Department of Materials Science and Engineering, College of Engineering and College of Science, Texas A&M University , College Station, Texas 77843, United States.
Nano Lett. 2017 Aug 9;17(8):5027-5034. doi: 10.1021/acs.nanolett.7b02268. Epub 2017 Jul 6.
Nonlinear optical properties of materials such as second and higher order harmonic generation and electro-optic effect play pivotal roles in lasers, frequency conversion, electro-optic modulators, switches, and so forth. The strength of nonlinear optical responses highly depends on intrinsic crystal symmetry, transition dipole moments, specific optical excitation, and local environment. Using first-principles electronic structure theory, here we predict giant second harmonic generation (SHG) in recently discovered two-dimensional (2D) ferroelectric-ferroelastic multiferroics-group IV monochalcogenides (i.e., GeSe, GeS, SnSe, and SnS). Remarkably, the strength of SHG susceptibility in GeSe and SnSe monolayers is more than 1 order of magnitude higher than that in monolayer MoS, and 2 orders of magnitude higher than that in monolayer hexagonal BN. Their extraordinary SHG is dominated by the large residual of two opposite intraband contributions in the SHG susceptibility. More importantly, the SHG polarization anisotropy is strongly correlated with the intrinsic ferroelastic and ferroelectric orders in group IV monochalcogenide monolayers. Our present findings provide a microscopic understanding of the large SHG susceptibility in 2D group IV monochalcogenide multiferroics from first-principles theory and open up a variety of new avenues for 2D ferroelectrics, multiferroics, and nonlinear optoelectronics, for example, realizing active electrical/optical/mechanical switching of ferroic orders in 2D multiferroics and in situ ultrafast optical characterization of local atomistic and electronic structures using noncontact noninvasive optical SHG techniques.
材料的非线性光学性质，如二次谐波产生和电光效应，在激光、频率转换、电光调制器、开关等方面起着关键作用。非线性光学响应的强度高度依赖于内在的晶体对称性、跃迁偶极矩、特定的光激发和局部环境。在这里，我们使用第一性原理电子结构理论，预测了最近发现的二维（2D）铁电-铁弹多铁体-第 IV 主族单卤化物（即 GeSe、GeS、SnSe 和 SnS）的巨大二次谐波产生（SHG）。值得注意的是，单层 GeSe 和 SnSe 的 SHG 电导率强度比单层 MoS 高一个数量级，比单层六方 BN 高两个数量级。它们非凡的 SHG 主要由 SHG 电导率中两个相反的内带贡献的残余主导。更重要的是，SHG 极化各向异性与第 IV 主族单卤化物单层中的固有铁弹和铁电序密切相关。我们的研究结果从第一性原理理论上提供了对二维第 IV 主族单卤化物多铁体中大 SHG 电导率的微观理解，并为二维铁电体、多铁体和非线性光电学开辟了多种新途径，例如在二维多铁体中实现铁电序的主动电/光/机械切换，以及使用非接触非侵入式光学 SHG 技术原位超快光学表征局部原子和电子结构。
