Resonance-enhanced optical nonlinearity in the Weyl semimetal TaAs
Resonance-enhanced optical nonlinearity in the Weyl semimetal TaAs
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DOI：
10.1103/physrevb.98.165113
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发表时间：
2018-10-10
影响因子：
3.7
通讯作者：
Torchinsky, D. H.
中科院分区：
文献类型：
--
作者：
Patankar, Shreyas;Wu, Liang;Torchinsky, D. H.
与传统的线性电导率不同，材料的二阶电导率 sigma((2))（将电流与电场的平方相关）仅在反演对称性被破坏时才非零。因此，二阶非线性光学响应是基础研究中作为对称破缺探针的有力工具。它们也是光学技术的核心，是产生光电流和倍频的基础。最近人们对具有非中心晶体结构的外尔半金属的兴趣激增，导致在此类材料中发现了许多与 sigma(2)) 相关的现象，例如光到直流电流的偏振选择性转换（光电池效应）以及在光子能量 1.5 eV 下观察到 TaAs 中巨大的二次谐波产生 (SHG) 效率。在这里，我们对 TaAs 的倍频光谱进行了测量，结果表明 1.5 eV 处的响应对应于 0.7 eV 处共振的高能尾部，此时二次谐波电导率大约比标准烛形非线性晶体 GaAs 中的电导率大 200 倍。这种非常大的倍频响应引发了 sigma((2)) 的最终极限问题，我们通过将频率积分非线性响应函数与基态偏振分布函数的第三累积量（或“偏度”）相关联的新定理来解决这个问题。
The second-order conductivity of a material, sigma((2)), relating current to the square of electric field, is nonzero only when inversion symmetry is broken, unlike the conventional linear conductivity. Second-order nonlinear optical responses are thus powerful tools in basic research as probes of symmetry breaking; they are also central to optical technology as the basis for generating photocurrents and frequency doubling. The recent surge of interest in Weyl semimetals with acentric crystal structures has led to the discovery of a host of sigma((2)) related phenomena in this class of materials, such as polarization-selective conversion of light to dc current (photogalvanic effects) and the observation of giant second-harmonic generation (SHG) efficiency in TaAs at photon energy 1.5 eV. Here, we present measurements of the SHG spectrum of TaAs, revealing that the response at 1.5 eV corresponds to the high-energy tail of a resonance at 0.7 eV, at which point the second harmonic conductivity is approximately 200 times larger than seen in the standard candle nonlinear crystal, GaAs. This remarkably large SHG response provokes the question of ultimate limits on sigma((2)), which we address by a new theorem relating frequency-integrated nonlinear response functions to the third cumulant (or "skewness") of the polarization distribution function in the ground state.