Nanoslits for Valleytronics

After excitation of the device with green laser light, coherent fluorescence occurs thanks to the silver saw tooth. (Source: Han & Ye, U. Groningen)

Scientists at the University of Groningen used a silver sawtooth nanoslit array to produce valley-coherent photo­luminescence in two-dimensional tungsten disulfide flakes at room temperature. Until now, this could only be achieved at very low temperatures. Coherent light can be used to store or transfer information in quantum electronics. This plasmon-exciton hybrid device is promising for use in integrated nano­photonics.

Tungsten disulfide has interes­ting electronic properties and is available as a 2D material. “The electronic structure of monolayer tungsten disulfide shows two sets of lowest energy points or valleys,” explains Associate Professor Justin Ye, head of the Device Physics of Complex Materials group at the University of Groningen. One possible application is in photonics, as it can emit light with valley-dependent circular polari­zation – a new degree of freedom to manipulate information. However, valleytronics requires coherent and polarized light. Unfor­tunately, previous work showed that photo­luminescence polari­zation in tungsten disulfide is almost random at room temperature.

“Tungsten disulfide is unique in that these two valleys are not identical,” says Ye. This means that to create linearly polarized light, both valleys must respond coherently to generate light in the photo­luminescence. “But the intervalley scattering at room tempera­ture largely destroys the coherence, so appreciable coherence is only achieved at very low temperatures that are close to zero.”

Ye and his postdoctoral researcher Chunrui Han therefore tried a different approach to create linearly polarized light by using a plasmonic meta­surface, in the form of a silver sawtooth nanoslit array. Such a material interacts strongly with tungsten disulfide and can transfer resonance induced by light in the form of an electro­magnetic field in the metal. “It enhances the light-material inter­action,” says Ye.

By adding a thin layer of silver meta­surface on top of a monolayer of tungsten disulfide, linear polari­zation induced by the valley coherence is increased to around 27 percent at room temperature. “This room temperature performance is even better than the valley polari­zation obtained in many previous reports measured at very low tempera­tures,” says Ye. The linear polarization could be further increased to 80 percent by adding the anisotropy of plasmonic resonance, in the form of the sawtooth pattern, to the optical response of the tungsten disulfide.

This means that Ye and Han are now able to induce linearly polarized photo­luminescence in this material. This accomplish­ment will make it possible to use both valley coherence of tungsten disulfide and plasmonic coherence of metasurfaces in opto­electronics at ambient temperatures. The next step is to replace the laser light that induced photo­luminescence with electrical input. (Source: U. Groningen)

Reference: C. Han & J. Ye: Polarized resonant emission of monolayer WS2 coupled with plasmonic sawtooth nanoslit array, Nat. Commun. 11, 713 (2020); DOI: 10.1038/s41467-020-14597-2

Link: Device Physics of Complex Materials, Zernike Institute for Advanced Materials, University of Groningen, Groningen, The Netherlands

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