Metamaterials: A double rift in time

In fact, there is another discovery that is even more important for society as a whole, Alo explains: The speed at which the optical properties of the material changed was much higher than scientists expected. The thin indium tin oxide layer became reflective within a few femtoseconds of the laser pulse striking it, and the effect faded almost as quickly. A femtosecond is one millionth of a billionth of a second. This means that the substance reacted 100 times faster than previously thought. “It’s an impressive result,” says Alo. “The selectivity of time periods is truly enormous.”

“There was evidence from previous experiments that indium tin oxide reacted quickly, but very quickly – it amazed us.”Stefan Mayer, physicist

Until now, there has been no known material whose refractive index can change so quickly, whether created artificially or naturally. “There was evidence from previous experiments that indium tin oxide reacted quickly, but so quickly, it astounded us,” says Stefan Mayer. Similar to spatially modified materials, where the structures must be smaller than the wavelength of light, with temporally modified materials, the speed of modification must be smaller than the period of oscillation of photons in order to see an interference effect. “This is very difficult to achieve,” says Mayer. The period of oscillation of near-infrared light, the longer wavelength light that is still visible, is in the femtosecond range. To illustrate: If the entire history of the universe from the Big Bang to today lasted one second, then one light oscillation is equivalent to the duration of only one day. Therefore the change in refractive index should occur faster. “Our double-slit experiment is the first step on the path toward very complex temporal modifications of the optical properties of materials,” says Mayer.

These properties are related to how light propagates through the material. Light or radiation generally consists of waves in which an electric field and a magnetic field oscillate at the same time. When such a wave hits a substance, the fields make its electrons move rhythmically. These elements act on fields and thus affect the direction and at what speed radiation propagates in matter, how its frequency changes and whether part of it is reflected. How this happens depends on two distinct properties of each material: permeability to electric fields, the permittivity ε, and permeability to magnetic fields, μ. Now if you want to influence the nature of a substance, you can adjust the chemical composition by changing the structure of the atoms or molecules. Or very specific geometric structures are introduced into the material that interact with electromagnetic waves and mimic atoms.

Calculations with light

In the case of indium tin oxide, experts did not provide any geometric structures. Instead, they specifically excite atoms with laser light of a specific wavelength, causing their refractive index to change suddenly. Indium tin oxide is a so-called near-zero epsilon substance, which means strong nonlinear interactions with light matter such as the Kerr effect occur.

The precise control of light observed in the experiment is one of the many promises of time-varying metamaterials. “Such the ability to quickly switch could enable components to reflect signals in time — the visual analogue of playing a piece of music backwards,” says Andrea Allo. In other words, the material in question undergoes a sudden change in its properties such that the light wave is reflected back in time and the frequency of the wave changes.

Instead of the face, one can see the back of the head in such a temporal mirror. In addition, blonde hair, for example, will appear blue and a green jacket will appear red. This makes time-varying synthetic materials attractive as optical switches for signal processing and wireless communications or as components of optical computers. Computing using light is more energy efficient and allows very different speeds than electronic data processing.