J. B. Pendry

Magnetism from conductors and enhanced nonlinear phenomena

We show that microstructures built from nonmagnetic conducting sheets exhibit an effective magnetic permeability /spl mu//sub eff/, which can be tuned to values not accessible in naturally occurring materials, including large imaginary components of /spl mu//sub eff/. The microstructure is on a scale much less than the wavelength of radiation, is not resolved by incident microwaves, and uses a very low density of metal so that structures can be extremely lightweight. Most of the structures are resonant due to internal capacitance and inductance, and resonant enhancement combined with compression of electrical energy into a very small volume greatly enhances the energy density at critical locations in the structure, easily by factors of a million and possibly by much more. Weakly nonlinear materials placed at these critical locations will show greatly enhanced effects raising the possibility of manufacturing active structures whose properties can be switched at will between many states.

Controlling electromagnetic fields

Using the freedom of design that metamaterials provide, we show how electromagnetic fields can be redirected at will and propose a design strategy. The conserved fields—electric displacement field D, magnetic induction field B, and Poynting vector B—are all displaced in a consistent manner. A simple illustration is given of the cloaking of a proscribed volume of space to exclude completely all electromagnetic fields. Our work has relevance to exotic lens design and to the cloaking of objects from electromagnetic fields.

TRANSMISSION RESONANCES ON METALLIC GRATINGS WITH VERY NARROW SLITS

For decades, it has been thought that subwavelength apertures have a very low transmission efficiency of light [1]. However, very recently, several experiments have shown that, if holes are structured forming a 2D periodic array in a metallic film, extraordinary optical transmission can be obtained at wavelengths up to 10 times larger than the diameter of the holes [2]. It has already been proposed that this effect could be exploited in different important technological areas such as photolithography or near field microscopy, or even to extract light from light emitting diodes [3]. Although experiments suggest that the excitation of surface plasmon polaritons (SPPs) in the metallic interfaces of the film plays a crucial role in this effect, a detailed understanding of the physical mechanism behind the enhanced transmission has not been reported yet. In this Letter we propose a simpler alternative structure in which similar extraordinary optical transmission effects can also been found and hence used for practical purposes: transmission metallic gratings with very narrow slits. We will show how, for particular wavelengths, incident light can excite surface electromagnetic modes of the gratings that are able to reemit the absorbed light in the forward direction with almost 100% efficiency. Moreover, a detailed study of these transmission resonances will provide physical insight into the mechanism of the extraordinary transmission in 2D hole arrays. Reflection metallic gratings have been analyzed for many years, mainly in connection with the study of SPPs andor localized electromagnetic modes of the grooves [4 ‐ 9]. With regard to transmission gratings, there have been some theoretical works in the past few years [10,11]. However, transmission gratings with very narrow slits remain unstudied, except for a very recent calculation of the optical transmission properties of a silver grating having the same geometrical parameters of the 2D hole arrays of Ref. [2] and with a special geometry for the slits [12]. The scope of this Letter is, however, different: we do not pretend to fit the experiments on 2D hole arrays because this is a different geometry. Instead, we are interested in analyzing the coupling between the SPPs of the two metallic interfaces of the grating as a possible mechanism to enhance optical transmission through perforated metallic films. We also study, for the first time, the transmission properties of waveguide modes excited in very narrow slits that are periodically structured. On top of Fig. 1 we show a schematic view of the structures under study with the definition of the different parameters: the period of the grating d, the width a, and height h of the slits. The substrate is characterized by a dielectric constant, e. Advances in material technology have allowed the production of transmission gratings with well-controlled profiles [13]. In this Letter we consider metal gratings made of gold and we use a fixed value for the grating period d 3.5 mm. We will only show the results for a 0.5 mm, although the dependence of the transmission resonances on a is also addressed. The thickness of the metallic grating h will be varied between 0 and 4 mm. We believe this range of geometrical values can be reached using present day technology, as reflection gratings with similar parameters have already been prepared [9]. Nevertheless, it should be pointed out that the effects discussed in this Letter do appear for any other range provided a is very small in comparison to d and the frequency of the incident light is well below the plasma frequency of the metal. The dielectric function of gold is described using the tables reported in Ref. [14]. We have analyzed the electromagnetic properties of these gratings by means of a transfer matrix formalism [15]. Within this formalism it is possible to calculate transmission and reflection coefficients for an incoming plane wave. Subsequently, the transmittance and reflectance of the grating as well as real-space electromagnetic fields can be calculated. Figure 1 shows zero-order transmittance for p-polarized normal incident radiation on metallic gratings in vacuum as a function of the wavelength of the incoming

Low frequency plasmons in thin-wire structures

A photonic structure consisting of an extended 3D network of thin wires is shown to behave like a low density plasma of very heavy charged particles with a plasma frequency in the GHz range. We show that the analogy with metallic behaviour in the visible is rather complete, and the picture is confirmed by three independent investigations: analytic theory, computer simulation and experiments on a model structure. The fact that the wires are thin is crucial to the validity of the picture. This new composite dielectric, which has the property of negative below the plasma frequency, opens new possibilities for GHz devices.

