Observation of strong and anisotropic nonlinear optical effects through polarization-resolved optical spectroscopy in the type-Ⅱ Weyl semimetal T_d-WTe_2

摘要

The unique bulk crossing points in the band structure of topological Weyl semimetals have been shown to enhance nonlinear optical and optoelectronic properties, even for light at optical wavelengths. Such pronounced nonlinear optical effects have been studied in type-Ⅰ Weyl semimetals, yet few studies have quantified these effects in type-Ⅱ Weyl semimetals. We here present an optical experimental study with polarization resolution of two nonlinear optical effects in the type-Ⅱ Weyl semimetal T_d-WTe_2. We begin by investigating the bulk optical second-harmonic response of this material using the rotational anisotropy of the second-harmonic generation. This technique allows us to simultaneously investigate the dependence of the second-harmonic response on the symmetry of the crystal and to quantify the size of that response, which we compare to other nonlinear crystals. We then use polarized time-resolved optical reflectivity spectroscopy to identify the nonlinear optical effect of impulsive stimulated Raman scattering as the origin of the coherent oscillations of the 0.25-THz shear mode. We find that the strength of this response in T_d-WTe_2 is enhanced compared with the observation of other modes excited through the displacive excitation of coherent phonons. Notably, both the second-harmonic response and the coherent excitations of the phonons demonstrate strong anisotropy, displaying a clear dependence on the polarization of the light used in the experiments. We use point-symmetry analyses of the material to guide our understanding of these observations and perform fluence-dependent measurements to investigate the electron-phonon coupling.