Could laser-like light be squeezed from salt?© Punchstock

Physicists in the United States have discovered a way to make what is essentially laser light, without using a laser. All you need to do, they say, is give a crystal of table salt a sharp knock.

It was previously assumed that this would do nothing more than squeeze sparks and ordinary light out of the crystal. But Evan Reed of the Lawrence Livermore National Laboratory in California and his co-workers say that the shock will also generate a small amount of 'coherent' light¹, the stuff that comes from lasers.

This unexpected source of laser-like light is not just an academic oddity. The new work shows that the coherent light coming from shocked salt should be in a fequency band called terahertz radiation, which cannot be generated by ordinary lasers, says Marin Soljacic, one of Reed's collaborators.

Terahertz radiation, with wavelengths of about a millimetre to tens of micrometres, is increasingly in demand for biomedical and technological applications. In particular, the radiation can peer through human flesh to image the anatomy beneath, without harming tissues the way X-rays do.

You might not get much of this kind of light out of a grain of salt, the team notes. But it could also provide a useful diagnostic tool for studying the effects of shock waves on crystalline materials, says Soljacic, who is based at the Massachusetts Institute of Technology (MIT).

The light fantastic

Light is composed of packets of energy called photons, each of which can be described as a wave with a particular frequency and wavelength. In ordinary (incoherent) light, like that from a light bulb, these waves rise and fall independently of one another. In coherent light, on the other hand, the waves are all synchronized, rather like a group of soldiers marching in step.

It is this characteristic that gives laser light its special properties. The beam stays bright and does not spread out, for example, enabling the fine focus necessary in CD and DVD players.

Making coherent light in lasers is a feat of coordination: all the photons are released together thanks to a positive feedback process through which one photon stimulates the release of another. This process gives 'laser' its name: it's an acronym of Light Amplification by Stimulated Emission of Radiation.

Reed and colleagues say that the coherence of light emitted from a shocked crystal comes instead from the way that a shock wave passing through the material can induce the regular rows of atoms to move in synchrony. This motion produces an electromagnetic wave - in other words, light.

Look and you shall find

Countless experiments have investigated the effects of shock waves on materials; the Lawrence Livermore lab is one of the world leaders in this area. But Reed's team says that coherent light emission has never been seen before, simply because no one thought to look for it.

His team tested their idea in a computer model, which predicted how sodium chloride (common salt) should behave when exposed to the kind of shock wave produced by an explosion or a bright laser pulse. The result was spikes of radiation emitted at extreme-infrared (terahertz) frequencies, corresponding to bands of coherent emission.

So much for the theory; will it work in practice? Reed says he is hoping to find out, in an experimental collaboration between the Livermore lab and Los Alamos National Laboratory in New Mexico. "The big difficulty is detecting the coherent signal. It'll be relatively weak," explains his MIT colleague John Joannopoulos. 

Massachsuetts Institute of Technology

• References

  1. Reed E. J., et al. Phys. Rev. Lett., 96. 013904 (2006). | Article |