On a hillside above Stanford University, the SLAC National Accelerator
Laboratory operates a scientific instrument nearly 2 miles long. In this giant
accelerator, a stream of electrons flows through a vacuum pipe, as bursts of
microwave radiation nudge the particles ever-faster forward until their
velocity approaches the speed of light, creating a powerful beam that scientists
from around the world use to probe the atomic and molecular structures of
inorganic and biological materials.
Now, for the
first time, scientists at Stanford and SLAC have created a silicon chip that
can accelerate electrons – albeit at a fraction of the velocity of that massive
instrument – using an infrared laser to deliver, in less than a hair’s width,
the sort of energy boost that takes microwaves many feet.
Writing in the Jan. 3 issue of Science, a team led by electrical
engineer Jelena Vuckovic explained how they carved a nanoscale channel out of
silicon, sealed it in a vacuum and sent electrons through this cavity while
pulses of infrared light – to which silicon is as transparent as glass is to
visible light – were transmitted by the channel walls to speed the electrons
along.
The accelerator-on-a-chip demonstrated in Science is just a prototype,
but Vuckovic said its design and fabrication techniques can be scaled up to
deliver particle beams accelerated enough to perform cutting-edge experiments
in chemistry, materials science and biological discovery that don’t require the
power of a massive accelerator.
“The largest
accelerators are like powerful telescopes. There are only a few in the world
and scientists must come to places like SLAC to use them,” Vuckovic said. “We
want to miniaturize accelerator technology in a way that makes it a more
accessible research tool.”
Team members liken their approach to the way that computing evolved from
the mainframe to the smaller but still useful PC. Accelerator-on-a-chip
technology could also lead to new cancer radiation therapies, said physicist
Robert Byer, a co-author of the Science paper. Again, it’s a
matter of size. Today, medical X-ray machines fill a room and deliver a beam of
radiation that’s tough to focus on tumors, requiring patients to wear lead
shields to minimize collateral damage.
“In this paper
we begin to show how it might be possible to deliver electron beam radiation
directly to a tumor, leaving healthy tissue unaffected,” said Byer, who leads
the Accelerator on a Chip International Program, or ACHIP, a broader effort of
which this current research is a part.
Inverse design
In their paper,
Vuckovic and graduate student Neil Sapra, the first author, explain how the
team built a chip that fires pulses of infrared light through silicon to hit
electrons at just the right moment, and just the right angle, to move them
forward just a bit faster than before.
To accomplish
this, they turned the design process upside down. In a traditional accelerator,
like the one at SLAC, engineers generally draft a basic design, then run
simulations to physically arrange the microwave bursts to deliver the greatest
possible acceleration. But microwaves measure 4 inches from peak to trough,
while infrared light has a wavelength one-tenth the width of a human hair. That
difference explains why infrared light can accelerate electrons in such short
distances compared to microwaves. But this also means that the chip’s physical
features must be 100,000 times smaller than the copper structures in a
traditional accelerator. This demands a new approach to engineering based on
silicon integrated photonics and lithography.
Vuckovic’s team
solved the problem using inverse design algorithms that her lab has developed.
These algorithms allowed the researchers to work backward, by specifying how
much light energy they wanted the chip to deliver, and tasking the software
with suggesting how to build the right nanoscale structures required to bring
the photons into proper contact with the flow of electrons.
“Sometimes, inverse designs can produce solutions that a human engineer
might not have thought of,” said R. Joel England, a SLAC staff scientist and
co-author on the Science paper.
The design
algorithm came up with a chip layout that seems almost otherworldly. Imagine
nanoscale mesas, separated by a channel, etched out of silicon. Electrons
flowing through the channel run a gantlet of silicon wires, poking through the
canyon wall at strategic locations. Each time the laser pulses – which it does
100,000 times a second – a burst of photons hits a bunch of electrons,
accelerating them forward. All of this occurs in less than a hair’s width, on
the surface of a vacuum-sealed silicon chip, made by team members at Stanford.
The researchers
want to accelerate electrons to 94 percent of the speed of light, or 1 million
electron volts (1MeV), to create a particle flow powerful enough for research
or medical purposes. This prototype chip provides only a single stage of
acceleration, and the electron flow would have to pass through around 1,000 of
these stages to achieve 1MeV. But that’s not as daunting at it may seem, said
Vuckovic, because this prototype accelerator-on-a-chip is a fully integrated
circuit. That means all of the critical functions needed to create acceleration
are built right into the chip, and increasing its capabilities should be
reasonably straightforward.
The researchers
plan to pack a thousand stages of acceleration into roughly an inch of chip
space by the end of 2020 to reach their 1MeV target. Although that would be an
important milestone, such a device would still pale in power alongside the
capabilities of the SLAC research accelerator, which can generate energy levels
30,000 times greater than 1MeV. But Byer believes that, just as transistors
eventually replaced vacuum tubes in electronics, light-based devices will one
day challenge the capabilities of microwave-driven accelerators.
Meanwhile, in
anticipation of developing a 1MeV accelerator on a chip, electrical engineer
Olav Solgaard, a co-author on the paper, has already begun work on a possible
cancer-fighting application. Today, highly energized electrons aren’t used for
radiation therapy because they would burn the skin. Solgaard is working on a
way to channel high-energy electrons from a chip-sized accelerator through a
catheter-like vacuum tube that could be inserted below the skin, right
alongside a tumor, using the particle beam to administer radiation therapy
surgically.
“We can derive
medical benefits from the miniaturization of accelerator technology in addition
to the research applications,” Solgaard said.
Journal article: https://science.sciencemag.org/content/367/6473/79
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