An international research team led by The University of Texas at Austin Department of Electrical and Computer Engineering’s Andrea Alù, an associate professor at the Cockrell School of Engineering, and assistant professor at the ECE and CSE Mikhail Belkin, in collaboration with colleagues from the Technische Universitaet Muenchen (Technical University of Munich, Germany), has created thin-film nonlinear metamaterials with optical response many orders of magnitude larger than that of traditional nonlinear materials.
Lasers have a fixed place in many fields of application, but there are still wavelengths for which either no systems exist, or at best only large and expensive ones. On the other hand remote sensing and medical applications call for compact laser systems, for example with wavelengths from the near infrared to the Terahertz region.
Nonlinear optical materials are widely used in laser systems. However, high light intensity and long propagation are required to produce strong nonlinear optical effects. The UTA/TUM researchers have created metamaterials with a million times stronger nonlinear optical response, compared to the traditional nonlinear materials, and demonstrated frequency conversion in films 100 times thinner than human hair using light intensity comparable to that of a laser pointer.
The scientists demonstrated this functionality by realizing a 400-nanometer-thick nonlinear mirror that reflects radiation at twice the input light frequency, and that could one day enable miniaturization of laser systems. For the given input intensity and structure thickness, the new nonlinear metamaterial produces approximately 1 million times larger frequency-doubled output, compared with similar structures based on conventional materials, according to a UTA release.
Furthermore, because the frequency conversion happens over subwavelength scales, the demonstrated nonlinear mirrors are free from the stringent requirement of matching the phase velocities of the input and output waves, which complicates nonlinear optical experiments with bulk nonlinear crystals. The new structures can be tailored to work at various frequencies from near-infrared to mid-infrared to terahertz and can be designed to produce giant nonlinear response for different nonlinear optical processes, such as second harmonic, sum- and difference-frequency generation, as well a variety of four-wave mixing processes.
The invention, called a “nonlinear mirror” by the researchers, could also help advance nonlinear laser systems that are used for chemical sensing, explosives detection, biomedical research and potentially many other applications. The researchers’ findings are reported in a study published in the July 3 issue of Nature, entitled “Giant nonlinear response from plasmonic metasurfaces coupled to intersubband transitions” (Nature 511, 65–69 (03 July 2014) doi:10.1038/nature13455), coauthored by Department of Electrical and Computer Engineering, Jongwon Lee, Mykhailo Tymchenko, Christos Argyropoulos, Pai-Yen Chen, Feng Lu, Andrea Alù, and Mikhail A. Belkin of The University of Texas at Austin, and Frederic Demmerle, Gerhard Boehm, and Markus-Christian Amann of the Walter Schottky Institut, Technische Universität München, Munich, Germany.
The coauthors propose and report experimental realization of metasurfaces with a record-high nonlinear response based on the coupling of electromagnetic modes in plasmonic metasurfaces with quantum-engineered electronic intersubband transitions in semiconductor heterostructures. They demonstrate that it is possible to engineer almost any element of the nonlinear susceptibility tensor of these structures, and experimentally verify this concept by realizing a 400-nm-thick metasurface with nonlinear susceptibility of greater than 5 x 10/4 picometres per volt for second harmonic generation at a wavelength of about 8 micrometres under normal incidence — a susceptibility they say is many orders of magnitude greater than any second-order nonlinear response in optical metasurfaces measured so far. The proposed structures can act as ultrathin highly nonlinear optical elements that enable efficient frequency mixing with relaxed phase-matching conditions, ideal for realizing broadband frequency up- and down-conversions, phase conjugation and all-optical control and tunability over a surface.
The metamaterials were created with nonlinear optical response a million times as strong as traditional nonlinear materials and demonstrated frequency conversion in films 100 times as thin as human hair using light intensity comparable with that of a laser pointer.
The UTA release notes that nonlinear optical effects are widely used by engineers and scientists to generate new light frequencies, perform laser diagnostics and advance quantum computing. Due to the small extent of optical nonlinearity in naturally occurring materials, high light intensities and long propagation distances in nonlinear crystals are typically required to produce detectable nonlinear optical effects.
“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,” comments Professor Alù.
The metamaterial at the basis of this unusual optical response consists of a sequence of thin layers made of indium, gallium and arsenic on the one hand and aluminum, indium and arsenic on the other. The researchers stacked approximately 100 of these layers, each between 1 nanometer and 12 nanometers thick, and sandwiched them between a layer of gold at the bottom and a pattern of asymmetric gold nanocrosses on top. The thin semiconductor layers confine electrons into desired quantum states, and gold nanocrosses resonate at input and output frequencies to enable the nonlinear optical response of the mirror.
