Efficient Higher Order Full-Wave Numerical Analysis of 3-D Cloaking Structures

Highly efficient and versatile computational electromagnetic analysis of 3-D transformation-based metamaterial cloaking structures based on a hybridization of a higher order finite element method for discretization of the cloaking region and a higher order method of moments for numerical termination of the computational domain is proposed and demonstrated. The technique allows for an effective modeling of the continuously inhomogeneous anisotropic cloaking region, for cloaks based on both linear and nonlinear coordinate transformations, using a very small number of large curved finite elements with continuous spatial variations of permittivity and permeability tensors and high-order p-refined field approximations throughout their volumes, with a very small total number of unknowns. In analysis, there is no need for a discretization of the permittivity and permeability profiles of the cloak, namely for piecewise homogeneous (layered) approximate models, with material tensors replaced by appropriate piecewise constant approximations. Numerical results show a very significant reduction (three to five orders of magnitude for the simplest possible 6-element model and five to seven orders of magnitude for an h-refined 24-element model) in the scattering cross section of a perfectly conducting sphere with a metamaterial cloak, in a broad range of wavelengths. Given the introduced explicit approximations in modeling of the spherical geometry and continuous material tensor profiles (both by fourth-order Lagrange interpolating functions), and inherent numerical approximations involved in the finite element and moment method techniques and codes, the cloaking effects are shown to be predicted rather accurately by the full-wave numerical analysis method.     

Electrodynamic eigenmodes in cellular morphology

Eigenmodes of the spherical and ellipsoidal dielectric electromagnetic resonator have been analysed. The sizes and shape of the resonators have been chosen to represent the shape of the interphase and dividing animal cell. Electromagnetic modes that have shape exactly suitable for positioning of the sufficiently large organelles in cell (centrosome, nucleus) have been identified. We analysed direction and magnitude of dielectrophoretic force exerted on large organelles by electric field of the modes. We found that the TM(1m1) mode in spherical resonator acts by centripetal force which drags the large organelles which have higher permittivity than the cytosol to the center of the cell. TM-kind of mode in the ellipsoidal resonator acts by force on large polarizable organelles in a direction that corresponds to the movement of the centrosomes (also nucleus) observed during the cell division, i.e. to the foci of the ellipsoidal cell. Minimal required force (10(-16)N), gradient of squared electric field and corresponding energy (10(-16)J) of the mode have been calculated to have biological significance within the periods on the order of time required for cell division. Minimal required energy of the mode, in order to have biological significance, can be lower in the case of resonance of organelle with the field of the cellular resonator mode. In case of sufficient energy in the biologically relevant mode, electromagnetic field of the mode will act as a positioning or steering mechanism for centrosome and nucleus in the cell, thus contribute to the spatial and dynamical self-organization in biological systems.     

Cloaking cytolytic peptides for liposome-based detection and potential drug delivery

Potent cytolytic peptides with specific tethering and cloaking sites have been synthesised and used to release payload from liposomes in a quantitative manner. A functionally located cloaking site has been modified specifically by simple conjugation without adversely affecting the cytolytic properties of the peptide. The cytolytic activity of modified peptides was then efficiently (>98%) cloaked and uncloaked by ligand-protein or hapten-antibody interactions. The principle of a dual response peptide has been demonstrated using an avidin-cloaked pH-sensitive peptide. Biospecific cloaking/uncloaking provided a new sensitive (approximately 12 pmol) homogeneous diagnostic and also appears potentially suited to bioresponsively targeted release of antimicrobial, anticancer and other drugs now delivered using liposomes.     

