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06 May 2009
Swiss-roll-like structure could boost resolution of optical microscopes.
Physicists in Germany have devised a new way to make metamaterials that could be used to boost the resolution of optical microscopes. The technique involves depositing alternating layers of semiconductor and metal on a flat surface and then rolling up the layers into a tube that resembles a hollow Swiss or jelly roll.
Preliminary measurements and computer simulations suggest that the rolls could be used to create a hyperlens – a device that can image objects much smaller than is possible using an optical microscope.
Novelty structures
Metamaterials are specially engineered structures that respond to light and other electromagnetic radiation very differently than conventional materials. They have been used to create not only hyperlenses but also other novel devices such as invisibility cloaks and superlenses.
The problem is that it is difficult to make metamaterials for visible light. This is because the structures in the metamaterials must be about the same size as the wavelength of the radiation – and for light this is hundreds of nanometres.
Thanks to ongoing innovations in the semiconductor industry, however, it is fairly easy to make nanometre-sized features. Unfortunately, these tend to be flat and an effective lens must have the appropriate curvature in 3D.
Roll me up
Now, Stefan Mendach, Stephan Schwaiger, Markus Broell and colleagues at the University of Hamburg have worked out a way to make a flat metamaterial "self-roll" itself into a tube that could be used as a hyperlens.
The team begins with a gallium arsenide (GaAs) substrate that has a 40 nm coating of aluminium arsenide. Then a sequence of three layers – indium gallium arsenide (InGaAs); GaAs; and silver – is laid on top several times over. Each layer is about 20 nm thick.
The InGaAs and GaAs layers have slightly different atomic spacings, which means that the layers are strained. When the aluminium arsenide layer is chemically removed, the structure – in an attempt to relieve the strain – automatically rolls up into a tube with an outer radius of about 2 µm. The rolling-up process takes about 30 seconds.
Radial focusing
The permittivity of the metamaterial — its ability to transmit electromagnetic waves — is related to its plasma frequency. The researchers found that the plasma frequency — and hence the permittivity — can be changed over a broad range from green light to the infrared simply by adjusting the ratio of the thicknesses of the metal and semiconductor layers.
Because the metamaterial is a cylinder, the path taken by light moving outward along a radius of the tube (and therefore the permittivity) is very different the path taken by light moving tangentially to the layers. For light at the plasma frequency radial light experiences a relatively large permittivity, while light moving tangentially experiences a relatively small permittivity. As a result, the light is focussed into the radial direction.
Capturing evanescence
Mendach told physicsworld.com that the magnification of the tubes was not great enough to confirm that they could be used as hyperlenses. Instead, they fed their optical measurements into computer simulations, which suggested that they could.
According to Mendach, the magnification can be boosted by creating tubes with a greater ratio of outer and inner diameter – something that the team is working on, along with an invisibility cloak.
Mendach believes that the hyperlenses could be used to create imaging systems in which tiny amounts of fluid containing living cells are pumped into tubes, which would image the cells. He also said that the tubes could be used to focus a laser beam to a tiny spot, which could be handy in performing space resolved spectroscopy.
The work was published in Physical Review Letters.
• This article first appeared on our sister website physicsworld.com
The team made several different tubes with different aluminium layer thicknesses. They then placed a tiny source of light inside the centre of each tube and measured how much light is transmitted through the wall as a function of frequency. This allowed the team to measure the plasma frequency of the metamaterial.
This means that the tube could be used as a hyperlens, which captures "evanescent" light from tiny objects and focuses it into an image that can be further magnified by conventional optics. Evanescent light can resolve features much smaller than wavelength-limited conventional optics – however, it does not travel far from the surface of the object and cannot be seen by a conventional microscope.
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