Shocked Crystals May Emit Coherent T-Rays

Numerical experiments indicate that mechanically shocking crystalline materials may cause them to generate coherent radiation in the 1- to 100-THz frequency range, potentially enabling the development of novel sources of terahertz rays for medical and biological imaging and for security applications.

Reporting their findings in the Jan. 13 issue of Physical Review Letters, investigators at MIT in Cambridge, Mass., and at Lawrence Livermore National Laboratory in Livermore, Calif., predict that the coherent radiation should be observable in real-world material systems and heretofore has escaped detection because researchers were not looking for it.

To model the effect of planar shock waves propagating in a crystal of NaCl and oriented to the [111] or [100] directions, the scientists used Lawrence Livermore’s Thunder, a massively parallel supercomputer, to solve classical equations describing the motion of 2 million to 3 million atoms subjected to thermal effects and to deformations of the crystal lattice. They found that the coherence length of the emitted terahertz radiation was on the order of a few millimeters, which the researchers propose would be improved by increasing the shock propagation time.

They suggest that the periodicity of the crystal lattice is behind the coherence. The propagating shock wave generates polarization currents that emit radiation, the frequencies of which are determined by the time it takes the shock wave to pass through a lattice unit.

Experimental efforts to detect coherent radiation from shocked crystals are under way in a joint project of Lawrence Livermore and Los Alamos National Laboratory in Los Alamos, N.M.