Showing papers by "Bae-Ian Wu published in 2008"

Extraordinary surface voltage effect in the invisibility cloak with an active device inside.

Abstract: The electromagnetic field solution for a spherical invisibility cloak with an active device inside is established. Extraordinary electric and magnetic surface voltages are induced at the inner boundary of a spherical cloak, which prevent electromagnetic waves from going out. The phase and handness of polarized waves obliquely incident on such boundaries are kept in the reflected waves. The surface voltages due to an electric dipole inside the concealed region are found equal to the auxiliary scalar potentials at the inner boundary, which consequently gain physical counterparts in this case.

Application of coordinate transformation in bent waveguides

Abstract: We apply coordinate transformations in the design of waveguides with bent geometry to reduce reflection at the incident port. It was found that in the case of metallic waveguides, by applying the transformation to the medium inside a right-angle bent waveguide, low reflection can be obtained. For the case of dielectric waveguides, this can also be done by applying the same transformation to the bent region containing both the dielectric core and its claddings. Our proposed technique provides an alternative method for designing bent waveguides with low insertion loss.

Directive emission obtained by coordinate transformation

Abstract: We use coordinate transformation theory to realize substrates that can modify the emission of an embedded source. Simulation results show that with proper transformation functions the energy radiated by a source embedded in these space variant media will be concentrated in a narrow beam. The thickness of the slab achieved with our transformations will no longer be restricted by the evanescent modes and the source can be placed at any position along the boundary of the substrate without affecting the radiation pattern. We also discuss the case where reduced parameters are used, which still performs well and is physically realizable.

Cherenkov radiation in anisotropic double-negative metamaterials

Abstract: We theoretically study reversed Cherenkov radiation (CR) in anisotropic double-negative metamaterials (DNMs) in general, and particularly in detail for one of the most practical cases, i.e., CR in a waveguide partially filled with anisotropic DNMs. The theory presented here provides a theoretical basis for possible experiments and potential applications. As an example, we discuss the physical properties of CR and the potential applications such as particle detectors and high-power sources.

Cloak for multilayered and gradually changing media

Abstract: Coordinate transformation approach has previously been used for designing a cloak which can shield an interior region in a homogeneous medium. In this paper, we consider the case where the background is no longer a homogeneous medium and determine the relative constitutive parameters of the cloak according to the multilayered and gradually changing background. We propose the parameters of cylindrical cloak structures working in multilayered and gradually changing media and the scheme of specifying these parameters could also be applied to the design of a cloak in an arbitrary isotropic background. The theoretical analysis based on coordinate transformation is given and numerical simulations are performed to illustrate these properties. The simulation results show that the cloaking with the proposed parameters performs well in these inhomogeneous background medium. Potential applications are also discussed.

Cylindrical cloak with axial permittivity/permeability spatially invariant

Abstract: In order to reduce the difficulties in the experimental realizations of the cloak but still keep good performance of invisibility, we proposed a perfect cylindrical invisibility cloak with spatially invariant axial material parameters. The advantage of this kind of cloak is that only ρ and φ components of μ (or e) are spatially variant, which makes it possible to realize perfect invisibility with two-dimensional metamaterials. The effects of perturbations of the parameters on the performance of this cloak are quantitatively analyzed by scattering theory. Our work provides a simple and feasible solution to the experimental realization of cloaks with ideal parameters.

Reversed Cherenkov radiation in a waveguide filled with anisotropic double-negative metamaterials

Abstract: The physical properties of Cherenkov radiation (CR) are theoretically investigated for a charged particle traveling along the axis of a cylindrical waveguide filled with anisotropic double-negative metamaterials (DNMs) The reversed CR and CR conditions are obtained using analytical method The influence of the particle velocity, the waveguide radius, and the constitutive parameters of the anisotropic DNMs are discussed A numerical example illustrates that the total radiated energy increases with increasing particle velocity, the radiated energy spectral density has different poles at the different frequencies for different anisotropic DNMs when the loss of the anisotropic DNM is smaller, and when the radius has the same order as the operating wavelength, the influence of the waveguide radius on the total radiated energy is smaller on the whole Since most of the metamaterials realized so far are anisotropic, our theoretical work based on anisotropic DNMs will be helpful for future experimental realizations

Cylindrical Cloak with Axial Permittivity/Permeability Spatially Invariant

Abstract: In order to reduce the difficulties in the experimental realizations of the cloak but still keep good performance of invisibility, we proposed a perfect cylindrical invisibility cloak with spatially invariant axial material parameters. The advantage of this kind of TE (or TM) cloak is that only rho and phi components of mu (or epsilon) are spatially variant, which makes it possible to realize perfect invisibility with two-dimensional (2D) magnetic (or electric) metamaterials. The effects of perturbations of the parameters on the performance of this cloak are quantitatively analyzed by scattering theory. Our work provides a simple and feasible solution to the experimental realization of cloaks with ideal parameters.

