A unique optical metamaterial with a refractive index of zero was used to generate light waves that gain strength as they move through the material in all directions. This demonstration of “phase mismatch-free nonlinear light” holds promise for quantum computing and networking, and for future light sources based on nonlinear optics. In a zero-index metamaterial, the phases of propagating light waves are mismatch-free in both directions, whereas in a positive/negative-index material, only the waves propagating in a forward direction are phase mismatch-free. Courtesy of Zhang group, UC Berkeley/Berkeley Lab. Future quantum computers will need fast and efficient multidirectional light sources, and this work at Lawrence Berkeley National Laboratory is an important step toward this goal. “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,” said Xiang Zhang, a faculty scientist with Berkeley Lab’s Materials Sciences Div. “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. A metamaterial can have a negative index of refraction, for example, which gives it the ability to bend light back towards the source, unlike materials found in nature, which always bend light forward, away from the source. From left, Xiang Zhang, Haim Suchowski, Zi Jing Wong, Kevin O’Brien and Alessandro Salandrino have created a nonlinear light-generating zero-index metamaterial that holds promise for future quantum networks and light sources. Courtesy of Roy Kaltschmidt, Berkeley Lab. In their previous metamaterials work, Zhang and his group have generated the first optical invisibility cloak, mimicked black holes, and created the first plasmonic nanolasers. In this latest study, they focused on the nonlinear properties of metamaterials. “Nonlinear optics phenomena play important roles in materials sciences, physics and chemistry,” Zhang said. “Frequency conversion, where photons of different energies merge or divide, is an especially important application of nonlinear optics because it allows the generation of new light sources.” 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 reabsorption 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.” “Phase mismatch is one reason why nonlinear optical processes are not common in everyday life,” said Haim Suchowski, a member of Zhang’s group. “In the past 60 years, since the beginning of nonlinear optics, scientists have been developing techniques to compensate this lack of momentum conservation in order to achieve phase matching. However, all of these techniques have limitations and present their own challenges.” In this graphic showing four-wave mixing in a positive/negative-index (upper) and zero-index (lower) metamaterial, forward-propagating FWM is much stronger than backward FWM for the positive/negative-index material but about the same in both directions for the zero-index metamaterial. Courtesy of Zhang group, UC Berkeley/Berkeley Lab. “Moreover, all phase mismatch compensation schemes work only in one specific direction, either forward or backward, but not both,” said group member Kevin O’Brien. “This restriction arises because the phase-matching process represents a balance between the momenta of the photons involved in the nonlinear interactions, a balance that is disturbed when the momentum of one photon changes sign because of a direction change.” 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 nm 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 metamaterial is always preserved regardless of the direction in which the light waves are generated,” Suchowski said. “We call the interactions of light and our metamaterial phase mismatch-free because the nonlinear light emission is equal in all directions.” The researchers tested their metamaterial using four-wave mixing, in which three beams of light mix in a nonlinear medium to create a fourth. Equal amounts of nonlinearly generated waves were observed in both forward and backward propagation directions. “The concept of phase mismatch-free nonlinear interactions provides a new degree of freedom in controlling the nonlinear dynamics in a metamaterial,” O’Brien said. “In addition to entangled photon generation, we could see the realization of other exotic effects such as bidirectional coherent Raman scattering for remote sensing applications.” Zhang is the corresponding author of a paper in Science on the work (doi: 10.1126/science.1244303) Suchowski, O’Brien, Zi Jing Wong, Alessandro Salandrino and Xiaobo Yin are co-authors. For more information, visit: www.lbl.gov