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August 01, 2011

Manipulating Light at Will

Duke electrical engineers have developed a man-made material that they say literally allows them to manipulate light at will.

They say that the results of their latest proof-of-concept experiments could lead to the replacement of electrical components with those based on optical technologies, which should allow for faster and more efficient transmission of information, much in the same way that replacing wires with optical fibers revolutionized the telecommunications industry.

The breakthrough revolves around a novel man-made structure known as a metamaterial. These exotic composite materials are not so much a single substance, but an entire structure that can be engineered to exhibit properties not readily found in nature. The structure used in these experiments resembles a miniature set of tan Venetian blinds.

Physical Review Letters - Controlling the Second Harmonic in a Phase-Matched Negative-Index Metamaterial



Nonlinear metamaterials have been predicted to support new and exciting domains in the manipulation of light, including novel phase-matching schemes for wave mixing. Most notable is the so-called nonlinear-optical mirror, in which a nonlinear negative-index medium emits the generated frequency towards the source of the pump. In this Letter, we experimentally demonstrate the nonlinear-optical mirror effect in a bulk negative-index nonlinear metamaterial, along with two other novel phase-matching configurations, utilizing periodic poling to switch between the three phase-matching domains.

When light passes through a material, even though it may be reflected, refracted or weakened as it passes through, it is still the same light coming out. This is known as linearity.

“For highly intense light, however, certain 'nonlinear' materials violate this rule of thumb, converting the incoming energy into a brand new beam of light at twice the original frequency, called the second-harmonic,” said Alec Rose, graduate student in the laboratory of David R. Smith, William Bevan Professor of electrical and computer engineering at Duke’s Pratt School of Engineering.

As an example, he used the crystal in some laser pointers, which transforms the normal laser light into a beam – the output can’t be any stronger than the input beam -- in another color, such as green, which would be the second-harmonic. Though they contain nonlinear properties, designing such devices requires a great deal of time and effort to be able to control the direction of the second harmonic, and natural nonlinear materials are quite weak, Rose said.

“Normally, this frequency-doubling process occurs over a distance of many wavelengths, and the direction in which the second-harmonic travels is strictly determined by whatever nonlinear material is used,” Rose said. “Using the novel metamaterials at microwave frequencies, we were able to fabricate a nonlinear device capable of 'steering' this second-harmonic. The device simultaneously doubled and reflected incoming waves in the direction we wanted.”

an have important consequences in all-optical communications, where the ability to manipulate light is crucial,” Rose said.

The device itself, which measures six inches by eight inches and about an inch high, is actually made up of row upon row of individual pieces arranged in parallel rows. Each piece is made of the same fiberglass material used in circuit boards and is etched with copper circles. Each copper circle has a tiny gap that is spanned by a diode, which when excited by light passing through it, breaks its natural symmetry, creating non-linearity.

“The trend in telecommunications is definitely optical,” Rose said. “To be able to control light in the same manner that electronics control currents will be an important step in transforming telecommunications technologies.
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