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# Liang Wu, University California Berkeley, MPPL2,Giant nonlinear optical responses in Weyl semimetals

Date: Tue. September 12th, 2017, 11:30 pm-12:30 pm
Location: Rockefeller 221 (Les Foldy Room)

Giant nonlinear optical responses in Weyl semimetals

Recently Weyl quasi-particles have been observed in transition metal monopnictides (TMMPs) such as TaAs, a class of noncentrosymmetric materials that heretofore received only limited attention. The question that arises now is whether these materials will exhibit novel, enhanced, or technologically applicable properties. The TMMPs are polar metals, a rare subset of inversion- breaking crystals that would allow spontaneous polarization, were it not screened by conduction electrons. Despite the absence of spontaneous polarization, polar metals can exhibit other signatures, most notably second-order nonlinear optical polarizability, leading to phenomena such as second-harmonic generation (SHG). In my talk, I will present a new perspective of nonlinear optical responses in probing Berry connection and Berry curvature and our discovery of a giant, anisotropic SHG in the TMMPs TaAs, TaP and NbAs[1]. With the fundamental and second- harmonic fields oriented parallel to the polar axis, the value of $\chi^{2}$ is larger by one order of magnitude than its value in the archetypal electro-optic materials GaAs and ZnTe, and in fact larger than reported in any crystal to date [1]. In order to fully understand whether this large SHG comes from the singularity of Berry curvature near the Weyl point, we lowered the photon energy and find that SHG signal increases at lower energies, which is attributed to a combination of Berry curvature near the Weyl points and van-hove singularities. [2].

References:

1. Wu, et al. Nature Physics 13, 350-355 (2017)
2. Work in preparation.

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