Metamaterials for magnifying superlenses

Materials World magazine
,
1 May 2007
Array of superlenses

Advances in the field of magnifying superlenses have been reported by two separate US research teams.

Conventional lenses are limited by the diffraction limit of light, which prevents high resolution imaging of features smaller than its wavelengths. The new superlenses are made from metamaterials designed to capture the evanescent waves that exist close to the surface of an object. These waves can resolve surface features much smaller than normal propagating waves, but decay too quickly for conventional lenses to capture.

Igor Smolyaninov, Associate Research Scientist at the University of Maryland, USA, and his team have created a magnifying superlens based on the propagation of surface-plasmon polaritons (SPPs).

The lens was made by depositing a gold film onto a glass slide and then spin coating the film with a polymethyl methacrylate photoresist that had been patterned using E-beam lithography. Polymethyl methacrylate is a good photoresist and can be patterned easily, but ‘any dielectric material will work,’ says Smolyaninov.

The object to be imaged and magnified is placed in the central region of the lens. The light scatters off the object and creates SPPs in the gold film that propagate radially outwards. ‘The lenses are used in succession so that the sub-wavelength information carried by one superlens is picked up by the next one,’ he explains. At the outer layers of the device, a magnified image of the object can be viewed using a conventional microscope.

The superlens was inspired by earlier research using droplets of glycerine to image sub-wavelength structures, Smolyaninov says. ‘Since the shape of the droplets may be ambiguous, we wanted to create a well-defined plasmonic superlens.’

The new device operates at a resolution of 70nm, ‘however, we believe it can be improved by another factor of two or three’.

Smolyaninov hopes to see the instrument used by biologists to visualise viruses and DNA molecules, and he believes it may also be used in reverse for nanoscale lithography.

A team of scientists from the University of California, Berkeley, USA, have simultaneously created a ‘hyperlens’.

Fabricated by forming a half-cylindrical cavity in a transparent quartz substrate, the device’s capabilities were demonstrated by shaping nanowires into the letters O and N. The cavity was obtained by opening a narrow slit in a chromium etching mask and using an isotropic quartz wet etching process. The chromium mask was then removed. Multiple thin films of silver and aluminium oxide were directly deposited on top of the substrate with the cavity. The shape of the thin film followed the geometry of the substrate, forming a hyperlens.

When an object is illuminated, its evanescent waves travel through the lens. As the wave vectors move outward, they become increasingly compressed, allowing the object’s image to be magnified. When the waves reach the outer layers of the lens, the image can be captured by a conventional optical lens and then projected onto a far-field plane.

‘To realise the special functionality of the hyperlens, the materials [have] to be special. Unfortunately, no natural materials fulfill the requirements, so we had to build an artificial material,’ explains Dr Zhaowei Liu, co-creator of the imaging tool. Silver was chosen for its relatively small loss of visible light, while aluminium oxide provided a high refractive index.

‘So far we have demonstrated the resolution to about 130nm, but there are no fundamental constraints to improve the resolution by up to a few tens of nanometres.’