Power/Performance Bits: Dec. 10

Optical Metamaterial with a Refractive Index of Zero
Most of the time we hear about the need for coherent light sources, such as those produced by lasers, but there may be equal promise looking in the other direction.

Quantum processors promise to be many times faster and more powerful than today’s supercomputers, but to get to that point they will need fast and efficient multi-directional light sources. Researchers at the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have taken an important step toward efficient light generation, the foundation for future quantum networks.

In a study led by Xiang Zhang, a faculty scientist with Berkeley Lab’s Materials Sciences Division, the 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 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. (Image courtesy of Zhang group)

“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.

“Nonlinear optics phenomena play important roles in materials sciences, physics and chemistry,” Zhang says. “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 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.

“Phase mismatch is one reason why nonlinear optical processes are not common in everyday life,” says Haim Suchowski, a member of Zhang’s research 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.”
Adds O’Brien, “Moreover, all phase mismatch compensation schemes work only in one specific direction, either forward or backward but not both. 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.”

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.

Nature Provides New Battery Material
Researchers from Carnegie Mellon University have found that ink from the cuttlefish, a close relative of the squid, provides the perfect chemistry and nanostructure to power tiny electronic devices that can be either ingested or implanted into the body for applications ranging from biosensing to drug delivery.

Image courtesy of CMU.

“We found that the melanin pigments in cuttlefish ink make it a perfect fit for use in battery electrodes that would ultimately be used in devices that operate in close proximity to sensitive living tissue,” said Bettinger, an assistant professor in the departments of Materials Science and Engineering (MSE) and Biomedical Engineering. Melanin is the pigment responsible for the dark color of skin, hair, scales, and is also found in animals.

CMU researchers have shown that naturally occurring melanins exhibit higher charge storage capacity compared to other synthetic melanin derivatives when used as anode materials. Pigment-based anodes are an important component in sodium-ion batteries, a battery technology that has been pioneered by Whitacre, an associate professor of materials science and engineering and engineering and public policy.

At present, high-performance energy storage systems for medical devices are designed to supply power to semi-permanent devices that are often encapsulated. These scenarios permit the use of potentially toxic electrode materials and electrolytes. Electronically active medical devices that are either biodegradable or ingestible require new energy storage materials that are benign and can operate in hydrated environments. Melanin-based electrodes represent a step closer to this goal.

“Our research shows that alternative systems that use biocompatible electrode materials with aqueous sodium-ion batteries could provide onboard energy sources for a variety of temporary medical devices including biodegradable electronic implants and ingestible systems,” Bettinger said.