A new discovery by U-M researchers could see energy from
the sun harnessed without solar cells (Image: dingbat2005 via
Flickr)
It has long been thought that, even though light has electric
and magnetic components, the effects of the magnetic field are
so weak that they could effectively be ignored. Now researchers
at the
University of Michigan (U-M) have discovered that under the
right conditions, a light field can generate magnetic effects
that are 100 million times stronger than previously expected.
The researchers say the discovery paves the way for the creation
of an "optical battery" that could harness power from the sun
without the use of
solar cells.
Stephen Rand, a professor in the departments of Electrical
Engineering and Computer Science, Physics and Applied Physics,
and his colleagues found that if light focused to an intensity
of 10 million watts per square centimeter (W/cm2) is traveling
through a material that does not conduct electricity, such as
glass, the light field can generate magnetic effects with the
strength equivalent to a strong electric effect.
"This could lead to a new kind of solar cell without
semiconductors and without absorption to produce charge
separation," Rand said. "In solar cells, the light goes into a
material, gets absorbed and creates heat. Here, we expect to
have a very low heat load. Instead of the light being absorbed,
energy is stored in the magnetic moment. Intense magnetization
can be induced by intense light and then it is ultimately
capable of providing a capacitive power source."
William Fisher, a doctoral student in applied physics at U-M,
says that a previously undetected brand of "optical
rectification" is what makes this possible. In traditional
optical rectification, light's electric field causes a pulling
apart of the positive and negative charges in a material, which
sets up a voltage similar to that in a battery. This electric
effect had previously been detected only in crystalline
materials that possessed a certain symmetry, but Rand and Fisher
found that light's magnetic field can also create optical
rectification in other types of materials, under the right
circumstances.
"It turns out that the magnetic field starts curving the
electrons into a C-shape and they move forward a little each
time," Fisher said. "That C-shape of charge motion generates
both an electric dipole and a magnetic dipole. If we can set up
many of these in a row in a long fiber, we can make a huge
voltage and by extracting that voltage, we can use it as a power
source."
Although the light must be focused through a non-conductive
material to an intensity of 10 million W/cm2, which is much
higher than the roughly 0.136 W/cm2 intensity of sunlight on its
own, the researchers are looking for new materials that would
work at lower intensities.
"In our most recent paper, we show that incoherent light like
sunlight is theoretically almost as effective in producing
charge separation as laser light is," Fisher said.
The researchers predict that with improved materials, they
could achieve 10 percent efficiency, which is comparable to
today's commercial-grade solar cells. They add that, because
their technique doesn't require the extensive semiconductor
processing required for traditional solar cells, it could also
make solar power much cheaper.
"All we would need are lenses to focus the light and a fiber
to guide it. Glass works for both. It's already made in bulk,
and it doesn't require as much processing. Transparent ceramics
might be even better," said Fisher.
Over the summer, the researchers will first work on
harnessing power with laser light, and then sunlight.
The researcher's paper is titled, "Optically-induced charge
separation and terahertz emission in unbiased dielectrics," and
the University
of Michigan is pursuing a patent for the technology.
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