A Superlens That Assembles Itself

Easily made nanolenses can perform superhigh-resolution lithography and imaging.

By Katherine Bourzac

Korean researchers have created nanoscale lenses with superhigh resolution using a novel self-assembly method. So far, they've demonstrated that the tiny lenses can be used for ultraviolet lithography, for imaging objects too tiny for conventional lenses, and for capturing individual photons from a light-emitting nanostructure called a quantum dot.

Self-assembly line: The spherical nanolenses shown in this electron microscope image have separated from the crystalline nanotubes underneath them. The structures spontaneously form when a solution of cup-shaped organic molecules is allowed to evaporate. Credit: Nature

The limits on the resolution of both light microscopes and the photolithographic instruments used by the semiconductor industry are a consequence of light's fundamental properties. Because of the way light scatters, or diffracts, even a perfect lens cannot distinguish two objects that are closer together than half the wavelength of the light used to image them.

Other researchers are making devices that overcome the diffraction limit using so-called metamaterials, which bend light in unnatural ways, or nanoscale metal gratings, which capture light through surface interactions. The new lenses, developed by researchers at the Pohang University of Science and Technology in Korea, overcome the diffraction limit because of their size. The lenses are flat on one side and spherical on the other and range in diameter from about 50 nanometers to three micrometers.

The size of each lens is on the same length scale as the wavelength of light that it interacts with, meaning that "the usual optics don't hold," says Chee Wei Wong, head of the Optical Nanostructures Laboratory at Columbia University in New York, who helped evaluate the lenses' performance. And it is the first time the properties of a spherical lens this small have been tested, says Kwang Kim, head of the Center for Superfunctional Materials at Pohang University, who led the research. "No ideal nanoscale lens was available in the past," says Kim.

Kim's team makes the tiny spherical lenses by evaporating a solution containing cup-shaped organic molecules. First, the molecules, which are based on carbon rings, are dissolved in an organic solvent; then water is added, and the solution is allowed to slowly evaporate. During the evaporation process, the organic molecules form crystalline nanotubes that form the lenses. By changing the temperature and the evaporation rate, Kim says, it is possible to control the lenses' ultimate size. Once the lenses have formed, they're stable. The work is described in a paper published today in the journal Nature.

"They found a nice way of building a lens," says Nicholas Fang, assistant professor of mechanical science and engineering at the University of Illinois at Urbana-Champaign. Spherical lenses are ordinarily made using multistep lithography to create a mold that is then patterned with polymers and heated, he says. The nanolenses' index of refraction--how the speed of light changes as it moves through them--is also impressive, says Fang.

To examine the properties of the lenses, the Korean researchers manipulated them using the tip of an atomic-force microscope, placing them on various surfaces for imaging or lithography. To demonstrate imaging beyond the diffraction limit, the lenses were used in conjunction with an optical microscope to resolve the details of a chip patterned with metallic stripes 220 nanometers apart. Without the lenses, this microscope, with a resolution limited to about 320 nanometers, couldn't resolve the same stripes. Further imaging studies showed that the lenses could be used to magnify objects by about 2.5 times. And when the lenses were used to focus ultraviolet light for lithography, they could resolve spots as small as 100 nanometers in the open air (the normal limit is about 192 nanometers).

The lenses also work well for near-infrared light, which is used for telecommunications. Infrared-light detectors generally aren't as sensitive as those for other wavelengths, says Columbia's Wong. To demonstrate the sensitivity of their lenses in this range, the Korean team placed a lens on top of a near-infrared light-emitting nanoparticle called a quantum dot and demonstrated improved detection efficiency.

A limitation of these nanolenses, as for other superlenses, is that they work only in what's known as the near field. That is, they can only focus light onto or gather light from objects in extremely close physical proximity, and must be placed on top of the surface or held just hundreds of nanometers from it. Before the nanolenses can be made into practical devices, this problem will need to be addressed.

http://www.technologyreview.com/computing/23040/