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A laser that sees through solid objects

Physicists from the U.K. and Switzerland have shown that a laser can work even if less than half of the light-amplifying material is in an 'excited' state and that solids can became transparent. This discovery could lead to a world where x-rays are no longer needed by doctors and where rescuers could find people buried after an earthquake.

It's always fascinating to see that scientists still want -- and sometimes succeed -- to challenge an Einstein's theory. This time, physicists from the U.K. and Switzerland have shown that a laser can work even if less than half of the light-amplifying material is in an 'excited' state. For their experiments, the researchers have created nanocrystals behaving like 'artificial atoms.' When they sent a laser beam through the crystals, the nanostructures became transparent. This discovery could lead to a world where x-rays are no longer needed by doctors and where rescuers could find people buried after an earthquake.

Here is the introduction of the Imperial College London news release.

Researchers from Imperial College London and the University of Neuchatel, Switzerland, have pioneered the technique which could be used to see through rubble at earthquake sites, or look at parts of the body obscured by bone. The effect is based on the development of a new material that exploits the way atoms in matter move, to make them interact with a laser beam in an entirely new way.

Below is a picture of Professor Chris Phillips, of the Department of Physics, in the group's laser laboratory (Credit: Cheryl Apsee/Imperial College London).

Imperial College London laser laboratory

Now here is a short scientific background.

The work is based on a breakthrough which contradicts Einstein's theory that in order for a laser to work, the light-amplifying material it contains, usually a crystal or glass, must be brought to a state known as 'population inversion'. This refers to the condition of the atoms within the material, which must be excited with enough energy to make them emit rather than absorb light.

As quantum physicists have suspected for a while, this condition was not always mandatory, but it's the first time it is demonstrated with solids. So here it is what did the researchers.

In order to make this breakthrough, the team created specially patterned crystals only a few billionths of a metre in length that behaved like 'artificial atoms'. When light was shone into the crystals, it became entangled with the crystals at a molecular level rather than being absorbed, causing the material to become transparent.

And this is a "schematic of the 'artificial atom' layered semiconductor nanostructure used in these experiments (Credit: Imperial College London/Nature Materials).

Imperial College London 'artificial atom'

Apparently the scientists are there for some more surprises. As said Phillips, "The results can be pretty weird at times, but it's very exciting and so fundamental."

And the team discovered that not only their laser could see through solid objects, but it would also be possible to slow down and even stop the propagation of light. This obviously could have important implications for our information networks.

This research work has been published by Nature Materials as an advanced online publication under the title "Gain without inversion in semiconductor nanostructures" (February 19, 2006). Here are two links to the abstract of this paper and to a page containing several figures and tables, including the diagram above. Below is the beginning of the abstract, which makes a good description of what the researchers have achieved.

When Einstein showed that light amplification needed a collection of atoms in 'population inversion' (that is, where more than half the atoms are in an excited state, ready to emit light rather than absorb it) he was using thermodynamic arguments. Later on, quantum theory predicted that matter–wave interference effects inside the atoms could, in principle, allow gain without inversion (GWI).
The coherent conditions needed to observe this strange effect have been generated in atomic vapours, but here we show that semiconductor nanostructures can be tailored to have 'artificial atom' electron states which, for the first time in a solid, also show GWI.
In atomic experiments, the coherent conditions, typically generated either by coupling two electron levels to a third with a strong light beam or by tunnel coupling both levels to the same continuum (Fano effect), are also responsible for the observation of 'electromagnetically induced transparency' (EIT). In turn, this has allowed observations of markedly slowed and even frozen light propagation.

If this discovery leads to a way to stop light propagation -- and to store the information carried by it -- does this mean it will help to secure our optical fiber networks by catching eavesdroppers? It's definitively a possibility.

Sources: Imperial College London news release, via EurekAlert!, February 19, 2006; and various web sites

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