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FINE-TUNING MATERIALS [Mechanical Engineering](Mechanical Engineering Via Acquire Media NewsEdge) Metamaterials have garnered attention in recent years. Because their properties come from their physical structures-minuscule components arranged in a specific pattern-rather than from their chemical makeup, metamaterials can be engineered to act in ways impossible for natural materials. This month, we look at two labs working with metamaterials and how their research could lead to future applications in quantum computing and nanoscale electronics. Researchers at the University of California, San Diego have found that a beam of light can make waves in crystals and those waves can be tuned-a phenomenon that might lead to the transmission of information in computer chips, better management of heat flow in nanoscale devices, or the creation of higher resolution images than now possible, said UCSD physicist Dimitri Basov. Basov's team fired a beam of infrared light at an atomic force microscope as it scanned across a 2-D material, hexagonal boron nitride. The microscope uses a needle at the end of an arm to probe atomic and molecular surfaces. The microscope transferred momentum from the light to the crystal, causing the light to generate waves in the boron nitride. The waves, called phonon polaritons, had wavelengths as short as those of ultraviolet light, at about 300 to 400 nanometers. The waves generated can be maintained long enough to be usable for practical applications, Basov said. "A wave on the surface of water is the closest analogy," he said. "You throw a stone and you launch concentric waves that move outward. This is similar. Atoms are moving. The triggering event is illumination with light." The atoms of boron nitride form layers that are stacked on top of one another and held together by forces between molecules. By adjusting the wavelength of the light and varying the number of layers of the boron nitride, the researchers were able to adjust the shape and size of the polaritons, or tune them to specific frequencies or amplitudes. Controlling the wave size means the crystal could be used to transmit information. Also, control of waves could be important to building nanometer-scale circuits. "You can direct information where you want it at the nanoscale," Basov said. The next frontier in computing involves quantum processors, which are expected to be many times faster and more powerful than today's supercomputers. Such computers could manipulate bits of quantumly entangled light rather than electrical signals to process information. One problem with using photons to carry information is they hardly interact with each other. While adding nonlinearity to the linear optical network may help realize quantum computation, implementing nonlinear optical effects isn't easy. But in a study led by Xiang Zhang, a faculty scientist in the materials sciences division at Lawrence Berkeley National Laboratory, a research team used an optical metamaterial with a refractive index of zero to generate "phase mismatch-free nonlinear light." The phase mismatch-free quality holds promise for optical quantum computing and networking. Nonlinear optical processes are a challenge to achieve and maintain because of the phase-mismatch problem, Zhang said. The interaction of intense laser light with a nonlinear material can generate light of a different color. But if the phases of the new and old photons aren't exactly aligned, the waves can produce destructive interference. In a zero-index metamaterial, the phases of propagating light waves are mismatch-free in both directions, Zhang said, which eliminates the problem of destructive interference. Researchers had previously shown that a metamaterial could be engineered to yield an index of zero. A beam of light shining through this zero-index metamaterial was unaffected, as if it had passed through a vacuum. Building on this, the Berkeley researchers engineered a zero-index metamaterial that generates light, Zhang said. "In our demonstration, we observed equal amounts of nonlinearly generated waves in both forward and backward directions," he said. "The removal of phase matching in nonlinear optical metamaterials may lead to the generation of entangled photons for quantum networking." The metamaterial created in the lab features a fishnet structure-a stack of metal-dielectric multilayers with perforated holes. The fishnet consists of 20 alternating layers of gold films 30 nanometers thick and magnesium fluoride films 50 nanometers thick on a 50-nanometer thick silicon nitride membrane. ME SOLID LIGHT WAVES THE LAB Basov Infrared Laboratory, University of California, San Diego; Dimitri Basov, principal investigator. THE OBJECTIVE The use of infrared spectroscopy to investigate novel physics of electronic and magnetic materials. THE DEVELOPMENT Wave generation and size adjustment within a crystal. LIGHT FOR QUANTUM NETWORKS THE LAB Lawrence Berkeley National Laboratory, materials sciences division; Berkeley, Calif.; Paul Alivisatos, laboratory director. THE OBJECTIVE Development of experimental techniques to understand new materials and phenomena at the scale of time and length. THE DEVELOPMENT Moving light through a metamaterial without destructive interference. The graphic shows four-wave mixing in a positive-index, upper, and zero-index, lower, metamaterial. FWM is about the same in both directions for the zero-index metamaterial. (c) 2014 American Society of Mechanical Engineers |