A physics advance introduced by Purdue University researchers paves the way toward the creation of more efficient metamaterials. By using semiconductors and by amplifying electron activity, the researchers achieved a 60% improvement in operational frequency, in pursuit of creating a metamaterial capable of functioning very high frequencies, including up to the wavelengths of the visible light spectrum. The advance is poised to significantly increase the resolution of medical scanning and scientific imaging, and could additionally reduce the size of supercomputers. The researchers set out to solve problems in the efficiency of nanophotonic materials, which bridge the gap between optics and electronics. As electrons operate at a much smaller scale than light, signal conversion contributes to inefficiencies. Newly developed ballistic optical materials consist of a composite of two transparent materials, creating a plasmonic material. Courtesy of Evan Simmons and Kun Li. To solve this problem, scientists working in nanophotonics have previously turned to hyperbolic materials to shrink photons by compressing the light, making it easier to interface with electrical systems. “The most important thing about hyperbolic materials is that they can compress light to almost any scale. When you make light small, you solve the problem of the disconnect between optics and electronics,” said Evgenii Narimanov, a theoretical physicist and professor of electrical and computer engineering at Purdue. “Then you can make very efficient optoelectronics.” Hyperbolic materials typically consist of interwoven metal and dielectric layers; each surface must be as smooth and defect-free as possible, down to the atomic level, to optimize the usefulness of the material in optoelectronic applications. The materials can be expensive and involve time-consuming processes. To overcome that, Narimanov turned to semiconductors. Though Narimanov said semiconductors do not necessarily make optical materials of an inherently desirable quality, as they do possess enough electrons, they do function at relatively low frequencies in the mid- to far-infrared scale. To improve imaging and sensing technologies, scientists need metamaterials that work in the visible or near-infrared spectrum, at much shorter wavelengths than the mid- and far-infrared. Narimanov and his team experimented with an optical phenomenon known as “ballistic resonance.” In these semiconductor-based optical metamaterials, free (ballistic) electrons interact with an oscillating optical field. By synchronizing the optical field with the frequency of motion of the free electrons as they bounce within the thin conducting layers, the electrons resonated. This intensified the reaction of each electron and created a metamaterial that worked at higher frequencies — about 60% of the way to the visible spectrum. “We showed that there is a physics mechanism that makes this possible,” Narimanov said. “Before, people did not realize this was something that could be done. We have opened the way. We showed it is theoretically possible, and then we experimentally demonstrated 606% improvement in the operational frequency over existing materials.” Narimanov teamed with Kun Li, Andrew Briggs, Seth Bank, and Daniel Wasserman at the University of Texas, and with Evan Simmons and Viktor Podolskiy at the University of Massachusetts Lowell. The University of Texas researchers developed the fabrication technology, and the UMass Lowell team contributed to the full quantum theory and performed numerical simulations to be sure everything would function as planned. “We will keep pushing this frontier,” Narimanov said. “Even if we are extremely successful, nobody is going to get semiconductor metamaterials to the visible and near-infrared spectrum within a year or two. It may take about five years. But what we have done is provide the material platform. The bottleneck for photonics is in the material where electrons and photons can meet on the same length scale, and we have solved it.” The research was published in Optica (www.doi.org/10.1364/OPTICA.402891).