An Optical Superlens That Is Easy on the Eyes

Hank Hogan

Superlenses are super because they can image below the diffraction limit of the light passing through them. However, current incarnations of superlenses — thin silver or silicon carbide slabs — are not very super when it comes to use: They can image below the diffraction limit but only in the near field, which means that they cannot be used as far-field detectors, like an eye or a camera.

Now a team of researchers from the University of California, Berkeley, has developed a far-field optical superlens and used it to image subwavelength objects, turning what appears with conventional optics as an indistinct blur into two clear lines (see figure). Their experiment demonstrated resolution improvement in only one direction, but the concept nonetheless is not limited.

Researchers have demonstrated far-field imaging below the diffraction limit using a far-field superlens (FSL) for subwavelength imaging. The FSL comprises a subwavelength grating built atop a thin layer of silver. The grating selectively enhances the evanescent waves from the object and converts evanescent waves into propagating waves. The FSL fits between the specimen and the objective of a regular optical microscope. Reprinted with permission of the American Chemical Society.

“We are trying to develop a general optical imaging tool like a conventional optical microscope but with much higher spatial resolution,” said postdoctoral researcher Zhaowei Liu.

If successful, the investigators’ work will have a significant effect on a variety of nanoscience and nanotechnology endeavors. For example, semiconductor circuit manufacturing makes use of photolithography, which would benefit greatly if the minimum feature size were smaller than currently achievable.

The researchers created a far-field superlens by stacking a subwavelength diffraction grating on top of a thin layer of silver. This arrangement enhanced the evanescent waves — the type confined to the near field — and converted them into propagating waves, the type found in the far field. The conversion process was designed to strongly enhance the first diffraction order while the rest were suppressed. Calculations showed that this approach would resolve objects as small as one-eighth the illumination wavelength.

A scanning electron microscope image shows a pair of 50-nm nanowires with a 70-nm gap between them (top). A conventional optical microscope image — restricted by the diffraction limit — cannot adequately resolve wires (center). A novel far-field superlens (FSL) does resolve the wires’ features (bottom). Image courtesy of Zhaowei Liu, University of California, Berkeley.

For a demonstration of the technique, they constructed a far-field superlens with a corrugated slab of silver that was 35 nm thick. They covered this layer with a 100-nm-thick spacer of polymethylmethacrylate (PMMA) and then topped everything with a silver grating 55 nm thick and a periodicity of 150 nm. With this construction held 35 nm away from test objects by a PMMA spacer, the investigators showed that they could image 50-nm-wide lines separated by a 70-nm gap using a 377-nm light source.

With the concept proven, the scientists are moving on to the next task. “Our next step is to design and demonstrate 2-D imaging ability,” Liu said.

Nano Letters, ASAP Edition, Jan. 17, 2007, doi: 10.1021/nl062635n.