Second-order nonlinear frequency generation in nanostructured surfaces

Nanostructured surfaces, which drastically manipulate light on a very small spatial scale were investigated over the last decades to develop compact optical devices. However, research on nonlinear properties in such systems is still in a mainly fundamental stage. Here, different schemes and materials are used to develop second-order nonlinear nanostructured surfaces which support strong second-harmonic generation under controlled radiation. A gallium arsenide metasurface is investigated whose intrinsic second-order nonlinearity is used to generate second-harmonic light propagating out of the metasurface plane. The periodic design results in second-harmonic diffraction into the first diffraction order, since the second-harmonic emission normal to the metasurface plane is forbidden for the crystal orientation. In contrast, the intrinsic second-order nonlinear susceptibility of ultrathin molybdenum disulfide monolayers enables the emission of the second-harmonic light normal to the surface plane. Here, such monolayers are patterned, to observe the diffracted second-harmonic generation in the zero and first diffraction orders. However, in a more elaborate design a topological dislocationis used to generate second-harmonic vortex beams in the first diffraction order. Unfortunately, the conversion efficiencies of the patterned two-dimensional materials are relatively low. To enhance the second-harmonic generation, a hybrid system of a multi-resonant silicon metasurface coupled to a molybdenum disulfide monolayer is investigated. By measuring and simulating the linear and second-order nonlinear properties, enhanced second-harmonic generation is observed and guidelines for the design are developed. The here found results support the development of nonlinear functionalized surfaces for second-order frequency generation. To understand such systems opens a path to the process of spontaneous parametric down-conversion enabling entangled-photon pair generation.


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