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Silver Slab Promises Subwavelength Imaging

Photonics Spectra
Jul 2004
Daniel S. Burgess

A team of scientists at the University of Canterbury in Christchurch, New Zealand, has demonstrated that a planar slab of silver can be used as a lens in a near-field lithography setup at 365 nm. If the approach can be refined to enable the l/9 sub-diffraction-limit resolutions predicted, it may find applications in high-density optical data storage as well as in the lithographic patterning of integrated circuits.

In 2000, based on the predictions of Victor G. Veselago in the 1960s, John B. Pendry of Imperial College in London published a series of equations that suggested that materials with negative refractive indices could be used to create flat-surfaced "superlenses" with perfect resolution . Since then, numerous experiments have established the existence of negative refraction and of the ability of flat slabs of such "left-handed" materials to focus electromagnetic radiation at microwave frequencies.

Despite the potential utility of the phenomenon in this regime, researchers have sought materials for optical frequencies. But these materials have been elusive. Luckily, Pendry's calculations also suggested that a thin slab of silver, while not a left-handed material, could display negative refraction at optical wavelengths and thus could be used to build a flat superlens. The current work supports this proposition.

Richard J. Blaikie of the University of Canterbury explained that silver exhibits negative dielectric permeability near its 330-nm plasma frequency that causes it to act like a negative-refractive-index material. Incident radiation scattered from a silver slab induces plasmons on the surface. If the incident radiation is near the plasma frequency, the plasmons induce corresponding surface charge oscillations on the opposite side of the slab.

"These then act as tiny radiation sources -- antennae -- that are phased in such a way as to form a real 1:1 magnified image on the other side of the silver slab," he said. "Magic, really."

Silver Slab Promises Subwavelength Imaging
Figure 1. In the experimental setup, a pattern in tungsten served as the object, poly(metylmethacrylate) (PMMA) functioned as spacers and a 120-nm-thick layer of silver acted as the lens. The stack was brought into proximity to a photoresist, and a mercury lamp served as the exposure source. Images courtesy of Richard J. Blaikie.

In the work, Blaikie and his colleagues fabricated a pattern of 120-nm to 2-µm lines and dots with tungsten coated with poly(methylmethacrylate) (PMMA) on a glass coverslip. To this, they added a 120-nm-thick layer of silver with a surface roughness of 2 nm rms and then another layer of PMMA, although another dielectric such as SiO2 could be used as the final spacer (Figure 1). They then brought the stack into proximity to a photoresist, exposed it through the stack with 365-nm radiation from a 350-W mercury lamp and developed the resist. They used an atomic force microscope to characterize the results.

Silver Slab Promises Subwavelength Imaging
Figure 2. The approach has produced approximately 250-nm features from an exposure wavelength of 365 nm.

Initially, the technique yielded imaged lines and spaces with widths on the order of the exposure wavelength. Subsequent experiments, Blaikie said, have produced approximately 250-nm features (Figure 2). Simulations suggest that imaging features as small as 40 nm -- a resolution of λ/9 -- may be possible with thinner silver and spacer layers, which the researchers hope to test in future setups.

Potential applications include data storage and lithography, for which a silver superlens would ease the need for sources of ever-shorter wavelengths. The team is considering its application particularly for high-resolution proximity lithography, which would eliminate the damage that masks can suffer in today's contact lithography.

A transparent optical component consisting of one or more pieces of optical glass with surfaces so curved (usually spherical) that they serve to converge or diverge the transmitted rays from an object, thus forming a real or virtual image of that object.
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