Fractal Drive - Photons In The Fast Lane

Researchers at the University of Rostock have developed a new type of microstructured material that transports light signals at higher speeds while shielding them from scattering and external sources of interference. This discovery will be published online by the renowned journal Science on May 12, 2022.

So-called photonic topological insulators (PTIs) are artificial materials in which light particles are guided in channels close to the surface without being able to be scattered into the interior. These "superconductors for photons" have long fascinated Professor Alexander Szameit from the University of Rostock. "Ever since we first succeeded in realizing such a system, we have been working on new ways of making these unique materials technologically usable," recalls the head of the solid-state optics working group. While topological insulators can protect light as it propagates along defined paths without being scattered by imperfections or external influences, this can also become a problem. "Regular structures, as they are normally used in the design of PTIs, slow down the propagation of light significantly," reports the first author of the study, Tobias Biesenthal. "So when we try to protect light signals, we load them with unnecessary ballast."

To solve this problem, the research team made an excursion into the strange and aesthetic world of fractals. As a mathematical concept, these were developed in 1967 by Benoit Mandelbrot, who was investigating why the British Isles seemed to gain hundreds of kilometers in length when higher-resolution maps were used to survey them. Fractal structures are ubiquitous in nature. Thus, the smallest branches of a tree are similar in arrangement to the larger branches that emerge from the main trunk. So fractals are characterized by self-similarity, which repeats features of the overall system in its subsections, regardless of the level of magnification. If a structure is repeated not only similarly but identically, one speaks of "exact" fractals. The most well-known example of this is the Sierpinski triangle – an equilateral triangle whose interior is subdivided by nested copies of itself. Paradoxically, although this object is easy to sketch on a piece of paper, it does not contain any surface, since each of its points can be mathematically associated with one of the many edges.

Working closely with partners from Haifa (Israel) and Zhejiang (China), the Rostock researchers solved the long-standing question of whether topological insulators can also be constructed without bulk material, and used their findings to reduce signals traveling on their edges from their load to free. "Like a stone that is jumped over the waves of the Baltic Sea, the light particles race along the outer edges of our structure without being slowed down by its interior," explains Dr. Matthias Heinrich, co-author and initiator of the work. “The crucial difference is that after a few jumps, a stone inevitably loses momentum and sinks. Since our novel fractal material effectively protects light from scattering, it can remain in the fast lane over the long term.”

The result of this successful international cooperation represents a significant advance in basic research in the field of topological photonics. Although there are still a number of hurdles to be overcome before the knowledge gained can find its way into our everyday lives, it opens up a wide range of fascinating possibilities such as topologically protected high-performance circuits for light; and a new class of versatile synthetic materials.

The work was funded by the German Research Foundation (DFG) and the Alfried Krupp von Bohlen und Halbach Foundation.