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New Optical Method to Differentiate Between Topological and Trivial Materials

Topological insulators can be defined as quantum materials that conduct electric current on edges and surfaces, just like metals, but behave as an insulator in bulk. This is attributed to their unusual electronic structure.

The direction of rotation of the light (green for circular clockwise, purple for circular anticlockwise) maps the topological phase diagram of the system, distinguishing between its trivial phase (above the black curve) and its topological phase (below the black curve). (Image credit: MBI)

Now, for the first time, researchers at the Max-Born Institute for Nonlinear Optics and Short Pulse Spectroscopy (MBI) have illustrated a new method to distinguish topological materials from their normal yet insignificant equivalents within a millionth of a billionth of a second. In this latest technique, an ultrafast laser light is used to probe the topological materials.

The researchers’ method could make it possible to use these materials as logic elements in light-controlled electronics with the ability to process data tens of thousands of times faster than possible at present. The study has been reported in Nature Photonics.

When it comes to the concept of topology, the most standard illustration involves an elastic pretzel that can be twisted, bent, or expanded in various ways; irrespective of the deformation, a bagel can be made out of a pretzel or holes can be added to it but without tearing it apart. As a result, the number of holes present in a pretzel is invariant and offers topological data about the shape of the pretzel.

In the case of a solid material, quantum-mechanical laws limit the type of energies that can be carried by electrons. This results in the development of bands that have either forbidden or allowed energies.

Physicists can use the topology concept to elucidate the intricate shapes of the allowed energy bands and thus give them a particular topological number. Within a material system, a unique topology of the band structure manifests itself in unusual characteristics that can be seen—for example, the surface conductivity observed in topological insulators.

The most remarkable aspect of topology is its robustness: properties induced by topology are protected by it.

Dr Álvaro Jiménez-Galán, Study Main Author, Max-Born Institute for Nonlinear Optics and Short Pulse Spectroscopy

In the same way that humans cannot alter the number of holes present in a pretzel without tearing it, various perturbations, including impurities, that often disrupt the material’s ability to conduct electricity do not influence high electron mobility on the topological insulator surface. This resistance to impurities is the reason why the electronic sectors strongly prefer topological materials.

Making Electrons “Speak” About Topology

Even though the system’s topology is strongly associated with the behavior of electrons in it, the imprint of topological characteristics on electron dynamics at the time scale of a millionth of a billionth of a second has not been identified until now.

By utilizing theoretical analysis and numerical simulations, the MBI group has demonstrated that data about system topology is certainly encoded in this highly fast electron dynamics and can be recovered by observing the light produced by electrons as they are activated with laser light.

If we imagine the electrons in a solid moving within energy bands as runners on the racing track, then our method allows to learn about the topology of this racing track, by simply measuring the acceleration of the runners.

Prof. Dr Olga Smirnova, Head, MBI Theory Group

The system’s electrons, when excited by the ultra-short laser pulses, hop from one energy band to a higher one and move rapidly on the new track. Subsequently, the accelerated electrons produce light and rapidly fall back to the lower position.

Although this process lasts only a minuscule part of a second, it is sufficient for an electron to “feel” the minute variation between the energy structures of topological and trivial insulators and “encode” this data into the produced light.

On the Way Toward Ultrafast Lightwave Electronics

This study illustrates how to differentiate topological insulators from trivial counterparts at an ultrafast rate, that is, to “read out” the topological data of the system through laser spectroscopy.

For the subsequent step, the MBI team has planned to apply this understanding to change a trivial insulator into a topological one and the other way around using laser light—in other words, to “write” the topological data into a material at a similar speed.

The hypothetical proof of this effect could advance the use of topological materials in optically controlled electronics. In such electronics, only the speed of electronic reaction to light establishes the limit for the data processing speed.


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