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Nano-architectural material refracts light backwards; an important step towards creating photonic circuits

Nano-architectural material refracts light backwards;  an important step towards creating photonic circuits
Scanning Electron Microscopy (SEM) image of the nanoscale grid. Credit: California Institute of Technology

A newly created nano-architectured material exhibits a property that was previously only theoretically possible: it can refract light backwards, regardless of the angle at which the light strikes the material.

This property is known as negative refraction, and it means that the refractive index - the speed at which light can travel through a given material - is negative across part of the electromagnetic spectrum at all angles.

Refraction is a common property in materials; think about how a straw in a glass of water looks staggered to the side, or how lenses in glasses focus the light. But negative refraction does not only involve moving the light a few degrees to one side. Instead, the light is sent at an angle exactly opposite that where it entered the material. This has not been observed in nature, but from the 1960s it was theorized to occur in so-called artificially periodic materials - that is, materials designed to have a specific structural pattern. Only now have manufacturing processes caught up with the theory to make negative refraction a reality.

"Negative refraction is crucial to the future of nanophotonics, which seeks to understand and manipulate the behavior of light when interacting with materials or solid structures on the smallest possible scales," said Julia R. 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. The magazine was published in Nano letters on October 21st.

The new material achieves its unusual property through a combination of nano- and micro-scale organization and the addition of a coating of a thin metal-germanium film through a time-consuming and labor-intensive process. Greer is a pioneer in the creation of such nanoarchitectured materials or materials whose structure is designed and organized on a nanometer scale, and which consequently exhibit unusual, often surprising properties - for example, unusually light ceramics that jump back to their original form, such as a fungus, after being compressed.

Under an electron microscope, the structure of the new material resembles a lattice of hollow cubes. Each cube is so small that the width of the beams that make up the structure of the cube is 100 times smaller than the width of a human hair. The grid is constructed of a polymeric material that is relatively easy to work with in 3D printing, and then coated with the metal germanium.

"The combination of structure and coating gives the grid this unusual property," says Ryan Ng (MS '16, Ph.D. '20), the corresponding author of the Nano Letters paper. Ng conducted this research while a graduate student in Greer's laboratory and is now a postdoc researcher at the Catalan Institute of Nanoscience and Nanotechnology in Spain. The research team focused on the cube lattice structure and the material as the right combination through a careful computer modeling process (and knowledge that geranium is a high-index material).

In order to get the polymer coated evenly on that scale with a metal, the research team required to develop a completely new method. Finally, Ng, Greer, and their colleagues used an atomization technique in which a disk of germanium was bombarded with high-energy ions that blew germanium atoms out of the disk and onto the surface of the polymer lattice. "It's not easy to get an even coating," says Ng. "It took a long time and a lot of effort to optimize this process."

The technology has potential applications for telecommunications, medical imaging, radar camouflage and data processing.

In a 1965 observation, Caltech alumni Gordon Moore (Ph.D. '54), a lifetime member of the Caltech Board of Trustees, predicted that integrated circuits would become twice as complicated and half as expensive every two years. However, due to the fundamental limits of power loss and transistor density allowed by current silicon semiconductors, the scaling predicted by Moore's law should soon end. "We are nearing the end of our ability to follow Moore's law; making electronic transistors as small as they can be," says Ng. The current work is a step towards demonstrating optical properties that would be necessary to activate 3D photonic circuits. Because light moves much faster than electrons, 3D photonic circuits would in theory be much faster than traditional ones.

That Nano letters the paper is entitled "Dispersion Mapping in 3-Dimensional Core-Shell Photonic Crystal Lattices Capable of Negative Refraction in the Mid-Infrared."

New materials show a split personality

More information: Victoria F. Chernow et al., Scattering mapping in 3-dimensional core-shell photonic crystal lattices capable of negative refraction in the mid-infrared, Nano letters (2021). DOI: 10.1021 / acs.nanolett.1c02851
Provided by the California Institute of Technology

Citation: Nano-architectural material refracts light backwards; an important step towards creating photonic circuits (2022, 28 January) retrieved 28 January 2022 from

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