Extreme conditions in semiconductors

July 30, 2018, University of Konstanz
Close-up of the experimental setup in the University of Konstanz’s high-field Terahertz lab. Under the extreme conditions of the experiment, a bright red glow can be seen to emanate from the gallium arsenide crystal used as a semiconductor. This is due to the system's extremely high optical nonlinearity, which occurs when Wannier-Stark localization sets in. Credit: Leitenstorfer research team

Scientists from the University of Konstanz and Paderborn University have succeeded in producing and demonstrating what is known as Wannier-Stark localization for the first time. In doing so, the physicists managed to overcome obstacles that had so far been considered insurmountable in the field of optoelectronics and photonics. Wannier-Stark localization causes extreme imbalance within the electric system of crystalline solids. "This fundamental effect was predicted more than 80 years ago. But it has remained unclear ever since whether this state can be realized in a bulk crystal, that is, on the level of chemical bonds between atoms," says Professor Alfred Leitenstorfer, professor of experimental physics at the University of Konstanz. Analogues of the effect have so far been demonstrated only in artificial systems like semiconductor superlattices or ultracold atomic gases. In a bulk solid, Wannier-Stark localization can only be maintained for an extremely short period of time, shorter than a single oscillation of infrared light. Using the ultrafast laser systems at the University of Konstanz, Wannier-Stark localization has now been demonstrated for the first time. The experiment was conducted in a high-purity gallium arsenide crystal grown at ETH Zurich using epitaxial growth. The research results were published in the scientific journal Nature Communications on 23 July 2018.

A crystal can be visualized as a three-dimensional grid composed of small beads that repel each other and are only kept together by rubber bands. The system remains stable as long as the rubber band is as strong as the repulsion is. If this is the case, the beads neither move closer to each other, nor do they move away from each other – the distance between them remains about the same. Wannier-Stark occurs when the rubber bands are removed abruptly. It is the electronic state that happens at the precise moment in time when the rubber bands have already gone but the beads still remain in place: The that hold the crystal together have been suspended.

If this state is maintained for too long, the beads will break apart and the crystal dissolves. To analyze Wannier-Stark localization, the physicists had to remove the stabilizing structures, capture the system within a fraction of a light oscillation using , and finally to stabilize it again to prevent the atoms from breaking apart. The experiment was made possible through the highly intense electric field of an ultrashort infrared light pulse, which is present in the crystal for a few femtoseconds only. "This is what we specialize in: studying phenomena that only exist on very short time scales," explains Alfred Leitenstorfer.

"In perfect insulators and semiconductors, electronic expand throughout the entire crystal. According to an 80-year-old prediction, this changes as soon as electrical voltage is applied," says Professor Torsten Meier from Paderborn University. "If the electric field inside the crystal is strong enough, the electronic states can be localized to a few atoms. This state is called the Wannier-Stark ladder," explains the physicist.

"A system that deviates so extremely from its equilibrium has completely new characteristics," says Alfred Leitenstorfer about why this state is so interesting from a scientific perspective. The short-lived Wannier-Stark localization correlates with drastic changes to the electronic structure of the crystal and results, for example, in extremely high optical nonlinearity. The scientists also assume that this state is chemically particularly reactive.

The first-ever experimental realization of Wannier-Stark localization in a was made possible through highly intense Terahertz radiation with field intensities of more than ten million volts per centimetre. The application of more ultrashort optical light pulses resulted in changes to the crystal's optical characteristics, which was instrumental to proving this state. "If we use suitably intense light pulses consisting of a few oscillations lasting some ten femtoseconds only, we can realize the Wannier-Stark localization for a short period of time," says Alfred Leitenstorfer. "Our readings match the theoretical considerations and simulations carried out both by my own research team and by that of my colleague, Professor Wolf Gero Schmidt," adds Torsten Meier. The researchers are planning to study the extreme state of Wannier-Stark localization on the atomic scale in more detail in the future and intend to make its particular characteristics usable.

Explore further: Ultracold atoms and ultrafast lasers

More information: C. Schmidt et al. Signatures of transient Wannier-Stark localization in bulk gallium arsenide, Nature Communications (2018). DOI: 10.1038/s41467-018-05229-x

Related Stories

Ultracold atoms and ultrafast lasers

July 6, 2018

Two separate research fields have been united in Hamburg for the very first time. Ultrashort laser pulses enable us to observe and manipulate matter on very short time scales, whereas ultracold atoms permit experiments with ...

Studying the quantum vacuum: Traffic jam in empty space

January 18, 2017

An important step towards a completely new experimental access to quantum physics has been made at University of Konstanz. The team of scientists headed by Professor Alfred Leitenstorfer has now shown how to manipulate the ...

Recommended for you

Custom-made artificial mother-of-pearl

December 10, 2018

Natural mother-of-pearl, such as mussels, is one of the hardest, most stable and stiff natural materials. Researchers have always been fascinated by it. The structure of mother-of-pearl is exquisite under the electron microscope; ...

Engineers repurpose wasp venom as an antibiotic drug

December 7, 2018

The venom of insects such as wasps and bees is full of compounds that can kill bacteria. Unfortunately, many of these compounds are also toxic for humans, making it impossible to use them as antibiotic drugs.

Researchers probe hydrogen bonds using new technique

December 7, 2018

Researchers at Carnegie Mellon University have used nuclear resonance vibrational spectroscopy to probe the hydrogen bonds that modulate the chemical reactivity of enzymes, catalysts and biomimetic complexes. The technique ...

Are amorphous solids elastic or plastic?

December 7, 2018

In a crystalline solid, the atoms form an ordered lattice. Crystalline solids respond elastically to small deformations: When the applied strain is removed, the macroscopic stress, as well as the microscopic configuration ...

Molecular insights into spider silk

December 7, 2018

Spider silk is one of the toughest fibres in nature and has astounding properties. Scientists from the University of Würzburg discovered new molecular details of self-assembly of a spider silk fibre protein.

0 comments

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.