Cloaking cytolytic peptides for liposome-based detection and potential drug delivery

Potent cytolytic peptides with specific tethering and cloaking sites have been synthesised and used to release payload from liposomes in a quantitative manner. A functionally located cloaking site has been modified specifically by simple conjugation without adversely affecting the cytolytic properties of the peptide. The cytolytic activity of modified peptides was then efficiently (>98%) cloaked and uncloaked by ligand-protein or hapten-antibody interactions. The principle of a dual response peptide has been demonstrated using an avidin-cloaked pH-sensitive peptide. Biospecific cloaking/uncloaking provided a new sensitive (∼12 pmol) homogeneous diagnostic and also appears potentially suited to bioresponsively targeted release of antimicrobial, anticancer and other drugs now delivered using liposomes.     

Measurements of the cross-bridge attachment/detachment process within intact sarcomeres by surface plasmon resonance.

We have developed a surface plasmon resonance (SPR) system to monitor the cross-bridge attachment/detachment process within intact sarcomeres from mouse heart muscle. SPR occurs when laser light energy is transferred to surface plasmons that are resonantly excited in a metal (gold) film. This resonance manifests itself as a minimum in the reflection of the incident laser light and occurs at a characteristic angle. The angle of the SPR occurrence depends on the dielectric permittivity of the sample medium adjacent to the gold film. Purified sarcomeric preparations are immobilized onto the gold film in the presence of a relaxing solution. Replacement of the relaxing solution with increasing Ca(2+) concentration solution activates the cross-bridge interaction and produces an increase in the SPR angle. These results imply that the interaction of myosin heads with actin within an intact sarcomere changes the dielectric permittivity of the sarcomeric structure. In addition, we further verify that SPR measurements can detect the changes in the population of the attached cross-bridges with altered concentrations of phosphate, 2,3-butanedione monoxime, or adenosine triphosphate at a fixed calcium concentration, which have been shown to reduce the force and increase the cross-bridge population in attached state. Thus, our data provide the first evidence that the SPR technique allows the monitoring of the cross-bridge attachment/detachment process within intact sarcomeres.     

Zirconium Nitride for Plasmonic Cloaking of Visible Nanowire Photodetectors

Light scattered by a photodetector disturbs the probing field, resulting in noise. Cloaking is an effective method to reduce this noise. Here we investigate theoretically an emerging plasmonic material, zirconium nitride (ZrN), as a plasmonic cloak for silicon (Si) nanowire-based photodetectors and compare it with a traditional plasmonic material, gold (Au). Using Mie formalism, we have obtained the scattering cancelation across the visible spectrum. We found that ZrN cloaks produce a significant decrease in the scattering from bare Si nanowires, which is 40% greater than that obtained with Au cloaks in the wavelength region of 400–500 nm. The scattering cancelations become comparable at 550 nm, with Au providing a better scattering cancelation compared to ZrN over the wavelength region of 600–700 nm. To include the absorption and provide a measure of overall performance on noise reduction, a figure of merit (FOM), defined as the ratio of the absorption efficiency and the scattering efficiency of the cloaked nanowire to that of the bare Si nanowire, was calculated. We show that the optimized ZrN cloak provides up to 3 times enhancement of the FOM over a bare Si NW and a 60% improvement over an optimized Au-cloaked NW, in the wavelength region of 400–500 nm. An optimized Au-cloaked NW shows up to 17.69 times improvement in the wavelength region of 600–700 nm over a bare Si NW and up to a 2.7 times improvement over an optimized ZrN-cloaked NW. We also predicted the optimal dimensions for the cloaked NWs with respect to the largest FOM at various wavelengths between 400 and 650 nm.

     

Plasmon Resonances in V-Shaped Gold Nanostructures

Using numerical simulations, we examine the change in plasmon resonance behavior in gold nanorod structures that have a V shape. The reduction in symmetry compared to linear rods causes two different longitudinal-type resonances to appear in a single structure, and the relative intensity and hybridization of these can be controlled by varying the angle of the arms of the ??V.?? The resonances may also be selectively excited by controlling the polarization of the incident light, thereby providing a convenient way to control a nanoscale optical electric field using far-field parameters. For example, the wavelength at which a strong resonance occurs in the V-shaped structures studied can be switched between 630 and 900?nm by a 90° rotation of the polarization of the incident light. Due to the symmetry of the targets, there will be three types of special near-field location; a location at which the electric field intensity is enhanced by either resonance, a location at which the electric field intensity is enhanced by the 630?nm resonance but not by the 890?nm resonance, and a location at which the electric field intensity is enhanced by the 890?nm resonance but not by the 630?nm one.     

