Metamaterials are not found in nature, but they do have the potential for a world of applications with their ability to bend light to keep satellites cooler, make ultra-efficient cell phone antennas and ultra-thin lenses, and allow photovoltaics to absorb more energy.
These materials are formed by nanostructures that behave as “atoms,” arranged on a substrate to modify light’s path in unique ways not possible by ordinary material. These surrogate substances can control an incoming light beam to facilitate the development of more efficient versions of ubiquitous, helpful devices - lasers, frequency converters, optical filters, and devices that steer beams, for instance.
However widespread commercial use of metamaterials has been limited by the restrictions imposed by the materials comprising them. At shorter wavelengths, metal-based metamaterials are “lossy” (lose energy) and can function efficiently only at low frequencies, for instance radio frequencies used by radar, prior to being overwhelmed by their own absorption.
Silicon does not discharge light and can onlyconvey it in a limited wavelength range due to its narrow working range (bandgap). Therefore neither class of material can form a metamaterial that will function in the optical and infrared ranges, where most commercial and military applications would occur.
Optical Metamaterials Enter the Arena
Researchers at Sandia National Laboratories are leading the way to the utilization of III-V semiconductors as the building blocks of metamaterials. (III-V is a reference to the elements in those columns in the periodic table.)
The researchers have published technical papers, including three in the last year, on work highlighting materials like aluminum-arsenide and gallium-arsenide, which are more efficient than metals for optical metamaterial applications, with broader bandgap ranges compared to that of silicon. The research holds enough promise to be featured on the covers of two technical journals.
There is very little work worldwide on all-dielectric metamaterials using III-V semiconductors. Our advantage is Sandia’s vast access to III-V technology, both in growth and processing, so we can move pretty fast.
Igal Brener, Researcher, Sandia National Laboratories
Brener leads the work with researchers Mike Sinclair and Sheng Liu.
Shinier than Gold
The new Sandia dielectric materials, which are a type of electrical insulator, provide a lot more than just efficiency. They lose little incoming energy and can even be fabricated in numerous layers to form intricate, 3D meta-atoms that reflect more light than shimmering gold surfaces, typically said to be the ultimate in infrared reflectivity.
The III-V materials also discharge photons when excited, which is something silicon cannot do despite being able to reflect, transmit and absorb.
Another benefit is their highly unpredictable outputs, across the color spectrum, therefore they might be used for producing “entangled photons” for quantum computing or to lengthen the wavelength range of lasers.
Sandia’s strategy also is appealing due to its comparatively simple technique of creating the artificial atoms, called resonators that are the backbone of the metamaterial.
The meta-atoms developed under the guidance of Liu measure a few hundred nanometers in diameter and are made of several actual atoms. One of Liu’s enhancements was to oxidize these miniature groupings around their perimeters to form layered coatings with a low index of refraction, instead of using a more expensive, time-consuming “flip-chip” bonding procedure.
The complexity of former techniques was a barrier to cost- and time-efficiency. Other Sandia researchers had employed a variant of his simplification formerly to make lasers, but not metamaterials, he said.
The oxidized, low-index surface encloses the high-index core “like in wintertime, you have a coat surrounding you,” Liu said. “To confine light, you need a high refractive-index contrast.” To state it differently, interior light bumping into the low-indexed oxide surface is guided back by the refractive difference so it moves alongside the high-index core.
Liu’s Sandia colleague Gordon Keeler realized controlled oxidation just by placing III-V materials in a hot oven and flowing water vapor over the sample.
It will oxidize at a certain rate. The more material, the longer it takes.
Sheng Liu, Sandia National Laboratires
The synthetic meta-atoms are sculpted in place during a lithographic procedure that allows researchers to create any pattern they want for the positioning of the metamaterial components. “We use simulations to direct us,” Liu said. The size of the synthetic atoms, to a certain extent, determines spacing.
Fractured Cubic Nanostructures Store Unusually Large Amounts of Energy
The researchers experimented with cubic and cylindrical nanostructures, decreasing the symmetry of the former to attain even better properties.
Cylinders are much easier to fabricate and typically can be used for conventional metasurfaces. But broken-symmetry cubes are crucial to obtain very sharp resonances. That’s the key issue of the paper.
Igal Brener, Researcher, Sandia National Laboratories
The plan of deliberately decreasing the cubic resonator nanostructure’s symmetry originated five or six years ago, said Sinclair, with an unexpected design that happened to break the purposely symmetrical shape of the meta-atoms when the team tried to copy a particular manufacturing defect.
“During a Laboratory Directed Research and Development [LDRD] Metamaterials Grand Challenge, when we were first fabricating cubic resonators in our effort to see if we could get beyond microwaves into infrared and optical metamaterials, we were playing with the shape of resonators to try to simulate the effect of lithography errors. In one simulation, we happened to cut a corner of the cube and all of a sudden very sharp reflection bands appeared,” Sinclair said.
Before that discovery, dielectric resonator metamaterials only displayed broad bands that did not capture a lot of energy. The team discovered that the new sharp resonances permitted greater energy storage - advantageous for efficient frequency conversion, and perhaps even for lasing and light emission.
Investigation of the crimped resonator will have to wait for a later project, funded by the Department of Energy’s Office of Science. Salvatore Campione, adding to an earlier study by Sandia researchers Lorena Basilio, Larry Warne and William Langston, employed electromagnetic simulations to decode precisely how the cubes capture light.
Sandia’s Willie Luk measured the reflective properties of the cubes. Another LDRD grant presently sponsors research into metamaterial lasing.
“We feel we’ve created a pretty flexible platform for a lot of different kinds of devices,” Sinclair said.
The continuing research work is assisted by Sandia’s John Reno, nationally recognized for growing very precise crystalline structures, who contributed the III-V wafers.
Three patents on aspects of the research have been submitted.
A major part of the research took place at the Sandia/Los Alamos Center for Integrated Nanotechnologies, a DOE Office of Science User Facility.