Giant anisotropic nonlinear optical response in transition metal monopnictide Weyl semimetals

TL;DR

This paper reports the discovery of a giant, anisotropic second-order nonlinear optical response in transition metal monopnictide Weyl semimetals, surpassing known materials and revealing potential for advanced optical applications.

Contribution

It demonstrates that TMMPs like TaAs, TaP, and NbAs exhibit unprecedented second-harmonic generation, indicating strong nonlinear optical properties linked to their Weyl semimetal nature.

Findings

Giant, anisotropic second-order nonlinear optical susceptibility in TMMPs

Value of χ^{(2)} exceeds that of GaAs and ZnTe by nearly an order of magnitude

Potential for novel optical device applications based on Weyl semimetals

Abstract

Although Weyl fermions have proven elusive in high-energy physics, their existence as emergent quasiparticles has been predicted in certain crystalline solids in which either inversion or time-reversal symmetry is broken\cite{WanPRB2011,BurkovPRL2011, WengPRX2015,HuangNatComm2015}. Recently they have been observed in transition metal monopnictides (TMMPs) such as TaAs, a class of noncentrosymmetric materials that heretofore received only limited attention \cite{XuScience2015, LvPRX2015, YangNatPhys2015}. The question that arises now is whether these materials will exhibit novel, enhanced, or technologically applicable electronic 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 \cite{anderson1965symmetry,shi2013ferroelectric,kim2016polar}. Despite the absence of spontaneous polarization, polar metals can exhibit other signatures of inversion-symmetry breaking, most notably second-order nonlinear optical polarizability, $\chi^{(2)}$, leading to phenomena such as optical rectification and second-harmonic generation (SHG). Here we report measurements of SHG that reveal a giant, anisotropic $\chi^{(2)}$ in the TMMPs TaAs, TaP, and NbAs. With the fundamental and second harmonic fields oriented parallel to the polar axis, the value of $\chi^{(2)}$ is larger by almost one order of magnitude than its value in the archetypal electro-optic materials GaAs \cite{bergfeld2003second} and ZnTe \cite{wagner1998dispersion}, and in fact larger than reported in any crystal to date.