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Highly non-linear metamaterials for laser technology

Technische Universitaet Muenchen (Walter Schottky Institute) : 03 July, 2014  (Special Report)
Non-linear optical materials are widely used in laser systems. However, high light intensity and long propagation are required to produce strong non-linear optical effects. Researchers at The University of Texas at Austin and the Technische Universitaet Muenchen have created metamaterials with a million times stronger non-linear optical response, compared to traditional non-linear materials, and demonstrated frequency conversion in films 100 times thinner than a human hair, using light intensity comparable to that of a laser pointer.
Lasers have a fixed place in many fields of application. Yet, 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.
A team of researchers at the Technische Universitaet Muenchen (Germany) and the University of Texas Austin (USA) has now developed a 400nm thick non-linear mirror that reflects frequency-doubled output using input light intensity as small as that of a laser pointer. For a given input intensity and structure thickness, the new non-linear metamaterials produce approximately one million times higher intensity of frequency-doubled output, compared to the best traditional non-linear materials.
Furthermore, because the frequency conversion happens over subwavelength scales, the demonstrated non-linear mirrors are free from the stringent requirement of matching the phase velocities of the input and output waves, which complicates non-linear optical experiments with bulk non-linear 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 non-linear response for different non-linear optical processes, such as second harmonic, sum- and difference-frequency generation, as well a variety of four-wave mixing processes.
The material created comprises a sequence of thin layers made of indium, gallium and arsenic on the one hand and aluminium, indium and arsenic on the other. About 100 of these layers are stacked, each between 1-12nm thick, on top of each other and then sandwiched between a layer of gold at the bottom and a pattern of asymmetrical, crossed gold nanostructures on top.
Tuning the semiconductor layers thicknesses and the gold surface nanostructures geometry, the researchers have two possible ways to adjust the structure to resonate optimally with the desired wavelengths. For the initial demonstration, the material converts light with a wavelength of 8000n to 4000nm. “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.
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 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.”
Using the semiconductor material grown at TU Muenchen, a team of researchers at the University of Texas, led by Prof. Mikhail Belkin and Prof. Andrea Alu, designed a pattern of crossed gold structures tailored to have resonances at particular input and output frequencies and fabricated then on top of the semiconductor layer. It is this specific combination of semiconductor material and gold nanostructures engineering that produces giant non-linear response. 
Although the patterns are considerably smaller than the wavelength of the incoming light, the metallic structures ensure that the light is optimally coupled to the material. Their special design also causes a strong increase in field strength at specific locations, which further amplifies the non-linear response.
In the future, the team envisions using new materials realized along these lines for other non-linear effects. “Alongside frequency doubling, our structures may be designed for sum- or difference-frequency generation,” says graduate student Jongwon Lee, at the University of Texas, the lead author on the paper. “These kinds of elements could be used to produce and detect terahertz radiation – which is of interest for sensing and imaging applications, for example in medicine, because it does not harm biological tissue.” 
“This work opens a new paradigm in non-linear optics by exploiting the unique combination of exotic wave interaction in metamaterials and of quantum engineering in semiconductors.” says Professor Andrea Alu. 
“On the applications side, our work unveils a pathway towards the development of ultra-thin non-linear optical elements for efficient frequency conversion that will operate without stringent phase-matching constrains of currently-used bulk non-linear crystals,” says Professor Mikhail Belkin. 
The research was funded by the National Science Foundation of USA, the US Air Force Office of Scientific Research, and the US Office of Naval Research, as well as the German Research Foundation in the context of the Excellnce Initiative (Cluster of Excellence Nanosystems Initiative Munich, NIM).
Giant non-linear response from plasmonic metasurfaces coupled to intersubband transitions Jongwon Lee, Mykhailo Tymchenko, Christos Argyropoulos, Pai-Yen Chen, Feng Lu, Frederic Demmerle, Gerhard Boehm, Markus-Christian Amann, Andrea Alù, and Mikhail A. Belkin Nature, 04.07.2014, DOI: 10.1038/nature13455
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