A new type of device could amplify the light emitted by a nanometer-scale object as much as 10,000 times, improving low-light photography and bringing previously hard-to-see items into view.
“It actually scatters more light. It also can absorb more light,” says Ming Zhou, a doctoral candidate in Zongfu Yu’s Nanophotonics Lab at the University of Wisconsin, Madison.
The device is based on a nanoresonator, which is similar to a laser cavity in that it bounces photons back and forth between mirrors, allowing them to amplify each other. The trouble is that the nanoresonator is so small (only a few hundred nanometers in cross-section for visible wavelengths) that even the amplified light is not bright enough to be easily seen.
The nanoresonator, which is embedded in a material with an index of 0.02, is small compared with the light from a laser beam. But when the beam moves over it, the resonator scatters the light at many times its own diameter, creating a bright circle.Illustration: University of Wisconsin-Madison
But the light can be amplified by a far greater amount if the nanoresonator is embedded in a material with an extremely low—but greater than zero—index of refraction. In vacuum, light bouncing off an object can scatter in any direction, spreading it thin. But when it moves through a material with a very low index of refraction, the options for scattering become much fewer, concentrating the light in a particular direction. Inside the material, the wavelength of light becomes much larger, leading it to resonate much more strongly. The concentrated light that emerges, back at its normal wavelength, is far brighter than what started out. A material with a refractive index of 0.01, for instance, would amplify the light 10,000-fold.
The low-index material could be built by creating a structure that alternates between two different materials; each layer would be 10 to 20 nanometers thick if the wavelength in question is 1000 nm, and even thinner for shorter wavelengths. A structure of this type, Zhou says, is not easy to fabricate. So far, the team has tested its theory only in computer simulations, the results of which the researchers explained in a recent paper published in the journal Physical Review Letters. They are now working on a physical demonstration.
Enhancing light by the amount they describe could magnify objects, and could make photography under extreme low-light conditions easier. Another possible use for such a device would be as a solar concentrator, directing vastly more sunlight onto a given area of a solar cell. “We can absorb much more light than a current device,” says Zhou. “The performance will be better.”