近日，瑞典查尔默斯理工大学Timur O. Shegai&George Zograf及其研究团队取得而一项新进展。经过不懈努力，他们实现共振过渡金属二硫化物纳米片超高折射率与奇异非线性的结合。相关研究成果已于2024年6月13日在国际知名学术期刊《自然—光子学》上发表。

该研究团队利用二硫化钼（MoS₂）多层膜的3R相，在共振非线性纳米光子学领域取得了重要进展。即便是在大块材料中，3R-MoS₂因其缺乏反演对称性而展现出显著优势，包括大量的二阶磁化率、近红外区域极高的各向异性折射率（n>4.5）以及低吸收损失。这些特性使得3R-MoS₂在非线性纳米光子学应用中极具吸引力。研究人员制造了不同半径的3R-MoS₂纳米片，这些纳米片能够支持共振anapole态。

实验结果显示，与相同厚度的未成形薄片相比，单个共振纳米片中的二次谐波产生得到了显著（>100倍）增强。特别地，当二级磁化率的材料共振与圆盘的anapole态的谱重叠时，增强效果达到最大。 这一方法不仅为增强纳米结构范德华材料中光学二阶非线性过程提供了强大的工具，还为非线性和量子高折射率过渡金属二硫族纳米光子学领域的发展铺平了道路。

据悉，固体中的二阶非线性引起了大量独特的物理现象，从压电和光学整流到光学参量放大、自发参量下转换和纠缠光子对的产生。单层过渡金属二硫化物，如二硫化钼，表现出已知的最高二阶非线性系数之一。然而，这些材料的单层性质阻止了仅从材料本身制造共振物体，需要使用外部结构来实现非线性过程的光学增强。

附：英文原文

Title: Combining ultrahigh index with exceptional nonlinearity in resonant transition metal dichalcogenide nanodisks

Author: Zograf, George, Polyakov, Alexander Yu., Bancerek, Maria, Antosiewicz, Tomasz J., Kucukoz, Betul, Shegai, Timur O.

Issue&Volume: 2024-06-13

Abstract: Second-order nonlinearity in solids gives rise to a plethora of unique physical phenomena ranging from piezoelectricity and optical rectification to optical parametric amplification, spontaneous parametric down-conversion and the generation of entangled photon pairs. Monolayer transition metal dichalcogenides, such as MoS₂, exhibit one of the highest known second-order nonlinear coefficients. However, the monolayer nature of these materials prevents the fabrication of resonant objects exclusively from the material itself, necessitating the use of external structures to achieve the optical enhancement of nonlinear processes. Here we exploit the 3R phase of a molybdenum disulfide multilayer for resonant nonlinear nanophotonics. The lack of inversion symmetry—even in the bulk of the material—provides a combination of massive second-order susceptibility, extremely high and anisotropic refractive index in the near-infrared region (n>4.5) and low absorption losses, making 3R-MoS₂ highly attractive for nonlinear nanophotonics. We demonstrate this by fabricating 3R-MoS₂ nanodisks of various radii, which support resonant anapole states, and observing substantial (>100-fold) enhancement of second-harmonic generation in a single resonant nanodisk compared with an unpatterned flake of the same thickness. The enhancement is maximized at the spectral overlap between the anapole state of the disk and the material resonance of the second-order susceptibility. Our approach unveils a powerful tool for enhancing the entire spectrum of optical second-order nonlinear processes in nanostructured van der Waals materials, thereby paving the way for nonlinear and quantum high-index transition metal dichalcogenide nanophotonics.

DOI: 10.1038/s41566-024-01444-9

Source: https://www.nature.com/articles/s41566-024-01444-9