Light guidance in photonic band gap guiding dual-ring light cages implemented by direct laser writing

Fiber-interfaced hollow-core light cage: a platform for on-fiber-integrated waveguides

Here, we demonstrate the realization of hollow-core light cages (LCs) on commercial step-index fibers using 3D nanoprinting, resulting in fully fiber-integrated devices. Two different light cage geometries with record-high aspect ratio strands and unique sidewise access to the core have been implemented, exhibiting excellent optical and mechanical properties. These achievements are based on the use of 3D nanoprinting to fabricate light cages and stabilize them with customized support elements. Overall, this approach results in novel, to the best of our knowledge, fiber-interfaced hollow-core devices that combine several advantages in a lab-on-a-fiber platform that is particularly useful for diffusion-related applications in environmental sciences, nanosciences, and quantum technologies.

Coupled waveguide model for computing phase and transmission through nanopillar-based metasurfaces

Dielectric metasurfaces are important in modern photonics due to their unique beam shaping capabilities. However, the standard tools for the computation of the phase and transmission through a nanopillar-based metasurface are either simple, approximating the properties of the surface by that of a single cylinder, or use full 3D numerical simulations. Here we introduce a new analytical model for computing metasurface properties which explicitly takes into account the effect of the lattice geometry. As an example we investigate silicon nanopillar-based metasurfaces, examining how the transmission properties depend on the presence of different modes in the unit cell of the metasurface array. We find that the new model outperforms the isolated cylinder model in predicting the phase, and gives excellent agreement with full numerical simulations when the fill fraction is moderate. Our model offers a waveguide perspective for comprehending metasurface properties, linking it to fiber optics and serving as a practical tool for future metasurface design.

3D-nanoprinted on-chip antiresonant waveguide with hollow core and microgaps for integrated optofluidic spectroscopy

Here, we unlock the properties of the recently introduced on-chip hollow-core microgap waveguide in the context of optofluidics which allows for intense light-water interaction over long lengths with fast response times. The nanoprinted waveguide operates by the anti-resonance effect in the visible and near-infrared domain and includes a hollow core with defined gaps every 176 µm. The spectroscopic capabilities are demonstrated by various absorption-related experiments, showing that the Beer-Lambert law can be applied without any modification. In addition to revealing key performance parameters, time-resolved experiments showed a decisive improvement in diffusion times resulting from the lateral access provided by the microgaps. Overall, the microgap waveguide represents a pathway for on-chip spectroscopy in aqueous environments.

Locally Structured On-Chip Optofluidic Hollow-Core Light Cages for Single Nanoparticle Tracking

Nanoparticle tracking analysis (NTA) is a widely used methodology to investigate nanoscale systems at the single species level. Here, we introduce the locally structured on-chip optofluidic hollow-core light cage, as a novel platform for waveguide-assisted NTA. This hollow waveguide guides light by the antiresonant effect in a sparse array of dielectric strands and includes a local modification to realize aberration-free tracking of individual nano-objects, defining a novel on-chip solution with properties specifically tailored for NTA. The key features of our system are (i) well-controlled nano-object illumination through the waveguide mode, (ii) diffraction-limited and aberration-free imaging at the observation site, and (iii) a high level of integration, achieved by on-chip interfacing to fibers. The present study covers all aspects relevant for NTA including design, simulation, implementation via 3D nanoprinting, and optical characterization. The capabilities of the approach to precisely characterize practically relevant nanosystems have been demonstrated by measuring the solvency-induced collapse of a nanoparticle system which includes polymer brush-based shells that react to changes in the liquid environment. Our study unlocks the advantages of the light cage approach in the context of NTA, suggesting its application in various areas such as bioanalytics, life science, environmental science, or nanoscale material science in general.

