Higher-order mode manipulation through fiber-interfaced metastructures for nonlinear frequency conversion applications

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Higher-order modes (HOMs) in fibers have recently attracted substantial attention due to their unique potential in highly topical areas. One of these areas is nonlinear frequency conversion, in which HOMs allow, for example, access to hard-to-reach dispersion landscapes or the exploration of previously inaccessible nonlinear effects. The key challenge here is to efficiently excite or convert HOMs to achieve the highest possible mode purity or to exclusively excite a desired mode. The slightest deviation in modal properties can lead to erroneous results, making precise control of beam properties essential. A cutting-edge approach for shaping beams is based on dielectric nanostructures. The two concepts followed in this project are holograms and metasurfaces, both of which allow shaping of intensity and phase profiles, with metasurfaces additionally capable of controlling polarization. Thus, the combination of dielectric nanostructures and optical fibers suggest a unique control over HOMs in the context of fiber-based nonlinear frequency conversion. This project is dedicated to dielectric metastructures located on the end faces of optical fibers, used in the context of HOM-based ultrafast nonlinear frequency conversion. The unique beam shaping capabilities of holograms and dielectric metasurfaces are exploited here to shape and transform optical beams in terms of intensity, phase and polarization. This is applied in the project to two nonlinear effects, namely third harmonic generation (THG) and supercontinuum generation (SCG). In addition to intensity, shaping phase and polarization is essential, which directly manipulates the electric field as opposed to purely intensity modulating concepts. The THG experiments address the transformation of complex phase-matched HOMs at visible wavelengths into linearly polarized beams with Gaussian profiles. Furthermore, fiber-interfaced metastructures are used for broadband SCG in desired HOMs with complex polarization distributions. The project includes design, simulation, implementation and optical characterization. For the realization of the metastructures on fiber end faces, two methods established and successfully used at the applicant's institute (3D nanoprinting, modified electron beam lithography) are applied. Overall, the project defines a novel photonic platform that not only opens a new field of application for metastructures - metastructure-enabled nonlinear fiber photonics - but also enables the investigation of otherwise difficult-to-access nonlinear effects in fibers with application in a variety of cutting-edge fields.