Scientists from the Max Planck Institute for Polymer Research, the University of Paderborn and the University of Konstanz have achieved a rare quantum state: they have demonstrated Wannier-Stark localization in a polycrystalline substance for the first time. The effect predicted about 80 years ago was previously proven only in single crystals.

The atoms of a crystal are arranged in a three-dimensional grid held together by chemical bonds. These bonds can be broken by very strong electric fields that displace the atoms. For example, a similar effect can be observed during a lightning strike when materials liquefy, evaporate or ignite. To demonstrate Wannier-Stark localization, the scientists created electrical fields of several million volts per centimeter for their experiments, which are much stronger than the fields produced by lightning strikes.

In the course of this process, the electronic system of a solid body – in this case a polycrystal – for a very short time moves away from the state of “equilibrium”.

“Wannier-Stark localization involves temporarily cutting off some chemical bonds. This state can be maintained for less than a picosecond—one millionth of one millionth of a second—without destroying matter. As soon as the electric field inside the crystal becomes strong enough, the chemical bonds directed towards the field are deactivated, causing the crystal to briefly turn into a system of unbound layers. We can say that chaos reigns. This phenomenon correlates with abrupt changes in the electronic structure of the crystal, which leads to abrupt changes in the optical characteristics, in particular, to high optical nonlinearity,” explained Torsten Meyer, professor at the University of Paderborn, who was in charge of the theoretical analysis of the experiments.

Nonlinear effects can lead to the emergence of new frequencies, such as those without which the targeted control of light needed for modern telecommunications would not be possible.

This effect was first demonstrated in 2018 using intense terahertz radiation in a specific crystal structure, suggesting the exact arrangement of atoms in a gallium arsenide crystal.

“This exact location was necessary so that we could observe field-induced localization,” Meyer said.

Now physicists have taken it one step further.

“We wanted to find out if polycrystalline perovskite, commonly used in solar cells and LEDs, could be used as an optical modulator,” said Heejae Kim, team leader at the Max Planck Institute for Polymer Research.

Optical modulators are used in telecommunications, LCDs, diode lasers, and materials processing. However, until now their production has been not only expensive, but also limited almost exclusively to the field of single crystals. Polycrystals such as perovskite could make a difference as they can be used as affordable modulators with a wide range of applications in the future.

“Despite the random orientation of individual crystallites—small building blocks within a polycrystal—we were able to observe clear results consistent with Wannier-Stark localization characteristics,” said Kim.

Simulations carried out at Paderborn later confirmed these findings. The future plans of the researchers include a more complete study of this extreme state of matter at the atomic level, the study of additional substances and further applications of this effect.