In recent years, due to the discovery of interesting properties such as extreme magnetoresistance (XMR) and a possible Type-II Weyl semimetal behaviour, the semi-metallic transition-metal dichalcogenide tungsten ditelluride (WTe2) has been the focus of many studies. In this work, we focus on the study of its out-of-equilibrium optical properties by using the pump-and-probe technique.
The main objective is to investigate the mechanisms leading to the realization of a perfect electron-hole balance in WTe2, considered as the prerequisite for XMR, and eventually control it via a non-equilibrium approach. This ambitious goal is realized by connecting the results from time-resolved optical spectroscopy to the important features of the material through the study of dielectric function modifications. We use an ultrafast Ti:Sapphire laser producing pulses at 780 nm, used as a pump-beam. As a probe, we generate a supercontinuum beam, extending over a wide region (500-1300 nm) including the visible and the near-IR, that is simultaneously sampled. We observe the generation of two coherent A1 optical phonons at ~8 cm-1 and ~80 cm-1, that display different damping time and couple to different Lorentz oscillators contributing to the dielectric function.
We reveal a marked anisotropy of the in-plane (ab plane) out-of-equilibrium properties, and we repeat the experiment at different temperatures to study the influence of XMR on the optical properties. Our findings constitute an important step towards the comprehension of the mechanism leading to a large and non-saturating magnetoresistance in WTe2.