Remote mismatch: Sensing opportunities

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  • Published: Jan 1, 2014
  • Author: David Bradley
  • Channels: Raman
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Light sources

Bi-directional coherent Raman scattering for remote sensing applications might now be possible thanks to US work into a unique optical metamaterial with a refractive index of zero that generates

Bi-directional coherent Raman scattering for remote sensing applications might now be possible thanks to US work into a unique optical meta material with a refractive index of zero that generates "phase mismatch-free non-linear light," wherein the light waves move through the material gaining strength in all directions.

Quantum computation has been touted as the "next big thing" in the information age, a potential "version 2.0" development, some pundits suggest. Quantum computers will operate many times faster and be far more powerful than today's supercomputers, although they will not be the simple number crunchers we know computers to be today, they will be problem solvers. However, technology is yet to catch up with the concept. Nevertheless, researchers are taking small steps towards that quantum leap of faith particularly in the development of the requisite fast and efficient multi-directional light sources.

Now, researchers at the Lawrence Berkeley National Laboratory led by materials scientist Xiang Zhang, have used a unique optical meta material with a refractive index of zero to generate "phase mismatch-free non-linear light." In this phenomenon, light waves propagate through the material and gain strength in all directions as they do so. This phase mismatch-free quality holds promise for quantum computing and networking, and future light sources based on non-linear optics.

Non-linear applications

"In our demonstration of non-linear dynamics in an optical meta material with zero-index refraction, equal amounts of non-linearly generated waves are observed in both forward and backward propagation directions," Zhang explains. "The removal of phase matching in non-linear optical meta materials may lead to applications such as efficient multidirectional light emissions for novel light sources and the generation of entangled photons for quantum networking."

Zhang has worked on these novel meta materials systems with Haim Suchowski, Kevin O'Brien, Zi Jing Wong, Alessandro Salandrino and Xiaobo Yin. Meta materials are artificial nanofabricated systems the optical properties of which are determined by the physical structure of their superlattices rather than their chemical composition - as with a butterfly wing or a colourful feather, where pigment does not produce the pattern, rather iridescence. Meta materials have gained much focus from researchers recently for microwave "invisibility cloaking" and other such behaviour. Their unique structures offer a way to attain electromagnetic behaviour in a material that cannot be accessed with conventional synthetic materials. For example, a meta material might have a negative index of refraction, so that it can essentially "bend" a beam of light back towards its source; known conventional materials and those that have been found in nature so far, always bend light forward away from the source.

In their work with meta materials, Zhang and his research group have generated the world's first optical invisibility cloak, mimicked black holes, and created the first plasmonic nanolasers. In this latest study, he and his group focused on the non-linear properties of meta materials.

Meta material swings both ways

Non-linear optics always comes to a barrier in development - the phase-mismatch problem. When laser light interacts with a non-linear material light of a different wavelength might be generated or photos might be absorbed; it depends on the relative phase between the two. Different phase velocities lead to destructive interference due to the lack of optical momentum conservation between the photons, "phase mismatch", in other words.

"Phase mismatch is one reason why non-linear optical processes are not common in everyday life," explains team member Suchowski. "In the past 60 years, since the beginning of non-linear optics, scientists have been developing techniques to compensate for this lack of momentum conservation in order to achieve phase matching. However, all of these techniques have limitations and present their own challenges." A solution that addresses the phase mismatch problem forwards and backwards has been keenly sought.

The Berkeley team has now created a zero-index meta material that might just be the solution scientists have been looking for. Their meta material features a fishnet structure - a stack of metal-dielectric multilayers with perforated holes. The fishnet consists of twenty alternating layers of gold films thirty nanometres thick and magnesium fluoride films 50 nm thick on a 50 nm thick silicon nitride membrane. "We've shown that optical momentum conservation in our meta material is always preserved regardless of the direction in which the light waves are generated," Suchowski explains.

Related Links

Science, 2013, 342, 1223: "Phase Mismatch – Free Nonlinear Propagation in Optical Zero-Index Materials"

Article by David Bradley

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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