Manipulating the directivity of antennas with metamaterial.

Abstract: In this paper we use spatially variant metamaterial substrate to manipulate the directivity of antennas. We show theoretically that by embedding a dipole at different locations inside this substrate, the emitted rays can be directed to different orientations as required. As a result, spatial multiplexing can be realized by carefully selecting proper parameters of this substrate. It can also be observed that the electric field received in this antenna system is enhanced when it is used for reception. Simulations based on finite element method are used to validate our theoretical analysis, showing a controllable high directive property. In order to simplify the physical realization process, we propose the reduced parameters for practical design and also study it with numerical simulations.

Analysis of the Surface Magnetoplasmon Modes in the Semiconductor Slit Waveguide at Terahertz Frequencies

Abstract: The propagation properties of surface plasmon polaritons (SPP) modes and surface magnetoplasmon polaritons (SMP) modes in a semiconductor slit waveguide are analyzed by the effective dielectric constant approach, and the interaction of the external magnetic field with the dispersion properties and field distributions of SMP modes in the Voigt configuration are emphasized in our analysis. Both the symmetric structure and the asymmetric structure are discussed in details. In contrast to the SPP modes which have one propagation band below the plasmon frequency only, the SMP modes have both the low-frequency propagation band below the plasmon frequency and the high-frequency propagation band above the plasmon frequency. When the external magnetic field increases, the two bands of the SMP modes will separate further in frequency, and the even symmetric distribution of the fundamental mode, which usually associates with the SPP mode, will be destroyed. These results can provide some guidance for the design of the tunable semiconductor waveguide in the terahertz regime.

Experimental Confirmation of Guidance Properties Using Planar Anisotropic Left-Handed Metamaterial Slabs Based on S-Ring Resonators

Abstract: We experimentally studied the guidance properties of the S-shaped metamaterial slabs. A peek of transmitted power due to the bulk guidance modes is observed in the negative band of the metamaterial,which is larger than a conventional dielectric waveguide made of FR4. The peek transmission frequency is shown related with the change of the negative band of the S-shaped metamaterial slab. Our results show good agreement with the theoretical predictions.

Interaction of an electromagnetic wave with a cone-shaped invisibility cloak and polarization rotator

Abstract: A cone-shaped cloak whose cross section gradually increases along the axial direction ($z$ direction) is proposed in this paper. We present full wave analysis of this cloak in response to electromagnetic waves, showing that a perfect conical cloak can support the propagation of any kind of fields. In addition, the reduced set of cloaking parameters is derived for azimuthally invariant $(\ensuremath{\partial}/\ensuremath{\partial}\ensuremath{\varphi}=0)$ incident fields. The advantage of this simplified cloaking structure is that all the components of material parameters are spatially invariant with relative magnitude larger than one. Hence, conical cloak with small scattering is physically realizable within a wide band of frequency for this specific type of incident fields. Finally, we apply similar transformation to achieve a polarization rotator which can arbitrarily control the polarization of the electromagnetic wave getting through. The proposed design provides a practical way to realize invisible cloak and some other electromagnetic devices, especially in the conditions that the source distribution is rotationally symmetric.

Rainbow and Blueshift Effect of a Dispersive Spherical Invisibility Cloak Impinged On by a Nonmonochromatic Plane Wave

Abstract: We demonstrate some interesting phenomena associated with a nonmonochromatic plane wave passing through a spherical invisibility cloak whose radial permittivity and permeability are of Drude and Lorentz types. We observe that the frequency center of a quasimonochromatic incident wave will suffer a blueshift in the forward scattering direction. Different frequency components have different depths of penetration, causing a rainbowlike effect within the cloak. The concept of group velocity at the inner boundary of the cloak needs to be revisited. Extremely low scattering can still be achieved within a narrow band.

Cloak of arbitrary shape

Abstract: We propose a segmentation design approach for a cloak of arbitrary shape. We suggest that polygonal 2D cloaks with any number of sides, which do not require geometrical symmetry, can be achieved by dividing the polygon into many triangular regions and applying a spatial transformation on each triangle. This idea is verified by full-wave finite-element-based simulations, showing electromagnetic waves smoothly guided around cloaked perfect-conducting cylinders with different shapes of cross sections. This segmentation-transformation method can be well extended to cloaking of arbitrary shapes, including 3D cases. As an example, we simulate a cylindrical cloak whose cross section is very close to the contour of a woman, which demonstrates the validity of our method and further confirms the power of transformation optics.

Free-standing THz electromagnetic metamaterials.

