Even at school, you learn that every material, especially solids, can be classified as either metal or insulator. When connecting poles of a battery to a piece of metal, electrons flow from the negative pole to the positive pole, i. the applied voltage generates an electric current. On the other hand, if you do the same experiment with a piece of non-conductive material, you will not measure any electric current. One might wonder, then, if the electrons in an insulator move at all when exposed to a strong field (voltage). And if they move: how far and how fast?

To answer this fundamental question, one has to measure the position of the electrons in the material with a spatial accuracy of 0.1 nm (0.1 nm = 10^-10 m), which roughly corresponds to the distance between neighboring atoms. This is possible by imaging the material with X-rays, which are scattered by electrons and provide information about their spatial distribution. In addition, one must create a very strong electric field to pull the electrons away from their origin atoms. Extremely strong electric fields can be generated for very short times (50 fs, 1 fs = 10^-15 s) by means of optical light pulses. In the current issue of Physical Review Letters (PRL 109, 147402, 2012), Johannes Stingl, Flavio Zamponi, Benjamin Freyer, Michael Woerner, Thomas Elsaesser and Andreas Borgschulte report on the first in-situ X-ray imaging of electron and atomic motions strong optical field were triggered. They recorded a time-dependent "electron density map" for LiBH₄ prototype material obtained from a series of snapshots using ultra-short x-ray flashes (100 fs). Snapshots at different times during and after the light pulse form an "X-ray film" that visualizes the atomic and electronic movements in the LiBH₄ crystal.

To the great surprise of the researchers, during the overlap between optical pulse of light and X-ray flash, an extremely fast electron transfer took place from the BH₄^-- to the neighboring Li + ion, which is about 0.25 nm away. Since the electric field of light reverses its direction every 1.3 fs, the electron is moved between two places at very high speed, about 1% of the speed of light (c = 300,000 km / s). After the light pulse, the electron returns to the BH₄ ion and the original electron distribution is restored. Besides this instantaneous and reversible electron transfer, there are no macroscopic currents, i. The material behaves like an insulator. Quantitative analysis shows that the large deflection of electrons between neighboring ions is the major contributor to electrical polarization and is the cause of many nonlinearities at optical frequencies. In addition to new insights into fundamental electrical and optical properties of insulators, the experiments on LiBH₄ offer high application potential for the temporal characterization of ultracure X-ray pulses.

Movie: The cartoon linked here shows the electron movement between neighboring atoms in a LiBH₄ crystal. The red curve in the upper part of the picture shows the electric field of the laser light as a function of time. The moving blue dot marks the strength and direction of the electric field for the corresponding snapshot in the lower part of the image. This shows an "electron density map" of the unit cell of a LiBH₄ crystal. Without applying an electric field, the BH₄ anions (very bright regions) have a larger electron density than the Li + cations (darker spots). During the laser pulse, the oscillating electric field drives strong electric currents between the BH₄ and Li + ions, which are indicated by the intensity of the triangular arrows that occur.