A nanostructure produces nonlinear effects a million times greater than traditional, macroscale nonlinear crystals, according to a team of researchers from Texas and Germany. This “metamirror” could enable miniaturized laser systems and enhance chemical sensing, explosives detection and biomedical research. Incident light with intensity as small as that of a laser pointer strikes a 400-nm-thick nonlinear mirror and is reflected back at double its original frequency. Courtesy of the University of Texas at Austin. Part of the mirror is a 400-nm-thick semiconductor structure, made up of about 100 alternating layers of indium-gallium-arsenide and aluminum-indium-arsenide, grown by molecular beam epitaxy at the Technical University of Munich. “This kind of structure is called a coupled quantum well,” said Munich professor Dipl.-Ing. Frederic Demmerle. “Now, when we stack a further thin layer at a precisely defined distance from the first layer, we can push these electron states closer together or pull them apart, adjusting them precisely to the desired wavelength.” A layer of gold lines the back face of the semiconductor nanostructure, and the front is covered with a plasmonic metasurface of asymmetric gold nanocrosses manufactured at the University of Texas at Austin. The ultra-thin layers of the metamaterial were produced with this molecular beam epitaxy system. Courtesy of W. Hoffmann/Technical University of Munich. The researchers said the structure has a nonlinear susceptibility of > 5 × 104 picometres per volt for second-harmonic generation. They demonstrated conversion of an 8 µm beam into a 4 µm beam, but said their device could be tailored to work across wavelengths from the near-infrared to the terahertz. Besides frequency doubling, the device could also be used for sum- or difference-frequency generation and four-wave mixing, the researchers said. “This work opens a new paradigm in nonlinear optics by exploiting the unique combination of exotic wave interaction in metamaterials and of quantum engineering in semiconductors,” said Texas professor Andrea Alu. The research was funded by the National Science Foundation, the U.S. Air Force Office of Scientific Research and the Office of Naval Research, as well as the German Research Foundation. The work was published in Nature (doi: 10.1038/nature13455). For more information, visit www.utexas.edu and www.tum.de.