Solid State Physics: Trembling crystals under the X-ray lens

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Solid State Physics: Trembling crystals under the X-ray lens
Solid State Physics: Trembling crystals under the X-ray lens

Trembling crystals under the X-ray lens

When light falls on a crystal, there are sometimes not only the classic reflections and refractions that we all know from school lessons. In special specimens, the atoms oscillate, creating their own electromagnetic pattern. The beauty of jelly is that it jiggles. Children in particular enjoy its trembling and shivering as soon as they hit its surface with a spoon, fingers or nose. Very curious young explorers might even prick the defenseless pudding and observe how its wobbling changes due to the structural defect. But it's not just kids who like jello, and it's not just pudding that jiggles. However, it is not known whether there was often jello for dessert in the canteen at Oxford University, where Andrea Cavalleri conducts research. On the other hand, it is well known that Cavalleri and an international team of physicists deal with scientifically trembling bodies. In an absolutely serious and even quite sophisticated way.

The researchers' "jelly substitutes" are crystals that, due to their strange optical properties, may have a future in optronics - the branch of technology that will one day do with light what computers do today still do with electrons. In these crystals, metal ions are embedded in a reference of oxygen ions, in which they have a small range of motion. For example, in the case of lithium tantalate (LiTaO3), the lithium and tantalum atoms can oscillate slightly around their rest position. It is precisely this mobility that gives the crystals their abilities and interests physicists around the world.

In order to put lithium tantalate into action, however, the right light is required. Its energy must roughly correspond to the resonance frequency of the metal atoms. Then there is a complex interplay between the electromagnetic field of the incident light, which causes the ions to vibrate, which in turn triggers further radiation with a slightly lower energy. Cavalleri's team has now observed for the first time how this terahertz radiation, which is somewhere between infrared light and microwaves, propagates in the crystal.

Exciting the terahertz radiation represented the smallest challenge. A laser was used for this purpose, the light of which was emitted in short pulses of 70 billionths of a second (70 femtoseconds, i.e. 7010-15seconds) with a wavelength of 800 nanometers (near infrared) hit the crystal surface at the optimal angle. There, the electromagnetic waves shifted the ions - what physicists call polariton and treat them like a particle for the sake of simplicity. In order to track the spread of these quasiparticles, which are the originators of terahertz radiation, the scientists bombarded the crystal with precisely tuned X-ray light, which is deflected slightly differently depending on the position of the ions in the lattice, before it falls on a detector. A setup whose feat consisted above all in precisely coordinating the pulses of the laser light with the X-ray light flashes.

Finally, the analysis showed that three rays passed through the crystal. First of all, of course, the laser light, whose direction of propagation was bent at the interface between vacuum and crystal according to optical laws. It was followed by terahertz radiation, which to a certain extent followed in the wake of light. The carriers were the tantalum atoms, which, as charged particles, produced the electromagnetic field through their vibrations. As a third component, the researchers found another terahertz radiation that propagated at only a third of the speed and stayed close to the surface.

Although the scientists were able to convincingly simulate their results with a theoretical model, their measurements still have a disadvantage: They literally only scratch the surface. The X-rays did not penetrate deeper than a few millionths of a meter into the crystals. Nevertheless, the method is likely to find its imitators. Because if one day light is really supposed to replace the usual electrons as mediators of information, knowledge of the interactions between electromagnetic waves and the atoms of the carrier materials is of crucial importance. The X-ray light offers a view into the interior of the crystals. Even if that doesn't make them quite as transparent as the classic jelly.

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