Microscope Slide Enables ~70-nm Resolution
David L. Shenkenberg
Conventional optical microscopes can resolve objects on the order of only 200 nm because they are limited by the diffraction of light waves, meaning that small things such as DNA and viruses cannot be seen. Although both scanning probe and electron microscopes can image biological samples smaller than the diffraction limit, the devices are incompatible with living cells.
From metamaterials, researchers made a superlens in the form of a microscope slide. This atomic force microscopy image shows an array of several superlenses.
With an ordinary optical microscope, Igor I. Smolyaninov and colleagues at the University of Maryland, College Park, have achieved a resolution below the diffraction limit, thanks to a unique microscope slide called a “superlens” that was made with metamaterials and that is based on surface plasmon resonance.
Smolyaninov said that his team ultimately wants to see molecules in living cells. “With a scanning probe microscope, you cannot see changes in the system, and you cannot take movies,” he explained.
He added that the group developed a microscope slide instead of an objective lens because the former can be mass-produced. “It may be much cheaper and easier to go with a glass slide.”
Making a slide lens
To create the superlens, the researchers used thermal evaporation to deposit a gold film on a glass substrate, and they used electron beam lithography to deposit concentric rings of polymethyl methacrylate, or PMMA, on the gold film. “PMMA-like compounds are often used in biological sample preparation, which makes our approach very compatible with biological applications,” Smolyaninov said.
Researchers used a laser to excite surface plasmons of a superlens. The doublets shown in the atomic force microscopy image (bottom figure) give rise to the divergent plasmon beams indicated by arrows in the top image, which was photographed using an ordinary optical microscope.
To produce superlens imaging, the scientists needed materials with negative and positive refractive indices relative to the plasmons, as PMMA and gold, respectively, have at an excitation range of ~500 nm. The investigators also needed to match the phase of the incident light to that of the plasmons, so they deposited doublets of PMMA dots 0.5 μm apart, near the inner ring of the circle and in the radial direction. They employed an argon-ion laser to excite the plasmons, and the plasmon rays propagated in a radial direction upon excitation, giving the superlens its magnification.
To demonstrate the efficacy of the superlens, the researchers visualized the doublets with a conventional optical microscope. Without activating the plasmons, they could not resolve the doublets; however, once the plasmons were excited, they observed two divergent plasmon beams generated by the doublets. They reached a resolution of ~70 nm, well below the ~200 nm diffraction barrier. Images taken with the superlens overlaid on ones obtained with atomic force microscopy showed that both can produce the same information.
The slide can theoretically resolve objects down to 10 nm, according to Smolyaninov, so it may have an even greater resolution than demonstrated in the study. “We will work on improvement of the lens design and use it on real-life biological samples,” he said.
Science, March 23, 2007, pp. 1699-1701.
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