Science

Metamaterials breakthrough could lead to the first wide-spectrum optical invisibility cloak

Metamaterials breakthrough cou...
A Stanford breakthrough in optical metamaterials could enable fabrication of a wide-spectrum invisibility cloak (Image: Shutterstock)
A Stanford breakthrough in optical metamaterials could enable fabrication of a wide-spectrum invisibility cloak (Image: Shutterstock)
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Exotic optical properties seen in the left glass, filled with negative refractive index water (Photo: Stanford University)
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Exotic optical properties seen in the left glass, filled with negative refractive index water (Photo: Stanford University)
Electric field response of the conformally mapped nanocrescents used in the Stanford metamaterial (Photo: Stanford University)
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Electric field response of the conformally mapped nanocrescents used in the Stanford metamaterial (Photo: Stanford University)
The Stanford metamaterial shows negative refractive index over a broad band of optical wavelengths (Photo: Stanford University)
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The Stanford metamaterial shows negative refractive index over a broad band of optical wavelengths (Photo: Stanford University)
A Stanford breakthrough in optical metamaterials could enable fabrication of a wide-spectrum invisibility cloak (Image: Shutterstock)
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A Stanford breakthrough in optical metamaterials could enable fabrication of a wide-spectrum invisibility cloak (Image: Shutterstock)
A prism made of the Stanford metamaterial shows negative refractive index by refracting a beam of light backwards compared to how a normal prism would (blue line) (Photo: Stanford University)
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A prism made of the Stanford metamaterial shows negative refractive index by refracting a beam of light backwards compared to how a normal prism would (blue line) (Photo: Stanford University)
An invisibility cloak works by bending light around a central region. As no light enters the central region, no one outside the cloak can see the cloaked object (Image: Brian Dodson)
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An invisibility cloak works by bending light around a central region. As no light enters the central region, no one outside the cloak can see the cloaked object (Image: Brian Dodson)
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An invisibility cloak works by bending light around a central region (Image: Brian Dodson)
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An invisibility cloak works by bending light around a central region (Image: Brian Dodson)

To make a Harry Potter-style invisibility cloak requires the use of materials that have what's known as a negative refractive index over all optical wavelengths, from red to violet. However, the artificially-structured optical materials from which cloaks are made thus far have been restricted to a very narrow range of optical wavelengths, limiting their ability to cloak over a range of colors. That obstacle to progress looks to be at an end, as a group of optical engineers at Stanford has succeeded in designing a broadband metamaterial that exhibits a negative refractive index over nearly the entire rainbow.

The first invisibility cloaks, made at Duke University, worked by bending light around an object to be cloaked, as illustrated below. However, it is not quite this simple. The light exiting the cloak must also match the polarization and the phase of the light that travels past the cloak, or it will show a visible presence. You wouldn't know what was being cloaked, but you could tell there was a cloak.

An invisibility cloak works by bending light around a central region (Image: Brian Dodson)
An invisibility cloak works by bending light around a central region (Image: Brian Dodson)

To make an effective cloak over all optical wavelengths requires a remarkable level of control over the optical properties of the materials which make up the cloak. This control is supplied by optical metamaterials, which are (usually) periodic nanostructured materials, where the periodic cells are in essence tiny electromagnetic circuits that interact with both the electric and magnetic fields of light.

This is a trick that very few natural materials can accomplish. While previous attempts to create metamaterials have involved the creation of artificial "atoms" that are composed of one constituent that interacts with the electric field, and one that interacts with the magnetic field, the individual constituents interact with different colors of light, and it is typically difficult to make them overlap over a broad range of wavelengths.

As a result, their bandwidth, or the range of wavelengths over which they function, is typically quite limited. A cloak that only works for a particular color of yellow light would not be terribly useful, unless all you want to cloak is bananas.

Stanford Assistant Professor of Materials Science and Engineering Jennifer Dionne and her co-workers have designed a new type of metamaterial with a unified structure that allows it to efficiently interact with both the electric and magnetic fields of light over a broad range of colors. It was created using a design technique called conformal transformation, which involved "folding" a two-dimensional metamaterial with known properties into a three-dimensional nanoscale object shaped like a crescent moon that preserves those original optical properties.

Electric field response of the conformally mapped nanocrescents used in the Stanford metamaterial (Photo: Stanford University)
Electric field response of the conformally mapped nanocrescents used in the Stanford metamaterial (Photo: Stanford University)

The new Stanford metamaterial consists of a three-dimensional periodic array made up of three of these artificial nanocrescent atoms. When tuned for visible light, the material would exhibit a negative refractive index over a band from blue to red, only missing the very extremes of the visible spectrum. However, the researchers claim that a few tweaks to its structure would make this metamaterial useful across the entire visible spectrum.

The broad bandwidth of the new Stanford metamaterial suggests that this new class of materials will one day allow the fabrication of invisibility cloaks that are truly invisible, at least to the human eye. Beyond this, the extraordinary freedom to control light with metamaterials is likely to lead to hordes of applications never previously imagined.

The team's work was recently reported in Advanced Optical Materials.

Source: Stanford University

11 comments
Scion
I'm still waiting for transparent aluminium so I can transport my humpback whale into the future to save the human race. Also and invisibility cloak to hide my jetpack in.
Charles Bosse
You say "artificial atoms" in this article but I think you must mean something else. I'm guessing that the materials used are artificial compounds - the atoms themselves most likely coming by way of solar fusion and not "artificial" means such as particle accelerators - but if I'm wrong, please be more specific.
Max Mocchi
I think that by "artificial atoms" they mean quantum dots. Odd that they would not explain that.
Onihikage
Transparent Aluminum already exists, Scion. It's called Aluminum Oxynitride.
Gregory Gannotti
no more complaining about the eye sores like cell phone towers and wind mills on the ocean. just make them invisible.
sk8dad
One clarification to the "Harry Potter" style cloak...you cannot see the outside world from inside a perfect cloak, since light from all directions are routed around the cloak.
StWils
Greg G.: Not all innovations & applications are free of issues. Invisible cell towers and Wind Mills would be fairly tough on inattentive private pilots.
James T. Kirk
Cloaking devices are forbidden by treaty with the Romulan Star Empire.
-J.
Hey SCION, they already have it. From Wikipedia with many other search engine options to confirm description: Aluminium oxynitride or AlON is a transparent polycrystalline ceramic with cubic spinel crystal structure composed of aluminium, oxygen and nitrogen. It is currently marketed under the name ALON by Surmet Corporation. Can be made into windows and other things, at the price of synthetic emerald or sapphire or something.
M. Rajput
Yah..Its a nice work...But it is not the FIRST time..researchers R. K. Sinha and group (Monika Rajput) in University of Delhi (Delhi College of Engineering) have already achieved negative index for entire visible region in 2010. This paper is published in Applied Physics B.....