A Novel Type of Nonlinear Photonic Circuitry

Illustration of a nonlinearity-Induced topological insulator: The synthetic photonic material composed of complex interwoven waveguides allows light to protect itself from external perturbations. (Source: L. Maczewsky, U. Rostock)

Researchers from the Univer­sity of Rostock have developed a novel type of nonlinear photonic circuitry in which intense light beams can define their own path and, in doing so, render themselves impervious to external pertur­bations. “Photons are an unruly bunch,” explains Alexander Szameit, whose group carried out the ground­breaking experiments. “As soon as one manages to herd them towards one specific point in space and time, they immediately disperse once again in all directions.” Indeed, centuries of research have been devoted to shaping the flow of light by a number of means: Lenses and curved mirrors can tightly focus rays from the sun. Powerful lasers generate coherent beams and short pulses of intense light. And fiber-optic cables deliver staggering amounts of optically encoded data across the world wide web. Yet, light waves are sur­prisingly delicate entities: A small crack in a lens, a mote of dust drifting through a laser beam, or a kink in the fiber can upset the intri­cate mechanisms that transform light into perhaps the most versatile tool ever harnessed by humanity.

Electronic topo­logical insu­lators – solids that do not conduct elec­tricity inside their bulk, yet at the same time are perfectly conductive along their surface – have been experi­mentally realized for the first time in 2007 by Laurens Molenkamp and his team at the University of Würzburg. Their photonic counter­parts have fascinated Szameit for a long time. “Ever since our first imple­mentation of a topo­logical insulator for light, we have strived to discover how these peculiar systems can be utilized,” the physicist remembers. While photonic topological insulators can guide light along precisely defined paths, and the mathe­matical framework under­pinning their design endows them with an unpre­cedented degree of robustness towards imper­fections or external pertur­bations, these sought-after properties also present a formidable obstacle. “Once injected into a topological channel, light pulses do not suffer from scattering losses, but this insulation also makes them virtually impossible to control without breaking them out of their protec­tive environment”, Matthias Heinrich summarizes the challenge currently faced by the scientific community.

Of course, on paper, the solution may seem obvious. “In principle, it’s easy. All you need is a switch that you can flip at will to instantly change the topo­logical properties of the system between two light pulses,” quips Szameit. However, topology is inex­tricably linked to the physical arrangement of the waveguide circuit, while ultrashort laser pulses are measured in femto­seconds – many orders of magnitude beyond the reach of even the fastest electronic modulators. In close colla­boration with theorists from the University of Rostock, the ICFO in Barcelona, the University of Lisbon and the Moscow Institute for Science and Techno­logy, the team of young researchers found a way to instead let the light itself decide whether to engage topo­logical protection or to behave as if in a conventional medium.

“Depending on their peak intensity, optical pulses can behave in funda­mentally different ways,” elaborates Ph.D. student Lukas Maczewsky. “Nonlinearity is the magic word: In photonics, sometimes two plus two really is more than just four.” After two years of intense research and countless hours in the labs of the Institute of Physics at the University Rostock, these efforts came to fruition. The non­linearity-induced topo­logical insulator allows light pulses above a certain threshold intensity to establish a transient topological domain in their immediate vici­nity. “Just like the U.S.S. Enterprise raises its shields, the self-generated protec­tive cocoon follows the light pulses and preserves them along their chosen path” the self-avowed Star Trek fan Szameit paints a vivid picture of the complex physics at play.

The successful inter­national colla­boration has sub­stantially advanced fundamental science in the field of quantum optics and in particular the research into photonic topo­logical insulators. Until these pieces can be assembled into a workable optical quantum computer several challenges remain to be resolved. Never­theless, the physicists’ newest discovery holds great promise for numerous innovative appli­cations such as topo­logically protected all-optical signal processing and self-improving photonic neuronal nets. Given the rapid pace of progress, these ideas that today may seem like science fiction, could soon become reality. (Source: U. Rostock)

Reference: L. J. Maczewsky et al.: Nonlinearity-induced photonic topological insulator, Science 370, 701 (2020); DOI: 10.1126/science.abd2033

Link: Experimental Solid-State Optics Group, Institute for Physics, University of Rostock, Rostock, Germany

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