Tunable Scattering Cancellation of Light Using Anisotropic Cylindrical Cavities

Engineered core-shell cylinders are good candidates for applications in invisibility and cloaking. In particular, hyperbolic nanotubes demonstrate tunable ultra-low scattering cross section in the visible spectral range. In this work, we investigate the limits of validity of the condition for invisibility, which was shown to rely on reaching an epsilon near zero in one of the components of the effective permittivity tensor of the anisotropic metamaterial cavity. For incident light polarized perpendicularly to the scatterer axis, critical deviations are found in low-birefringent arrangements and also with high-index cores. We suggest that the ability of anisotropic metallodielectric nanocavities to dramatically reduce the scattered light is associated with a multiple Fano-resonance phenomenon. We extensively explore such resonant effect to identify tunable windows of invisibility.     

New Type Design of the Triple-Band and Five-Band Metamaterial Absorbers at Terahertz Frequency

A new scheme to achieve a simple design of triple-band metamaterial absorber at terahertz frequency is presented. In this scheme, we employ a traditional sandwich structure, which is consisted of a metallic resonator and an appropriate thickness of the dielectric layer backed with an opaque metallic board, as the research object. Three strong but discrete resonance peaks with the narrow bandwidths and high absorptivities are realized. The combination of the dipolar resonance, LC (inductor-capacitor circuit) resonance, and the surface resonance of the metallic resonator determines the triple-band absorption. Numerical results also show that the frequencies of the three absorption bands and the number of the resonance peaks can be effectively tuned by adjusting or changing the geometric parameters of the metallic resonator. Moreover, we present a simple design of five-band terahertz absorber by further optimizing the sizes of the metallic elements in the top layer of the metamaterial. The design of the unit structures will assist in designing innovative absorbing devices for spectroscopy imaging, detection, and sensing.     

Broadband Optical Reflection Modulator in Indium-Tin-Oxide-Filled Hybrid Plasmonic Waveguide with High Modulation Depth

A surface plasmon resonance (SPR)-based optical reflection modulator consisting of vertically stacked silica-silicon-HfO₂-ITO-HfO₂-Ag-prism multilayer is proposed and numerically investigated. The free carrier-concentration-dependent permittivity of indium-tin-oxide (ITO) at the HfO₂/ITO interface induces an epsilon-near-zero (ENZ) effect contributing to strong field enhancement and modifies the SPR condition of incident light. With optimal geometry parameters and proper design of carrier concentration at the accumulation layer, modulation depth (MD) of ~100% and insertion loss (IL) of 3.7% can be simultaneously achieved. The IL can be further reduced by engineering silicon layer thickness. Moreover, the device offers a broadband operation wavelength from 1.5 to 1.6 μm with the variations of MD and IL smaller than 4 and 3%, respectively.

     

Frequency-Dependent Polarization Properties of Local Electric Field in Gold–Dielectric Multi-Nanoshells

The polarization properties of the local electric field in the gold–dielectric–gold multilayer nanoshells are investigated by theoretical calculation based on the quasi-static approximation. The calculation results show that the complete polarized incident light does not only stimulate the same directional polarized local electric field. The polarized angle of the local field may changes from 0° to 90° as the wavelength and location are changed. The distributions of local field polarization are different in dielectric layer or gold shell and display different features in different plasmonic hybridization mode. As the incident wavelength is increased, the hot spot of polarizing angle moves monotonously in the middle dielectric shell, whereas moves nonmonotonously in the gold shell and surrounding environment. In the gold shell, the gap between hot spots of polarizing angle may occur at the resonance frequency. However, the hot spots of polarizing angle always occur at the resonance frequencies in the surrounding environment. These interesting results show that the single-molecule detection based on metal nanostructure induced surface-enhanced Raman scattering and surface enhanced fluorescence could be optimized by adjusting the incident light polarization and frequency.     

