Broadband polygonal invisibility cloak for visible light

Abstract

Invisibility cloaks have recently become a topic of considerable interest thanks to the theoretical works of transformation optics and conformal mapping. The design of the cloak involves extreme values of material properties and spatially dependent parameter tensors, which are very difficult to implement. The realization of an isolated invisibility cloak in the visible light, which is an important step towards achieving a fully movable invisibility cloak, has remained elusive. Here, we report the design and experimental demonstration of an isolated polygonal cloak for visible light. The cloak is made of several elements, whose electromagnetic parameters are designed by a linear homogeneous transformation method. Theoretical analysis shows the proposed cloak can be rendered invisible to the rays incident from all the directions. Using natural anisotropic materials, a simplified hexagonal cloak which works for six incident directions is fabricated for experimental demonstration. The performance is validated in a broadband visible spectrum.

Photorealistic ray tracing of free-space invisibility cloaks made of uniaxial dielectrics

“…In particular, the performance goes well beyond that of recent impressive experiments [6] in that it leads to the correct time-of-flight ("wave cloak"), at least in the center of the image, whereas the piece-wise homogeneous calcite polygonal structure in Ref. 6 is expected to exhibit huge time-of-flight differences compared to empty space and is hence only a "ray cloak".…”

“…Moreover, in contrast to Ref. 6, our cloak and the resulting performance are rotationally invariant, i.e., the performance does not depend on the viewing direction. The price to pay in our case is that two components of the local permittivity tensor assume positive values below one, i.e., we need superluminal light propagation.…”

“…Recent experiments [6] on macroscopic broadband visible-frequency free-space invisibility cloaks made of standard uniaxial calcite have used purely dielectric anisotropic materials (calcite), i.e., the magnetic permeability is set to unity everywhere. This means that the response becomes polarization dependent and not impedance-matched.…”

A THIRD WAY TO CLOAK AN OBJECT: COVER-UP WITH A BACKGROUND OBJECT (Invited Paper)

“…Invisibility, which has been a long-time goal, has received many studies with transformation optics (TO), meta-materials and other methods in recent years [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Readers can get some background on this topic with some recent reviews (see e.g., [17,18]).…”

“…Many researchers try to make such OICs easier to construct (e.g., triangle transformations to eliminate the inhomogeneity of the OIC [14], Eikonal approximation to reduce the inhomogeneity [3]). Many methods have been proposed to remove the singularity at the inner surface of the point-expanded OIC, including sacrificing the performance of the cloak (e.g., extend an extremely small volume but not a point to a finite volume [15]), abolishing the requirement for phase preservation [16], or introducing Non-Euclidean space in the reference space (see e.g., [4] and references therein). However there is always an essential problem in an OIC: the detecting wave doesn't touch the hidden object (it is simply guided around it), which leads to the fact that the cloaked object cannot communicate with the outside world in the same frequency band.…”