Nano-architected materials have a new property that can reflect light backwards, no matter what angle it is being viewed.
Negative refraction is a property that allows light to travel at a slower speed through certain materials.
Refraction is a common property of materials. Think about how a straw appears to be shifted to one side in glass water or how lenses in eyeglasses focus light. Negative refraction is more than shifting light to one side. Instead, the light is sent at an angle opposite to the one it entered into the material. Although this has never been observed in nature, it was first suggested that artificially periodic materials, which are materials designed to follow a particular structural pattern, may exhibit this phenomenon. Negative refraction is only now possible because fabrication processes are better than the theory.
“Negative refraction is essential to the future in nanophotonics,” Julia R. Greer, Caltech’s Ruben F. and Donna Mettler professor of Materials Science and Mechanics and Medical Engineering and one of the senior writers of a paper describing this new material, says. The paper was published in the journal Nano letters.
This new material has an unusual property because it is a combination of organization at both the micro- and nanoscales and the addition of a thin metal-germanium film. This was a labour-intensive and time-consuming process. Greer is a pioneer in creating nano-architected materials. These materials are designed and organized at nanometer scales and have unusual, sometimes surprising, properties. For example, extremely lightweight ceramics can be compressed and then reshaped to their original form, similar to a sponge.
The structure of the new material looks like a lattice made up of hollow cubes when examined under an electron microscope. The cubes are so small that the widths of the beams which make up the cube’s structure are 100 times smaller than a human hair. The lattice was made using a polymer material that is easy to use in 3-D printing and then coated with metal germanium.
Ryan Ng (MS’16, PhD’20), co-author of the Paper on Nano Letters, said that the lattice’s unusual properties are due to the combination of the structure with the coating. Ng was a graduate student at Greer’s laboratory and is currently a postdoctoral researcher at the Catalan Institute of Nanoscience and Nanotechnology, Spain. Through a meticulous computer modelling process, the research team found the perfect combination of cube-lattice structure & material. Geranium is a high-index material.
The research team had to devise a new way to coat the polymer evenly with a metal. Ng, Greer and their collaborators used a sputtering method in which high-energy ions bombarded a disk of Germanium. This blasted the germanium atoms from the disk onto the surface of a polymer lattice. Ng states that it is difficult to achieve an even coating. It took a lot of time and effort to optimize this process.
This technology could be used for computing, medical imaging, radar camouflaging and telecommunications.
Caltech alumnus Gordon Moore, PhD. ’54 predicted in a 1965 observation that integrated circuits would become twice as complex and half as costly every two years. He was also a life member of Caltech’s Board of Trustees. Moore’s Law predicts that the current silicon semiconductors will soon allow for a fundamental limit on power dissipation, transistor density, and other limitations. Ng states that we are at the end of our ability to follow Moore’s Law and make electronic transistors as small as possible. This work is important in demonstrating the optical properties required for 3-D photonic circuits. The theory is that 3-D photonic circuits would move light faster than electrons.
