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The behaviour of Tungsten ditelluride (WTe2) in few-layer form is not yet fully characterized. Here the authors use a near-field terahertz microscopy technique to observe the electromagnetic responses of WTe2 flakes from one to several layers and to study their semimetallic/ semiconducting behavior.

The authors use time-resolved scanning near-field optical microscopy to probe the ultrafast excitonic processes and their impact on waveguide operation in transition metal dichalcogenide crystals. They observe significant modulation of the complex index by monitoring waveguide modes on the fs time scale, and identify both coherent and incoherent manipulations of WSe2 excitonic resonances.

Optoelectronics: guided hyperbolic phonon-polaritons in h-BN boost mid-infrared graphene photodetectors h-BN hyperbolic phonon-polaritons can be probed electrically in a van der Waals photodetector by guiding them towards a graphene junction. A team led by F.H.L. Koppens at ICFO developed a nano-optoelectronic device whereby light from a laser beam, shone on a heterostructure of monolayer graphene encapsulated in h-BN, is converted to hyperbolic phonon-polaritons. Once the latter are launched at the edge of a metallic bottom split gate, they propagate as highly confined and directional rays towards graphene, where they are absorbed. This results in the generation of hot carriers which diffuse spatially towards the graphene junction, giving rise to an inhomogeneous temperature distribution which, in turn, leads to a strong photo-response. Besides enhanced responsivity and room temperature operation, this mid-infrared photodetector possesses tunable frequency selectivity, making it appealing for imaging and sensing applications.