Caltech scientists make reverse refraction

Scientists at the California Institute of Technology have created a nano-architected material that exhibits a property that was previously only theoretically possible – refract light backward, regardless of the angle at which the light strikes the material.

This unique and previously theoretical property, known as negative refraction, could have major technological applications.

"Negative refraction is crucial to the future of nanophotonics, which seeks to understand and manipulate the behavior of light when it interacts with materials or solid structures at the smallest possible scales," said Julia Greer, Caltech's Ruben F. and Donna Mettler Professor of Materials Science, Mechanics and Medical Engineering, and one of the senior authors of a paper describing the new material.

Refraction is a common property; think of how a straw in a glass of water appears shifted out of alignment from what is seen above the liquid or the way lenses in eyeglasses focus light. Negative refraction, however, does not just involve shifting light a few degrees to one side; instead, the light is sent in an angle completely opposite from the one at which it entered the material.

This has not been observed in nature but, beginning in the 1960s, was theorized to occur in so-called artificially periodic materials-that is, materials constructed to have a specific structural pattern. Only now have fabrication processes caught up to theory to make negative refraction a reality.

The new material achieves its unusual property through a combination of organization at the nano- and microscale and the addition of a coating of a thin metal germanium film through a time- and labor-intensive process.

Greer is a pioneer in the creation of such nano-architected materials or materials whose structure is designed and organized at a nanometer scale and that consequently exhibit unusual properties. The materials often demonstrate surprising properties, such as exceptionally lightweight ceramics that spring back to their original shape, like a sponge, after being compressed.

Under an electron microscope, the new negative refractive material's structure resembles a lattice of hollow cubes.

Each cube is so tiny that the width of the beams that make up the cube's structure is 100 times smaller than the width of a human hair. The lattice was constructed using a polymer material, which is relatively easy to work with in 3D printing and then coated with germanium.

"The combination of the structure and the coating give the lattice this unusual property," said Ryan Ng, corresponding author of the paper.

Ng conducted this research while a graduate student in Greer's lab and is now a postdoctoral researcher at the Catalan Institute of Nanoscience and Nanotechnology in Spain.

The research team zeroed in on the cube-lattice structure and material as the right combination through a painstaking computer modeling process.

To get the polymer coated evenly at that scale with a metal required the research team to develop an entirely new process. In the end, Ng, Greer, and their colleagues used a sputtering technique in which a disk of germanium was bombarded with high-energy ions that blasted germanium atoms off the disk and onto the surface of the polymer lattice.

"It isn't easy to get an even coating," said Ng. "It took a long time and a lot of effort to optimize this process."

The technology in question has potential applications for telecommunications, medical imaging, radar camouflaging, and computing.

The paper was published in "Nano Letters" and is entitled "Dispersion Mapping in 3-Dimensional Core-Shell Photonic Crystal Lattices Capable of Negative Refraction in the Mid-Infrared."

With the ability to turn light almost in on itself, perhaps we are one step closer to developing the kind of invisibility so often seen in science fiction. Regardless, it's one step closer to understanding mechanisms of light and perhaps the first steps toward mastering light itself.