Synopsis: Charged shock waves

Shock waves, familiar from hydrodynamics, acoustics, and optics, have been observed in the changing charge state of iron defects in lithium niobate crystals upon application of even a modest voltage across the crystal.
Synopsis figure

A shock wave is a propagating discontinuity in an otherwise linear and well-behaved medium. Shock fronts form whenever a disturbance is driven through a medium faster than it would normally move and are commonly seen in hydrodynamic or acoustic settings. Now, in a paper appearing in Physical Review Letters, Stephan Gronenborn, Matthias Falk, Daniel Haertle, and Karsten Buse, of the University of Bonn, Germany, together with Boris Sturman in Russia, have observed shock-wave behavior in iron-doped ferroelectric lithium niobate crystals in an electric field—an unexpected phenomenon in a solid-state system.

The shock wave takes the form of a traveling change of transparency of the doped lithium niobate crystal. While undoped lithium niobate is transparent, defects created by iron doping absorb light shone through the crystal. The iron defects exist in two charge states: Fe2+ and Fe3+, and the crystal becomes increasingly transparent as the ratio of Fe2+ to Fe3+ decreases. When a voltage is applied to the crystal, a transparent region appears by the cathode, and this region increases in the wake of a propagating front as the ion ratio changes across the sample, in a fashion reminiscent of a traveling shock wave.

The shock wave is not fast: it takes several hours for it to travel one centimeter, but its observation may have important consequences for the fast-moving field of optoelectronics. – Daniel Ucko


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