For centuries, scientists and engineers have pushed the
limits of materials to make better lenses. Inventions such as the Fresnel lens
made lighthouses visible from further away and plastics made coke-bottle
eyeglasses a thing of the past. Now a research team at Harvard has made another
leap: a tiny cone-shaped lens that eliminate distortions in everything from cell phone cameras to ligh signals that travel through fiber optic cables.
Ultra-precise lenses are used in telecommunications to focus the beams in fiber-optic systems and in some cell phone cameras. Making them smaller and flatter frees up space and reduces the weight of devices. But existing solid lenses aren't distortion-free, however, and fixing that usually means using multiple lenses, which adds to weight and size.
The Harvard team, led by Frederico Capasso, used a kind of
lens called an axicon. A normal lens takes on a curved shape, but an axicon is
more like a cone. Axicons are used in situations where one needs a steady beam
of light that doesn't spread out.
The axicon lens is made of 60-nanometer thick bits of gold, each
shaped like a tiny "V". The gold changes the amount of time it takes
for the light to pass through the silicon lens. In fact, the amount they change the focus of
the beam is precisely tuned across the surface of the lens.
The precise tuning eliminates the distortions that would
ordinarily accompany any lens. For example, a wide-angle camera lens has a "fish
eye" effect towards the edge of the image. This design would get rid of
that problem.
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Lenses like this could show up in cell phone cameras,
where the space is small and it's difficult to make a lens that gathers enough light, or in fiber-optic communications, where focused light has
to travel long distances without being distorted.
The work appeared in the journal Nano Letters.
Credit: The Capasso Group / Harvard University
