The Berkeley Science Review: Articles

Fishy Physics
Metamaterials bend light backward (view PDF)
by Aaron Lee

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Bending light is nothing new; anyone who wears glasses is grateful for this phenomenon. Scientists use microscopes to guide light and achieve remarkable focus of the microscopic world. But their tools have an unavoidable limit in resolution set by the wavelength of visible light: a typical optical microscope cannot resolve details smaller than 200 nanometers (billionths of a meter) in size, a phenomenon known as the diffraction limit. To achieve resolution beyond this limit, large and costly devices are necessary. But Professor Xiang Zhang of the UC Berkeley Nanoscale Science and Engineering Center and Department of Mechanical Engineering thinks there is an easier way to beat this diffraction limit: gain complete control over the movement of light.

The direction light bends when passing between two surfaces (e.g., water and air) depends on how each material interacts with light, measured by a value called the index of refraction. For example, water has an index of about 1.33 and air has an index of 1.0. Since water has a higher index, reflected light off an underwater fish always bends toward the surface of the lake. Similarly if someone were to shine a flashlight on the lake, the light would always bend towards the line perpendicular to the surface of the water. All naturally occurring materials have a positive index, limiting the number of possible directions light can travel.
Zhang has overcome this limitation by engineering materials that have negative indices of refraction. In this case the light appears to bounce off the perpendicular line and bend backwards. If water instead had a negative index, the fish would appear to be flying through the air.

These often-called "metamaterials" gain their properties from their nanoscale structure rather than their molecular composition. As described in the August 2008 edition of Science, Zhang and coworkers uniformly arranged straight silver nanowires in non-conducting aluminum oxide to achieve a negative index for red light. The nanowires are spaced 100 nanometers apart, a distance one thousand times smaller than a human hair and nearly seven times smaller than the wavelength of red light. Since the incoming wave is larger than this spacing, it is "unaware" of this structure and sees a uniform material. "The light effectively sees a material composed of nanosized Ômeta-atoms' that we can engineer to control the path of light," says Jie Yao, a graduate student in Zhang's lab and coauthor on the Science paper.

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Fabricating negative index materials for visible light has been a challenge due to its short wavelengths. Previous metamaterials have achieved negative indices only at microwave and infrared wavelengths, which are at least one and a half times that of red light. These metamaterials have also been relatively thin, consisting of a single to a few meta-atomic layers. Increasing the thickness of the material resulted in considerable energy loss due to absorption in the material. Zhang has also overcome this limitation and achieved effective three-dimensional metamaterials measuring tens of meta-atomic layers thick.

Previous metamaterials were constructed so electrons in the metamaterial would oscillate at the frequency of the incoming wave (see "Metamaterials World," BSR Fall 2006). This allowed the wave to be absorbed and re-emitted in another direction. Zhang's metamaterial takes a new approach by relying on silver's conductive properties. Light is an electromagnetic wave, consisting of an oscillating electric field with a perpendicular magnetic field. The incoming electric field, when aligned with the silver wires, induces a current in the wires that emits a new electromagnetic wave propagating in another direction. The incoming magnetic field is unaware of the wires and follows the new electric field, resulting in a new electromagnetic wave. Since the magnetic field is not absorbed in the process, less energy is lost as the wave moves through the material.

An alternative optical metamaterial was devised by stacking alternating layers of silver and insulating magnesium cut into a nanoscale fishnet pattern. As described in their August 2008 Nature paper, these layers form small circuits that induce an electromagnetic wave moving in the opposite direction as the original. "This new design has incredible flexibility and it is simple," says Jason Valentine, graduate student and coauthor of the article. "You want the simplest design possible when dealing with fabrication." This design also demonstrated better energy efficiency compared to previous metamaterials.

Such metamaterials can enable researchers to overcome the diffraction limit and resolve details smaller than the wavelength of the incident light. When light waves impact a material, a pattern of waves is formed near the material's surface. Details smaller than the wavelength of light are in these waves, yet they decay and vanish almost instantly. Negative index materials placed close to the object can amplify these evanescent waves, allowing them to propagate far enough to be resolved. Zhang's group demonstrated a working "superlens" in 2005 ("The Sharpest Image," BSR Fall 2005), being able to resolve two lines separated by one-tenth the wavelength of red light. Superlenses could lead to inexpensive optical microscopes that would be able to resolve structures as small as living viruses. "The idea is simple enough that it could one day be used even in high school biology classrooms," explains Leo Zeng, postdoctoral researcher and lab manager of Zhang's group.

In fact, overcoming the diffraction limit is just one of several applications achievable by mastering the manipulation of light. These materials may one day play a role in high speed optical computing, cavities that can trap light in a way that mimics the environment around a black hole, and cloaking devices. Zhang's remarkable achievement of bending red light the "wrong" way has laid the necessary groundwork for the development of practical application of metamaterials. "The results demonstrated in these two papers seemed impossible in 2003, but we did it," says Zeng. "We are limited only by our imaginations of what is possible."

Aaron Lee is a graduate student in astronomy.

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