Coherent Optical Photons from Shock Waves in Crystals

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. General analytical theory and NaCl molecular dynamics simulations demonstrate coherence lengths on the order of mm (around 20 THz) and potentially greater. The emission frequencies are determined by the shock speed and the lattice constants of the crystal and can potentially be used to determine atomic-scale properties of the shocked material.

Figure 1

Fourier transform of the electric polarization surface current component in the shock propagation direction for molecular dynamics simulations of a shock propagating through NaCl in the $[111]$ direction (left) and $[100]$ direction (right). Narrow bandwidth, coherent peaks exist in the shocked simulations (black) that do not exist in the simulations without shocks. Gray arrows are emission frequencies predicted by Eq. (3). The coherence length for emission from the 16 THz peak in the $[111]$ case is about 5 mm, comparable to that of some commonly used lasers. Thermal noise gives rise to an incoherent background.