Research Highlight

Subject Category: Optics, photonics and optoelectronics

NPG Asia Materials research highlight; doi:10.1038/asiamat.2011.35
Published online 7 March 2011

Photonics: Rounding up the light

A plasmonic ‘nanocorral’ with a broken-symmetry ring structure focuses light to subwavelength spots.

For centuries, glass lenses have done a great job in focusing light, whether in Galileo’s first telescope or in Abbe’s microscopes. However, these optical instruments all suffer from an intrinsic limitation: they cannot focus light into spots smaller than the wavelength of the light used. Xing Zhu and co-workers from Peking University in Beijing, China, in collaboration with colleagues from Tsinghua University in Beijing and Rice University in Houston, Texas, USA, have now developed a nanoscale lens that is able to beat this limit.1 This lens design could prove useful for sensing applications or as a small source of light.

As light cannot be focused down to such small spots using conventional optics, researchers have investigated other effects, such as plasmonics, which rely on the collective motions of electrons on the surface of metals. These surface plasmons, which act as an antenna for concentrating light on a small scale, can be excited by using a nanoscale ring structure known as a nanocorral.

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Fig. 1: Near-field optical intensity distribution for a surface plasmon lens based on a broken-symmetry nanocorral.

In previous designs, the ring structures used only generated surface plasmons when excited with circularly polarized light — a particular form of light with polarization that rotates with time. As this behavior is too restrictive for most applications, Zhu and his colleagues tested an alternative geometry in which the two halves of the ring have different diameters, effectively ‘breaking’ the ring symmetry. The revised design now works with more useful linearly polarized light. “This provides a much simpler and faster way to realize surface plasmon focusing,” says Zhu.

The researchers fabricated their nanocorral structure on a flexible plastic substrate. Great care has to be taken during fabrication, explains Zhu. “The quality of the gold films, the accuracy of the height, width and curvature of the corral are all crucial.” The researchers were able to focus surface plasmons down to spots of about 320 nm (see image), which is about half the wavelength of the laser used for excitation.

Further advances can be expected from extensions of this proof of principle. “We have already tried to improve the quality of the surface plasmon nanofocusing by using multi-corrals and other more complex structures,” says Zhu. Useful conventional lens designs have finally arrived at the nanoscale.

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References

  1. Fang, Z., Peng, Q., Song, W., Hao, F., Wang, J., Nordlander, P. & Zhu, X. Plasmonic focusing in symmetry broken nanocorrals. Nano Lett. 11, 893 (2011). | Article | PubMed | ISI |

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.

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