Terahertz response of monolayer and few-layer WTe2 at the nanoscale

Abstract

Tungsten ditelluride (WTe2) is an atomically layered transition metal dichalcogenide whose physical properties change systematically from monolayer to bilayer and few-layer versions. In this report, we use apertureless scattering-type near-field optical microscopy operating at Terahertz (THz) frequencies and cryogenic temperatures to study the distinct THz range electromagnetic responses of mono-, bi- and trilayer WTe2 in the same multi-terraced micro-crystal. THz nano-images of monolayer terraces uncovered weakly insulating behavior that is consistent with transport measurements. The near-field signal on bilayer regions shows moderate metallicity with negligible temperature dependence. Subdiffractional THz imaging data together with theoretical calculations involving thermally activated carriers favor the semimetal scenario with $$\Delta \approx -10\,{{{\rm{meV}}}}$$ Δ ≈ − 10 meV over the semiconductor scenario for bilayer WTe2. Also, we observed clear metallic behavior of the near-field signal on trilayer regions. Our data are consistent with the existence of surface plasmon polaritons in the THz range confined to trilayer terraces in our specimens. Finally, data for microcrystals up to 12 layers thick reveal how the response of a few-layer WTe2 asymptotically approaches the bulk limit.

In-Plane Anisotropy in Biaxial ReS2 Crystals Probed by Nano-Optical Imaging of Waveguide Modes

“…For background-free imaging, the signal amplitudes s n of the scattered radiation are additionally demodulated at harmonics n of the tapping frequency. We found that, in contrast to the terahertz − and mid-infrared ranges, , n ≥ 4 is required for the visible and near-infrared spectral range in most experiments. For s -polarized light, the signal-to-noise ratio is decreased due to a weaker coupling to the evanescent fields at the tip apex.…”

Investigating active area dependent high performing photoresponse through thin films of Weyl Semimetal WTe2

“…Nanosheets, microcrystals of WTe 2 have been successfully studied for broad spectral/terahertz photodetection 7 , 15 . Compared to nanostructures, thin films of WTe 2 are better for technological applications due to less complexity in device fabrication but the optoelectronics properties of sputtered WTe 2 thin films and their nanoscale active area dependent photodetection remain relatively unexplored.…”

Local Fluctuations in Cavity Control of Ferroelectricity

“…In the future it would be extremely interesting to extend this approach to include more complex heterostructures which feature multiple types material [65,66] including metals [67], insulators [68], superconductors [69,70], ferroelectrics [41][42][43][44][45], magnets [71][72][73][74][75][76][77], and multiferroics [78]-all of which are phases of matter which can be characterized by their couplings to electromagnetic fields and which are now realizeable down to the twodimensional limit [79]. In the future it would be very interesting to consider the mutual coupling of different stacked phases through their shared quantum electrodynamic environment.…”