Wang Jia-Peng, Chen Xinfeng, Zhao Qiyi, Fang Yuqiang, Liu Quan, Fu Jierui, Liu Yue, Xu Xinlong, Zhang Jia, Zhen Liang, Xu Cheng-Yan, Huang Fuqiang, Meixner Alfred J, Zhang Dai, Gou Gaoyang, Li Yang
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China.
Frontier Institute of Science and Technology & State Key Laboratory of Electrical Insulation and Power Equipment, Xi'an Jiaotong University, Xi' an 710049, China.
ACS Nano. 2024 Jun 25;18(25):16274-16284. doi: 10.1021/acsnano.4c02854. Epub 2024 Jun 13.
Integration of atomically thin nonlinear optical (NLO) devices demands an out-of-plane (OP) emission dipole of second harmonic generation (SHG) to enhance the spontaneous emission for nanophotonics. However, the research on van der Waals (vdWs) materials with an OP emission dipole of SHG is still in its infancy. Here, by coupling back focal plane (BFP) imaging with numerical simulations and density functional theory (DFT) calculations, we demonstrate that vdWs Janus NbSeI, ranging from bulk to the monolayer limit, exhibits a dominant OP emission dipole of SHG owing to the breaking of the OP symmetry. Explicitly, even-layered NbSeI with symmetry is predicted to exhibit a pure OP emission dipole attributed to the only second-order susceptibility coefficient χ. Meanwhile, although odd-layered NbSeI with symmetry has both OP and IP dipole components (χ and χ), the value of χ is 1 order of magnitude greater than that of χ, leading to an approximate OP emission dipole of SHG. Moreover, the crystal symmetry and OP emission dipole can be preserved under hydrostatic pressure, accompanied by the enhanced χ and the resulting 3-fold increase in SHG intensity. The reported stable OP dipole in 2D vdWs NbSeI can facilitate the rapid development of chip-integrated NLO devices.
原子级薄的非线性光学（NLO）器件的集成需要二次谐波产生（SHG）的面外（OP）发射偶极子来增强纳米光子学中的自发发射。然而，关于具有SHG的OP发射偶极子的范德华（vdWs）材料的研究仍处于起步阶段。在这里，通过将背焦平面（BFP）成像与数值模拟和密度泛函理论（DFT）计算相结合，我们证明了从体相到单层极限的vdWs Janus NbSeI，由于OP对称性的破坏，表现出占主导地位的SHG的OP发射偶极子。具体而言，预测具有 对称性的偶数层NbSeI表现出归因于唯一二阶极化率系数χ的纯OP发射偶极子。同时，尽管具有 对称性的奇数层NbSeI具有OP和IP偶极子分量（χ和χ），但χ的值比χ大1个数量级，导致SHG的近似OP发射偶极子。此外，晶体对称性和OP发射偶极子在静水压力下可以保持，同时χ增强，SHG强度增加3倍。报道的二维vdWs NbSeI中稳定的OP偶极子可以促进芯片集成NLO器件的快速发展。
