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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