Topological phase controlled nonlinear optical processes at metasurfaces

由于纳米结构表面具有很高的应用潜力和与标准CMOS工艺兼容的简单制造方法，近年来用于光学应用的纳米结构表面引起了人们的极大兴趣。在我们以前的工作中，我们证明了纳米结构表面，即所谓的亚表面，可以为圆偏振光提供拓扑(Berry)相，这种拓扑(Berry)相仅依赖于单元单元的取向。由于这种拓扑阶段可以在设计过程中自由设计，因此可以通过单个界面获得透镜、光束整形器甚至全息图等任意功能。随着我们的提议，我们将超越这一最新进展，并首次将拓扑相的概念扩展到非线性光学和非线性亚表面。我们提出的概念将允许在非线性光学过程中随意定制非线性材料偏振的空间位相。在这里，我们计划在等离子体纳米结构的基础上实现沿表面具有各种精心设计的光学相位分布的非线性光学准表面。由于等离子体纳米结构与光的相互作用强度很大，因此非常适合这种结构。空间相分布可以通过单元格相对于实验室框架的某一方向来引入。样品将被制作成标准电子束光刻，并用非线性光学光谱进行测量。关于非线性过程，我们将研究从这些表面产生的二次和三次谐波。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对非线性相位的完全控制，以便操纵产生的非线性光的传播特性。作为项目的一部分，我们将分析具有高度非线性光学二向色性的单元设计。这些材料自然使用圆偏振光作为描述的基础。在我们的实验中，我们将通过测量产生的非线性光束的方向和轮廓与引入的相位的关系来分析传输特性。这样的几何位相操纵将允许设计具有明确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，可以得到非线性过程的完美相位匹配条件，而这在传统的非线性光学框架中是不可能的。此外，拓扑相的概念本质上是无色散的，因此在很宽的波长范围内应该非常健壮。我们期待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.