Tracking and analyzing the individual diffusion of nanoscale objects such as proteins and viruses is an important methodology in life science. Here, we show a sensor that combines the efficiency of light line illumination with the advantages of fluidic confinement. Tracking of freely diffusing nano-objects inside water-filled hollow core fibers with core diameters of tens of micrometers using elastically scattered light from the core mode allows retrieving information about the Brownian motion and the size of each particle of the investigated ensemble individually using standard tracking algorithms and the mean squared displacement analysis. Specifically, we successfully measure the diameter of every gold nanosphere in an ensemble that consists of several hundreds of 40 nm particles, with an individual precision below 17% (±8 nm). In addition, we confirm the relevance of our approach with respect to bioanalytics by analyzing 70 nm λ-phages. Overall these features, together with the strongly reduced demand for memory space, principally allows us to record thousands of frames and to achieve high frame rates for high precision tracking of nanoscale objects.

We experimentally observe an effective PT -phase transition through the exceptional point in a hybrid plasmonic-dielectric waveguide system. Transmission experiments reveal fundamental changes in the underlying Eigenmode interactions as the environmental refractive index is tuned, which can be unambiguously attributed to a crossing through the plasmonic exceptional point. These results extend the design opportunities for tuneable non-Hermitian physics to plasmonic systems.Hermitian systems and their operators are used to describe a wide range of physical phenomena to predict the evolution of Eigenstates via unitary operations containing purely real Eigenvalues [1,2]. Recently, increasing attention has been dedicated to non-Hermitian systems, which are non-conservative and generally yield complex Eigenvalue spectra [3]. Non-Hermitian systems are typically created by opening a Hermitian system to the environment by including dissipation and/or gain. Interestingly, coupled non-Hermitian systems can yield purely real Eigenvalues which respect parity-time (PT ) symmetry via an appropriate balance of gain and loss [4,5]. In an unbalanced scenario, a PT -broken system shows complex conjugate Eigenvalue pairs. The PT -symmetric (PTS) and PT -broken (PTB) regimes are separated by the exceptional point (EP), where the Eigenvalues coalesce, which is associated with several interesting physical phenomena, such as level repulsion [6], reflectionless propagation [7], and topological phase transitions [8].

Fiber Bragg gratings inscribed in the waist of tapered photosensitive fibers offer specific attractive properties for sensing applications. A small-diameter fiber reduces structural influences for imbedded fiber sensing elements. In the case of application as a force-sensing element for tensile forces, sensitivity scales inversely with the fiber cross-sectional area. It is therefore possible to increase force sensitivity by several orders of magnitude compared to Bragg grating sensors in conventionally sized fibers. Special requirements for such Bragg grating arrangements are discussed and experimental measurements for different fiber taper diameters down to 4 µm are presented.

Due to the ongoing improvement in nanostructuring technology, ultrathin metallic nanofilms have recently gained substantial attention in plasmonics, e.g. as building blocks of metasurfaces. Typically, noble metals such as silver or gold are the materials of choice, due to their excellent optical properties, however they also possess some intrinsic disadvantages. Here, we introduce niobium nanofilms (~10 nm thickness) as an alternate plasmonic platform. We demonstrate functionality by depositing a niobium nanofilm on a plasmonic fiber taper, and observe a dielectric-loaded niobium surface-plasmon excitation for the first time, with a modal attenuation of only 3–4 dB/mm in aqueous environment and a refractive index sensitivity up to 15 μm/RIU if the analyte index exceeds 1.42. We show that the niobium nanofilm possesses bulk optical properties, is continuous, homogenous, and inert against any environmental influence, thus possessing several superior properties compared to noble metal nanofilms. These results demonstrate that ultrathin niobium nanofilms can serve as a new platform for biomedical diagnostics, superconducting photonics, ultrathin metasurfaces or new types of optoelectronic devices.

We discuss a fiber-integrated refractive index sensor with strongly improved detection performance. The resonator has been implemented by means of focused-ion beam milling of a step index fiber and shows a sensitivity of about 1.15µm/RIU. Coating the resonator walls led to a strongly improved mirror reflectivity by a factor of about 26. Design rules for device optimization and a detailed mathematical analysis are discussed, revealing that the sensor operates as an optimized Fabry-Perot resonator. We also show that the performance of such kind of Fabry-Perot sensors is, in general, limited by the detection limit function - a quantity depending on the cavitiy's finesse and on the measurement capabilities used.

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Coupling of light from free space to optical fibers is essential for many applications, while commonly used step-index optical fibers provide insufficient coupling efficiencies especially at large angles of incidence. Here, we demonstrate record-high coupling efficiencies achieved with dielectric nanostructures located on single-mode fiber end faces. We introduce a novel approach that allows fabricating dielectric nanostructures at the facet of a step-index optical fiber via an extended version of planar electron-beam based lithography. We demonstrate polarization-and angle-independent coupling of light into the fiber across a wide range of angles as large as 80°. We support our experimental results with an analytical model and extensive numerical simulations. Our results reveal the key properties of nanostructure-empowered fibers that may improve the performance of many optical devices requiring efficient collection of light, including quantum technologies (single-photon collection) or biophotonics (in vivo imaging). Our approach can be extended to other materials and geometries, merging fiber optics with high-index dielectric metasurfaces, allowing for unprecedented functionalities for the efficient control of light.

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