博士生张贤同学在国际纳米权威期刊Nano Letters上发表论文

      近日，课题组成员张贤同学在刘老师的指导下，以第一作者在国际纳米权威期刊《纳米快报》(Nano Letters，影响因子12.94)上发表题为 “Magnetic-order-mediated second harmonic generation in a room-temperature van der Waals ferromagnet Fe₃GaTe₂”(室温范德华铁磁体 Fe₃GaTe₂ 中磁有序介导的二次谐波的特性)的实验论文。

图：室温范德华铁磁体 Fe₃GaTe₂ 中磁有序介导的二次谐波的特性

      二次谐波产生 (second-harmonic generation，SHG)是一种典型的二阶非线性光学过程，其中两个频率为 ω 的光子结合产生一个频率为 2ω 的光子。作为探测对称性演化和局域电偶极构型的有力手段，SHG 已被广泛应用于相关研究领域。在非磁性或弱磁性的二维材料中，SHG 响应主要由时间反演对称的 i 型分量 χ(i) 主导。该分量仅在空间反演对称性被破缺时才非零，因此对层数、堆垛方式以及局域晶格畸变等结构特征高度敏感。当体系中形成磁有序时，时间反演对称性的破缺会引入额外的时间反演反对称分量 χ(c)，其符号随磁化方向发生反转。该磁性分量使 SHG 对磁畴构型、自旋取向以及磁对称性变化表现出高度敏感性。近年来，磁性 SHG 已成功揭示了 CrI₃、CrSBr、CrSP₄ 和 MnPS₃ 等低温二维磁体中的多种隐含磁对称性和磁畴动力学行为，凸显了其在磁态分析和对称性分辨表征方面的独特优势。然而，尽管 SHG 具有内在的对称性灵敏性，其在解析空间变化的磁畴织构、刻画畴壁边界以及追踪磁场驱动演化方面仍有待深入探索。

      Fe₃GaTe(FGT)凭借其独特的结构与磁对称性条件，为突破 SHG 在磁畴研究中的局限性提供了重要机遇。FGT 是一种范德华铁磁体，在 350–380 K 仍保持稳定的垂直磁各向异性和显著的自旋极化特性，使其成为室温磁性非线性光学研究的理想平台。块体 FGT 属于中心对称的 P6₃/mmc 空间群，而在奇数层结构中空间反演对称性被完全破缺，在较厚或偶数层样品的表面和界面处也会发生局域破缺，从而产生可测量的结构 i 型二阶非线性响应 χ(i)。与此同时，FGT 稳定的面外磁化破坏了时间反演对称性，使磁性 c 型分量 χ(c) 得以产生。空间反演与时间反演对称性的协同破缺，为研究 χ(i)–χ(c) 的相干干涉、分辨局域磁畴对比度以及关联 SHG 偏振变化与磁场驱动畴重构过程提供了理想环境。因此，对 FGT 中 SHG 响应进行系统研究，对于推动磁性 SHG 在室温磁光耦合领域的应用具有重要意义。

    在本工作中，我们通过实验与理论相结合的方法系统研究了室温铁磁体 FGT 的非线性光学响应。通过机械剥离制备不同厚度的 FGT 薄片，我们发现 SHG 强度随厚度呈现显著的非单调变化规律，即先增强后减弱。SHG 显微成像进一步实现了对室温迷宫状磁畴结构的全光学直接成像，其畴形貌与磁力显微镜测量结果高度一致，为范德华铁磁体中无接触磁畴成像迈出了重要一步。磁场依赖的 SHG 实验揭示了结构偶极贡献(χ_even)与磁化耦合项(χ_odd)之间可调控的相干干涉效应，该效应表现为 SHG 强度中的可逆磁滞行为以及磁场控制的偏振图样旋转，表明非线性光学响应可实现有效的磁调控。第一性原理计算表明，Fe 自旋不对称性与层间杂化是调制 χ^(2) 张量的关键微观因素，为实验观测到的非线性响应提供了明确的电子结构起源解释。 本研究揭示了 FGT 中厚度和磁场可调的磁性 SHG 特性，克服了传统 SHG 在解析磁畴结构方面的关键限制，并进一步拓展了磁性 SHG 在高温磁光耦合与自旋光子器件集成领域的应用前景。

文章链接：https://pubs.acs.org/doi/full/10.1021/acs.nanolett.5c06338?_gl=1*1gmuxec*_gcl_au*NjkzMTcxMDUyLjE3NzA4OTU0MDM.*_ga*MTQwNDE0NTk2LjE3NzA4OTU0MDM.*_ga_XP5JV6H8Q6*czE3NzMxMDU4MTAkbzc1JGcxJHQxNzczMTA1ODM5JGozMSRsMCRoMA..

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This work represents an important advance in nonlinear magneto-optics by establishing second harmonic generation (SHG) as a domain-resolved, fully optical probe of magnetic order in a room-temperature van der Waals ferromagnet. By demonstrating SHG imaging of labyrinthine magnetic domains in Fe₃GaTe₂ (FGT), the authors overcome a long-standing limitation of magnetic SHG, which has historically been restricted to symmetry analysis and bulk-averaged magnetic responses rather than spatially resolved domain textures.

A key strength of the study lies in the clear identification of coherent interference between structural and magnetization-coupled second-order susceptibilities. The observation of magnetic-field-induced SHG hysteresis and polarization rotation provides compelling evidence that the nonlinear optical phase can be actively tuned by magnetic order. This insight elevates SHG from a passive symmetry probe to an active platform for magneto-optical control, with direct relevance to spin–photon coupling and nonlinear photonic functionality.

Equally significant is the demonstration of room-temperature operation. Most prior magnetic SHG studies in two-dimensional magnets have been limited to cryogenic conditions, severely constraining their applicability. The robust perpendicular magnetic anisotropy and high Curie temperature of FGT make it an ideal material system in which to extend magnetic SHG into technologically relevant regimes. The close correspondence between SHG domain imaging and magnetic force microscopy further validates SHG as a non-contact, all-optical alternative for probing magnetic domain structures in van der Waals ferromagnets.

The inclusion of first-principles calculations provides valuable microscopic insight, linking the magnetization-coupled χ^(2) response to Fe orbital spin asymmetry and interlayer hybridization. This connection between electronic structure and nonlinear optical response lays an important foundation for materials-by-design approaches to magnetic nonlinear optics.

Looking forward, this work opens several promising directions. Extending SHG-based domain imaging to dynamical regimes could enable ultrafast tracking of domain-wall motion and magnetization switching. Moreover, applying this approach to heterostructures and moiré-engineered magnetic systems may reveal new opportunities for controlling nonlinear optical phases through interfacial coupling and symmetry engineering.

Overall, this study convincingly establishes magnetic-order-mediated SHG as a powerful and versatile tool for symmetry-resolved and spatially resolved magnetic characterization at room temperature, marking a significant step toward integrating nonlinear optics with two-dimensional spintronic and spin-photonic platforms.