— Concentric rings of plastic on gold allow an optical microscope to resolve objects too small to otherwise be seen (Image: Science/Maryland University)
A "superlens" that refracts light in unconventional ways to let an optical microscope see beyond the normal limit of its vision has been created by US researchers. They hope to develop a cheaper, mass-produced version that could upgrade the microscopes used in research laboratories worldwide.
Optical lenses can only resolve details down to those that are half the wavelength of light in size a few hundred nanometres. Light waves carrying information about these tiny features do not travel more than a few hundred nanometres because of interference and diffraction.
"By the time light reaches the lens a great deal of information has been lost the information about sub-wavelength features," explains Allan Boardman, an expert on negative refractive materials at the Salford University in the UK, who was not involved with the US research. "A superlens can collect that."
The superlens created by Igor Smolyaninov, Yu-Ju Hung, and Christopher Davis at Maryland University, US, magnifies this light so it can be seen by a conventional microscope.
The lens is made from an arrangement of concentric plastic rings, spaced about 500 nanometres apart, on top of a gold surface. In experiments, this surface was used to image a pattern of plastic dots, deposited in the centre of all the rings like a bull's-eye. The dots are too small and too close together to be distinguished with an optical microscope, but the superlens makes it possible.
A laser was shone onto the dots, exciting electrons from the gold surface into waves called plasmons. These waves ripple through electrons on the surface at the speed of light and, when they reach the concentric plastic rings, the waves are refracted. "But they don't experience it like a normal lens," Smolyaninov explains. "They are refracted the opposite way to usual."
As a result, the plasmon rays reflected by the central, nanoscopic dots diverge, which effectively magnifies the image they carry. Since the electrons in the plasmon wave also emit light, the resulting image can then be observed on the outer rings by a conventional optical microscope.
Using this method Smolyaninov and colleagues achieved a resolution of 70nm, or one-seventh the wavelength of the light used. This is four times better than would be possible with light alone.
The team are now altering the shape of the rings to improve the quality of images. "By stacking multiple copies of the flat devices it should be possible to use it in 3D," says Smolyaninov.
The team hope in particular to make a version that could offer an instant upgrade to biologists. "We are interested in developing a slide for a normal microscope that will allow samples like viruses or DNA to be imaged below the diffraction limit," says Davis.
In the same issue of the journal Science another team, from the University in California, Berkeley, also in the US, reveal a similar "hyperlens" also based on plasmons. This operates in a similar way to earlier versions (see 'Superlens' has its reach extended), but produces slightly better images.
However, the Maryland researchers claim their device should easier to mass produce since it is made using electron beam lithography, a process already widely used in the electronics industry.
Journal references: Science (vol 315, pps 1686, 1689)