Transparency on Demand: New Process Can Render Artificial Materials Entirely Invisible

Scientists at the University of Rostock have devised a revolutionary technology that may make artificial materials translucent or even completely invisible on command, in partnership with researchers from the Vienna University of Technology. Their finding was just published in Science Advances, a prestigious magazine.

In science fiction, such as Harry Potter's Cloak of Invisibility, making anything invisible is a recurring motif. It certainly sounds nice, but the reason it appears so frequently in stories is because it would be a really valuable piece of technology. There are obvious applications for espionage and the military, but there are many more.

Given its great utility, it should come as no surprise that scientists and engineers have been working on this for some time. They've made significant progress as well, constructing invisibility cloaks out of molybdenum trioxide, metamaterials, metascreens, and dielectric materials. It all boils down to properly manipulating light, and what's even more amazing is that advancements in this field may improve sensors, telecommunications, encryption, and a variety of other technologies.

The last frontier is space... a spaceship When an impenetrable nebula cuts off all contact links, Enterprise continues on its mission to explore the cosmos. Many episodes of the renowned television series Star Trek need the brave crew to 'tech the tech' and'science the science' within 45 minutes of broadcast in order to aid their escape from this or a similar situation before the end credits roll. A team of scientists from the University of Rostock succeeded in inventing an entirely new technique for the construction of artificial materials that can transfer light signals without distortions using precisely calibrated energy flows despite spending much more time in their laboratory.

“When light spreads in an inhomogeneous medium, it undergoes scattering. This effect quickly transforms a compact, directed beam into a diffuse glow, and is familiar to all of us from summer clouds and autumn fog alike,” Professor Alexander Szameit of the University of Rostock's Institute for Physics explains where his team's thinking began. The microscopic density distribution of a material, in particular, determines the characteristics of scattering. “The fundamental 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," Szameit explains.

The movement of energy, or, more correctly, the amplification and attenuation of light, is described by the second attribute, known in the field of photonics as non-Hermiticity. Intuitively, the accompanying consequences may appear to be undesired — fading of a light beam owing to absorption, for example, would appear to be very detrimental to the objective of increasing signal transmission. Non-Hermitian effects, on the other hand, have become an important part of contemporary optics, and an entire field of study is dedicated to harnessing the complex interplay of losses and amplification for new functionality.

“This approach opens up entirely new possibilities,” says Andrea Steinfurth, a PhD student and the paper's first author. To combat any beginning of deterioration, it becomes feasible to deliberately amplify or dampen certain components of a beam of light at the tiny level. The nebula's light-scattering qualities could be entirely subdued in order to stay in the image. “We are actively modifying a material to tailor it for the best possible transmission of a specific light signal,” Steinfurth continues. “To this end, the energy flow must be precisely controlled, so it can fit together with the material and the signal like pieces of a puzzle.”

The researchers in Rostock successfully handled this difficulty in close collaboration with collaborators from the Vienna University of Technology. They were able to generate and monitor tiny light signal interactions with their newly designed active materials in networks of kilometer-long optical fibers in their studies.

Induced transparency is only one of the many possibilities that these discoveries open up. The avoidance of dispersion is insufficient if an object is to genuinely vanish. Instead, light waves must emerge entirely undisturbed from behind it. Even in the vacuum of space, however, diffraction alone guarantees that every signal will change form. “Our research provides the recipe for structuring a material in such a way that light beams pass as if neither the material, nor the very region of space it occupies, existed. Not even the fictitious cloaking devices of the Romulans can do that,”  adds co-author Dr. Matthias Heinrich, circling back to Star Trek's last frontier.

The discoveries described in this paper mark a watershed moment in non-Hermitian photonics research, paving the way for new methods to active fine-tuning of sensitive optical systems, such as medical sensors. Optical encryption and secure data transfer, as well as the creation of adaptable artificial materials with specific characteristics, are further possible uses.