A newly created nano-architected material exhibits a property that
previously was just theoretically possible: it can refract light backward,
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 that light can travel through a given material—is
negative across a portion of the electromagnetic spectrum at all angles.
Refraction is a common property in materials; think of the way a straw in a
glass of water appears shifted to the side, or the way lenses in eyeglasses
focus light. But negative refraction does not just involve shifting light a
few degrees to one side. Rather, 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 have
caught up to 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 it interacts with
materials or solid structures at the smallest possible scales," says 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 paper was published in Nano Letters
on October 21.
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, often surprising properties—for example,
exceptionally lightweight ceramics that spring back to their original shape,
like a sponge, after being compressed.
Under an electron microscope, the new material's structure resembles a
lattice of hollow cubes. Each cube is so tiny that the width of the beams
making 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 the metal
germanium.
"The combination of the structure and the coating give the lattice this
unusual property," says Ryan Ng (MS '16, Ph.D. '20), corresponding author of
the Nano Letters 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 (and the knowledge that geranium is a
high-index material).
To get the polymer coated evenly at that scale with a metal required the
research team to develop a wholly new method. 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 of the
disk and onto the surface of the polymer lattice. "It isn't easy to get an
even coating," Ng says. "It took a long time and a lot of effort to optimize
this process."
The technology has potential applications for telecommunications, medical
imaging, radar camouflaging, and computing.
In 1965 observation, Caltech alumnus Gordon Moore (Ph.D. '54), a life member
of the Caltech Board of Trustees, predicted that integrated circuits would
get twice as complicated and half as expensive every two years. However,
because of the fundamental limits on power dissipation and transistor
density allowed by current silicon semiconductors, the scaling predicted by
Moore's Law should soon end. "We're reaching the end of our ability to
follow Moore's Law; making electronic transistors as small as they can go,"
Ng says. The current work is a step towards demonstrating optical properties
that would be required to enable 3D photonic circuits. Because light moves
much more quickly than electrons, 3D photonic circuits, in theory, would be
much faster than traditional ones.
The Nano Letters paper is titled "Dispersion Mapping in 3-Dimensional
Core–Shell Photonic Crystal Lattices Capable of Negative Refraction in the
Mid-Infrared."
Reference:
Victoria F. Chernow et al, Dispersion 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
Tags:
Physics