Using "superlenses" that can amplify rapidly decaying light that normal lenses cannot, light microscopes can now see details smaller than a wavelength of light buried below a surface. Researchers say their new method could help peer into artificial or biological structures and scan the molecular compositions of objects.
Normally light microscopy can at most resolve details roughly half the wavelength of the light used. If light is squeezed past this limit, it flares out from an aperture as two portions: a far-field part that spreads out, and a near-field fraction that remains close, decaying rapidly once emitted. Conventional lenses can capture far-field light, but they lose all the information about details smaller than a wavelength contained in near-field light. Metal probe tips scanning over a surface can detect this near-field light, but until now they could only "see" what they touch and not below.
To get a better glimpse of features below the skin of an object, physicist Rainer Hillenbrand at the Max Planck Institute for Biochemistry in Martinsreid, Germany, and his colleagues used superlenses. These novel devices exploit a property called permittivity, which describe a substance's ability to transmit an electric field. Specifically, the superlenses combine materials with negative and positive electric permittivities, so that internal electric fields align in opposite directions from each other in response to external fields. The result: superlenses can recover near-field as well as far-field light.
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The researchers placed a superlens made of crystalline silicon carbide 440 nanometers thick between the scanning microscope tip and a gold film patterned with holes of different sizes. They showed they were able to image details using infrared light that were 1/15 the size of the wavelength used and 880 nanometers below the surface.
A limitation of this new technique, reported in the September 15 Science, is that the maximum depth it can see below the surface of an object and the size of the tiniest details it can resolve are the same--the width of the superlens. In other words, this method cannot realistically image relatively tiny features that are buried relatively deep below a surface at the same time, explains researcher Gennady Shvets, a physicist at the University of Texas at Austin.
In comparison, ultrasound can simultaneously image details as little as 20 nanometers in size and as much as 1,000 nanometers below the surface. In addition, it can also probe mechanical properties. Still, superlenses could help determine chemical properties of scanned features much better than ultrasound, suggests metrologist Alain Diebold, a senior fellow at semiconductor consortium SEMATECH in Austin, Tex., who did not participate in the study.
Moreover, past light microscopy techniques using scanning probes involved mechanical contact between the tip and the sample, "and fragile samples often prevented a high-resolution measurement process, making, for example, the optical probing of living cells nearly impossible," Hillenbrand says. Superlenses could help probe cells a distance from their surface "in their natural environment," he explains, and could also help examine defects in electronics.