An Invisibility Cloak Using Silver Nanowires

In this paper, we use the parameter retrieval method together with an analytical effective medium approach to design a well-performed invisible cloak, which is based on an empirical revised version of the reduced cloak. The designed cloak can be implemented by silver nanowires with elliptical cross sections embedded in a polymethyl methacrylate host. This cloak is numerically proved to be robust for both the inner hidden object as well as incoming detecting waves and is much simpler, thus easier to manufacture when compared with the earlier one proposed by Cai et al. (Nat Photon 1: 224, 2007).     

A Design for Band Enhanced Dielectric Absorber Based on Fractal-Like Structure

A dielectric metamaterial absorber has been proposed, which consists of fractal-like structure and conductive sheet. The fractal-like structure is made by the high permittivity dielectric and also is covered by the conductive sheet. Absorptivity of such a dielectric metamaterial absorber is 99.1%, which can be found at 10.196 GHz; meanwhile, the absorber is polarization insensitive. To enhance the bandwidth of absorber, a novel absorber also is proposed, whose bandwidth is 0.566 GHz, which ranges from 9.752 to 10.318 GHz, and relative bandwidth is 5.64%. The maximum absorptivity can reach to 99.8%, and the proposed absorber also is polarization insensitive. In the meantime, the absorber shows excellent performance which is incident angle insensitive; when the incident angle is increased to 70°, the absorptivity is larger than 75%.

     

Synchronous Tuning of Twined Resonance Modes with Controllable Spectral Separation in Plasmonic Gratings

We investigated angle-resolved tuning performance of the double resonance modes of waveguide metallic grating structures, where the incident angle was changed in the plane formed by the extending direction of the grating lines and the wave vector with the grating plane tilted with respect to the vertical axis. Double resonance modes were observed due to tilting of the grating, which were tuned simultaneously to the blue with increasing the angle of incidence. The spectral separation between the resonance modes can be adjusted simply by changing the tilting angle of the grating. Such a double resonance device is important for exploring multichannel optical filters, optical switching device, or sensors.     

An Exterior Anti-Cloak

Based on transformation optics, we propose an exterior anti-cloak design in bipolar coordinates. The anti-cloak is a three-layered structure used to cancel the function of an exterior cloak. When the exterior cloak is placed close to a macroscopic object that can be seen with human eyes, the object is hidden from the outside electromagnetic detection. However, when the exterior anti-cloak is applied, the hidden object becomes visible to the electromagnetic waves. Simulation results demonstrate excellent performance of the exterior cloak and anti-cloak. Both the cloak and anti-cloak are exterior ones, which are convenient and flexible for practical application.     

Wide-Angle Near-Perfect Absorber Based on Sub-Wavelength Dielectric Grating Covered by Continuous Thin Aluminum Film

We design and numerically investigate an optical absorber consisting of the sub-wavelength dielectric grating covered by continuous thin aluminum film. In this absorber, the aluminum film act as an efficient absorbing material because of the enhanced electric field in the air nano-grooves, and the absorption spect+rum can be manipulated by Fabry-Perot cavity mode resonance. According to the spectrum manipulation mechanism, the wavelength of absorption peak can be tuned by changing the heights and widths of the air nano-grooves. More importantly, the high absorption is very robust to the incident angle around the designed wavelength. From the nanofabrication point of view, the light absorber can be fabricated more easily without the need for ion or electrochemical etching of metal and it is easy to be integrated into complex photonic devices.     

