Invisibility Cloak for Near-Infrared Light
Xiang Zhang [Photo courtesy: Roy Kaltschmidt/ Lawrence Berkeley National Laboratory]
In a paper published in 'Nature Materials' , a team led by Xiang Zhang, a principal investigator with Lawrence Berkeley National Laboratory’s Materials Sciences Division and director of UC Berkeley’s Nano-scale Science and Engineering Center, reported the creation of a “carpet cloak” from nanostructured silicon that conceals the presence of objects placed under it from optical detection in near-infrared region at a wavelength range of 1,400–1,800 nm.
Previous demonstrations of cloaking, where objects are rendered invisible at certain frequencies, have been limited to the microwave regime. This new development takes a significant step closer to achieving invisibility in a region that can be seen (no ... not 'seen' ...must be changed to 'experienced'!) by human eye.
In recent years, several theories for 'invisibility' schemes have been proposed for cloaking devices using transformation optics and conformal mapping. The necessary medium for enabling precise control over the flow of electromagnetic waves was provided by Metamaterial which gains its properties from its spatially tailored structure rather than directly from its composition. The first microwave cloaking with the use of metamaterials was demonstrated in 2006 by Duke University's Pratt School of Engineering  but the realization of cloaking at optical frequencies, a key step towards achieving actual invisibility, has eluded scientists mainly because the metal elements absorb too much light.
Previous work by Zhang and his group with invisibility devices involved complex metamaterials - composites of metals and dielectrics whose extraordinary optical properties arise from their unique structure rather than their composition. They constructed one material out of an elaborate fishnet of alternating layers of silver and magnesium fluoride, and another out of silver nanowires grown inside porous aluminum oxide. With these metallic metamaterials, Zhang and his group demonstrated that light can be bent backwards, a property unprecedented in nature.
“We have come up with a new solution to the problem of invisibility based on the use of dielectric (nonconducting) materials,” says Zhang. “Our optical cloak not only suggests that true invisibility materials are within reach, it also represents a major step towards transformation optics, opening the door to manipulating light at will for the creation of powerful new microscopes and faster computers.”
[Image courtesy: Thomas Zentgraf] These three images depict how light striking an object covered with the carpet cloak acts as if there were no object being concealed on the flat surface. In essence, the object has become invisible.
The optical 'carpet' cloak designed by the Berkeley team used quasi-conformal mapping to conceal an object that is placed under a curved reflecting surface by imitating the reflection of a flat surface. The cloak consists only of isotropic dielectric materials, which enables broadband and low-loss invisibility at a wavelength range of 1,400–1,800 nm. While the carpet itself can still be seen, the bulge of the object underneath it disappears from view. Shining a beam of light on the bulge shows a reflection identical to that of a beam reflected from a flat surface, meaning the object itself has essentially been rendered invisible.
[Video by Jensen Li] This video shows how a beam of light is obstructed by an object in a flat surface and casts a shadow until the object is cloaked, at which point the light is reflected as if the surface were still perfectly flat.
The new cloak created by Zhang and his team is made exclusively from dielectric materials, which are often transparent at optical frequencies. The cloak was demonstrated in a rectangular slab of silicon (250 nanometers thick) that serves as an optical waveguide in which light is confined in the vertical dimension but free to propagate in the other two dimensions. A carefully designed pattern of holes - each 110 nanometers in diameter - perforates the silicon, transforming the slab into a metamaterial that forces light to bend like water flowing around a rock. In the experiments reported in Nature Materials, the cloak was used to cover an area that measured about 3.8 microns by 400 nanometers. It demonstrated invisibility at variable angles of light incident.
[Image courtesy: Xiang Zhang] Image (a) is a schematic diagram showing the cloaked region (marked with green) which resides below the reflecting bump (carpet) and can conceal any arbitrary object by transforming the shape of the bump back into a virtually flat object. Image (b) was taken with a scanning electron microscope image of the carpet coated bump.
Right now the cloak operates for light between 1,400 and 1,800 nanometers in wavelength, which is the near-infrared portion of the electromagnetic spectrum, just slightly longer than light that can be seen with the human eye. However, because of its all dielectric composition and design, Zhang says the cloak is relatively easy to fabricate and should be upwardly scalable. He is also optimistic that with more precise fabrication this all dielectric approach to cloaking should yield a material that operates for visible light - in other words, true invisibility to the naked eye.
“Even with the advances that have been made in optical metamaterials, scaling sub-wavelength metallic elements and placing them in an arbitrarily designed spatial manner remains a challenge at optical frequencies", says Zhang,"In this experiment, we have demonstrated a proof of concept for optical cloaking that works well in two dimensions. Our next goal is to realize a cloak for all three dimensions, extending the transformation optics into potential applications.”
 "An optical cloak made of dielectrics"
Jason Valentine, Jensen Li, Thomas Zentgraf, Guy Bartal & Xiang Zhang,
Nature Materials, Published online: 29 April 2009 doi:10.1038/nmat2461. Abstract
 "Metamaterial Electromagnetic Cloak at Microwave Frequencies"
D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr, D. R. Smith,
Science, Vol. 314, pp. 977 - 980 (2006). Abstract
[We thank Lawrence Berkeley National Laboratory media relations for materials used in this posting]