Coupled periodic nanowaveguides for the generation of photon pairs

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One of the central challenges of integrated optics is the complete control of the spatial and spectral properties of guided optical fields. The spatial distribution of guided fields is usually controlled by coupling of several waveguides using evanescent energy exchange between them. Although this approach is well understood, it is usually implemented using very similar waveguides with low dispersion. Strong dispersion, on the other hand, enables spectral control and is usually achieved in single nanosized waveguides with a large refractive index contrast between wave-guiding core and the surrounding media, which additionally may be periodically structured to introduce resonances in the dispersion.By forming coupled waveguide systems from structured and unstructured nanowaveguides with large refractive index contrast we aspire to simultaneously enable far reaching control of the spatial and spectral properties of the propagating fields. Thus, we are combining the favorable properties of periodically structured waveguides for dispersion control with the inherent possibilities of coupled waveguide systems for spatial control. Furthermore, such systems of very different coupled waveguides additionally enhance the possibilities for controlling light propagation by unlocking new degrees of freedom.The aim of this project is to establish a fundamental understanding of how the properties of such coupled periodic nanowaveguides depend on their parameters, e.g. the size of the waveguides, period of the structuring or overall symmetry of the system. Such insight will enable the systematic design of coupled periodic nanowaveguides for specific purposes.As a relevant application case we will study the generation of photon pairs with tailorable properties by spontaneous parametric down-conversion in coupled periodic nanowaveguides manufactured in lithium niobate. The spectral and spatial properties of the generated two-photon quantum states sensitively depend on the respective properties of the generating structures, opening up the possibility for controlling them by the structure design. In this project we plan to realize sources for photon pairs with tunable spectrum and entanglement as well as two-photon states with complex spectral profile needed to realize specific temporal waveforms. Furthermore, we will demonstrate spatially engineered two-photon sources, allowing for e.g. the deterministic separation of the generated photons or the routing depending on their spectral properties.By completing this project, we will introduce a new type of structure to integrated optics and will deliver an understanding of the interplay of coupling and periodic structuring on the nanoscale. Finally, the project will open up new perspectives for quantum optics due to a larger variety of experimentally available light states.