Super Imaging at a Distance
Combining SNOM with a superlensing material enables contact-free imaging.
In conventional microscopy, resolution is limited to about λ/2 of the light source because of diffraction. Scanning near-field microscopy (SNOM), in which a nanometer-scale probe with a tiny aperture is raster-scanned across an object of interest, and scattering SNOM, in which an apertureless metallic probe is brought into close contact with the object, enable imaging below the diffraction limit. However, these methods can damage delicate specimens — making live-cell imaging, for example, challenging — and the technique is limited to the specimen’s surface.
Now investigators at Max Planck Institute for Biochemistry in Martinsried, Germany, and the University of Texas at Austin have visualized subwavelength objects at a distance by combining SNOM with a superlens, a thin slab of material that, at specific wavelengths, exhibits negative electric permittivity, enabling imaging below the diffraction limit.
A group at the university, led by Gennady Shvets, fabricated a superlens that operates near 11 μm by sandwiching a 440-nm-thick layer of silicon carbide between two 220-nm-thick layers of silicon dioxide. “Superlensing is a resonant phenomenon, and to observe it, one needs to find two materials with opposite dielectric permittivities. We were fortunate to find such a pair,” he said.
For a specimen to image, the researchers placed a 60-nm-thick gold film against one oxide layer and milled several 1200-, 860- and 540-nm-diameter holes into it. They then used a homebuilt atomic force microscope to scan a probe tip in tapping mode across the oxide layer on the opposite side. As the tip moved, light from a 10.84-μm carbon dioxide laser from MPB Communications Inc. of Montreal scattered off the superlens, which the scientists measured. Because the oxide surface was homogeneous, changes detected by the tip were from the superlens.
They imaged features on the far side, 880 nm away, including the smallest holes, which were about 1/20 the size of the source wavelength. To confirm the effect of the superlens, they tuned the laser to a smaller wavelength, and the features disappeared.
Use of the technique depends upon finding or constructing materials that act as a superlens at the right wavelength without too much loss. The task of finding appropriate material may become easier because SNOM offers a way to study the basic phenomena of superlensing. “We think it is a highly valuable optical tool to obtain detailed experimental insights,” said the institute’s Rainer Hillenbrand.
He also noted that pairing SNOM with superlenses opens up the possibility of novel applications, such as imaging of biological objects in their natural environment — safely separating the specimen from the probe tip with a superlens.
“Another application of an infrared superlens could be found in nondestructive probing of metallic interconnects buried under a layer of glass or another dielectric, as commonly used in semiconductor devices,” he said.
Science, Sept. 15, 2006, p. 1595.
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