Controlling valley polarization in 2D transition metal dichalcogenides with chiral plasmonic nanostructures – 2D-CHIRAL

Transition metal dichalcogenides (TMDs) are emerging materials exhibiting a range of intriguing properties. In particular, materials like MoS2 or WSe2, which are indirect gap semiconductors when bulk, start to exhibit a direct gap and hence become luminescent when they are reduced to a single layer. Moreover, due to the presence of non-equivalent valleys in their Brillouin zone, the polarization of emitted photons depends on the excited valley. As a consequence, right (resp. left) circularly polarized light will be emitted under right (resp. left) circularly polarized excitation. However, the emission rate of those materials remains weak, and the observed degree of circular polarization drops for non-cryogenic temperatures.

In this project, we propose to couple chiral metallic nanostructures (such as a metasurface consisting of an array of chiral metal nanoparticles) to single layers or flakes of TMD in order to generate plasmon-enhanced valleytronics. Chiral metal nanostructures supporting localized surface plasmon resonances have the ability to generate “super-chiral” electromagnetic fields that we propose to use to control valley polarization inside the TMDC layer (either at the excitation wavelength, emission wavelength, or both). We expect an unprecedented control over the polarization state of the emitted luminescence.

To achieve this goal, we will develop 2D and 3D chiral metamaterials. 2D-chiral metamaterials are relatively easy to make, exhibit circular dichroism and can be easily coupled with a layer of TMD. On the other hand, 3D-chiral metamaterials are more complex to make, but allow for unique geometries with highly chiral electromagnetic fields on the nanoscale. In this project we will consider a 3D structure consisting of two stacked chiral nanostructures, with a slight rotation angle between the two. The resulting nanogap (or nanocavity) will host a single- or multi-layer TMD, allowing for a simultaneous enhancement of the number of fluorescent photons (plasmon-enhanced luminescence) and for enhancement of the degree of circular polarization of the emitted light – hence creating a new emission regime that pristine TMD samples cannot offer.