New insights into the energy states of quantum dots

Scientists have experimentally proven the theoretically predicted Auger effect in quantum dots.

Scientists from Basel, Bochum, and Copenhagen have gained new insights into the energy states of quantum dots. Through their experiments, scientists have provided evidence on theoretically predicted Auger effect in quantum dots.

Scientists have confirmed that certain energy confirmed specific energy transitions in quantum dots that had been proven theoretically by the Auger effect.

Scientists used self-organizing processes in crystal growth to create a quantum dot. This process produces lots of nanometer-sized crystals, such as indium arsenide. In these, they can trap charge carriers, such as a single electron.

This construct feasible for quantum communication as it makes it possible to encode using charge carrier spins. For this coding, it is necessary to be able to manipulate and read the spin from the outside. During readout, quantum information can be imprinted into the polarization of a photon, for example. This then carries the information further at the speed of light and can be used for quantum information transfer.

That’s the reason; scientists are keen to know what exactly happens in the quantum dot when energy is irradiated from outside onto the artificial atom.

In a multi-electron atom, an excited electron can decay by emitting a photon. Typically, the leftover particles are in their ground state. In a radiative Auger process, the leftover electrons are in an excited state, and a redshifted photon is created. In a semiconductor quantum dot, radiative Auger is predicted for charged excitons.

Now, the experimental observation has been achieved by scientists from Basel. Together with their colleagues from Bochum and Copenhagen, the Basel-based researchers Dr. Matthias Löbl and Professor Richard Warburton, have observed the radiative Auger process in the limit of just a single photon and one Auger electron.

An electron inside a quantum dot is raised by a photon (green waveform) to a higher energy level. The result is a so-called exciton, an excited state consisting of two electrons and one hole. By emitting a photon (green waveform), the system returns to the ground state (green path). In rare cases, a radiative Auger process takes place (red arrow): an electron stays in the excited state, while a photon of lower energy (red waveform) is emitted.
An electron inside a quantum dot is raised by a photon (green waveform) to a higher energy level. The result is a so-called exciton, an excited state consisting of two electrons and one hole. By emitting a photon (green waveform), the system returns to the ground state (green path). In rare cases, a radiative Auger process takes place (red arrow): an electron stays in the excited state, while a photon of lower energy (red waveform) is emitted. Credit: RUB, Arne Ludwig

For the first time, the scientists demonstrated the connection between the radiative Auger process and quantum optics. They show that quantum optics measurements with the radiative Auger emission can be used as a tool for investigating the dynamics of the single electron.

The radiative Auger effect can help scientists determine the structure of the quantum mechanical energy levels available to a single electron in the quantum dot. Until now, this was only possible indirectly via calculations in combination with optical methods. Now a direct proof has been achieved. This helps to understand the quantum mechanical system better.

Scientists observed the effect not only in quantum dots in indium arsenide semiconductors but also in the semiconductor gallium arsenide.

In both material systems, the team from Bochum has achieved very stable surroundings of the quantum dot, which has been decisive for the radiative Auger process. For many years now, the group at Ruhr-Universität Bochum has been working on the optimal conditions for stable quantum dots.

Journal Reference:
  1. Löbl, M.C., Spinnler, C., Javadi, A. et al. Radiative Auger process in the single-photon limit. Nat. Nanotechnol. (2020). DOI: 10.1038/s41565-020-0697-2

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