‘Metamirror’ Doubles Incident Light Frequency

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.

Published: July 2014

Glossary

four-wave mixing: A phenomenon that occurs in WDM and DWDM systems when three closely spaced signal wavelengths near the zero-dispersion wavelength interact with each other, producing a fourth wavelength that interferes with the original signal. A moderate amount of dispersion can be designed into some systems to ensure that this effect does not take place.
indium: Metal used in components of the crystalline semiconductor alloys indium gallium arsenide (InGaAs), indium gallium arsenide phosphide (InGaAsP), and the binary semiconductor indium phosphide (InP). The first two are lattice-matched to InP as the light-emitting medium for lasers or light-emitting diodes in the 1.06- to 1.7-µm range, and the last are used as a substrate and cladding layer.
nano: An SI prefix meaning one billionth (10-9). Nano can also be used to indicate the study of atoms, molecules and other structures and particles on the nanometer scale. Nano-optics (also referred to as nanophotonics), for example, is the study of how light and light-matter interactions behave on the nanometer scale. See nanophotonics.
near-infrared: The shortest wavelengths of the infrared region, nominally 0.75 to 3 µm.
nonlinear optics: Nonlinear optics is a branch of optics that studies the optical phenomena that occur when intense light interacts with a material and induces nonlinear responses. In contrast to linear optics, where the response of a material is directly proportional to the intensity of the incident light, nonlinear optics involves optical effects that are not linearly dependent on the input light intensity. These nonlinear effects become significant at high light intensities, such as those produced by...
second-harmonic generation: Second-harmonic generation (SHG) is a nonlinear optical process that occurs when two photons with the same frequency combine within a nonlinear material, resulting in the generation of a new photon with twice the frequency (and therefore half the wavelength) of the original photons. This phenomenon is a specific case of second-order nonlinear optical effects. Key points about second-harmonic generation include: Nonlinear optical process: SHG is a nonlinear optical effect, meaning that the...
terahertz: Terahertz (THz) refers to a unit of frequency in the electromagnetic spectrum, denoting waves with frequencies between 0.1 and 10 terahertz. One terahertz is equivalent to one trillion hertz, or cycles per second. The terahertz frequency range falls between the microwave and infrared regions of the electromagnetic spectrum. Key points about terahertz include: Frequency range: The terahertz range spans from approximately 0.1 terahertz (100 gigahertz) to 10 terahertz. This corresponds to...

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