Tunable Dielectric Properties of Ferrite-Dielectric Based Metamaterial

A ferrite-dielectric metamaterial composed of dielectric and ferrite cuboids has been investigated by experiments and simulations. By interacting with the electromagnetic wave, the Mie resonance can take place in the dielectric cuboids and the ferromagnetic precession will appear in the ferrite cuboids. The magnetic field distributions show the electric Mie resonance of the dielectric cuboids can be influenced by the ferromagnetic precession of ferrite cuboids when a certain magnetic field is applied. The effective permittivity of the metamaterial can be tuned by modifying the applied magnetic field. A good agreement between experimental and simulated results is demonstrated, which confirms that these metamaterials can be used for tunable microwave devices.     

Extraordinary Transmission of Three-Dimensional Crescent-like Holes Arrays

We developed a method to fabricate a periodic array of three-dimensional crescent-like holes (3DCLH) via an inverted hemispherical colloidal lithography. It is found that there exists an extraordinary optical transmission in this non-planar perforated periodic array of 3DCLH when the electric field of the incident light is perpendicular to the cross-line of the crescent-like hole. This extraordinary optical peak is insensitive with the incident angles and sensitive with the angle between the electric field of the incident light to the cross-line of the 3DCLH. Numerical simulation based on finite-difference time-domain method reveals that this peak is caused by an asymmetric localized surface plasmon resonance. This structure might be useful for the optical sensing and optical-integrated circuits.     

Beamline characterization of a dielectric-filled reentrant cavity resonator as beam current monitor for a medical cyclotron facility

At PSI (Paul Scherrer Institute), Switzerland, a superconducting cyclotron called “COMET” delivers proton beam of 250 MeV pulsed at 72.85 MHz for proton radiation therapy. Measuring proton beam currents (0.1–10nA) is of crucial importance for the treatment safety and is usually performed with invasive monitors such as ionisation chambers (ICs) which degrade the beam quality. A new non-invasive beam current monitor working on the principle of electromagnetic resonance is built to replace ICs in order to preserve the beam quality delivered. The fundamental resonance frequency of the resonator is tuned to 145.7 MHz, which is the second harmonic of the pulse rate, so it provides signals proportional to beam current. The cavity resonator installed in the beamline of the COMET is designed to measure beam currents for the energy range 238–70 MeV. Good agreement is reached between expected and measured resonator response over the energy range of interest. The resonator can deliver beam current information down to 0.15 nA for a measurement integration time of 1 s. The cavity resonator might be applied serving as a safety monitor to trigger interlocks within the existing domain of proton radiation therapy. Low beam currents limit the abilities to detect sufficiently, however, with the potential implementation of FLASH proton therapy, the application of cavity resonator as an online beam-monitoring device is feasible.     

Direct measurement of DNA bending by type IIA topoisomerases: implications for non-equilibrium topology simplification

Type IIA topoisomerases modify DNA topology by passing one segment of duplex DNA (transfer or T–segment) through a transient double-strand break in a second segment of DNA (gate or G–segment) in an ATP-dependent reaction. Type IIA topoisomerases decatenate, unknot and relax supercoiled DNA to levels below equilibrium, resulting in global topology simplification. The mechanism underlying this non-equilibrium topology simplification remains speculative. The bend angle model postulates that non-equilibrium topology simplification scales with the bend angle imposed on the G–segment DNA by the binding of a type IIA topoisomerase. To test this bend angle model, we used atomic force microscopy and single-molecule Förster resonance energy transfer to measure the extent of bending imposed on DNA by three type IIA topoisomerases that span the range of topology simplification activity. We found that Escherichia coli topoisomerase IV, yeast topoisomerase II and human topoisomerase IIα each bend DNA to a similar degree. These data suggest that DNA bending is not the sole determinant of non-equilibrium topology simplification. Rather, they suggest a fundamental and conserved role for DNA bending in the enzymatic cycle of type IIA topoisomerases.