Scientists have achieved a seemingly unattainable energy transition

A group of scientists from Basel and Bochum have successfully conducted an apparently unattainable energy transition in an artificial atom, using laser light.

Making use of the radiative Auger process, a team of scientists have become the first team to specifically excite a seemingly impossible energy transition in an artificial atom, using laser light.

Ultrapure semiconductor samples for the experiment were produced by Dr. Julian Ritzmann at Ruhr-Universität Bochum, under the supervision of Dr. Arne Ludwig at the Chair for Applied Solid State Physics headed by Professor Andreas Wieck. The measurements were carried out by a team from the University of Basel, run by Clemens Spinnler, Liang Zhai, Giang Nguyen and Dr. Matthias Löbl in the group led by Professor Richard Warburton.

The radiative Auger process consists of an electron falling from a higher to a lower energy level and, as a result, emits its energy partly in the form of light and partly by transferring it to another electron. The artificial atoms are narrowly defined areas in semiconductors that could one day form the basis for quantum communication. The findings are illustrated by the collaborative team from the University of Basel and Ruhr-Universität Bochum together with colleagues from Münster and Wroclaw in “Nature Communications”, published online on 12 November 2021.

Electrons change energy levels

Atoms consist of a nucleus and electrons that travel around within the nucleus. These electrons can assume different energy levels. For example, electrons that are more tightly bound to the nucleus, meaning those that are closer to it, have a lower energy than electrons that are further away from the nucleus. However, the electrons are unable to assume any arbitrary energy levels, only specific levels are possible.

If an electron acquires energy, for example by absorbing a light particle, a photon, it can be increased to a higher energy level. Whereas if an electron falls to a lower energy level, energy is then released. This energy can be emitted in the form of a photon. Although, it can also be transferred to one of the other electrons. In this particular case, only some of the energy is released as light and the rest is absorbed by the other electron. This process is known as the radiative Auger process.

Conducting a unique energy transition with two lasers

By exposing light particles, electrons can be lifted to a higher energy level; however, they can also be stimulated to emit energy by an incident light particle. The energy of the incident light particle must always correspond exactly to the difference in the two energy levels between which the electron is to be transferred. The researchers utilised two lasers: one moved electrons between a low and a high energy level, while the other between a high and medium energy level. This middle energy level corresponds to a non-equilibrium level, meaning the transfer to the middle level doesn’t exist without a radiative Auger process.

Additionally, a transition between the low and the medium energy level shouldn’t have occurred because the relevant light was not irradiated. However, this seemingly impossible transition was successful due to the energy transfer from one electron to another in the radiative Auger process.

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