Combining Spintronics With Nanophotonics

From spintronics to nanophotonics in a 2D-material made of tungsten disulfide. (Source: TU Delft / Scixel)

Spintronics in materials of just a few atoms thick is an emerging field in which the spin of electrons is used to process data, rather than the charge. Unfor­tunately, the spin only lasts for a very short time, making it difficult to exploit in elec­tronics. Researchers from the Kavli Institute of Nano­science at TU Delft, working with the Nether­lands Orga­nisation for Scientific Research’s AMOLF institute, have now found a way to convert the spin infor­mation into a predictable light signal at room tempera­ture.

The discovery brings the worlds of spin­tronics and nano­photonics closer together and might lead to the develop­ment of an energy-efficient way of proces­sing data, in data centres, for example. The research revolved around a nano-con­struction consisting of two components: an extremely thin silver thread, and a 2D material made of tungsten disul­fide. The researchers attached the silver thread to a slice of tungsten disul­fide measuring just four atoms in thickness. Using circu­larly polarised light, they created excitons with a specific rota­tional direction. The direction of that spin could be initia­lized using the rota­tional direction of the laser light.

Excitons are actually electrons that have bounced out of their orbit. With this technique, the laser beam ensures that the electrons are launched into a wider orbit around a positively charged hole, in much the same way as a hydrogen atom. The excitons thus created want to return to their original state. On their return to the smaller orbit, they emit an energy package in the form of light. This light contains the spin infor­mation, but it emitted in all directions.

To enable the spin infor­mation to be put to use, the researchers returned to an earlier disco­very. They had shown that when light moves along a nanowire, it is accom­panied by a rotating electro­magnetic field very close to the wire: it spins clockwise on one side of the wire, and anti-clock­wise on the other side. When the light moves in the opposite direction, the spin direc­tions change too. So the local rotational direction of the electro­magnetic field is locked one-to-one to the direc­tion with which the light travels along the wire. “We use this pheno­menon as a type of lock combi­nation,” explains Kuipers. “An exciton with a parti­cular rota­tional direction can only emit light along the thread if the two rota­tional directions correspond.”

And so a direct link is created between the spin information and the propa­gation direction of the light along the nano­wire. It works almost per­fectly: the spin infor­mation is launched in the right direction along the thread in 90% of cases. In this way, fragile spin infor­mation can be care­fully converted into a light signal and trans­ported over far greater distances.

Thanks to this technique, which works at room tempera­ture, you can easily make new opto­electronic cir­cuitry. Kuipers: “You don’t need a stream of electrons, and no heat is released. This makes it a very low-energy way of trans­ferring infor­mation.” The discovery clears the way for combining the worlds of spin­tronics and nano­photonics. Kuipers: “This combi­nation may well result in green infor­mation processing strategies at the nano­scale.” (Source: TU Delft)

Reference: S.-H. Gong et al.: Nanoscale chiral valley-photon interface through optical spin-orbit coupling, Science 359, 443 (2018); DOI: 10.1126/science.aan8010

Link: Kavli Inst. of Nanoscience, Dept. of Quantum Nanoscience, Delft Univ. of Technology, Delft, Netherlands

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