A new process can make man-made materials transparent or even entirely invisible

Space, the ultimate frontier. The Enterprise spacecraft continues its mission to explore the galaxy, when all communication channels are suddenly cut off by an impenetrable nebula. In many episodes of the iconic television series, the valiant crew must “tech the tech” and “science the science” in just 45 minutes of airtime in order to facilitate their escape from this or a similar situation before the credits. Although they spent much more time in their laboratories, a team of scientists from the University of Rostock managed to develop an entirely new approach to designing artificial materials capable of transmitting light signals without any distortion by means of precisely tuned energy flows. They published their results in Scientists progress.

“As light propagates through an inhomogeneous medium, it undergoes scattering. This effect rapidly transforms a compact, directed beam into a diffused glow, and is familiar to us all from summer clouds and autumn fog” , said Professor Alexander Szameit of the Institute for Physics at the University of Rostock describes the starting point of his team’s thoughts. Notably, it is the microscopic density distribution of a material that dictates the specifics of scattering. Szameit continues, “The basic idea of ​​induced transparency is to take advantage of a much lesser-known optical property to clear a path for the beam, so to speak.”

This second property, known in the field of photonics under the mysterious title of non-hermiticity, describes the flow of energy, or more precisely the amplification and attenuation of light. Intuitively, the associated effects may seem undesirable – in particular fading of a light beam due to absorption would seem highly counterproductive to improving signal transmission. Nevertheless, non-Hermitian effects have become a key aspect of modern optics, and an entire field of research strives to exploit the sophisticated interplay of loss and amplification for advanced functionality.

“This approach opens up entirely new possibilities,” reports PhD student Andrea Steinfurth, first author of the paper. With respect to a beam of light, it becomes possible to selectively amplify or attenuate specific portions of a beam at the microscopic level to counter any incipient degradation. To stay in the picture of the nebula, its light-scattering properties could be completely removed. “We actively modify a material to adapt it to the best possible transmission of a specific light signal,” explains Steinfurth. “To do this, the flow of energy must be precisely controlled, so that it can fit into the material and the signal like pieces of a puzzle.” In close cooperation with partners from the Vienna University of Technology, the Rostock researchers have successfully met this challenge. In their experiments, they were able to recreate and observe the microscopic interactions of light signals with their new active materials in kilometer-long fiber optic arrays.

In fact, induced transparency is just one of the fascinating possibilities that arise from these discoveries. If an object really must be made to disappear, preventing diffusion is not enough. Instead, light waves should emerge behind it undisturbed. Yet even in the vacuum of space, diffraction alone guarantees that any signal will inevitably change shape. “Our research provides the recipe for structuring a material in such a way that light beams pass through as if neither the material, nor the very region of space it occupies, existed. Even the fictional Romulan cloaking devices do not can’t do that,” says co-writer Dr. Matthias Heinrich, returning to Star Trek’s Last Frontier.

The results presented in this work represent a breakthrough in fundamental research on non-Hermitian photonics and provide new approaches for the active fine-tuning of sensitive optical systems, for example sensors for medical use. Other potential applications include optical encryption and secure data transmission, as well as the synthesis of versatile artificial materials with tailored properties.