Coherent interaction of atoms with a beam of light confined in a light cage

Controlling coherent interaction between optical fields and quantum systems in scalable, integrated platforms is essential for quantum technologies. Miniaturised, warm alkali-vapour cells integrated with on-chip photonic devices represent an attractive system, in particular for delay or storage of a single-photon quantum state. Hollow-core fibres or planar waveguides are widely used to confine light over long distances enhancing light-matter interaction in atomic-vapour cells. However, they suffer from inefficient filling times, enhanced dephasing for atoms near the surfaces, and limited light-matter overlap. We report here on the observation of modified electromagnetically induced transparency for a non-diffractive beam of light in an on-chip, laterally-accessible hollow-core light cage. Atomic layer deposition of an alumina nanofilm onto the light-cage structure was utilised to precisely tune the high-transmission spectral region of the light-cage mode to the operation wavelength of the atomic transition, while additionally protecting the polymer against the corrosive alkali vapour. The experiments show strong, coherent light-matter coupling over lengths substantially exceeding the Rayleigh range. Additionally, the stable non-degrading performance and extreme versatility of the light cage provide an excellent basis for a manifold of quantum-storage and quantum-nonlinear applications, highlighting it as a compelling candidate for all-on-chip, integrable, low-cost, vapour-based photon delay.

Orders of magnitude loss reduction in photonic bandgap fibers by engineering the core surround

We demonstrate how to reduce the loss in photonic bandgap fibers by orders of magnitude by varying the radius of the corner strands in the core surround. As a fundamental working principle we find that changing the corner strand radius can lead to backscattering of light into the fiber core. Selecting an optimal corner strand radius can thus reduce the loss of the fundamental core mode in a specific wavelength range by almost two orders of magnitude when compared to an unmodified cladding structure. Using the optimal corner radius for each transmission window, we observe the low-loss behavior for the first and second bandgaps, with the losses in the second bandgap being even lower than that of the first one. Our approach of reducing the confinement loss is conceptually applicable to all kinds of photonic bandgap fibers including hollow core and all-glass fibers as well as on-chip light cages. Therefore, our concept paves the way to low-loss light guidance in such systems with substantially reduced fabrication complexity.

The Optofluidic Light Cage - On-Chip Integrated Spectroscopy Using an Antiresonance Hollow Core Waveguide

Emerging applications in spectroscopy-related bioanalytics demand for integrated devices with small geometric footprints and fast response times. While hollow core waveguides principally provide such conditions, currently used approaches include limitations such as long diffusion times, limited light-matter interaction, substantial implementation efforts, and difficult waveguide interfacing. Here, we introduce the concept of the optofluidic light cage that allows for fast and reliable integrated spectroscopy using a novel on-chip hollow core waveguide platform. The structure, implemented by 3D nanoprinting, consists of millimeter-long high-aspect-ratio strands surrounding a hollow core and includes the unique feature of open space between the strands, allowing analytes to sidewise enter the core region. Reliable, robust, and long-term stable light transmission via antiresonance guidance was observed while the light cages were immersed in an aqueous environment. The performance of the light cage related to absorption spectroscopy, refractive index sensitivity, and dye diffusion was experimentally determined, matching simulations and thus demonstrating the relevance of this approach with respect to chemistry and bioanalytics. The presented work features the optofluidic light cage as a novel on-chip sensing platform with unique properties, opening new avenues for highly integrated sensing devices with real-time responses. Application of this concept is not only limited to absorption spectroscopy but also includes Raman, photoluminescence, or fluorescence spectroscopy. Furthermore, more sophisticated applications are also conceivable in, e.g., nanoparticle tracking analysis or ultrafast nonlinear frequency conversion.

Essentials of resonance-enhanced soliton-based supercontinuum generation

Supercontinuum generation is a key process for nonlinear tailored light generation and strongly depends on the dispersion of the underlying waveguide. Here we reveal the nonlinear dynamics of soliton-based supercontinuum generation in case the waveguide includes a strongly dispersive resonance. Assuming a gas-filled hollow core fiber that includes a Lorentzian-type dispersion term, effects such as multi-color dispersive wave emission and cascaded four-wave mixing have been identified to be the origin of the observed spectral broadening, greatly exceeding the bandwidths of corresponding non-resonant fibers. Moreover, we obtain large spectral bandwidth at low soliton numbers, yielding broadband spectra within the coherence limit. Due to the mentioned advantages, we believe the concept of resonance-enhanced supercontinuum generation to be highly relevant for future nonlinear light sources.