Tuning the semiconductor layers thicknesses and the gold surface nanostructures geometry, the researchers have two possibilities to adjust the structure to resonate optimally with the desired wavelengths. For the initial demonstration, the material converts light with a wavelength of 8000 nanometers to 4000 nanometers. “Laser light in this frequency range can be used in gas sensors for environmental technology,” says Frederic Demmerle, project member at the Walter Schottky Institute of the TU Muenchen in a TUM release.
The realized mirror converts light from a wavelength of 8 micrometers to 4 micrometers; however, the structures can be tailored to work at other wavelengths, from near-infrared to mid-infrared to terahertz.
The ability to double the frequency of a beam of light stems from the engineered electron states in the semiconductor material. When the semiconductor layers are only a few nanometers thick, the electrons can only occupy specific energy states and can be resonantly excited by the electromagnetic radiation.
“This kind of structure is called a coupled quantum well,” says Dr. 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.”
“Alongside frequency doubling, our structures may be designed for sum- or difference-frequency generation, as well as a variety of four-wave mixing processes,” notes UT Austin graduate student Jongwon Lee, the lead author on the Nature paper. “Alongside frequency doubling, our structures may be designed for sum- or difference-frequency generation. These kinds of elements could be used to produce and detect terahertz radiation – which is of interest for sensing and imaging applications, e.g., in medicine, because it does not harm biological tissue.”
“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 semicondcutors.” observes Professor Alu.
“On the applications side, our work unveils a pathway towards the development of ultrathin nonlinear optical elements for efficient frequency conversion that will operate without stringent phase-matching constrains of currently-used bulk nonlinear crystals,” says Professor Mikhail Belkin.
Dr. Belkin from 2004 to 2008 worked in Federico Capasso’s group in the School of Engineering and Applied Sciences at Harvard University, first as a postdoctoral fellow and later as a research associate. He joined the faculty of the University of Texas at Austin in the fall of 2008. Dr. Belkin is a recipient of the AFOSR Young Investigator Program Award (2009), NSF CAREER Award (2012), and DARPA Young Faculty Award (2012). His current research interests include giant nonlinear optical effects in metamaterials, quantum cascade lasers, and other engineered and/or low-dimensional structures, sub-wavelength resolution microscopy in mid- and far-infrared, development and application of mid-IR and THz radiation sources and detectors, and plasmonic devices. The Belkin Lab’s work is supported by AFOSR, ARO, DARPA, DOE, NSF, the Robert A. Welch Foundation, and the Texas Board of Higher Education Norman Hackerman Advanced Research Program.
Dr. Andrea Alù is an Associate Professor and the David & Doris Lybarger Endowed Faculty Fellow in Engineering in the Department of Electrical and Computer Engineering at The University of Texas at Austin. He is affiliated with the Wireless Networking and Communications Group, an interdisciplinary center for research and education at The University of Texas at Austin with an emphasis on industrial relevance, and with the Applied Research Laboratories at the University of Texas at Austin.
Dr. Alù is also an Associate Editor of IEEE Antennas and Wireless Propagation Letters and Optics Express, Editor of Metamaterials and Advanced Electromagnetics and guest editor of several special issues on metamaterials and plasmonics. His research specializes in metamaterials and plasmonics, with applications spanning microwaves, infrared, optical frequencies and acoustic waves, including nanooptics and nanophotonics, cloaking and transparency, nanocircuits and nanostructures modeling, miniaturized antennas and nanoantennas. His research has significantly contributed to the fields of plasmonic cloaking, optical nanoantennas and nanocircuits and to the theory and modeling of metamaterials. He is currently teaching graduate and undergraduate courses on electromagnetics and metamaterials, is currently seeking brilliant and highly motivated graduate students and postdocs to join his group at UT Austin.
Drs. Belkin and Alù’s team of researchers included electrical and computer engineering graduate students Jongwon Lee, Mykhailo Tymchenko and Feng Lu. Pai-Yen Chen and Christos Argyropoulos, who graduated from the Cockrell School in 2013, also contributed to the paper. The semiconductor material was grown at the Walter Schottky Institute, Technical University of Munich.
The research was funded by the National Science Foundation, the Air Force Office of Scientific Research, the Office of Naval Research, and the German Research Foundation. Drs.Alù and Belkin have both received research funding for other projects from the National Science Foundation and other major public science foundations. Belkin has also received research funding for other projects from the companies Omega Optics, Anasys Instruments andHamamatsu Photonics, and Alù from the AEgis Technologies Group. An alumnus and former doctoral student who worked on the project, Pai-Yen Chen, now works for Intellectual Ventures Inc.
The University of Texas at Austin
Technische Universitaet Muenchen Walter Schottky Institute
The University of Texas at Austin