Gold nanoparticles arranged into geometric patterns can control incoming light with light via a four-wave mixing process, transforming its output into a different color, Rice University theoretical and applied physicists say. The technique could be applied to the production of nonlinear media, each with tailored optical properties. Four-wave mixing has been widely studied, but Rice’s gold disc-patterning method is the first to produce materials tailored to perform four-wave mixing with a range of colored inputs and outputs. “Versatility is one of the advantages of this process,” said study co-author Naomi Halas, director of the university’s Laboratory for Nanophotonics (LANP) and Rice’s Stanley C. Moore Professor in Electrical and Computer Engineering and a professor of biomedical engineering, chemistry, physics and astronomy. “It allows us to mix colors in a very general way. That means not only can we send in beams of two different colors and get out a third color, but we can fine-tune the arrangements to create devices that are tailored to accept or produce a broad spectrum of colors.” By arranging optically tuned gold discs in a closely spaced pattern, Rice University scientists created intense electrical fields and enhanced the nonlinear optical properties of the system. Here a computer model displays the plasmonic interactions that give rise to the intense fields. Courtesy of Yu Zhang/Rice University. Information processing that takes place in today’s computers, tablets and smartphones is electronic. Each of the billions of transistors in a computer chip uses electrical inputs to act upon and modify the electrical signals passing through it. Processing information with light instead of electricity could yield faster and more energy-efficient computers, but building an optical computer is complicated by the quantum rules that light obeys. “In most circumstances, one beam of light won't interact with another,” said Peter Nordlander, a theoretical physicist at LANP and co-author of the new study. “For instance, if you shine a flashlight at a wall and you cross that beam with the beam from a second flashlight, it won't matter. The light that comes out of the first flashlight will pass through, independent of the light from the second. “This changes if the light is traveling in a ‘nonlinear medium,’ ” he said. “The electromagnetic properties of a nonlinear medium are such that the light from one beam will interact with another. So, if you shine the two flashlights through a nonlinear medium, the intensity of the beam from the first flashlight will be reduced proportionally to the intensity of the second beam.” The Rice investigators used electron-beam lithography to etch puck-shaped gold discs — a type of nonlinear media — that were then placed on a transparent surface for optical testing. Each was designed to harvest the energy from a particular light frequency; by arranging a dozen of the discs in a closely spaced pattern, the team was able to enhance the nonlinear properties of the system by creating intense electrical fields. Gold discs tuned to capture the energy from two incoming beams of light can produce output of a third color. Here a computer animation shows how the electromagnetic wave (red = positive, blue = negative) from the incoming light propagates through the system as a series of plasmonic waves. Courtesy of Yu-Rong Zhen/Rice University. “Our system exploits a particular plasmonic effect called a Fano resonance to boost the efficiency of the relatively weak nonlinear effect that underlies four-wave mixing,” Nordlander said. “The result is a boost in the intensity of the third color of light that the device produces.” Graduate student and co-author Yu-Rong Zhen calculated the precise arrangement of the 12 discs that would be required to produce two coherent Fano resonances in a single device, and graduate student and lead co-author Yu Zhang created the device that produced the four-wave mixing — the first such material ever created. “The value of this research goes beyond the design for this particular device,” Halas said. “The methods used to create this device can be applied to the production of a wide range of nonlinear media, each with tailored optical properties.” The technique was described in the Proceedings of the National Academy of Sciences (doi: 10.1073/pnas.1220304110). For more information, visit: www.rice.edu