Observation of strong and anisotropic nonlinear optical effects through polarization-resolved optical spectroscopy in the type-II Weyl semimetal Td−WTe2
Observation of strong and anisotropic nonlinear optical effects through polarization-resolved optical spectroscopy in the type-II Weyl semimetal Td−WTe2
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DOI：
10.1103/physrevb.104.064304
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发表时间：
2021-08
影响因子：
3.7
通讯作者：
E. Drueke;Junjie Yang;Liuyan Zhao
中科院分区：
文献类型：
--
作者：
E. Drueke;Junjie Yang;Liuyan Zhao
拓扑Weyl半金属带结构中独特的体交叉点已被证明可以增强非线性光学和光电子特性，即使对于光学波长的光也是如此。这种明显的非线性光学效应已经在i型Weyl半金属中得到了研究，但很少有研究对ii型Weyl半金属中的这些效应进行量化。本文对ii型Weyl半金属${T}_{\ mathm {d}}\text{\ensuremath{-}}\ mathm {W}{\ mathm {Te}}_{2}$中两种非线性光学效应的偏振分辨率进行了光学实验研究。我们首先利用二次谐波产生的旋转各向异性研究了这种材料的体光学二次谐波响应。这种技术允许我们同时研究二次谐波响应对晶体对称性的依赖，并量化该响应的大小，我们将其与其他非线性晶体进行比较。然后，我们使用偏振时间分辨光学反射率光谱来确定脉冲受激拉曼散射的非线性光学效应是0.25 thz剪切模式相干振荡的起源。我们发现在${T}_{\mathrm{d}}\text{\ensuremath{-}}\mathrm{W}{\mathrm{Te}}_{2}$中的响应强度比在相干声子的位移激发下观测到的其他模式的响应强度有所增强。值得注意的是，声子的二次谐波响应和相干激发都表现出很强的各向异性，显示出对实验中使用的光的偏振的明显依赖。我们使用材料的点对称分析来指导我们对这些观察结果的理解，并执行依赖于影响的测量来研究电子-声子耦合。
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 ${T}_{\mathrm{d}}\text{\ensuremath{-}}\mathrm{W}{\mathrm{Te}}_{2}$. 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 ${T}_{\mathrm{d}}\text{\ensuremath{-}}\mathrm{W}{\mathrm{Te}}_{2}$ 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.