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26 February 2011

Shaped pulses make opaque film see-through

A NEW way of providing crystal clear vision through an opaque layer could one day lead to a non-invasive technique for targeting cancer cells.
Some opaque materials will allow small amounts of light through if they are in a thin enough layer. But as light passes through the layer it is scattered in both time and space, so an image projected on one side emerges blurry and unfocused on the other.
Now Jochen Aulbach at the FOM Institute for Atomic and Molecular Physics in Amsterdam, the Netherlands, and colleagues, have found a way to sharpen things up. They figured that it should be possible to manipulate light so that the scattering it experiences as it passes through the layer leaves it focused.
The team achieved this through a trial and error process. They used a liquid crystal device which allows precise control of light, called a spatial light modulator (SLM), to manipulate 64-femtosecond-long laser pulses being projected onto a layer of paint. A detector measured the intensity and duration of the pulses that emerged from the other side. This information was then passed to a computer program that used it to tweak the SLM to make the next pulse arriving at the detector both brighter and less spread out in time.
In a paper to appear in Physical Review Letters, the team reports that it took about 10 minutes of repetition for the system to refine the tweaks sufficiently to create a coherent, bright pulse that was still just 115 femtoseconds long despite its tortuous route through the paint. By modifying the light pulse to travel through skin instead of paint it might be possible to deliver short, intense laser pulses to destroy cancer cells but leave nearby healthy cells intact.
Sylvain Gigan of the Langevin Institute in Paris, France, calls the time resolution impressive. "It is both very elegant and very effective," he says.

**Published in "New Scientist"

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