Abstract: Using micromanufactured S-shaped gold strings suspended in free space by means of window-frames, we experimentally demonstrate an electromagnetic meta-material (EM3) in which the metallic structures are no longer embedded in matrices or deposited on substrates such that the response is solely determined by the geometrical parameters and the properties of the metal. Two carefully aligned and assembled window-frames form a bi-layer chip that exhibits 2D left-handed pass-bands corresponding to two different magnetic resonant loops in the range of 1.4 to 2.2 THz as characterized by Fourier transform interferometry and numerical simulation. Chips have a comparably large useful area of 56 mm2. Our results are a step towards providing EM3 that fulfill the common notions of a material.

Reconfigurable cloak for multiple operating frequencies

Abstract: An invisible cloak based on active metamaterials whose permittivity can be controlled is proposed, and the performances of the cloak are verified in numerical simulations. Because of the tunability of the active metamaterials, the working frequency of the invisible cloak is reconfigurable, which is different from the traditional cloak designs based on passive metamaterials whose working frequencies are fixed.

Full-wave analysis of prolate spheroidal and hyperboloidal cloaks

Abstract: The electromagnetic behaviour of a prolate spheroidal cloak is investigated based on a full-wave scattering theory. We find that different physical behaviours related to surface displacement current will be induced at the inner boundary of the perfect spheroidal cloak, depending on the orientation of the cloak with respect to the direction of the incident wave. Such interesting phenomena can also be found in other cloaks, whose inner surface is transformed from a line in the initial coordinate system. A hyperbolidal cloak has also been proposed to further illustrate this point.

Enhancement of radiation properties of a compact planar antenna using transformation media as substrates

Abstract: In this paper we study the behavior of wave radiation of a horizontal electric dipole antenna with grounded metamaterial substrates formed by using coordinate transformation technology. From theoretical analysis and simulation results, we can find that such metamaterial substrates not only improve the directive emission but also enhance the radiation efficiency as the dipole antenna gets closer and closer to the metallic ground plane. We thus demonstrate that the transformation medium can offer a theoretical basis for designing a compact planar antenna with transformation media as substrates. † Also as a visiting scientist with Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

Limitations of high-order transformation and incident angle on simplified invisibility cloaks.

Abstract: We studied the scattering from the simplified cylindrical cloaks analytically at both normal and oblique incidences. We found that these simplified cylindrical cloaks may produce a larger scattering at nonnormal incidences than that from an object without any cloak, making this object more "visible". Even at normal incidence, the high-order transformation with impedance matched at the outer boundary can produce stronger scattering than the linear simplified one without matched impedance. This is due to the inefficiency of guided waves close to the inner boundary. Therefore, a square root transformation can improve scattering by guiding waves away from the inner boundary.

Tunable TE/TM Wave Splitter Using a Gyrotropic Slab

Abstract: A TE/TM wave splitter composed of a gyrotropic slab is proposed. We demonstrate theoretically that, when the working frequency is chosen to be within one of the two ranges, total reflection occurs at the boundary of a slab of gyrotropic medium for either TE or TM component of the incident waves. Tuning can be done by choosing the working frequency band or adjusting the applied magnetic field. Furthermore, within the TE-stop or TM-stop frequency region, if the incident angle is selected appropriately, the other polarized component of the wave is totally transmitted. And we also show that when the slab is thicker, there are more possibilities to satisfy the full-pass condition. Finite-element method simulations verified the theoretical results.

Experimental characterization and cell interactions of a two-dimensional isotropic left-handed metamaterial

Abstract: We experimentally realized a two-dimensional (2D) isotropic left-handed material based on a cross-embedded S-ring resonator. Experimental results show that the 2D metamaterial with cross embedded arrangement exhibits much better performance than that with conventional arrangement. Theoretical interpretations are proposed, showing a reduced coupling between the rings that are located in the two orthogonal directions.

Time domain simulation of electromagnetic cloaking structures with TLM method.

Abstract: The increasing interest in invisible cloaks has been prompted in part by the availability of powerful computational resources which permit numerical studies of such a phenomenon. These are usually carried out with commercial software. We report here a full time domain simulation of cloaking structures with the Transmission Line Modeling (TLM) method. We first develop a new condensed TLM node to model metamaterials in two dimensional situations; various results are then presented, with special emphasis on what is not easily achievable using commercial software.

Effects of Different Transformations on the Performance of Cylindrical Cloaks

Abstract: The cylindrical cloak created with nonlinear transformation is studied based on the analytical scattering theory. The equivalent surface displacement current at the inner boundary of an ideal cloak is shown to be independent of the transformation function. For the nonideal cloak with perturbation at the inner boundary, we show that the performance of the cloak with low loss can be greatly improved by using certain nonlinear transformations.

