A new kind of lens reaches an unprecedentedly sharp focus by giving up on being perfect. The lens is the first ever to help take visual light images of structures smaller than 100 nanometers, which could make it useful for nanotechnology and probing the insides of cells.
Ordinary lenses, like those used in magnifying glasses, have curved surfaces that bend light to a single point. A small object sitting at that point appears larger and sharply focused, helping myopic readers discern fine print and old-school detectives search for fingerprints. But conventional lenses need to be almost perfect to work. Scratches and roughness destroy the clear image.
“Every deviation from the perfect surface results in a deteriorated focus,” said Elbert van Putten, a graduate student at the University of Twente in the Netherlands. “And in practice you’ll always see surface defects.”
The smallest object that physicists have managed to focus a single conventional lens on is 200 nanometers across, just larger than the smallest known bacteria (although more complicated microscopy systems have reached down to 50 nanometers). But a lot of structures that physicists and chemists are interested in, like sub-cellular structures, nanoelectric circuits and photonic structures, are less than half that size.
To push the focal limit down below 100 nanometers, van Putten and colleagues abandoned the idea of a perfect lens.
“We took a completely different approach: We deliberately made the surface porous so that it strongly scatters light,” van Putten said. The results were published May 13 in Physical Review Letters.
The researchers started with a 400-nanometer thick wafer of gallium phosphide, a material that strongly slows light that travels through it. Then they etched a random pattern of scratches and holes into the wafer’s surface using sulfuric acid.
When light hits the holey wafer, it scatters off in all directions — exactly the opposite of what you normally want from a lens. But where ordinary lenses focus light after it passes through the glass, the scattering lens manipulates the light before it ever hits the rough surface.
The researchers analyzed the patterns the scattered light made and computed the pattern the incoming light waves would need to have in order for the lens to converge them to one spot. They then programmed a laser to send this adjusted light through the lens.
“Even though light is scattered into all directions, you can steer it into one spot again,” van Putten said.
To test their scattering lens, van Putten and colleagues took photographs of gold nanoparticles 97 nanometers across. The resulting image (above, right) was much sharper than the blurry print taken with a conventional lens (left).
“The focus is always at the theoretical limit, as sharp as it could be,” van Putten said. “We’re not hindered anymore by surface errors.”
Image: Courtesy of Elbert van Putten.
Citation:
Scattering Lens Resolves Sub-100 nm Structures with Visible Light. E.G. van Putten, D. Akbulut, J. Bertolotti, W.L. Vos, A. Lagendijk, and A.P. Mosk. Physical Review Letters, vol. 106, May 13, 2011. DOI: 10.1103/PhysRevLett.106.193905.
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