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.

/pdf/controlling-electromagnetic-fields-3ryxkryf1t.pdf

Controlling Electromagnetic Fields

A recently published theory has suggested that a cloak of invisibility is in principle possible, at least over a narrow frequency band. We describe here the first practical realization of such a cloak; in our demonstration, a copper cylinder was "hidden" inside a cloak constructed according to the previous theoretical prescription. The cloak was constructed with the use of artificially structured metamaterials, designed for operation over a band of microwave frequencies. The cloak decreased scattering from the hidden object while at the same time reducing its shadow, so that the cloak and object combined began to resemble empty space.

/pdf/metamaterial-electromagnetic-cloak-at-microwave-frequencies-1qj4zdgl8e.pdf

Metamaterial Electromagnetic Cloak at Microwave Frequencies

Conventional optical components rely on gradual phase shifts accumulated during light propagation to shape light beams. New degrees of freedom are attained by introducing abrupt phase changes over the scale of the wavelength. A two-dimensional array of optical resonators with spatially varying phase response and subwavelength separation can imprint such phase discontinuities on propagating light as it traverses the interface between two media. Anomalous reflection and refraction phenomena are observed in this regime in optically thin arrays of metallic antennas on silicon with a linear phase variation along the interface, which are in excellent agreement with generalized laws derived from Fermat’s principle. Phase discontinuities provide great flexibility in the design of light beams, as illustrated by the generation of optical vortices through use of planar designer metallic interfaces.

http://www.zenogaburro.it/papers/LightPropagation.pdf

Light Propagation with Phase Discontinuities: Generalized Laws of Reflection and Refraction

We present the design for an absorbing metamaterial (MM) with near unity absorbance A(omega). Our structure consists of two MM resonators that couple separately to electric and magnetic fields so as to absorb all incident radiation within a single unit cell layer. We fabricate, characterize, and analyze a MM absorber with a slightly lower predicted A(omega) of 96%. Unlike conventional absorbers, our MM consists solely of metallic elements. The substrate can therefore be optimized for other parameters of interest. We experimentally demonstrate a peak A(omega) greater than 88% at 11.5 GHz.

Perfect metamaterial absorber.

Metamaterials are artificially fabricated materials that allow for the control of light and acoustic waves in a manner that is not possible in nature. This Review covers the recent developments in the study of so-called metasurfaces, which offer the possibility of controlling light with ultrathin, planar optical components. Conventional optical components such as lenses, waveplates and holograms rely on light propagation over distances much larger than the wavelength to shape wavefronts. In this way substantial changes of the amplitude, phase or polarization of light waves are gradually accumulated along the optical path. This Review focuses on recent developments on flat, ultrathin optical components dubbed 'metasurfaces' that produce abrupt changes over the scale of the free-space wavelength in the phase, amplitude and/or polarization of a light beam. Metasurfaces are generally created by assembling arrays of miniature, anisotropic light scatterers (that is, resonators such as optical antennas). The spacing between antennas and their dimensions are much smaller than the wavelength. As a result the metasurfaces, on account of Huygens principle, are able to mould optical wavefronts into arbitrary shapes with subwavelength resolution by introducing spatial variations in the optical response of the light scatterers. Such gradient metasurfaces go beyond the well-established technology of frequency selective surfaces made of periodic structures and are extending to new spectral regions the functionalities of conventional microwave and millimetre-wave transmit-arrays and reflect-arrays. Metasurfaces can also be created by using ultrathin films of materials with large optical losses. By using the controllable abrupt phase shifts associated with reflection or transmission of light waves at the interface between lossy materials, such metasurfaces operate like optically thin cavities that strongly modify the light spectrum. Technology opportunities in various spectral regions and their potential advantages in replacing existing optical components are discussed.

Flat Optics With Designer Metasurfaces

We propose an analytical approach for far-field calculation of antenna arrays comprised of arbitrarily oriented, identical elements using a straightforward rotation matrix calculation method. In addition, here we present a Constructive Analytical Phasing (CAP) method for arbitrary configurations of linearly polarized array elements. Conformal spherical and randomly oriented planar arrays of microstrip patch antennas are given as examples, wherein analytical results are verified by full-wave simulations. This approach provides substantial speed up compared to full-wave simulations, facilitating significantly broader tradeoff studies and optimizations, for a variety of antenna performance parameters.

Analytical Phasing of Arbitrarily Oriented Arrays Using a Fast, Analytical Far-Field Calculation Method

Rolled resonant element

We present two techniques for optimizing the position of transmitter and receiver modules on a sparse aperture for a millimeter-wave computational imaging system. The first technique uses an easily computable spatial representation of the transmitter and receiver array, called the coarray, to ideally distribute the spatial frequency components probed by the imaging setup. The second approach involves maximizing the information added by a complete measurement of the scene by the system. This approach is analogous to the system capacity maximization frequently employed in wireless communication. We show that employing aperture configurations optimized using these two techniques over commonly used standard aperture configurations results in 30% less mean-squared error when used to reconstruct a particular ensemble of 400 arbitrary 2-D objects. Finally, we discuss the similarities and differences between the two optimization strategies in terms of imaging performance and computational speed, including a case when one strategy performs better than the other.

Optimization of a Sparse Aperture Configuration for Millimeter-Wave Computational Imaging

Exemplary methods, systems and components enable an enhanced direct-viewing optical device to make customized adjustments that accommodate various optical aberrations of a current user. In some instances a real-time adjustment of the transformable optical elements is based on known corrective optical parameters associated with a current user. In some implementations a control module may process currently updated wavefront measurements as a basis for determining appropriate real-time adjustment of the transformable optical elements to produce a specified change in optical wavefront at an exit pupil of the direct-viewing device. Possible transformable optical elements may have refractive and/or reflective and/or diffractive and/or transmissive characteristics that are adjusted based on current performance viewing factors for a given field of view of the direct-viewing device. Some embodiments enable dynamic repositioning and/or transformation of corrective optical elements based on a detected shift of a tracked gaze direction of a current user of the direct-viewing device.

Adjustable optics for ongoing viewing correction

A method is described for applying FDTD simulation to electromagnetic metamaterial slabs with off-normal incident plane wave radiation. Issues of incommensurate spatial periods and interpolation of irregularly spaced data are discussed. Validity of the method is tested on a dielectric slab with known analytical solutions and the method is used to analyse split ring resonators of current interest. Copyright © 2006 John Wiley & Sons, Ltd.

David Schurig

Papers

Analytical Phasing of Arbitrarily Oriented Arrays Using a Fast, Analytical Far-Field Calculation Method

Rolled resonant element

Optimization of a Sparse Aperture Configuration for Millimeter-Wave Computational Imaging

Adjustable optics for ongoing viewing correction

Off‐normal incidence simulations of metamaterials using FDTD