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We report on the first demonstration of a proof-of-principle optical fiber ‘meta-tip’, which integrates a phase-gradient plasmonic metasurface on the fiber tip. For illustration and validation purposes, we present numerical and experimental results pertaining to various prototypes implementing generalized forms of the Snell’s transmission/reflection laws at near-infrared wavelengths. In particular, we demonstrate several examples of beam steering and coupling with surface waves, in fairly good agreement with theory. Our results constitute a first step toward the integration of unprecedented (metasurface-enabled) light-manipulation capabilities in optical-fiber technology. By further enriching the emergent ‘lab-on-fiber’ framework, this may pave the way for the widespread diffusion of optical metasurfaces in real-world applications to communications, signal processing, imaging and sensing.

Soft self-assembling photonic materials such as cholesteric liquid crystals are attractive due to their multiple unique and useful properties, in particular, an optical band gap that can be continuously and dynamically tuned in response to weak external influences, easy device integration, compatibility with flexible architectures, and, as shown here, potential for submicrometer optical applications. We study such a system formed by a short-pitch cholesteric confined in the core of polymer fibers produced by coaxial electrospinning, showing that the selective reflection arising from the helical photonic structure of the liquid crystal is present even when its confining cavity is well below a micrometer in thickness, allowing as little as just half a turn of the helix to develop. At this scale, small height variations result in a dramatic change in the reflected color, in striking difference to the bulk behavior. These conclusions are made possible by combining focused ion beam (FIB) dissection and imaging of the internal fiber morphology with optical microscopy. The FIB dissection further reveals that the cross section of the cavity within the fiber can have a shape that is quite different from that of the outside fiber. This is critical for the photonic behavior of the composite fiber because different optical textures are generated not only by change in thickness but also by the shape of the cavity. Our results provide insights into the behavior of cholesterics in submicrometer cavities and demonstrate their potential at such dimensions.

Precision medicine is continuously demanding for novel point of care systems, potentially exploitable also for in-vivo analysis. Biosensing probes based on Lab-On-Fiber Technology have been recently developed to meet these challenges. However, devices exploiting standard label-free approaches (based on ligand/target molecule interaction) suffer from low sensitivity in all cases where the detection of small molecules at low concentrations is needed. Here we report on a platform developed through the combination of Lab-On-Fiber probes with microgels, which are directly integrated onto the resonant plasmonic nanostructure realized on the fiber tip. In response to binding events, the microgel network concentrates the target molecule and amplifies the optical response, leading to remarkable sensitivity enhancement. Moreover, by acting on the microgel degrees of freedom such as concentration and operating temperature, it is possible to control the limit of detection, tune the working range as well as the response time of the probe. These unique characteristics pave the way for advanced label-free biosensing platforms, suitably reconfigurable depending on the specific application.In biochemical sensing field, Lab-on -Fiber (LOF) based devices essentially consist on the combination of optical resonant nanostructures (typically patterned metallic slab supporting surface plasmon resonances (SPR)) and functional coating materials integrated on the optical fiber tip [1][2][3][4][5][6][7] . LOF technology is continuously leading to the development of novel biosensing probes with unique properties in term of size, weight and ease of interrogation 4,5 . In addition to point of care applications, LOF based devices seem to be particularly promising for in-vivo analysis systems, thanks to the intrinsic properties of optical fibers that make them easily integrable inside medical catheters or needles 8 . Typically, the working principle of LOF probes relies on the affinity interaction of a ligand attached to the sensor surface with the target molecule present in a liquid solution at a certain concentration. However, standard label-free approaches fail when target molecules are small, for example about a few hundreds of dalton. In that case, the ligand/analyte binding process produces a biological layer that is not thick enough for providing a local refractive index (RI) change that is optically detectable by the sensor. Analogous issues occur in such applications where the detection of larger analytes with very low limit of detection (LOD) is required. To enhance the sensitivity, gold and magnetic nanoparticles have been proposed as "molecular concentrators" able to localize multiple binding events on a single particle, and successively deliver target analyte from the solution to the sensor surface [9][10][11] . At the same time, approaches exploiting hydrogels (HGs) have been proposed 12,13 . HGs basically allow to: i) increase the analyte loading capacity by translating a conventional 2D interaction surface into a 3D volume inter...

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