博士生谢兴在国际权威期刊Advanced Functional Materials上发表论文

      近日，课题组成员谢兴同学在刘老师的指导下，以第一作者在国际权威期刊Advanced Functional Materials(影响因子：19.0)上发表题为 “Programmable Anisotropy in Low-Symmetry van der Waals Crystals via Moiré and Symmetry Engineering ”(基于莫尔与对称性工程的低对称性范德华异质结的可编程光学各向异性特性  )的实验论文。

图：基于莫尔与对称性工程的低对称性范德华异质结的可编程光学各向异性特性  

    各向异性是物理、化学和生物学中的一种基本属性，指材料的物理和化学性质随方向不同而表现出不同的行为。它在多种偏振敏感应用中发挥着关键作用，包括光探测器、偏振传感、成像和光学雷达等。低对称性范德华(vdW)材料的出现，极大推动了各向异性器件的研究与发展。尽管人们已经采用应变工程、异质界面调控和外场调控等方法来控制各向异性性质，但在复杂环境和多样化应用场景中，如何发现新的低对称性二维(2D)材料，并发展新的策略来精准调控其各向异性行为，仍然面临重要挑战。莫尔超晶格的兴起为凝聚态物理开辟了新的前沿，催生了强关联相变、超导、莫尔激子和莫尔准晶等一系列引人注目的新奇现象。然而，当前大多数研究主要集中于扭转石墨烯、过渡金属硫族化合物(TMDs)和六方氮化硼(hBN)等高对称性材料体系，这些体系通常保留其各向同性特征。相比之下，低对称性二维材料中的莫尔超晶格研究则提供了一个独特机遇。在这类体系中，层间扭转角和层数不对称性能够引发对称性破缺，从而重塑其各向异性行为。然而，如何利用莫尔工程对低对称性范德华材料的各向异性进行精确调控，仍是一个尚未充分探索的重要课题。

         在本工作中，我们研究了正交堆垛 PdSe₂/PdSe₂ 异质结构中，层间扭转角和层数不对称性对五角相 PdSe2  面内各向异性的调控作用。通过拉曼光谱和二次谐波产生(SHG)光谱，我们证明了 PdSe2 的各向异性性质可以通过调节层间扭转角和层数不对称性实现显著调控。当 PdSe₂ 处于正交堆垛时，其偏振图样发生明显变化：拉曼散射由二色性分布转变为四瓣花形，而 SHG 偏振图则由蝴蝶结状演化为哑铃状。这些偏振特征还可以通过进一步改变层数不对称性实现更精细的调控。值得注意的是，莫尔工程诱导出一种依赖扭转角的偏振行为，即随着扭转角变化，拉曼散射和 SHG 强度呈现相反的偏振演化趋势。特别地，在 90° 扭转角下，由于对称性破缺及极化率张量方向性的调制，SHG 偏振比显著提升至约 80。如此优异的偏振调控能力，使其在光通信中的光学编码以及偏振分辨光电流产生、进而用于高灵敏光探测和成像等方面展现出潜在应用前景。第一性原理计算进一步表明，层间扭转角能够改变 PdSe2  的电子结构，诱导其在间接带隙与直接带隙之间转变，并在小扭转角下形成平带。我们的研究表明，莫尔工程与对称性工程为调控低对称性范德华材料中的各向异性提供了强有力的策略，也为探索莫尔物理和量子现象开辟了新的平台。

文章链接: https://advanced.onlinelibrary.wiley.com/doi/10.1002/adfm.75529

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Twisting low-symmetry crystals unlocks programmable optical anisotropy

Controlling anisotropy—the dependence of a material’s response on direction—is central to polarization-sensitive technologies such as photodetectors, optical encoding, imaging, and radar. In this work, the authors show that anisotropy in the low-symmetry van der Waals crystal PdSe₂ can be programmed through a combination of moiré engineering and symmetry breaking. By introducing interlayer twist and layer-number asymmetry in PdSe₂/PdSe₂ homostructures, they demonstrate a new route to reshaping both linear and nonlinear optical responses in a highly controllable way.

Using polarization-resolved Raman and second-harmonic generation (SHG) spectroscopy, the study reveals that the two optical probes evolve in opposite ways as the twist angle is varied. Raman polarization patterns change from dichroic to four-petal shapes, while SHG transforms from bowknot-like to dumbbell-like profiles. Most strikingly, above a critical twist angle of 44°, the SHG response enters a strongly dichroic regime, culminating in a polarization ratio of about 80 at 90° twist. The authors attribute this behavior to twist-induced symmetry lowering, which activates and redistributes specific components of the second-order susceptibility tensor.

The work goes beyond spectroscopy by translating the anisotropy control into functional demonstrations. The large SHG contrast is used for binary optical encoding, while polarization-dependent photocurrent measurements show promise for polarization-sensitive photodetection and imaging. These experiments indicate that moiré-controlled anisotropy is not only a fundamental phenomenon but also a practical design handle for photonic and optoelectronic devices.

First-principles calculations further show that twisting also reconstructs the electronic structure of PdSe₂, including indirect-to-direct bandgap conversion at selected angles and the formation of flat bands at small twist angles. Together, these results establish low-symmetry moiré superlattices as a fertile platform for both programmable anisotropy and emergent quantum phenomena, extending moiré physics beyond the high-symmetry material systems that have dominated the field so far.