MICROSCOPY: Superlens enhances near-field scanning microscopy

While achieving negative refractive index requires negative permittivity as well as permeability, and typically requires the fabrication of a structured metamaterial, a proposed approximate solution consisting of a thin silver slab that shows negative permittivity (only) in the optical range can also have superresolution, and be easier to fabricate.¹

For example, if an object is positioned 20 nm in front of a 40-nm-thick silver slab, the object’s evanescent waves can excite surface-plasmon oscillations that couple to corresponding ones on the other side of the slab, producing an image 20 nm away from the slab’s far side. Experimental verification of this approach has shown that, by spinning a photoresist on the image side of the slab as a recorder and using 365 nm radiation for object illumination, λ/7 structures could be resolved in the developed photoresist.²

Now, a more-direct approach has been taken by researchers at Max-Planck-Institut für Biochemie (Martinsried, Germany) and the University of Texas, Austin, in which the optical image field created by a thin slab is sampled by scanning it with a near-field optical microscope in scattering mode (s‑SNOM).³ When compared to s-SNOM measurements taken directly at the sample, the superlens-mediated technique is less invasive, and hence might be applied even for biological samples.

The superlens consists of a 440-nm-thick single-crystalline silicon carbide (SiC) membrane coated on both sides with 220-nm-thick silicon dioxide (SiO₂) layers, the lower one covered by a 60-nm-thick gold film structured with holes of different sizes to form the object; the image is expected on the surface of the upper SiO₂ layer (see figure). For the spectral range 10.3 to 12.5 µm, SiC provides negative permittivity due to nearly resonant excitation of phonon surface polaritons. Superlensing of the SiO₂/SiC/SiO₂ slab is expected if the real parts of the permittivity of SiC and SiO₂ are equal and of opposite sign, which occurs at 10.85 µm. The light source was a tunable 13CO₂ laser, which served for object illumination via the superlens and for the generation of the signal of the s-SNOM. In this way also opaque samples could be investigated.

A conventional s-SNOM signal was generated by the optical field between the metallic needle and the surface. That field was locally strongly enhanced when the tip approached the surface so that an oscillatory backscatter signal could be recorded when the s-SNOM was driven in the tapping (vibrating) mode at 250 kHz. The additional optical field due to the superlens image was superposed, and hence could be traced on the SiO₂-induced constant background.

Recordings were performed at wavelengths of 10.85 µm for amplitude images and at 11.03 µm for phase-contrast images: both show clearly the holes in the underlying gold foil, indicating that λ/20-size objects at a distance of 880 nm could be imaged by the SiC superlens. In addition, when the wavelength was set to 9.25 µm where the permittivity is no longer negative, no structures could be seen.

The researchers foresee extension to shorter wavelengths so that structure sizes of some 50 nm might become recordable if appropriate superlens materials are used.

REFERENCES

1. J.B. Pendry, Phys. Rev. Lett. 85(18) 3966 (2000).

2. H. Lee et al., New J. Physics 7, 255 (2005).

3. T. Taubner et al., Science 313, 1595 (2006) and online material.