1.College of Physics， Sichuan University， Chengdu 610065， China;2.Department of Physics， Sichuan Normal University， Chengdu 610068;3.State Key Laboratory of Applied Surface Physics， Fudan University， Shanghai 200438， China;4.Department of Physics， Fudan University， Shanghai 200438， China
Project supported by the National Natural Science Foundation of China （Grant No. 11874271）.
Single crystal of tungsten ditelluride (WTe2) is a transition metal dichalcogenide which displays a wide range of electronic properties, such as non-saturable magnetoresistance. Both theoretical predictions and experimental measurements describe that WTe2 is a new topological state of matter called type-II Weyl semimetal with tilt Weyl cones. Things become even more interesting when its thickness is reduced to a single layer. For example, monolayer WTe2 with 1T’ structure is a class of large-gap quantum spin Hall insulators which exhibit a helical one-dimensional edge mode. This structural distortion causes an intrinsic band inversion between chalcogenide-p and metal-d bands, spin-orbit coupling then opens up a band gap large enough to realize quantum spin Hall effect at temperatures below about 100 K. Optical measurements provide a powerful tool to investigate electronic structure which complements photoemission and tunneling measurements due to its sensitivity to the role of electronic interactions in solids. Among many kinds of optical techniques, scattering-type scanning near-field optical microscopy (SNOM) is a powerful tool for nanostructure investigation, which consists of a laser coupled to a metal-coated tip and probe the local electric field in a close proximity to the sample. This technique provides information about the complex optical properties of the sample of nanoscale resolution. Here we use SNOM to study the near-field optical response of WTe2 thin films, we have observed bright fringes near the edge of the thin film sample and also a thickness dependence on optical contrast to the sample and substrate. To understand this behavior, first we obtain the dielectric function of WTe2 at room temperature by Drude-Lorentz model via fitting the infrared radiation (IR) reflectance and conductivity spectra, then the near-field ratio of thin film sample to the diamond substrate is calculated by the Finite-dipole model. The experimental result reveals that the behaviour of the sample cannot be fully described by the bulk properties. We assume that a decoupled thin layer exists on the surface of the bulk. There are two possible explanations for the observation of the near-field patterns of bright outside fringes. Firstly, a hot-spot field may be produced between the tip and the sample edge due to the enhancement of the local electric field under the IR illumination, a similar behavior has been revealed in surface-metallic black phosphorus. Another probability is that the topological edge states of top decoupled monolayer WTe2 lead to an enhancement of the local optical conductivity. This work provides a reference from the optical research of topological materials in the future.