Photonics: Shed a little light

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  • Published: May 1, 2016
  • Author: David Bradley
  • Channels: UV/Vis Spectroscopy
thumbnail image: Photonics: Shed a little light

Walk the line

Photonics can be an opaque field of endeavour. Now, work by scientists at the University of Rochester, New York, USA, could lend a little transparency to the science by shedding light an alternative approach to applications.

Photonics can be an opaque field of endeavour. Now, new research could lend a little transparency to the science by shedding light an alternative approach to applications.

At the centre of many photonics applications is nonlinear optics, the non-classical channelling of light through materials that gives rise to holography, second harmonic generation, frequency mixing, self-focusing, four-wave mixing, Raman amplification, multi-photon absorption, cross-polarised wave generation, optical solitons and the rest. The effects depend on polarisation, light intensity and the molecular or crystalline structure of the material in question. It might be said that the more prominent the effect, the greater the nonlinearity, the more promising the material will be for novel, real-life applications.

Now, Robert Boyd, who is Professor of Optics and Physics at the University of Rochester, New York, USA, and also the Canada Excellence Research Chair in Quantum Nonlinear Optics at the University of Ottawa, and his colleagues have demonstrated that the transparent, electrical conductor indium tin oxide (commonly known as ITO) can result in up to one hundred times greater nonlinearity than other known materials.

Game changer

"This result is a game-changer for photonics applications," Boyd enthuses. "It rests on the core of what I've worked on for over thirty years at Rochester. I find it very rewarding that even after all this time there are still fundamental questions to be answered in the field of nonlinear optics." Boyd and his colleagues published details of the latest ITO work in the journal Science at the end of April 2016.

Photonics can use light to transmit information and also to manipulate information through logical operations, just as electronics does with electrons. One factor on which many of the nonlinear optical phenomena hinge is the refractive index of the material and how it affects the transmission of light. Changing the refractive index of a material so that light travelling faster or slower through it, is the key to how photonics controls light but when the refractive index of that material is different for different light intensities, then that material is said to be a nonlinear optical material.

A pulse of light can change the refractive index of a material for mere femtoseconds, but that can be long enough for a second pulse to "see" the change and behave in a different way to how it would if it were itself the first and only pulse. The rapidity with which a material recovers from changes can also be exploited in switching, for instance.

Boyd and his graduate student at Ottawa, Zahirul Alam, and former research associate Israel De Leon (now a professor at the Tecnologico de Monterrey, Mexico) were able to boost the nonlinearity of a material one hundredfold above the previous record by exploiting the optical properties of ITO in the epsilon-near-zero region, the point at which the mathematically "real" part of the material's permittivity drops to close to zero. ENZ materials have been the focus of much research for their potential in coupling optical signals and other applications.

"It was surprising that showing such a strong optical nonlinearity in a known metal was this easy," explains Boyd. "This material has been around for many years, but until now the community had overlooked the potential that the 'epsilon-near-zero' region of materials offered."

Responsive

"The optical nonlinear response that we have observed introduces a new paradigm in nonlinear optics," adds De Leon. "The common knowledge had always been that nonlinear effects are tiny compared with the linear ones; but in our work we have measured a nonlinear response that is 170% larger than the linear response." This result opens the door for more careful study of this region of materials, with a view to finding a material that can offer the perfect characteristics specific photonic applications.

The 'epsilon-near-zero' region for this material is linked to light of a specific frequency, roughly a wavelength of 1.2 micrometres. This wavelength is of interest because it lies between that of visible light and light of wavelength 1.5 micrometres. This wavelength is of particular interest for technologists working on optical communications. It is possible that tweaks to the chemical composition of the material could be used to fine tune the frequency at which epsilon near zero occurs, thus bringing this frequency closer to that used by optical communications. De Leon points out that other metals, specifically gold and silver, have an ENZ region in the ultraviolet.

Related Links

Science 2016, online: "Large Optical Nonlinearity of Indium Tin Oxide in its Epsilon-Near-Zero Region"

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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