Molecular holograms are coming into focus
18:00 08 June 2007
NewScientist.com news service
3D images of individual molecules may soon be possible thanks to a breakthrough in holography by Swiss scientists. The technique would be useful to biologists interested in how the shapes of proteins and other components of life relate to their function.
A hologram records the complex interference pattern that is created when laser light reflects off an object. Bouncing light off the hologram recreates this pattern of waves giving the impression that the viewer is looking at the original object in 3D.
But for years physicists have grappled with a kind of double vision that has made using holography difficult.
The process results in an out-of-focus second image being superimposed on the main one, which can seriously degrade the result. "The twin image problem has existed since holography was conceived. People have always worried about it," says Hans-Werner Fink, a physicist at the University of Zurich in Switzerland.
In many applications, the ghostly twin image has a negligible effect and can be ignored. But when the source of light is close to the object being imaged, the twin can dramatically reduce the resolution of hologram. That’s a big problem for scientists trying to make holograms of tiny objects such as viruses and proteins.
Physicists have devised various optical techniques for removing the twin, but they do not always work for light of much shorter wavelengths such as X-rays.
Now Fink and his colleague Tatiana Latychevskaia have solved the problem in a way that should work, regardless of the source of illumination.
Latychevskaia says she realised how to do it after noticing that the blurry superimposed twin makes some areas of the light field brighter than would be possible were there a single image alone.
So she designed a computer program that identifies these regions, replaces them with a more realistic light level, and than calculates how this would affect the light field that created them.
The modified light field is then used to create a new image and the process begins again. Repeating this process many times removes the twin image entirely, dramatically sharpening the result.
Archie Howie, emeritus professor of physics at the University of Cambridge, UK, says the technique is an interesting development. "The authors provide convincing evidence that the method works," he says.
And he believes it could have much broader potential. "Although tested so far only in the optical region, there is no obvious reason why the authors' method should not also work with X-rays and electron waves," he told New Scientist. That would be important for using the technique on a microscopic scale.
Latychevskaia is excited by the technique’s potential. “It could help us produce images of biological molecules in 3D for the first time. And that would be very significant.”
Journal reference: Physical Review Letters (DOI: 0.1103/PhysRevLett.98.233901)
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