"Our research deals with nonlinear optics in metamaterials," explains team leader Wenshan Cai of Georgia Tech. "Nonlinear optics is a branch of optics that is important for actively controlling light and generating new laser frequencies (via so-called harmonic generation or light wave mixing). In our new work, published in Nature Materials, we provide the first experimental evidence for perhaps the most famous prediction in the entire field of nonlinear metamaterials: 'backward phase matching', or the 'nonlinear mirror'."

Phase matching is the standard way to efficiently generate certain frequencies of light in nonlinear optical materials – generally by fine-tuning how the crystals making up the photonic material are oriented, he adds. In second-harmonic generation (which is also a nonlinear process used to double the frequency of light), phase matching requires that the refractive index of the initial (or "fundamental") and the doubled (or "harmonic") frequencies are identical. "In this way, the fundamental light will gradually convert to its harmonic along the direction in which light propagates and the output harmonic light will also propagate in the same direction," explains Cai.

Man-made structures containing subwavelength metallic components

Metamaterials are man-made structures containing subwavelength metallic components that behave like meta-atoms, or building blocks. They have very different properties from those of naturally occurring materials. For example, negative-index materials (NIMs) are structures artificially engineered to have a negative index of refraction. This means that light travelling through such materials is bent the "wrong way", compared to that in normal materials, which have a positive index.

NIMs have a number of desirable properties that do not exist in normal materials, including the ability to focus light to a point smaller than its wavelength. Scientists have already used these structures to make novel devices such as "invisibility cloaks" and hyperlenses – devices that can image objects much smaller than is possible using an optical microscope. They could also be exploited to make better solar cells, faster computer chips and ultrasensitive sensors.

"Milestone" for understanding the fundamental physics of optical materials

"The unconventional behaviour of metamaterials has led scientists to rethink and re-evaluate many of the established rules of nonlinear optics," Cai tells nanotechweb.org. "Over 10 years ago, a number of theoretical physicists predicted that if the two waves (the fundamental and the harmonic) possess opposite indices of refraction, then the harmonic output will travel back towards the source of the fundamental wave. We have now seen this new type of backward phase-matching condition."

Although real-world applications are still some way off, this research represents a milestone for understanding the fundamental physics of optical materials, he says.

Designing custom-made nonlinear structures

Observing backward phase matching was no easy task, adds team member Shoufeng Lan. "It was daunting to produce a negative index material big enough to allow us to see the effect, not to mention the difficultly in tailoring the refractive index at both the fundamental and harmonic frequencies at the same time."

The team says that it would now like to explore other phenomena in nonlinear materials, not just backward phase matching. "These metamaterials could revolutionize the entire field of nonlinear optics," says team member Sean Rodrigues. "Previous work on these materials mainly focused on looking at various high-order processes in different nonlinear crystals, but being able to design custom-made nonlinear structures – which we can do with metamaterials – is bound to open up an entirely new arena in which to explore nonlinear light-matter interactions."

For more on the latest developments in nanophotonics, visit the Nanotechnology focus collection celebrating the 2015 International Year of Light.