous dielectric cylinders etched in the alternating graphene and dielectric layers and its inverse counterpart are considered. The transmittance of the photonic crystal is obtained. The waveguide due to the localization of the electromagnetic wave on the lattice defect that breaks the translational symmetry of the GPC of two different topologies is studied. The different topologies of GPC are characterized by different photonic band structures with different widths of photonic band gaps (PBG) and provide different frequencies for the localized electromagnetic wave due to the defect. The frequencies of the localized mode for both type of the GPC, located inside the lowest PBG, are in the range of THz or tens of THz depending on the topology of the GPC. It is shown that the photonic band gap always can be tuned by changing the chemical potential of graphene to provide formation of the localized photonic mode due to the defect. The technological advantages of the GPC, as well as the opportunity to tune the PBG and the frequency of the localized electromagnetic wave in the terahertz region of spectrum for the GPC are discussed.