Influence of External Magnetic Field on a Symmetrical Gyrotropic Slab in Terms of Goos-hänchen Shifts

Abstract: A detailed study on the influence of an external magnetic field on a symmetrical gyrotropic slab in terms of Goos-Hanchen (GH) phase shifts is presented. The GH phase shifts at both boundaries of the slab are calculated, and the guidance condition is explained by means of them. It is found that the external magnetic field destroys the spatial symmetry of the field distribution, and we use the concepts of ‘penetration’ distance as well as effective thickness to illustrate the phenomenon. In term of the GH phase shifts, the spatial distribution of the time-average Poynting power is also derived. We find that influenced by the external magnetic field, the positive and negative time-average Poynting power along the waveguide direction can exist simultaneously in the gyrotropic medium, depending on the transverse position. † Also with Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA02139, USA

Efficiency Improvement of Broadband Helix Traveling Wave Tubes Using Hybrid Phase Velocity Tapering Model

Abstract: A hybrid phase tapering model is proposed for a wideband (two-octave) vane-loaded helix traveling wave tube, of which the dispersion characteristic is anomalous over the operating frequency band. The small signal gain and electron efficiency is studied by using a novel nonlinear one-dimensional beam-wave interaction theory. Compared with a conventional helix traveling wave tube, of which the helix pitch is held constant in the output section, this hybrid phase velocity tapering model, which can be realized by changing the helix pitch, can provide a wideband efficiency improvement, especially in the lower or higher frequency band. It is concluded from our results that in order to effectively improve the electron efficiency, dispersion shaping and helix pitch tapering should be taken into account simultaneously.

Lateral Displacement of an Electromagnetic Beam Reflected from a Grounded Indefinite Uniaxial Slab

Abstract: A theoretical analysis of the lateral shift for an electromagnetic beam reflected from an uniaxial anisotropic slab coated with perfect conductor is presented. The analytic expression for the lateral shift is derived by using the stationary-phase approach, and the conditions for negative and positive lateral shifts are discussed. It is shown that the lateral shift depends not only on the slabthickness and the incident angle, but also on the constitutive parameters of the uniaxial medium. Enhancement and suppression of lateral shift are observed and are attributed to the interference between the reflected

Cloak Changing with Background

Abstract: Coordinate transformation approach has previously been used for designing cloak which can shield an interior region in a homogeneous medium In this letter, we consider the case where the background is no longer a homogeneous medium and determine the relative constitutive parameters of the cloak according to the multilayered background We propose the parameters of cylindrical cloak structures working in multilayered media and the scheme of specifying these parameters could also be applied to the design of cloak in arbitrary isotropic background The theoretical analysis based on coordinate transformation is given and numerical simulations are performed to illustrate these properties The simulation results show that the cloaking with the proposed parameters performs well in these inhomogeneous background medium Potential applications are also discussed

Imaging of Objects through Lossy Layer with Defects

Abstract: The imaging method when a lossy layer (e. g., a defected metallic slab or a plasma layer) is present between the target and the sensor is demonstrated using the concept of active left-handed material (LHM). The effect of the lossy layer to the reflection coefficients measured by the receiver can be cancelled by imaginatively adding an active LHM layer, which has a same thickness as the lossy layer but an opposite sign of the constitutive parameters. Therefore, the updated reflection coefficients obtained after this data process look like the lossy layer has been removed, which leads to a significant improvement of the target imaging. When the lossy layer is inhomogeneous due to the existence of small defects, we use a homogenization procedure based on the Drude model to characterize its effective constitutive parameters. Our simulation examples shows the effectiveness of the proposed method.

A new strategy to conceal an object from electromagnetic wave

Abstract: A new recipe for concealing objects from detection is suggested. Different with traditional cloak which deflects light around the core of the cloak to make the object inside invisible, our cloak guides the light to penetrate the core of the cloak but without striking some region of the cloak shell - the so called folded region. Full wave analytical calculation shows that this cloak will lead to a scattering enhancement instead of scattering reduction in contrast to the traditional cloak; the scattered field distribution can also be changed as if the scatterer is moved from its original position. Such interesting phenomenon indicates the proposed cloak can be used to disguise the true information of the object, e.g. the position, the size, etc, and further mislead the observer and avoid being detected.

Theoretical investigation into Cherenkov radiation in an anisotropic double-negative medium

Abstract: Cherenkov radiation for particles traveling in an unbounded anisotropic double-negative medium is theoretically investigated to obtain the analytical expressions for the Cherenkov radiation condition, Cherenkov radiation angle and inversed Cherenkov radiation. The total radiated energy (spectral density) per unit length of path is derived by using rigorous field theory. As a specific example, the effects of loss of the anisotropic double-negative medium and the effective plasma frequency on the total radiated energy per unit length of path are also discussed. It turns out that the wave vector and time-averaged Poynting vector in an anisotropic double-negative medium are, in generally, not exactly anti-parallel, and the Cherenkov radiation angle and the total radiated energy in this case is different from that in the isotropic double-negative medium.