Physicists from Hannover Predict Novel Light Molecules

Mobile communication, video streaming and satellite navigation would be inconceivable without light. It is light that enables the highest data transmission rates over long distances. Moreover, light is one of the most precise and efficient tools known to science.

A team of physicists from the PhoenixD Cluster of Excellence at Leibniz University Hannover predicts that in the future light will be used for numerous new functionalities that seemed unthinkable just a short while ago.

For these predictions to succeed, light must be precisely controlled and manipulated on the smallest possible scales. Achieving this goal is not easy. It requires short pulses, high intensities and suitable materials, such as optical glass fibres, through which the light can be directed and distributed in networks.

For this purpose, the PhoenixD research group as well as partners from ITMO University (St Petersburg) and University of Rostock focus on optical solitons. These are special light-wave packets that travel through optical fibres (glass fibre), essentially without changing their shape and properties.

The engineer John Scott Russell discovered this principle in 1834 in a completely different medium – water. Russell observed water waves in a Scottish canal, which propagated for kilometres without any losses.

The researchers from Hannover were now able to demonstrate for the first time that solitons of different-colour light exhibit a remarkable behaviour when colliding. They can be fused together, creating a new type of bound state: light molecules. According to the researchers, these molecules are very robust against disturbances.

Under certain conditions, they can even emit light themselves. They therefore fulfil special requirements for the realisation of optical functions such as optical switches or transmission networks for information.

“The transfer of simple quantum mechanical concepts allows us to efficiently describe and interpret the complex phenomena of purely classical non-linear optics found here”, says Prof Dr Ayhan Demircan from the Institute of Quantum Optics at Leibniz University Hannover, head of the research group from which the work originated.

In contrast to conventional matter molecules, which consist of individual atoms, light molecules are assemblages of individual solitons. This creates artificial light states that do not exist in nature. Controlling this newly discovered mechanism opens up a wide range of applications, including improved communication with much higher data transmission rates.

The knowledge gained could also be an essential basis for the internet of the future – the quantum internet, the quantum computer or new tap-proof encryption methods. Moreover, the effect may be applied to integrated optical microchips.

Research findings published in “Physical Review Letters”

The work published in the scientific journal “Physical Review Letters” initially provided the theoretical basis for these novel light molecules, enabling the researchers to describe the relevant processes efficiently and gain a better understanding of them. In this context, the experts describe a non-linear classical field theory specifically developed for this purpose. Previously, this mechanism was completely unknown.

“Our discovery opens up a promising new field in an established field of research. Numerous follow-up studies are expected to address many open questions”, says Dr Oliver Melchert, first author of the study and a member of research staff at the Institute of Quantum Optics at Leibniz University Hannover.

Due to the technical challenges, it would not have been possible to implement the discovered mechanism in laboratory experiments. However, through rapid progress in optics in recent years, suitable laser sources, materials and processes are now available to generate such interactions between light pulses and use them for specific applications.

The researchers intend to implement the results of the theoretical study in laboratory experiments and develop the necessary measurement technology. The work paves the way for further exciting findings. “Our work is a perfect example of research driven by pure scientific curiosity initially leading to new findings in basic areas but often resulting in new applications with enormous potential”, says Prof Dr Bernhard Roth from Hannover Centre for Optical Technologies (HOT) who was also involved in the study.

The PhoenixD Cluster of Excellence

Between 2019 and 2025, the Cluster of Excellence PhoenixD led by Leibniz University Hannover will receive approximately 52 million euros of funding from the federal government and the State of Lower Saxony via the German Research Foundation (DFG). The cluster is a collaboration of TU Braunschweig, Max Planck Institute for Gravitational Physics (Albert Einstein Institute), Physikalisch-Technische Bundesanstalt and Laser Zentrum Hannover e.V. Within the scope of the cluster, more than 100 scientists from the fields of physics, mechanical engineering, electrical engineering, chemistry, computer science and mathematics conduct interdisciplinary research. The cluster explores the possibilities offered by digitisation for novel optical systems as well as their production and application.

Prof Dr Ayhan Demircan, head of the research group and Task Group S3 – Micro and Nano Photonics in the Cluster of Excellence PhoenixD (Tel. + 49 511 762 17219, Email demircan@iqo.uni-hannover.de)

Prof Dr Bernhard Wilhelm Roth, head of Task Group F1 – Precision Metrology in the Cluster of Excellence PhoenixD and managing director of the Hannover Centre for Optical Technologies (Tel. +49 511 762 17907, Email bernhard.roth@hot.uni-hannover.de)

Dr Oliver Melchert, first author of the study and member of Task Group S3 – Micro and Nano Photonics in the Cluster of Excellence PhoenixD (Tel. +49 49 511 762 3381, Email melchert@iqo.uni-hannover.de)

O. Melchert, S. Willms, S. Bose, A. Yulin, B. Roth, F. Mitschke, U. Morgner, I. Babushkin, A. Demircan
Soliton Molecules with Two Frequencies
Phys. Rev. Lett. 123, 243905 (2019).
https://doi.org/10.1103/PhysRevLett.123.243905