Silver nanorod microscopy

Japanese researchers have developed a design concept for a light microscope which could in principle be used for imaging of nanoscale objects. The device would rely on a novel subwavelength imaging technique which allows for the visualization of objects that are smaller than the wavelength of the photons used in the device.

Once thought to be impossible, subwavelength imaging can now be performed because of the development of nanostructured metamaterials with a negative refractive index, which can act as a lens by focusing incident light. Until now though, such materials only worked at one wavelength and could only transfer images over short distances, so their potential for use as lenses was limited.

Reporting in a recent issue of Nature Photonics, Satoshi Kawata of the RIKEN Nanophotonics Laboratory and his colleagues propose a lens consisting of stacked silver nanorods, which would be capable not only of colour imaging at a resolution of nanometers using visible light waves, but also of transferring the images over much longer distances. Such a device could be used to directly image viruses or the distribution of protein molecules within cell membranes. 

The proposed device is based on a phenomenon called plasmonic resonance, which refers to the collective excitation of free electrons that occurs when light falls on a metallic surface. As well as amplifying the light waves in the near field, these resonances can be propagated through the metal. However, in the materials developed to date, the energy transmitted decays rapidly as it moves away from the surface.

The Japanese group suggests that this problem can be overcome using a stacked arangement of three layers of nanorod arrays, so that plasmonic resonances at the gaps between the nanorods would prevent the decay of the transmitted light field that would occur in a longer single nanorod. The researchers' simulations showed that such an arrangement could in theory transmit the light field for distances of more than one thousandth of a millimeter.

The simulations also showed that increasing the number of nanorod layers increases the number of resonant modes, thereby allowing for the transmission of large parts of the visible light spectrum, which would be necessary for colour imaging. The tapered structure of the nanorod arrays would expand the image of the object in the near field. The image could then be magnified further by a conventional optical device at the other end of the nanorods and then recorded with, for example, a charge-coupled device in the far field.

Kawata is sure that devices based on high resolution metallic nanolenses with extremely high resolution will eventually replace optical microscopes with conventional lenses. He and his colleagues have shown that long-distance colour image transfer at high resolution is feasible, but so far they have not demonstrated that this in practice. Building such a device will depend on the ability to accurately grow uniform nanorod arrays with the proposed design and to introduce gaps into them.

Kawata, S. (2008). Subwavelength colour imaging with a metallic nanolens. Nat. Photonics 2: 438-442 DOI: 10.1038/nphoton.2008.103