Giant anisotropic nonlinear optical response in transition metal monopnictide 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 timereversal symmetry is broken [1][2][3][4]. Recently they have been observed in transition metal monopnictides (TMMPs) such as TaAs, a class of noncentrosymmetric materials that heretofore received only limited attention [5][6][7]. 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 [8][9][10]. Despite the absence of spontaneous polarization, polar metals can exhibit other signatures of inversion-symmetry breaking, most notably second-order nonlinear optical polarizability, χ (2) , leading to phenomena such as optical rectification and second-harmonic generation (SHG). Here we report measurements of SHG that reveal a giant, anisotropic χ (2) in the TMMPs TaAs, TaP, and NbAs. With the fundamental and second harmonic fields oriented parallel to the polar axis, the value of χ (2) is larger by almost one order of magnitude than its value in the archetypal electrooptic materials GaAs [11] and ZnTe [12], and in fact larger than reported in any crystal to date.The last decade has witnessed an explosion of research investigating the role of bandstructure topology, as characterized for example by the Berry curvature in momentum space, in the electronic response functions of crystalline solids [13]. While the best established example is the intrinsic anomalous Hall effect in time-reversal breaking systems [14], several nonlocal [15,16] and nonlinear effects related to Berry curvature generally [17,18]and in WSM's specifically [19,20] have been predicted in crystals that break inversion symmetry. Of these, the most relevant to this work is a theoretical formulation [21] of SHG in terms of the shift vector, which is a quantity related to the difference in Berry connection between two bands that participate in an optical transition. * Electronic address: liangwu@berkeley.edu † Electronic address: jworenstein@lbl.gov Fig. 1a and caption provide a schematic and description of the optical set-up for measurement of SHG in TMMP crystals . Figs. 1b,c show results from a (112) surface of TaAs. The SH intensity from this surface is very strong, allowing for polarization rotation scans with signal-to-noise ratio above 10 6 . In contrast, SHG from a TaAs (001) surface is barely detectable (at least six orders of magnitude lower than (112) surface). Below, we describe the use of the set-up shown in Fig. 1a to characterize the second-order optical susceptibility tensor, χ ijk , defined by the relation,As a first step, we determined the orientation of the high-symmetry axes in the (112) surface, which are the [1,-1,0] and [1,1,-1] directions. To do so, we simultaneously rotated the linear polarization of th...

Towards properties on demand in quantum materials

“…Berry phase effects also underlie the phenomenon of 'shift currents'-an optically induced charge separation arising from asymmetry in the electronic wavefunctions-which provides a new paradigm for designing high-performance optical-frequency conversion and photovoltaic materials 28,29 . Recent studies of transition metal monopnictide-based Weyl semimetals (discussed in 'Topological phenomena under control') that exhibit the requisite band topology have revealed giant second-order nonlinear optical responses 30,31 . Another promising device concept is an optically pumped low-threshold laser based on a monolayer of the prototypical TMD material WSe 2 32 .…”

“…At present, state-of-the-art ultrafast methods offer a near-complete characterization of the transient state 152 . Indeed, tr-ARPES provides a direct probe of the transient electronic structure 66,67,120,153 , tabletop and accelerator-based X-ray free-electron lasers readily capture momentary crystal arrangements 118,154 , nonlinear optical methods can directly interrogate the symmetries of quantum phases and their host lattices 30,[155][156][157][158] , while optical and soft X-ray techniques document electronic excitations, in some cases with elemental and orbital specificity 159 . Importantly, mesoscopic phenomena that are prevalent in quantum materials lead to new time and energy scales, as documented in various ultrafast studies 110,160 .…”

Berry curvature dipole in Weyl semimetal materials: An ab initio study

“…These nonlinear effects originate from the intraband resonant transitions at a low frequency in a noncentrosymmetric metal. Although they have played an important role in predicting topological materials and estimating their linear-response properties, there is still a lack of ab initio studies on the nonlinear optical effects of WSMs to quantitatively reveal the role of the Weyl points in realistic materials [31].…”

“…Because Weyl points are monopole sources or drains of the Berry curvature of Bloch wave functions in momentum space, a WSM can exhibit an anomalous Hall effect when breaking the time-reversal symmetry (TRS) [17][18][19] or a spin Hall effect [20], as a linear response to an external electric field. Recent theoretical [21][22][23][24][25][26][27][28][29][30] and experimental [31][32][33][34] studies have revealed giant nonlinear optical responses in inversion-symmetry-breaking WSMs, such as the photocurrent from the circular photogalvanic effect (CPGE), second harmonic generation (SHG), and nonlinear Hall effect. These nonlinear effects can be much stronger in WSMs than traditional electro-optic materials owing to the large Berry curvature [22,35,36].…”

“…Corresponding nonlinear susceptibilities in several materials promise a nonlinear Hall effect in the dc field limit, which is within the experimentally detectable range. [31][32][33][34] studies have revealed giant nonlinear optical responses in inversion-symmetry-breaking WSMs, such as the photocurrent from the circular photogalvanic effect (CPGE), second harmonic generation (SHG), and nonlinear Hall effect. These nonlinear effects can be much stronger in WSMs than traditional electro-optic materials owing to the large Berry curvature [22,35,36].…”