Nanotowers Fire Off Photons

Physicists from the University of Würzburg are boasting research that could make tap-proof data transmission a possibility in the near future.

At the heart of the concept are tiny towers made from semiconducting material. They are around 10 µm in height with a diameter of just one to two micrometers (a human hair is roughly a hundred times thicker).

Tiny towers, 100 times thinner than a human hair, are made by the Department of Applied Physics at the University of Würzburg from semiconducting material and possess special properties. (Image: Monika Emmerling / Adriana Wolf)

The Würzburg physicists, along with a team from Institute of Semiconductor Optics and Functional Interfaces of the University of Stuttgart, dubbed these structures "nanotowers."

Contained inside these nanotowers are special structures capable of emitting light – these are known as quantum dots, and their electronic and optical properties can be customized during production. Quantum dots, in the same way as single atoms, possess precisely defined energy states. This enables them to send out photons (light particles) with an exact amount of energy.

Single photons can be generated

What is special about the Würzburg quantum dot towers is that "with them it is possible to 'fire off' single photons in a targeted fashion. It is structural elements like these that are needed for the tap-proof transmission of data in the field of quantum cryptography," explained Stephan Reitzenstein, a physicist at Würzburg.

However, to date, the production of single photons in these structures has only been achieved with temperatures well below minus 100 °C. So, there are still hurdles to overcome before the concept can be routinely applied.

New tool for analyzing quantum dots

The Stuttgart physicists studied the Würzburg nanotowers as part of a venture sponsored by the German Research Foundation (DFG).

"The towers serve as a new tool for analyzing the properties of quantum dots in a way never seen before," said Reitzenstein.

The Stuttgart team discovered an unexpected effect, known as non-resonant coupling. This suggests strong light-matter interactions in such solid-state systems.

"This will have major repercussions on the design and functionality of future quantum emitters that are based on quantum dots," explained Peter Michler, a professor at Stuttgart.

Structure of the Würzburg towers

The new insights were made possible by the special structure and highly optimized production of the towers.

The tiny structures consist of a sophisticated sequence of layers made from the semiconductors aluminum arsenide and gallium arsenide.

"Their special structure makes them into high-quality optical resonators, which confine single photons on a light wavelength scale in all three spatial dimensions," said Reitzenstein.

Embedded in the center of the towers are some 100 quantum dots made from the semiconducting material indium gallium arsenide.

"Using special spectroscopic procedures, however, a single quantum dot can purposefully be brought into resonance with the optical mode of a tower in order to conduct fundamental physics experiments on the interaction between light and matter," added Reitzenstein.

Physicists from the University of Würzburg and the University of Stuttgart jointly published their work in the journal Nature Photonics.

Those involved in the publication from Würzburg's Department of Applied Physics were Stephan Reitzenstein, Andreas Löffler, Sven Höfling and Prof. Alfred Forchel. The Stuttgart team included Serkan Ates, Sven M. Ulrich, Ata Ulhaq and Prof. Peter Michler.

For more information, visit: www.uni-wuerzburg.de/en