Metamaterials don’t exist in nature, but their ability to make ultra-thin lenses and ultra-efficient cell phone antennas, bend light to keep satellites cooler and let photovoltaics absorb more energy mean they offer a world of possibilities.
Formed by nanostructures that act as “atoms,” arranged on a substrate to alter light’s path in ways no ordinary material can achieve, these surrogate substances can manipulate an incoming light beam to enable the creation of more efficient versions of ubiquitous, valuable devices — optical filters, lasers, frequency converters and devices that steer beams, for example.
But extensive commercial use of metamaterials has been restrained by the limitations imposed by the materials comprising them. Metal-based metamaterials are “lossy” (lose energy) at shorter wavelengths and can operate effectively only at low frequencies, such as the radio frequencies used by radar, before being overwhelmed by their own absorption. Silicon doesn’t emit light and can transmit it only in a limited wavelength range because of its narrow working range (bandgap). So neither class of material can create a metamaterial that will operate in the infrared and optical ranges, where most military and commercial applications would take place.
Sandia National Laboratories researchers are helping lead the way to the use of III-V semiconductors as the building blocks of metamaterials. (III-V refers to elements in those columns in the periodic table.) Sandia researchers have published technical papers, including three in the past year, on work featuring materials like gallium-arsenide and aluminum-arsenide, which are more efficient than metals for optical metamaterial applications, with wider bandgap ranges than silicon. The work is promising enough to have been featured on the covers of two technical journals.
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.