The new technique they’ve developed is based on a phenomenon observed in metallic structures known as plasmon resonance. Plasmons are coordinated waves, or ripples, of electrons that exist on the surfaces of metals at the point where the metal meets the air.
While the plasmon resonances of metals are predetermined in nature, Atwater and his colleagues found that those resonances are capable of being tuned to other wavelengths when the metals are made into tiny nanostructures in the lab.
“Normally in a metal like silver or copper or gold, the density of electrons in that metal is fixed; it’s just a property of the material,” Atwater says. “But in the lab, I can add electrons to the atoms of metal nanostructures and charge them up. And when I do that, the resonance frequency will change.”
An ultra-sensitive needle measures the voltage that is generated while the nanospheres are illuminated. Credit: AMOLF/Tremani – Figure: Artist impression of the plasmo-electric effect
“We’ve demonstrated that these resonantly excited metal surfaces can produce a potential”—an effect very similar to rubbing a glass rod with a piece of fur: you deposit electrons on the glass rod. “You charge it up, or build up an electrostatic charge that can be discharged as a mild shock,” he says. “So similarly, exciting these metal nanostructures near their resonance charges up those metal structures, producing an electrostatic potential that you can measure.”
This electrostatic potential is a first step in the creation of electricity, Atwater says. “If we can develop a way to produce a steady-state current, this could potentially be a power source. He envisions a solar cell using the plasmoelectric effect someday being used in tandem with photovoltaic cells to harness both visible and infrared light for the creation of electricity.
Although such solar cells are still on the horizon, the new technique could even now be incorporated into new types of sensors that detect light based on the electrostatic potential.
“Like all such inventions or discoveries, the path of this technology is unpredictable,” Atwater says. “But any time you can demonstrate a new effect to create a sensor for light, that finding has almost always yielded some kind of new product.”
The conversion of optical power to an electrical potential is of general interest for energy applications, and is typically obtained via optical excitation of semiconductor materials. Here, we introduce a new method using an all-metal geometry, based on the plasmon resonance in metal nanostructures. In arrays of Au nanoparticles on an indium-tin-oxide substrate and arrays of 100-nm-diameter holes in 20-nm-thick Au films on a glass substrate, we show negative and positive surface potentials during monochromatic irradiation at wavelengths below or above the plasmon resonance respectively. We observe such plasmoelectric surface potentials as large as 100 mV under 100 mW/cm^2 illumination. Plasmoelectric devices may enable development of entirely new types of all-metal optoelectronic devices that can convert light into electrical energy.