- An array of tiny holes produces subwavelength resolution
Technique enables ablation of subdiffraction targets in cells
At one time, optical microscopists who wanted to see finer details in their samples were out of luck. They were constrained by the diffraction barrier, which limits resolution in the far field to about a half wavelength. Then investigators developed near-field scanning optical microscopes, breaching the diffraction barrier and making superresolution possible. However, the cost was low photon flux and the need to maintain a separation between sample and aperture of only a few nanometers.
Now researchers at Harvard Medical School in Boston have come up with another approach, one that literally falls in between the others. They showed that an array of subwavelength apertures in a metal film could achieve superresolution without some of the drawbacks associated with near-field imaging.
“Unlike near-field approaches, rougher surfaces can be imaged due to the long working distance,” said team leader Peter R.H. Stark.
He added that other benefits include a better signal-to-noise ratio and high irradiance on the target, making the collection of higher-quality images faster than is possible with other methods. For biological applications, besides imaging, the technique enables ablation of subdiffraction targets in cells or selective bleaching of individual fluorophores.
The nanometric aperture technique developed by the researchers arose from a study of the mesofield. This region lies between the near field -- about a half-wavelength distance from an object -- and the far field, where the distance is many times the wavelength. It isn’t well understood what happens to light in the mesofield, where it transitions from the evanescent waves that characterize the near field to the classical waves that dominate the far field.
In studying this region, the researchers perforated metallic films using a focused ion beam system from FEI Co. of Hillsboro, Ore. One group of films consisted of 100-nm-thick gold and a 4-nm-thick adhesion layer of chromium on borosilicate glass. Another group consisted of 110-nm silver layers sputtered onto low-stress silicon nitride films, with thickness between 75 and 500 nm.
The researchers carefully chose the size and spacing of the cylindrical holes milled into the metal so that the holes excited surface plasmon-photon coupling at the interface. They made the holes smaller than the classical resolution limit.
Thus, for the gold film, the holes were 150 nm in diameter and arranged in a hexagonal lattice with a spacing of 500 nm. In the case of the silver film, the openings were 60 nm and spaced twice their diameter apart.
The researchers illuminated the gold film arrays of subwavelength apertures using a focused white light source, created by sending the light of a xenon lamp through a double Jobin Yvon monochromator. Using a Zeiss CCD camera and a Nikon microscope, they collected far-field images of the lattice. They found that the images conserved the aperture size and spacing, a result indicating propagating light and subwavelength resolution.
Researchers used subwavelength apertures for far-field transmission. On the left is an image of a hexagonal aperture array in a composite metal film on an insulator (100-nm gold on 4-nm chromium on borosilicate cover glass). The apertures are 150 nm in diameter with a lattice constant of 500 nm. On the right is an image taken of the emission from the device with illumination on the gold side. The decrease in intensity at the bottom is the result of radiance variation. Images reprinted with permission of PNAS.
As for the silver films, the investigators coated the other side of the silicon nitride membrane with photoresist. This arrangement gave them a recording medium that was placed a known distance away from the metallic film. They thereby could test the performance of the nanometric apertures from the edge of the near field to the mesofield.
They used a laser operating at 410 nm as a light source and performed direct-write patterning of the photoresist. Because the index of refraction in the silicon nitride was 2.02, the equivalent wavelength in the spacer film was 203 nm.
On the left is a scanning electron micrograph, at 150 kX, of twinned holes in patterned photoresist. These were created at a distance of half a wavelength (100 nm) of the light source from 60-nm apertures spaced 120 nm apart in a silver film. The direction of the incident polarization is indicated by the arrow. The SEM image in the middle was taken at 35 kX and shows twinned holes in photoresist at a wavelength (200 nm) distance from the apertures. On the right, the SEM image, at 150 kX, shows twinned holes in photoresist at a distance of 2.5 wavelengths (500 nm) from the apertures. The twinned holes are on the left-hand side of the figure. The hole-set to hole-set interference is on the right-hand side of the figure.
After exposing the photoresist, the researchers measured the results using an atomic force microscope from Asylum Research of Santa Barbara, Calif. They found that they could reproduce twinned 60-nm-diameter holes spaced 120 nm apart at a distance to the recording photoresist of up to 2.5 wavelengths, or 500 nm. This proved that they were achieving subwavelength resolution at a distance well beyond the near field. Stark pointed out that one of the most interesting aspects of the investigation is that photons appear to follow ballistic trajectories over a short range of the mesofield.
Subwavelength imaging at these greater distances did require a greater dose, however. Tests showed that the dose needed in the near field at 0.5 wavelengths of separation had to be upped fivefold to obtain similar results at 2.5 wavelengths. Thus, higher fluxes seem to be required for subwavelength imaging at greater aperture-to-sample distances. The work was published in the Nov. 27 issue of PNAS.
Stark said that research in the meso-field continues, with some promising results. He noted that the nanometric aperture technique had attracted commercial attention, with the makers of the photolithography steppers used in semiconductor manufacturing and chip makers themselves especially interested. He also said that the technique could be used for superresolution fluorescence and brightfield microscopy.
“By scanning the hole array over a fluorescently labeled specimen and collecting the resulting fluorescence in the far field, an ultraresolution map can be made through reconstruction,” he explained.
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