Topological phase controlled nonlinear optical processes at metasurfaces

• Applicant: Professor Dr. Thomas Zentgraf
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2015 to 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 271596654

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.

• DFG Programme: Research Grants