35 million euros, 210 scientists, 12 years of research: after the maximum possible duration of three funding periods, the Collaborative Research Centre/TRR 142 "Tailor-made Nonlinear Photonics: From Basic Concepts to Functional Structures" officially came to an end in December 2025. With their research, the scientists at Paderborn University and TU Dortmund University have achieved a great deal and literally brought light into the darkness. The experts have developed materials that are smaller than the wavelength of light, precisely manipulated, controlled and even teleported tiny particles of light - photons. They have created quantum light sources - indispensable tools for quantum computers and ultra-fast communication - and low-temperature electronics for controlling quantum experiments. And these are just a few examples. Above all, however, they have made essential contributions to international basic research on optical systems. In the long term, the scientists have paved the way for more efficient optical components and new technologies.

Non-linear effects

The aim of the Collaborative Research Centre (CRC) was to research, develop and construct so-called non-linear photonic systems. In simple terms, these are changes in light waves that do not occur in nature, i.e. in our everyday lives. Natural optical phenomena are linear. When light and matter interact, linear effects change the incident light. Examples are reflection or scattering. However, the wavelengths always remain the same. This is different with a laser: it enables the generation of non-linear effects such as frequency doubling. This corresponds to half the wavelength of the original light. The TRR 142 has produced concepts for harnessing novel non-linear functionalities from the fields of material physics and quantum photonics for applications in the field of future information and communication technologies.

From the basics to application

The scientists have utilised state-of-the-art technological capabilities to explore new physical properties and devices based on tailored, strong nonlinearities and real quantum effects. "We wanted to bring non-linear optical and quantum effects from the stage of basic physical research into application," says Prof Dr Thomas Zentgraf from the Department of Physics at Paderborn University, spokesperson for the CRC. To this end, the core competences of Paderborn University in the fields of photonic materials, solid-state technology, quantum optics and theory have been brought together with those of TU Dortmund University in non-linear spectroscopy and instrumentation. The team focussed on tailoring nonlinear interactions, controlling quantum systems, light emission and propagation, and nonlinearities at the single-photon level - truly pioneering work, as it later turned out. "The TRR 142 has made a significant contribution to the further development of non-linear photonics and quantum optics and has laid the foundations for promising future technologies with its interdisciplinary cutting-edge research. By successfully linking theory, materials research and experimental practice, it has strengthened the scientific excellence of Paderborn University," says University President Prof Dr Matthias Bauer.

Tap-proof communication through manipulation

One prominent application of SFB research is encrypted, tap-proof communication. This is because the knowledge gained in the field of photonics research makes precisely this possible. To encode the transported data, the scientists have, among other things, specifically changed optical properties - i.e. the propagation and transmission of light. To do this, they have developed so-called meta-surfaces. These are artificial components that influence the light waves. Until now, these materials were neither designed nor sufficiently researched for efficient use. "They consist of artificially produced structures whose optical, magnetic or electrical properties do not occur in nature. Their advantage is that they can refract and even change radiation," says Prof Zentgraf. "This allows new frequencies to be reached without which targeted manipulation would not be possible."

Pioneering work in integrated optics

This arrangement of nanostructures on surfaces has made it possible to create artificial materials with adjustable linear and non-linear optical behaviour. Their functionality goes far beyond that of conventional materials. This enables compact optical components for frequency conversion or the control of light propagation. Research in the field of quantum photonics was focussed on quantum communication, quantum sensor technology and quantum information processing. An important basis for the implementation of the projects was pioneering technological work in the field of integrated optics, such as the development of efficient waveguides for frequency conversion. Through the targeted use of these technological developments, integrated optical frequency converters, quantum light sources and non-linear interferometers could be realised, which are indispensable key components for optical quantum technologies.

Milestones: quantum dots and quantum teleportation

Quantum technologies offer many new ways of processing and transmitting information and carrying out precise measurements. As the protection of sensitive data and information is becoming increasingly important, corresponding communication networks are gaining in significance. Semiconductor quantum dots also play an important role in this context. These are tiny structures that behave like artificial atoms. With precise laser excitation, they can precisely control individual photons and realise single photon sources, an essential basis for absolutely secure communication using quanta. In addition to producing such structures, the Paderborn physicists have also succeeded in realising quantum teleportation using "imperfect quantum dots", i.e. artificial material structures. This involves transferring the state of one photon to another. The transmitter and receiver are entangled with each other. This requires sources that produce indistinguishable photons.

Research for the photonics of the future

"The close cooperation between two outstanding partners from the fields of solid-state physics and optical spectroscopy has created a strong alliance dedicated to researching and developing the non-linear photonics of the future. Using state-of-the-art theoretical approaches and innovative experimental methods, we have been able to investigate fundamental physical issues and new device designs - based on customised nonlinearities and fundamental quantum effects," says Prof. Zentgraf. The jointly achieved results are milestones on the way to future information technologies or those already in the evaluation phase, such as quantum communication or optical quantum information processing. TRR 142 has thus laid the foundations for achieving technological sovereignty in order to open up new markets for future optical technologies.

Collaborative Research Centres are long-term university research institutions in which scientists work together as part of an interdisciplinary research programme. They make it possible to work on innovative, demanding, complex and long-term research projects by coordinating and concentrating people and resources at the applicant universities. They are funded by the German Research Foundation (DFG) for a period of up to 12 years, with each sponsorship period lasting four years.

This text was translated automatically.