New insights into the energy levels in quantum dots
Researchers have experimentally proven the theoretically predicted Auger effect in quantum dots.

Date:
June 25, 2020
Source:
Ruhr-University Bochum
Summary:
Researchers have gained new insights into the energy states of
quantum dots. They are semiconductor nanostructures and promising
building blocks for quantum communication. With their experiments,
the scientists confirmed certain energy transitions in quantum
dots that had previously only been predicted theoretically: the
so-called radiative Auger process.



FULL STORY ========================================================================== Researchers from Basel, Bochum and Copenhagen have gained
new insights into the energy states of quantum dots. They are
semiconductor nanostructures and promising building blocks for
quantum communication. With their experiments, the scientists confirmed
certain energy transitions in quantum dots that had previously only been predicted theoretically: the so-called radiative Auger process. For their investigations, the researchers in Basel and Copenhagen used special
samples that the team from the Chair of Applied Solid State Physics at Ruhr-Universita"t Bochum had produced. The researchers report their
results in the journal Nature Nanotechnology, published online on 15
June 2020.


==========================================================================
Lock up charge carriers In order to create a quantum dot, the Bochum researchers use self-organizing processes in crystal growth. In the
process, they produce billions of nanometer-sized crystals of, for
example, indium arsenide. In these they can trap charge carriers,
such as a single electron. This construct is interesting for quantum communication because information can be encoded with the help of 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.

This is why scientists are interested, for example, in what exactly
happens in the quantum dot when energy is irradiated from outside onto
the artificial atom.

Special energy transitions demonstrated Atoms consist of a positively
charged core which is surrounded by one or more negatively charged
electrons. When one electron in the atom has a high energy, it can reduce
its energy by two well-known processes: in the first process the energy
is released in the form of a single quantum of light (a photon) and the
other electrons are unaffected. A second possibility is an Auger process,
where the high energy electron gives all its energy to other electrons
in the atom.

This effect was discovered in 1922 by Lise Meitner and Pierre Victor
Auger.

About a decade later, a third possibility has been theoretically described
by the physicist Felix Bloch: in the so-called radiative Auger process,
the excited electron reduces its energy by transferring it to both, a
light quantum and another electron in the atom. A semiconductor quantum
dot resembles an atom in many aspects. However, for quantum dots,
the radiative Auger process had only been theoretically predicted so
far. Now, the experimental observation has been achieved by researchers
from Basel. Together with their colleagues from Bochum and Copenhagen,
the Basel-based researchers Dr. Matthias Lo"bl and Professor Richard
Warburton have observed the radiative Auger process in the limit of
just a single photon and one Auger electron. For the first time, the researchers 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.

Applications of quantum dots Using the radiative Auger effect, scientists
can also precisely 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 better understand the quantum mechanical system.

In order to find ideal quantum dots for different applications,
questions such as the following have to be answered: how much time
does an electron remain in the energetically excited state? What energy
levels form a quantum dot? And how can this be influenced by means of manufacturing processes? Different quantum dots in stable environments
The group observed the effect not only in quantum dots in indium arsenide semiconductors. The Bochum team of Dr. Julian Ritzmann, Dr. Arne Ludwig
and Professor Andreas Wieck also succeeded in producing a quantum dot
from 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-Universita"t Bochum has been working on
the optimal conditions for stable quantum dots.


========================================================================== Story Source: Materials provided by Ruhr-University_Bochum. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Matthias C. Lo"bl, Clemens Spinnler, Alisa Javadi, Liang Zhai,
Giang N.

Nguyen, Julian Ritzmann, Leonardo Midolo, Peter Lodahl, Andreas
D. Wieck, Arne Ludwig, Richard J. Warburton. Radiative Auger
process in the single- photon limit. Nature Nanotechnology, 2020;
DOI: 10.1038/s41565-020-0697-2 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2020/06/200625102515.htm

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