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Non-linear metamaterials deliver compact lasers

Million times higher intensity frequency-doubled output
Using layers of indium, gallium and arsenic alternated with aluminum, indium and arsenic in a coupled quantum well structure, scientists at the Technische Universitaet Muenchen, Germany and the University of Texas, Austin, USA have developed a way of making compact lasers at wavelengths for which either no laser systems exist or at best only large and expensive ones.

About 100 of these InGaAs and ALInAs layers (each between one and twelve nanometers thick) were stacked on top of each other and sandwiched between a layer of gold at the bottom and a pattern of asymmetrical, crossed gold nanostructures on top to make a 400nm thick nonlinear mirror. For a given input intensity and structure thickness, the new structure can produce approximately one million times higher intensity of frequency-doubled output, compared to the best traditional nonlinear materials, according to the researchers.

For the initial demonstration, the material converted 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.

Doubling the frequency of a beam of light in this way 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.It is possible to adjust the structure to resonate optimally with the desired wavelengths by tuning the semiconductor layers' thicknesses and the gold surface nanostructures geometry. The metallic structures ensure that the light is optimally coupled to the material. Their design also causes a strong increase in field strength at specific locations, which further amplifies the nonlinear response.

According to the researchers, because the frequency conversion happens over subwavelength scales, the 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. In the future, the team thinks the structures could be used for other nonlinear 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 'Giant nonlinear response from plasmonic metasurfaces coupled to intersubband transitions'  published today in Nature 2014, DOI: 10.1038/nature13455

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