Topological Insulators for Optoelectronics

Topo­logical insu­lators are materials with very special properties. They conduct elec­tricity or light particles on their surface or edges only but not on the inside. This unusual behaviour could even­tually lead to technical inno­vations which is why topo­logical insu­lators have been the subject of intense global research for several years. Physicists of Julius-Maximilians-Univer­sität Würzburg JMU in Germany with colleagues from the Technion in Haifa, Israel, and Nanyang Techno­logical Univer­sity in Singa­pore have success­fully built an exciton-polariton topo­logical insu­lator operating with both light and electronic exci­tations simul­taneously.

The novel topological insulator built in the Würzburg Institute of Physics: a controllable flow of hybrid optoelectronic particles travels along its edges. (Source: K. Winkler)

According to Sven Höfling, who heads the JMU Chair for Applied Physics, such topo­logical insu­lators have a dual benefit: “They could be used for both switched electronic systems and laser appli­cations.” The topo­logical insu­lators developed previously are based on either electrons or photons, allowing only one of these appli­cations to be imple­mented. Sebastian Klembt, group leader at Höfling’s chair, played a lead role in the project. He gives more details: The novel topo­logical insu­lator was built on a microchip and basically consists of the gallium arsenide semi­conductor compound. It has a honey­comb structure and is made up of many small pillars, each two micro­meters in diameter.

When exciting this micro­structure with laser light, light-matter particles form inside it, exclusively at the edges. The particles then travel along the edges and around the corners with rela­tively low loss. “A magnetic field enables us to control and reverse the propa­gation direc­tion of the particles,” Klembt says. It is a sophis­ticated systems which works in appli­cation-oriented dimensions – on a microchip – and in which light can be controlled. Usually, this is not so easy to accomplish: Pure light particles have no electric charge and therefore cannot be readily controlled with electric or magnetic fields. The new topo­logical insu­lator in contrast is capable of doing this by “sending light around the corner” in a manner of speaking.

This research success of Höfling’s team shows once more that the Univer­sity of Würzburg’s Institute of Physics is a leading center of topo­logical insu­lator research. In 2007, JMU Professor of Physics, Laurens Molen­kamp, pioneered the field by experi­mentally building the world’s first topo­logically insu­lator. The Würzburg physicists’ excellence in this field was again recog­nized recently in September 2018 with the awarding of a grant for the Excel­lence Cluster “Com­plexity and Topology in Quantum Materials” within the scope of the German Excel­lence Strategy compe­tition.

The Technion scientists have comple­mentary expertise: it is the group of Mordechai Segev which has demon­strated the first photonic topo­logical insu­lator, back in 2013, and launched the field of “Topo­logical Photonics”. The groups have now joined forces to demon­strate this first symbiotic light-matter topo­logical insulator, which holds great promise both as a funda­mental disco­very and by opening the door for exiting appli­cations in opto­electronics. (Source: JMU)

Reference: S. Klempt et al.: Exciton-polariton topological insulator, Nature, online 8 October 2018; DOI: 10.1038/s41586-018-0601-5

Links: Wilhelm-Conrad-Röntgen-Research Center for Complex Material Systems, University Würzburg, Würzburg, Germany • Solid State Institute (M. Segev), Technion, Haifa, Israel

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