Light cages meet metasurfaces: On-chip twisted light applications enabled by direct laser writing

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Due to its unique properties in terms of physics and applications, twisted (chiral) light with both spin and orbital angular momentum is a highly topical field of research. From a waveguide perspective, twisted optical fibers exhibit fascinating phenomena such as helical Bloch modes or strong circular dichroism. Twisting rates achieved, however, are rather small, only leading to a small perturbative-type influence on mode formation. From the perspective of wavefront shaping, geometric metasurfaces enable control over orbital angular momentum states, yet often suffer from significant cross talk for example in a context of multiplexing.A recently introduced implementation approach for sophisticated photonic structures is 3D nanoprinting using Direct Laser Writing (DLW), offering unique advantages for waveguides and metasurfaces. Here, novel on-chip hollow-core waveguides - so-called light cages - and metasurfaces with access to the additional height degree of freedom have been jointly demonstrated by the applicants.The proposed project targets to explore the generation, guidance and manipulation of light interacting with nanoprinted twisted photonic structures including light cages, chiral metasurfaces and their combination. The project aims to (i) understand the properties of light propagating inside twisted light cages and controlled via chiral metasurfaces, (ii) unlock the potential of DLW to realise twisted functional photonic structures, and (iii) to evaluate whether the combination of chiral metasurfaces and twisted light cages results in a novel photonic platform to reach previously inaccessible physics and applications. To give some examples, DLW principally enables light cages with exceptionally high twist rates and unexplored geometries. For metasurfaces, the height degree of freedom offers full control not only over geometric but also over the propagation phase and therefore the complex amplitude, and via sophisticated shapes also enables intrinsically chiral unit cells. Equally important for our work, DLW allows for direct implementation of metasurface-interfaced twisted light cages in a single fabrication step.Overall, the project will uncover novel physics concerning generation, guidance and manipulation of light interacting with twisted photonic structures. Potential applications that are evaluated within the context of this project will include chiral molecular on-chip sensing, OAM-assisted multiplexing or atomic magnetometry.