LIVERMORE, Calif., Jan. 17, 2006 -- For the last 50 years, the only fundamental ways to produce coherent light has been with lasers or free-electron lasers. But a group of researchers from Lawrence Livermore National Laboratory (LLNL) and the Massachusetts Institute of Technology have found a new source of coherent optical radiation that is unique.
Applications for this research are numerous, but the most immediate result may be a new diagnostic tool to determine the properties of shock waves, said Evan Reed, an E.O. Lawrence postdoctoral fellow at Lawrence Livermore and lead author of a paper that appeared in the Jan. 13 edition of Physical Review Letters. Other Livermore authors include Richard Gee of LLNL’s Chemistry and Chemical Engineering Div.
This figure 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). (Image: LLNL)
Through a series of theoretical calculations and experimental simulations, scientists generated a mechanical shock wave inside a dielectric crystalline material, in this case common table 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 makeup 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).
“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.”
Coherent light is very narrow bandwidth radiation; it is useful for interferometry (the measurement of two or more waves coming together at the same time and place, such as optical and shock waves) and is usually associated with lasers.
The invention of the laser in 1958 as a source of coherent light enabled a wide range of applications, including medical technologies and energy production, because of the coherence of the light they generate. However, producing coherent light from a source other than a laser can serve as a diagnostic for understanding shock waves, specifically providing information about shock speed and the degree of crystallinity, Reed said.
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. The intensive computer simulations were made possible by using LLNL’s Thunder parallel computer.
LLNL’s Laboratory Directed Research and Development program is funding an experiment to observe coherent radiation in the laboratory. Reed, Michael Armstrong (a chemistry and materials science postdoctoral researcher) and researchers from Los Alamos National Laboratory (LANL) will collaborate on the project, which will be conducted at LANL experimental facilities.
For more information, visit: www.llnl.gov