A 'salty' source of coherent light

Coherent light is today only produced by lasers. But by sending shock waves inside a humble crystalline material -- kitchen salt -- researchers from Lawrence Livermore National Laboratory (LLNL) have found a new way to produce coherent light for the first time in 50 years.

Coherent light is produced by a beam of photons that all have the same frequency and are all at the same phase. And today lasers are the only form of technology that we know able to create such light. But by sending shock waves inside a humble crystalline material -- kitchen salt -- researchers from Lawrence Livermore National Laboratory (LLNL) have found a new way to produce coherent light for the first time in 50 years -- at least in the terahertz frequency range. This could lead to applications in optical communications, quantum computing or shock diagnostics.

Here is how LLNL reports this discovery.

Through a series of theoretical calculations and experimental simulations, scientists generated a mechanical shock wave inside a dielectric crystalline material, in this case kitchen salt (NaCl). One might expect to see only incoherent photons and sparks from the shocked crystal.
But what they found was so much more. Weak yet measurable coherent light was seen emerging from the crystal. The emission frequencies are determined by the shock speed and the lattice make-up of the crystal. The team found that measurable coherent light can be observed emerging from the crystal in the range of 1 to 100 terahertz (THz).

[The figure below] "shows the emission of coherent light at 22 THz from a molecular dynamics simulation of shocked NaCl (table salt). The left panel shows the emission of the light as a function of time while the shock is propagating. The right panel shows the generated radiation as a function of location within the shocked crystal indicating the 22 THz coherent signal is generated at the shock front (between the white dotted lines)" (Credit for image and caption: LLNL).

Coherent light emitted through kitchen salt

Here is a quote from Evan Reed, a postdoctoral fellow at LLNL in the Chemistry and Chemical Engineering Division (CMS).

"To our knowledge, coherent light never has been seen before from shock waves propagating through crystals because a shocked crystal is not an obvious source to look for coherent radiation," Reed said. "The light and radiation was in a portion of the electromagnetic spectrum that is not usually observed in these types of experiments."

Interestingly enough, the researchers used the Thunder supercomputer, number 11 on the TOP500 List in November 2005, in parallel with their physical experiments.

In the computational experiments, the researchers observed the light generated by a shocked polarized material by performing molecular dynamics simulations of shock waves propagating through crystalline NaCl. The simulations solved the classical equations of motion for atoms that are subject to interaction, thermal effects and deformation of the crystal lattice.

As I mentioned above, there are many applications for such a source of coherent light, but according to Reed, one of the first to come could be a new diagnostic tool to determine the properties of shock waves.

For more information, this research work has been published by Physical Review Letters under the name "Coherent Optical Photons from Shock Waves in Crystals" (Volume 96, Number 1, Article 013904, January 13, 2006). Here is a link to the abstract which gives some additional details.

We predict that coherent electromagnetic radiation in the 1-100 THz frequency range can be generated in crystalline materials when subject to a shock wave or solitonlike propagating excitation. To our knowledge, this phenomenon represents a fundamentally new form of coherent optical radiation source that is distinct from lasers and free-electron lasers. The radiation is generated by the synchronized motion of large numbers of atoms when a shock wave propagates through a crystal.

And if you want to learn more about this subject, please read this page about coherent light at Wikipedia.

Sources: Lawrence Livermore National Laboratory news release, January 13, 2006; Will Knight, New Scientist, January 12, 2006; and various web sites

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