UW Team
Refrigerates Liquid
With a Laser
for the First Time
by Jennifer Langston,
University of Washington,
November 16, 2015
Since the first
laser was invented in 1960, they’ve almost always given off heat — either as a
useful tool, a byproduct or a fictional way to vanquish intergalactic enemies.
But those
concentrated beams of light have never been able to cool liquids. University of Washington researchers are the first to
solve a decades-old puzzle — figuring out how to make a laser refrigerate water
and other liquids under real-world conditions.
“Typically, when you
go to the movies and see Star Wars laser blasters, they heat things up. This is
the first example of a laser beam that will refrigerate liquids like water
under everyday conditions,” said senior author Peter Pauzauskie,
UW assistant professor of materials science and engineering. “It was really an
open question as to whether this could be done because normally water warms
when illuminated.”
The discovery could
help industrial users “point cool” tiny areas with a focused point of light.
Microprocessors, for instance, might someday use a laser beam to cool specific
components in computer chips to prevent overheating and enable more efficient
information processing.
Scientists could
also use a laser beam to precisely cool a portion of a cell as it divides or
repairs itself, essentially slowing these rapid processes down and giving
researchers the opportunity to see how they work. Or they could cool a single
neuron in a network — essentially silencing without damaging it — to see how
its neighbors bypass it and rewire themselves.
“There’s a lot of
interest in how cells divide and how molecules and enzymes function, and it’s
never been possible before to refrigerate them to study their properties,” said
Pauzauskie, who is also a scientist at the U.S. Department of Energy’s Pacific
Northwest National Laboratory in Richland, Washington.
“Using laser cooling, it may be possible to prepare slow-motion movies of life
in action. And the advantage is that you don’t have to cool the entire cell,
which could kill it or change its behavior.”
The UW team chose
infrared light for its cooling laser with biological applications in mind, as
visible light could give cells a damaging “sunburn.” They demonstrated that the
laser could refrigerate saline solution and cell culture media that are
commonly used in genetic and molecular research.
To achieve the
breakthrough, the UW team used a material commonly found in commercial lasers
but essentially ran the laser phenomenon in reverse. They illuminated a single
microscopic crystal suspended in water with infrared laser light to excite a
unique kind of glow that has slightly more energy than that amount of light
absorbed.
This higher-energy
glow carries heat away from both the crystal and the water surrounding it. The
laser refrigeration process was first demonstrated in vacuum conditions at Los
Alamos National Laboratory in 1995, but it has taken nearly 20 years to
demonstrate this process in liquids.
Typically, growing
laser crystals is an expensive process that requires lots of time and can cost
thousands of dollars to produce just a single gram of material. The UW team
demonstrated that a low-cost hydrothermal process can be used to manufacture a
well-known laser crystal for laser refrigeration applications in a faster,
inexpensive and scalable way.
The UW team also
designed an instrument that uses a laser trap — akin to a microscopic tractor
beam — to “hold” a single nanocrystal surrounded by liquid in a chamber and
illuminate it with the laser. To determine whether the liquid is cooling, the
instrument also projects the particle’s “shadow” in a way that allows the
researchers to observe minute changes in its motion.
As the surrounding
liquid cools, the trapped particle slows down, allowing the team to clearly
observe the refrigerating effect. They also designed the crystal to change from
a blueish-green to a reddish-green color as it cools, like a built-in color
thermometer.
“The real challenge
of the project was building an instrument and devising a method capable of
determining the temperature of these nanocrystals using signatures of the same
light that was used to trap them,” said lead author Paden Roder, who recently received his
doctorate from the UW in materials science and engineering and now works at
Intel Corp.
So far, the UW team
has only demonstrated the cooling effect with a single nanocrystal, as exciting
multiple crystals would require more laser power. The laser refrigeration
process is currently quite energy intensive, Pauzauskie said, and future steps
include looking for ways to improve its efficiency.
One day the cooling
technology itself might be used to enable higher-power lasers for
manufacturing, telecommunications or defense applications, as higher-powered
lasers tend to overheat and melt down.
“Few people have
thought about how they could use this technology to solve problems because
using lasers to refrigerate liquids hasn’t been possible before,” he said. “We
are interested in the ideas other scientists or businesses might have for how
this might impact their basic research or bottom line.”
The research was
funded by the Air Force Office of Scientific Research and the UW, and
benefitted from additional support from the National Science Foundation,
Lawrence Livermore National Laboratory and Pacific Northwest National Laboratory.
