Topological phase controlled nonlinear optical processes at metasurfaces

纳米结构表面的光学应用获得了最近很大的兴趣，由于他们的应用和简单的制造方法，是与标准的CMOS技术兼容的高潜力。在我们以前的工作中，我们证明了纳米结构表面，所谓的超颖表面，可以提供一个拓扑（Berry）相位的圆偏振光，仅取决于单位细胞的方向。由于这种拓扑相位可以在设计过程中自由设计，因此可以通过单个接口获得任意功能，如透镜，光束整形器，甚至全息图。通过我们的建议，我们将超越这一最新进展，并首次将拓扑相位的概念扩展到非线性光学和非线性超颖表面。我们提出的概念将允许定制的空间相位的非线性材料的偏振在非线性光学过程中的意愿。在这里，我们计划实现非线性光学超颖表面与各种精心设计的光学相位分布沿着基于等离子体纳米结构的表面。等离子体纳米结构由于其与光的大的相互作用强度而非常适合。空间相位分布可以通过相对于实验室框架的单元格的特定取向来引入。样品将通过标准电子束光刻法制造，并通过非线性光学光谱法测量。关于非线性过程，我们将研究从这些表面的二次和三次谐波产生。为了进一步增强表面的非线性，我们将把等离子体结构嵌入到高度非线性的聚合物中。我们的目标包括实现非线性相位的完全控制，以操纵所产生的非线性光的传播特性。作为项目的一部分，我们将分析具有高非线性光学二向色性的单胞设计。这些材料自然地使用圆偏振光作为描述的基础。通过实验，我们将通过测量所产生的非线性光束的方向和轮廓依赖于引入的相位来分析传输特性。这种几何相位操纵将允许设计具有良好定义的非线性光学性质的光学材料。例如，对于各向同性材料，可以获得非线性过程的完美相位匹配条件，这在传统的非线性光学框架中是不可能的。此外，拓扑相位的概念本质上是无色散的，因此应该在宽波长范围内非常稳健地工作。我们期望我们的实验研究将为非线性光学元件的设计开辟一个新的领域。

Nanostructured surfaces for optical applications gained recently much interest due to their high potential for applications and the simple fabrication methods that are compatible with standard CMOS technology. In our previous work we demonstrated that nanostructured surfaces, so-called metasurfaces, can provide a topological (Berry) phase for circularly polarized light that solely depends on the orientation of the unit-cells. Since this topological phase can be freely engineered in the design process arbitrary functionalities like lenses, beam shapers, or even holograms can be obtained by a single interface. With our proposal we will go beyond this recent progress and extent the concept of the topological phase for the first time to nonlinear optics and nonlinear metasurfaces. Our proposed concept would allow tailoring the spatial phase of the nonlinear material polarization in a nonlinear optical process at will. Here, we plan to realize nonlinear optical metasurfaces with various well-designed optical phase distributions along the surface based on plasmonic nanostructures. Plasmonic nanostructures are well-suited due to their large interaction strength with light. A spatial phase distribution can be introduced by a certain orientation of the unit-cell with respect to the laboratory frame. The samples will be fabricated be standard electron beam lithography and measured by nonlinear optical spectroscopy. Concerning the nonlinear processes we will investigate the second- and third-harmonic generation from these surfaces. To enhance the nonlinearity of the surfaces even further we will embed the plasmonic structures into a highly nonlinear polymer. Our goal includes the realization of a full control of the nonlinear phase in order to manipulate the propagation characteristics of the generated nonlinear light. As part of the project we will analyze unit-cell designs with high nonlinear optical dichroism. These materials naturally use circularly polarized light as basis for the description. With our experiments we will analyze the propagation characteristics by measuring the generated nonlinear beam direction and profile in dependence of the introduced phase. Such a geometrical phase manipulation would allow the design of optical materials with well-defined nonlinear optical properties. For example a perfect phase matching condition for nonlinear processes could be obtained for isotropic materials which would be impossible in the traditional framework of nonlinear optics. In addition, the concept of the topological phase is inherently dispersion-less and should work therefore very robust over a broad wavelength range. We expect that our experimental investigations will open a new field in the design of nonlinear optical elements.