“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.