A research team used a unique optical metamaterial with a refractive index of zero to generate “phase mismatch–free nonlinear light,” meaning the generated light waves move through the material gaining strength in all directions. This phase mismatch-free quality holds promise for quantum computing and networking, and future light sources based on nonlinear optics – the phenomena that occur when interactions with light modify a material’s properties.

“In our demonstration of nonlinear dynamics in an optical metamaterial with zero-index refraction, equal amounts of nonlinearly generated waves are observed in both forward and backward propagation directions,” says Zhang. “The removal of phase matching in nonlinear optical metamaterials may lead to applications such as efficient multidirectional light emissions for novel light sources and the generation of entangled photons for quantum networking.”

Metamaterials are artificial nanofabricated constructs whose optical properties arise from the physical structure of their superlattices rather than their chemical composition. They’ve garnered a lot of attention in recent years because their unique structure affords electromagnetic properties unattainable in nature. For example, a metamaterial can have a negative index of refraction, the ability to bend light back towards the source, unlike materials found in nature, which always bend light forward away from the source.

"The concept of phase-mismatch free nonlinear interactions provides a new degree of freedom in controlling the nonlinear dynamics in a metamaterial."Kevin O’Brien, co-lead author.

In their work with metamaterials, Zhang and his research group have generated the world’s first optical invisibility cloak, mimicked black holes, and created the first plasmonic nanolasers.

Nonlinear optical processes are always a challenge to achieve and maintain because of the phase-mismatch problem. The interaction of intense laser light with a nonlinear material can generate new light of a different color, but can also lead to the re-absorption of previously generated photons, depending 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, known as “phase mismatch” in the jargon of nonlinear optics.

Previously, it was demonstrated that a metamaterial could be engineered to yield a net refractive index of zero. A beam of light shined through the superlattice of this zero-index metamaterial was unaffected, as if it had passed through a vacuum. The Berkeley researchers surpassed this effort by engineering a zero-index metamaterial that actually generates light through a nonlinear process. This metamaterial features a fishnet structure – a stack of metal-dielectric multilayers with perforated holes. The fishnet consists of 20 alternating layers of gold films 30 nanometers thick and magnesium fluoride films 50 nanometers thick on a 50 nanometer thick silicon nitride membrane.

The researchers tested their metamaterial using a technique called four-wave mixing, in which three beams of light mix in a non-linear medium to create a fourth. Equal amounts of nonlinearly generated waves were observed in both forward and backward propagation directions.

O’Brien and Suchowski have compared the emission of light waves throughout their zero-index metamaterial to that of positive- and negative-index materials by drawing an analogy with the generation of water waves from rocks dropped in a pond.

This story is reprinted from material from Berkeley Lab, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.