Thin silver sheet makes superlens

May 18/25, 2005

An ordinary lens can focus light to about half the light's wavelength. Visible lightwaves range from 400 to 700 nanometers, and ultraviolet lightwaves range from 10 to 400 nanometers.

Researchers from the University Of California at Berkeley have fashioned a superlens from a thin sheet of silver that resolves images as small as 60 nanometers using 365-nanometer light.

The superlens could be used in super-resolution microscopes, and promises to extend the ability of existing chip-manufacturing plants to make the smaller transistors that lead to faster computer chips and smaller bits that increase storage capacity, according to the researchers.

Super resolution imaging is possible because the silver lens has a negative index of refraction. Ordinary lenses have a positive index of refraction. A positive index of refraction causes light to bend as it passes from one material to another; this is what causes the bent-drinking-straw illusion where air meets water. A negative index of refraction causes light to bend in the opposite direction.

Negative index of refraction materials also amplify evanescent light -- the light field immediately around an object that ordinary lenses can't pick up. The evanescent light conveys finer details about an object than ordinary, or propagating, light.

Existing near-field microscopes makes super resolution images by capturing evanescent light, but can only scan objects point-by-point. Superlenses image whole objects at once.

When the researchers shown 365-nanometer-wavelength ultraviolet light through a chromium mask containing 40-nanometer lines forming the word "nano" the superlens produced images focused to 89 nanometers, well below half the light's wavelength.

The lens could be used in practical applications in one to five years, according to the researchers. The work appeared in the April 22, 2005 issue of Science (Sub-Diffraction-Limited Optical Imaging with a Silver Superlens).

Page One

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