Uzundal Can B, Jamnuch Sasawat, Berger Emma, Woodahl Clarisse, Manset Paul, Hirata Yasuyuki, Sumi Toshihide, Amado Angelique, Akai Hisazumi, Kubota Yuya, Owada Shigeki, Tono Kensuke, Yabashi Makina, Freeland John W, Schwartz Craig P, Drisdell Walter S, Matsuda Iwao, Pascal Tod A, Zong Alfred, Zuerch Michael
Department of Chemistry, University of California, Berkeley, California 94720, USA.
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA.
Phys Rev Lett. 2021 Dec 3;127(23):237402. doi: 10.1103/PhysRevLett.127.237402.
Second harmonic generation (SHG) spectroscopy ubiquitously enables the investigation of surface chemistry, interfacial chemistry, as well as symmetry properties in solids. Polarization-resolved SHG spectroscopy in the visible to infrared regime is regularly used to investigate electronic and magnetic order through their angular anisotropies within the crystal structure. However, the increasing complexity of novel materials and emerging phenomena hampers the interpretation of experiments solely based on the investigation of hybridized valence states. Here, polarization-resolved SHG in the extreme ultraviolet (XUV-SHG) is demonstrated for the first time, enabling element-resolved angular anisotropy investigations. In noncentrosymmetric LiNbO_{3}, elemental contributions by lithium and niobium are clearly distinguished by energy dependent XUV-SHG measurements. This element-resolved and symmetry-sensitive experiment suggests that the displacement of Li ions in LiNbO_{3}, which is known to lead to ferroelectricity, is accompanied by distortions to the Nb ion environment that breaks the inversion symmetry of the NbO_{6} octahedron as well. Our simulations show that the measured second harmonic spectrum is consistent with Li ion displacements from the centrosymmetric position while the Nb─O bonds are elongated and contracted by displacements of the O atoms. In addition, the polarization-resolved measurement of XUV-SHG shows excellent agreement with numerical predictions based on dipole-induced SHG commonly used in the optical wavelengths. Our result constitutes the first verification of the dipole-based SHG model in the XUV regime. The findings of this work pave the way for future angle and time-resolved XUV-SHG studies with elemental specificity in condensed matter systems.
二次谐波产生（SHG）光谱技术广泛应用于固体表面化学、界面化学以及对称性性质的研究。可见到红外波段的偏振分辨SHG光谱技术常被用于通过晶体结构中的角向各向异性来研究电子和磁序。然而，新型材料和新出现现象的复杂性不断增加，阻碍了仅基于杂化价态研究对实验结果的解读。在此，首次展示了极紫外波段的偏振分辨SHG（XUV-SHG），实现了元素分辨的角向各向异性研究。在非中心对称的LiNbO₃中，通过能量依赖的XUV-SHG测量清晰地区分了锂和铌的元素贡献。这项元素分辨且对称敏感的实验表明，LiNbO₃中已知会导致铁电性的锂离子位移，伴随着铌离子环境的畸变，这也打破了NbO₆八面体的反演对称性。我们的模拟表明，测量得到的二次谐波光谱与锂离子从中心对称位置的位移一致，同时Nb─O键因氧原子位移而伸长和收缩。此外，XUV-SHG的偏振分辨测量结果与基于光学波长常用的偶极诱导SHG的数值预测结果高度吻合。我们的结果首次验证了XUV波段基于偶极的SHG模型。这项工作的发现为未来在凝聚态物质系统中进行具有元素特异性的角度和时间分辨XUV-SHG研究铺平了道路。
