Sandia develops 3D metasurfaces with optical possibilities

The work is promising enough to have been featured on the covers of two technical journals. "There is very little work worldwide on all-dielectric metamaterials using III-V semiconductors," said Sandia researcher Igal Brener, who leads the Sandia work with researchers Mike Sinclair and Sheng Liu. "Our advantage is Sandia's vast access to III-V technology, both in growth and processing, so we can move pretty fast."

The new Sandia dielectric materials—a kind of electrical insulator—offer more than just efficiency. They lose little incoming energy and can even be fabricated in multiple layers to form complex, three-dimensional meta-atoms that reflect more light than shiny gold surfaces, usually considered the ultimate in infrared reflectivity. The III-V materials also emit photons when excited—something that silicon, which can reflect, transmit and absorb—can't do.

Another advantage is their highly variable outputs, across the color spectrum so they might be used to extend the wavelength range of lasers or for generating "entangled photons" for quantum computing.

Sandia's approach also is attractive for its relatively simple method of forming the artificial atoms, known as resonators, that are the guts of the metamaterial. Created under the supervision of Liu, the meta-atoms are a few hundred nanometers in diameter and made of many actual atoms.

One of Liu's improvements was to oxidise these tiny groupings around their perimeters to create layered coatings with a low index of refraction, rather than use a more expensive, time-consuming "flip-chip" bonding process.

The complexity of previous methods was an obstacle to cost- and time-efficiency. Other Sandia researchers had used a variant of his simplification previously to make lasers, but not metamaterials, he said.

The oxidised, low-index surface surrounds 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." Put another way, interior light bumping into the low-indexed oxide surface is herded back by the refractive difference so it travels along the high-index core.

Liu's Sandia colleague Gordon Keeler achieved controlled oxidation simply by putting III-V materials in a hot oven and flowing water vapor over the sample. "It will oxidise at a certain rate," Liu says. "The more material, the longer it takes."

The man-made meta-atoms are sculpted in place during a lithographic process that permits researchers to make any pattern they chose for the placement of the metamaterial components. "We use simulations to direct us," Liu said. Spacing is determined to some extent by the size of the manmade atoms.

The researchers experimented with cylindrical and cubic nanostructures, reducing the symmetry of the latter to achieve even better properties. "Cylinders are much easier to fabricate and typically can be used for conventional metasurfaces," said Brener. "But broken-symmetry cubes are crucial to obtain very sharp resonances. That's the key issue of the paper."

The idea of intentionally reducing the symmetry of a cubic resonator nanostructure originated five or six years ago, said Sinclair, with a serendipitous design that happened to break the intentionally symmetrical shape of the meta-atoms when the team tried to mimic a particular manufacturing flaw.

"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.

Prior to that discovery, dielectric resonator metamaterials only showed broad bands that didn't trap much energy. The researchers found the new sharp resonances allowed greater energy storage—beneficial for efficient frequency conversion, and perhaps even for light emission and lasing.

Exploration of the crimped resonator had to wait for a later project, sponsored by the Department of Energy's Office of Science. Salvatore Campione, building on previous work by Lorena Basilio, Larry Warne and William Langston—all of Sandia—used electromagnetic simulations to unravel precisely how the cubes trap light. Sandia's Willie Luk measured the cubes' reflective properties. Another LDRD grant currently supports research into metamaterial lasing.

"We feel we've created a pretty flexible platform for a lot of different kinds of devices," Sinclair said. The ongoing work is aided by Sandia's John Reno, nationally known for growing extremely precise crystalline structures, who contributed the III-V wafers.