MNM – Cloaking
[As In Romulan/Klingon]
CLOAKING – Visualising invisibility – [PDF]
ABSTRACT – As a consequence of the wave nature of light, invisibility devices based on isotropic media cannot be perfect. The principal distortions of invisibility are due to reflections and time delays. Reflections can be made exponentially small for devices that are large in comparison with the wavelength of light. Time delays are unavoidable and will result in wave-front dislocations. This paper considers invisibility devices based on optical conformal mapping. The paper shows that the time delays do not depend on the directions and impact parameters of incident light rays, although the refractive-index profile of any conformal invisibility device is necessarily asymmetric. The distortions of images are thus uniform, which reduces the risk of detection. The paper also shows how the ideas of invisibility devices are connected to the transmutation of force, the stereographic projection and Escheresque tilings of the plane.
On Perfect Cloaking -[PDF]
ABSTRACT – We show in principle how to cloak a region of space to make its contents classically invisible or transparent to waves. The method uses sensors and active sources near the surface of the region, and could operate over broad bandwidths. A general expression is given for calculating the necessary sources, and explicit, fully causal simulations are shown for scalar waves. Vulnerability to broad-band probing is discussed, and any active scheme should detectable by a quantum probe, regardless of bandwidth.
Full-wave invisibility of active devices at all frequencies – [PDF]
ABSTRACT[REF] – There has recently been considerable interest in the possibility, both theoretical and practical, of invisibility (or “cloaking”) from observation by electromagnetic [EM] waves. Here, we prove invisibility, with respect to solutions of the Helmholtz and Maxwell’s equations, for several constructions of cloaking devices. Previous results have either been on the level of ray tracing [Le,PSS] or at zero frequency [GLU2,GLU3], but recent numerical [CPSSP] and experimental [SMJCPSS] work has provided evidence for invisibility at frequency ‘k\ne 0’. We give two basic constructions for cloaking a region ‘D’ contained in a domain ‘\Omega’ from measurements of Cauchy data of waves at ‘\p \Omega’; we pay particular attention to cloaking not just a passive object, but an active device within ‘D’, interpreted as a collection of sources and sinks or an internal current.
Engineers create ‘optical cloaking’ design for invisibility
ABSTRACT – Artificially structured metamaterials have enabled unprecedented flexibility in manipulating electromagnetic waves and producing new functionalities, including the cloak of invisibility based on coordinate transformation. Unlike other cloaking approaches, which are typically limited to subwavelength objects, the transformation method allows the design of cloaking devices to render a macroscopic object invisible. In addition, the design is not sensitive to the object that is being cloaked. The first experimental demonstration of such a cloak at microwave frequencies was recently reported. We note, however, that that design cannot be implemented for an optical cloak, which is certainly of particular interest because optical frequencies are where the word ?invisibility’ is conventionally defined. Here we present the design of a non-magnetic cloak operating at optical frequencies. The principle and structure of the proposed cylindrical cloak are analyzed, and the general recipe for the implementation of such a device is provided.
CAPTION – These two images (Cloak off, top. Cloak on, bottom) were taken from corresponding videos depicting scientific simulations performed at Purdue to show how objects might be “cloaked” to render them invisible. The new findings demonstrate how to cloak objects for any single wavelength, not for the entire frequency range of the visible spectrum. But the research represent a step toward creating an optical cloaking device that might work one day for all wavelengths of visible light. The videos show how light interacts with an uncloaked and cloaked object. When uncloaked, as depicted in the first image, light waves strike the object and bounce backward. As depicted in the second image, a cloaking device designed using nanotechnology guides light around anything placed inside this cloak.