Light has both electric and magnetic effects, but until a recent discovery by physicists at the University of Michigan (UM), scientists believed the magnetic properties of light were so weak that it would be useless for practical applications.
Stephen Rand, UM professor of applied physics, and his colleagues have turned that theory upside down. They discovered that, at the right intensity and when light is traveling through a nonconductive material, the isolated magnetic properties were enhanced as much as 100 million times than previously expected.
The researchers found a way to make an “optical battery” and, in the process, overturned a century-old tenet of physics. In a practical sense, the discovery opens a new way of sourcing solar energy less expensively than making traditional photovoltaic solar cells.
Rand, who authored the research paper “Optically Induced Charge Separation and Terahertz Emission in Unbiased Dielectrics,” which was recently published in the Journal of Applied Physics, said, “You could stare at the equations of motion all day, and you will not see this possibility. We’ve all been taught that this doesn’t happen. It’s a very odd interaction. That’s why it’s been overlooked for more than 100 years.”
UM is pursuing patent protection for the intellectual property.
The discovery has to do with optical rectification, where light’s electrical field causes positive and negative charges of a material to be forcibly tugged apart, causing an electrical charge. Until now, this had only been seen in crystalline materials that possessed a certain symmetry, such as monocrystalline and polycrystalline silicon used in solar cells. But under the right circumstances and with nonconductive materials, light’s magnetic field can create optical rectification in something as common as glass.
“It turns out that the magnetic field starts curving the electrons into a C shape, and they move forward a little each time,” said William Fisher, a doctoral student in applied physics at UM. “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.”
However, there’s one hurdle that must be overcome: the light must be focused at an intensity of 10 million watts-per-square-centimeter, but Fisher said new materials that would work at lower intensities are being developed.
Otherwise, the breakthrough could lead to a new breed of solar cells that are as powerful as thin-film solar panels but much cheaper. With improved materials, Rand and Fisher believe they could achieve 10 percent efficiency in converting solar power to usable energy. That’s equivalent to today’s commercial--grade solar cells.