Polarization Switching for Excitonic Circuits

A new 2D material combines tungsten diselenide with molybdenum diselenide to reveal new properties with an array of possible high-tech applications. (Source: EPFL)

They were the first to control exciton flows at room tempera­ture. And now, the team of scientists from EPFL’s Labora­tory of Nano­scale Elec­tronics and Structures (LANES) has taken their tech­nology one step further. They have found a way to control some of the properties of excitons and change the polari­zation of the light they generate. This can lead to a new gene­ration of electronic devices with transistors that undergo less energy loss and heat dissi­pation. The scientists’ disco­very forms part of the new field of research of valley­tronics.

Excitons are created when an electron absorbs light and moves into a higher energy level. This excited electron leaves behind an electron hole in its previous energy band. And because the electron has a negative charge and the hole a positive charge, the two are bound together by a Coulomb force. These excitons exist only in semi­conducting and insu­lating materials. Their extra­ordinary pro­perties can be easily accessed in 2D materials, which are materials whose basic structure is just a few atoms thick. The most common examples of such materials are carbon and molyb­denite.

When such 2D materials are combined, they often exhibit quantum pro­perties that neither material possesses on its own. The scientists thus combined tungsten diselenide with molyb­denum dise­lenide to reveal new pro­perties with an array of possible high-tech appli­cations. By using a laser to generate light beams with circular polari­zation, and slightly shifting the posi­tions of the two 2D materials so as to create a moiré pattern, they were able to use excitons to change and regulate the polari­zation, wave­length and intensity of light.

The scientists achieved this by mani­pulating one of the excitons’ properties: their “valley,” which is related to the extremes of energies of the electron and the hole. These valleys – which are where the name valley­tronics comes from – can be leveraged to code and process infor­mation at a nano­scopic level. “Linking several devices that incor­porate this tech­nology would give us a new way to process data,” says Andras Kis, who heads LANES. “By changing the polari­zation of light in a given device, we can then select a specific valley in a second device that’s connected to it. That’s similar to switching from 0 to 1 or 1 to 0, which is the funda­mental binary logic used in computing.” (Source: EPFL)

Reference: A. Ciarrocchi et al.: Polarization switching and electrical control of interlayer excitons in two-dimensional van der Waals heterostructures, Nat. Phot., online 31 December 2018; DOI: 10.1038/s41566-018-0325-y

Link: Laboratory of Nanoscale Electronics and Structures LANES, École Polytechnique Fédérale de Lausanne EPFL, Lausanne, Switzerland

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