Precision machining produces tiny, light-guiding cubes for advancing
info tech

Date:
January 28, 2022
Source:
DOE/Oak Ridge National Laboratory
Summary:
Drilling with the beam of an electron microscope, scientists have
precisely machined tiny electrically conductive cubes that can
interact with light and organized them in patterned structures
that confine and relay light's electromagnetic signal. This
demonstration is a step toward potentially faster computer chips
and more perceptive sensors.



FULL STORY ========================================================================== Drilling with the beam of an electron microscope, scientists at the
Department of Energy's Oak Ridge National Laboratory precisely machined
tiny electrically conductive cubes that can interact with light and
organized them in patterned structures that confine and relay light's electromagnetic signal. This demonstration is a step toward potentially
faster computer chips and more perceptive sensors.


==========================================================================
The seeming wizardry of these structures comes from the ability of their surfaces to support collective waves of electrons, called plasmons, with
the same frequency as light waves but with much tighter confinement. The
light- guiding structures are measured in nanometers, or billionths of
a meter - - 100,000 times thinner than a human hair.

"These nanoscale cube systems allow extreme confinement of light in
specific locations and tunable control of its energy," said ORNL's
Kevin Roccapriore, first author of a study published in the journal
Small. "It's a way to connect signals with very different length scales."
The feat may prove critical for quantum and optical computing. Quantum computers encode information with quantum bits, or qubits, determined
by a quantum state of a particle, such as its spin. Qubits can store
many values compared with the single value stored by a classical bit.

Light -- electromagnetic radiation that propagates by massless elementary particles called photons -- replaces electrons as the messenger in
optical computers. Because photons travel faster than electrons and do
not generate heat, optical computers could have performance and energy efficiency superior to classical computers.

Future technologies may use the best of both worlds.



========================================================================== "Light is the preferred way to communicate with qubits, but you cannot
connect contacts to them directly," said senior author Sergei Kalinin of
ORNL. "The problem with visible light is its wavelengths range from about
380 nanometers for violet to around 700 nanometers for red. That's too
big because we want to make devices only a few nanometers in size. This
work aims to create a framework to move technology beyond Moore's law and classical electronics. If you try to put 'light' and 'small' together,
that's exactly where plasmonics comes into play." And if there's a
great future in plasmonics, the ORNL-led achievement may help overcome a
signal size mismatch that threatens the integration of components made
of different materials. Those hybrid components will need to "talk"
to each other in next-generation optoelectronic devices. Plasmonics may
bridge the gap.

Plasmonic phenomena were first observed in metals, which are conductive
because of their free electrons. The ORNL team used cubes made of a
transparent semiconductor that behaves like a metal -- indium oxide
doped with tin and fluorine.

The fact that the cube is a semiconductor is the key to its energy
tunability.

The energy of a light wave is related to its frequency. The higher
the frequency, the shorter the wavelength. Wavelengths of visible light
appear to the human eye as colors. Because a semiconductor can be doped --
that is, a small impurity can be added -- its wavelength can be shifted
on the spectrum.

The study's cubes were each 10 nanometers wide, which is much smaller
than the wavelength of visible light. Synthesized at the University of
Texas at Austin by Shin-Hum Cho and Delia Milliron, the cubes were placed
in a detergent to prevent clumping and pipetted onto a substrate, where
they self-assembled into a two-dimensional array. A shell of detergent surrounded each cube, spacing them apart evenly. After the detergent
was removed, the arrays were sent to ORNL.



========================================================================== "That the cubes do not directly touch is important for the collective behavior," said Roccapriore, who organized the cubes into diverse
structures.

"Each cube individually has its own plasmon behavior. When we bring them together in geometries like a nanowire, they talk to one another and
produce new effects that are not typically seen in similar geometries
that aren't made up of individual elements." The study builds on prior
work to sculpt three-dimensional structures as small as a nanometer with
an electron beam. "The current paper proves that the plasmonic effect,
as well as the structure, can be sculpted," Roccapriore said.

"At the end of the day, we're interested in the electron wave -- where
is it and what is its energy? We're controlling those two things."
Kalinin added, "We want to transition from using what exists in nature by chance to fabricating materials with the right responses. We can take a
system of cubes, shine light on it and channel energy into small volumes localized exactly where we want them to be." The project was a natural
for Roccapriore, who conducted a lot of electron-beam lithography in
graduate school and even built a machine in his garage to make and mill 3D-printed structures. At ORNL, experimenting with the beam of an electron microscope, he adjusted its current to intentionally shift from imaging to modification mode. He found he could remove bits of cubes or entire cubes
from an array to make patterned objects at will. He also discovered that,
just like addition of chemical elements enables tuning of cube energies,
so too does selective removal of chemical elements. Such atomic precision
is possible with scanning transmission electron microscopy, or STEM.

The key to characterizing plasmonic behavior within single cubes and
among collective cube assemblies was a technique called electron energy
loss spectroscopy. It uses a STEM instrument with an electron beam
filtered to energies within a narrow range. The beam loses energy as
its electrons pass through the sample, interact with electrons in the
material and transfer a little energy to the system by exciting plasmons.

Electron energy loss spectroscopy provides deep insights into exotic
physics and quantum phenomena related to plasmonic behavior," said
co-author Andrew Lupini of ORNL, who helped map the energies of
electrons in the cubes and arrays of cubes. Lupini is one of the
developers of aberration-corrected STEM, which made pioneering
advances possible. "Electron energy loss spectroscopy lets us
analyze evolving plasmonic responses in real time as the cubes
are sculpted. We can figure out relationships between arrangements
of cubes and their plasmonic properties." The scientists plan to
create a library of relationships between materials, structures and
plasmonic properties. That new knowledge will provide the foundational understanding needed to eventually mass-produce structures that can direct
the flow of light in plasmonic nanocircuits. According to Roccapriore,
"the idea is to understand the relationships using machine learning
and then automate the process." Video: https://youtu.be/AUf7FW633n0 ========================================================================== Story Source: Materials provided by
DOE/Oak_Ridge_National_Laboratory. Note: Content may be edited for style
and length.


========================================================================== Related Multimedia:
* 10_nanometer_wide_cubes ========================================================================== Journal Reference:
1. Kevin M. Roccapriore, Shin‐Hum Cho, Andrew R. Lupini, Delia J.

Milliron, Sergei V. Kalinin. Sculpting the Plasmonic Responses of
Nanoparticles by Directed Electron Beam Irradiation. Small, 2021;
18 (1): 2105099 DOI: 10.1002/smll.202105099 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220128141327.htm

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