Hiding under the Carpet: A New Strategy for Cloaking

A new type of cloak is discussed: one that gives all cloaked objects the appearance of a flat conducting sheet. It has the advantage that none of the parameters of the cloak is singular and can in fact be made isotropic. It makes broadband cloaking in the optical frequencies one step closer.

Low-Energy Electron Diffraction

The essential elements are an ultrahigh vacuum (UHV) chamber to preserve surface cleanliness, an electron gun to produce a collimated beam of electrons in the energy range 0 to 500 eV, a crystal holder and manipulator, and some means of observing the diffracted electrons, typically a fluorescent screen. Further details may be found elsewhere.(1–3) The major difficulty is common to all surface experiments, namely, to keep the surface clean. The UHV chamber will normally contain an array of techniques for cleaning the surface (provision for heating the sample, ion bombardment) as well as some means of detecting impurities at the surface, usually by detection of Auger signals from adsorbed atoms. LEED is very sensitive to cleanliness of the surface and small amounts of contaminant can produce quite spurious results. Experiments conducted on clean, perfect, surfaces can produce a large amount of structural information of high precision. Obviously it is only possible to produce precise data for surfaces which are well defined in the first place.

Three-Dimensional Invisibility Cloak at Optical Wavelengths

Hidden Under the Carpet Transformation optics, combined with the ability to fabricate structures with complex refractive index profiles, allow materials to be formed with fascinating optical properties, such as cloaks where both the object and the cloak concealing the object are rendered invisible. To date, the cloaks have been restricted to two dimensions, which limits realistic applications. Based on a photonic crystal structure with a polymer filling the empty space, Ergin et al. (p. 337, published online 18 March) have designed, fabricated, and realized a three-dimensional cloak operating at optical wavelengths. A structured photonic crystal can be used to cloak an object at optical wavelengths and over a wide viewing angle. We have designed and realized a three-dimensional invisibility-cloaking structure operating at optical wavelengths based on transformation optics. Our blueprint uses a woodpile photonic crystal with a tailored polymer filling fraction to hide a bump in a gold reflector. We fabricated structures and controls by direct laser writing and characterized them by simultaneous high–numerical-aperture, far-field optical microscopy and spectroscopy. A cloaking operation with a large bandwidth of unpolarized light from 1.4 to 2.7 micrometers in wavelength is demonstrated for viewing angles up to 60°.

Full-wave simulations of electromagnetic cloaking structures

Pendry et al. have reported electromagnetically anisotropic and inhomogeneous shells that, in theory, completely shield an interior structure of arbitrary size from electromagnetic fields without perturbing the external fields. Neither the coordinate transformation-based analytical formulation nor the supporting ray-tracing simulation indicate how material perturbations and full-wave effects might affect the solution. We report fully electromagnetic simulations of the cylindrical version of this cloaking structure using ideal and nonideal (but physically realizable) electromagnetic parameters that show that the low-reflection and power-flow bending properties of the electromagnetic cloaking structure are not especially sensitive to modest permittivity and permeability variations. The cloaking performance degrades smoothly with increasing loss, and effective low-reflection shielding can be achieved with a cylindrical shell composed of an eight- (homogeneous) layer approximation of the ideal continuous medium. An imperfect but simpler version of the cloaking material is derived and is shown to reproduce the ray bending of the ideal material in a manner that may be easier to experimentally realize.

Surfaces with holes in them: new plasmonic metamaterials

In this paper we explore the existence of surface electromagnetic modes in corrugated surfaces of perfect conductors. We analyse two cases: one-dimensional arrays of grooves and two-dimensional arrays of holes. In both cases we find that these structures support surface bound states and that the dispersions of these modes have strong similarities with the dispersion of the surface plasmon polariton bands of real metals. Importantly, the dispersion relation of these surface states is mainly dictated by the geometry of the grooves or holes and these results open the possibility of tailoring the properties of these modes by just tuning the geometrical parameters of the surface.

Calculation of material properties and ray tracing in transformation media

Complex and interesting electromagnetic behavior can be found in spaces with non-flat topology. When considering the properties of an electromagnetic medium under an arbitrary coordinate transformation an alternative interpretation presents itself. The transformed material property tensors may be interpreted as a different set of material properties in a flat, Cartesian space. We describe the calculation of these material properties for coordinate transformations that describe spaces with spherical or cylindrical holes in them. The resulting material properties can then implement invisibility cloaks in flat space. We also describe a method for performing geometric ray tracing in these materials which are both inhomogeneous and anisotropic in their electric permittivity and magnetic permeability.