Researchers from the Technion in Haifa and Friedrich-Alexander-Universität (FAU) Erlangen-Nürnberg have, for the first time, demonstrated that electrons in a so-called Floquet topological insulator can be directed with light. The work, published in Nature Physics, combines theoretical foundations from Israel with experimental implementation in Germany and could pave the way for faster, more energy-efficient electronic components.
The theoretical foundation was provided by the working group led by Dr Ofer Neufeld at the Technion (Schulich Faculty of Chemistry, Helen Diller Quantum Center). The experiments were carried out at the Friedrich-Alexander University (FAU) of Erlangen-Nuremberg by Professor Peter Hommelhoff (Chair holder Laser Physics, now at LMU Munich) and Dr Daniel Lesko, postdoctoral researcher at the Chair of Laser Physics.
To set the scene: since 2007, solid-state physics has known a fourth group of materials beyond conductors, insulators, and semiconductors – topological insulators. They conduct electricity exclusively on their surfaces, and do so extremely efficiently, while their interior remains an electrical insulator. What makes them fascinating is that this surface conduction is protected by the intrinsic geometry of the electronic states. The catch: topological insulators are extremely difficult to produce, and only a handful of suitable compounds exist.
Graphene – a single layer of carbon atoms arranged in a honeycomb lattice – is different. It is comparatively easy to produce, several times stronger than a steel sheet of comparable thickness, nearly transparent, and an excellent electrical conductor. By default, however, it behaves like an ordinary conductor, without any exotic topological properties.
This is precisely where the joint work comes in: the researchers irradiated graphene with intense, ultrashort laser pulses. These create energy gaps in the material for a very brief period. For the duration of the pulses, graphene temporarily assumes the properties of a topological insulator. In physics, this light-induced, out-of-equilibrium state is called a “Floquet state.” It exists only as long as the laser pulses are acting on the material.
The crucial next step is provided by the polarisation of the light. The team used circularly polarised light, in which the electric field rotates continuously – somewhat like swinging a hose in circles while watering flowers. This rotating light forces the electrons in the graphene onto circular trajectories. FAU researcher Dr Daniel Lesko puts it this way: ‘similar to when ice skaters all move in the same direction on an ice rink’.
To steer the electrons in a targeted manner, the team fired a second laser beam with exactly twice the frequency – and thus twice the photon energy – onto the material. This “beamed” selected electrons onto a different circular track, depending on the point of the original circular path where the beam intercepted them. Professor Peter Hommelhoff, who held the Chair of Laser Physics at FAU (now at LMU Munich), describes it like this: ‘We were able to vary the location of the spotlight and therefore this Berry curvature as we liked and could do so extremely quickly.’
A second, equally remarkable effect: the electrons on the original circular track all move in the same direction and carry the same energy, yet they are not identical – they differ in a quantum parameter that Lesko aptly likens to blue or red team jerseys. With the second laser, the scientists were able to selectively “beam” only the “red” or only the “blue” electrons onto the new track. For computer components this would in principle mean that twice the amount of information could be processed, depending on which “team” is used.
Measuring the generated currents as a function of polarisation and pulse timing revealed that the quantum state produces chiral responses: the current behaves differently when the light is polarised clockwise versus counter-clockwise. This provides the first experimental evidence of active electron control in an out-of-equilibrium topological state – confirming theoretical ideas developed over recent years about quantum materials.
‘The implications of this research extend well beyond graphene,’ says Dr Neufeld. The findings could enable faster, more energy-efficient electronic components, as well as communication, sensing, and computing systems whose properties can be switched on and off with light.
The research was funded by the German Research Foundation (DFG), the EU FET Open programme under Horizon 2020, and the Gordon and Betty Moore Foundation.