Higher-order mode manipulation through fiber-interfaced metastructures for nonlinear frequency conversion applications

近年来，光纤中的高阶模因其在热点领域的独特潜力而引起了广泛的关注。其中一个领域是非线性频率转换，其中HOMs允许，例如，进入难以到达的色散景观或探索以前无法进入的非线性效果。这里的关键挑战是有效地激发或转换HOMs以达到最高的模式纯度或仅激发所需的模式。模态特性的最小偏差可能导致错误的结果，因此必须精确控制梁的特性。一种基于电介质纳米结构的尖端光束成形方法。在这个项目中遵循的两个概念是全息图和超表面，它们都允许塑造强度和相位轮廓，超表面还能够控制偏振。因此，介电纳米结构和光纤的结合为基于光纤的非线性频率转换提供了一种独特的控制HOMs的方法。本项目致力于研究位于光纤端面上的介电元结构，用于基于homm的超快非线性变频。本文利用全息图和介电超表面独特的光束整形能力，在强度、相位和偏振方面对光束进行整形和变换。该方法在工程中应用于三次谐波产生（THG）和超连续统产生（SCG）两种非线性效应。除了强度，相位和极化的塑造是必不可少的，这直接操纵电场，而不是纯粹的强度调制的概念。THG实验研究了可见光下复相位匹配HOMs向高斯型线偏振光束的转变。此外，在具有复杂极化分布的理想HOMs中，光纤接口元结构被用于宽带SCG。该项目包括设计、仿真、实现和光学表征。为了实现光纤端面上的元结构，应用了申请人研究所建立并成功使用的两种方法（3D纳米打印和改进电子束光刻）。总的来说，该项目定义了一个新的光子平台，不仅为元结构打开了一个新的应用领域-元结构支持的非线性光纤光子学-而且还使研究在各种前沿领域应用的光纤中难以获得的非线性效应成为可能。

Higher-order modes (HOMs) in fibers have recently attracted substantial attention due to their unique potential in highly topical areas. One of these areas is nonlinear frequency conversion, in which HOMs allow, for example, access to hard-to-reach dispersion landscapes or the exploration of previously inaccessible nonlinear effects. The key challenge here is to efficiently excite or convert HOMs to achieve the highest possible mode purity or to exclusively excite a desired mode. The slightest deviation in modal properties can lead to erroneous results, making precise control of beam properties essential. A cutting-edge approach for shaping beams is based on dielectric nanostructures. The two concepts followed in this project are holograms and metasurfaces, both of which allow shaping of intensity and phase profiles, with metasurfaces additionally capable of controlling polarization. Thus, the combination of dielectric nanostructures and optical fibers suggest a unique control over HOMs in the context of fiber-based nonlinear frequency conversion. This project is dedicated to dielectric metastructures located on the end faces of optical fibers, used in the context of HOM-based ultrafast nonlinear frequency conversion. The unique beam shaping capabilities of holograms and dielectric metasurfaces are exploited here to shape and transform optical beams in terms of intensity, phase and polarization. This is applied in the project to two nonlinear effects, namely third harmonic generation (THG) and supercontinuum generation (SCG). In addition to intensity, shaping phase and polarization is essential, which directly manipulates the electric field as opposed to purely intensity modulating concepts. The THG experiments address the transformation of complex phase-matched HOMs at visible wavelengths into linearly polarized beams with Gaussian profiles. Furthermore, fiber-interfaced metastructures are used for broadband SCG in desired HOMs with complex polarization distributions. The project includes design, simulation, implementation and optical characterization. For the realization of the metastructures on fiber end faces, two methods established and successfully used at the applicant's institute (3D nanoprinting, modified electron beam lithography) are applied. Overall, the project defines a novel photonic platform that not only opens a new field of application for metastructures - metastructure-enabled nonlinear fiber photonics - but also enables the investigation of otherwise difficult-to-access nonlinear effects in fibers with application in a variety of cutting-edge fields.