Light Cage Research Articles

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.

Inverse design of optical cages by using a fully connected neural network and time-reversed dipole radiation

In the conventional scheme, the focal field is designed by selecting appropriate dipole parameters based on theory of the time-revered dipole radiation. The time-revered method relies on empirical judgment and iterative parameter testing to create only a few special optical cages, involving vast time-consuming global parameter scanning and prescreening. In this work, a combination of a time-reversed dipole radiation model and a fully connected neural networks (FCNN) is introduced as the inverse design of optical cages. Flexibility and little time consumption in the design of the light cage are the two pivotal advantages. The dual light needle focusing fields with varying lengths and the synthesis of light cages featuring bubble focusing fields with distinct characteristics are attained, which are prescribed focal structures with freedom. With the help of the FCNN, the required dipole parameters for generating the expected focal fields are obtained quickly. This approach opens up possibilities for the utilization of neural networks in the design and modulation of more structured focusing fields. The achieved optical cage holds great potential for diverse applications, such as multiparticle trapping, manipulation, assembly, and enhanced nanoscale optical operations.

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.

On-chip fluorescence detection using photonic bandgap guiding optofluidic hollow-core light cage

The on-chip detection of fluorescent light is essential for many bioanalytical and life-science related applications. Here, the optofluidic light cage consisting of a sparse array of micrometer encircling a hollow core represents an innovative concept, particularly for on-chip waveguide-based spectroscopy. In the present work, we demonstrate the potential of the optofluidic light cage concept in the context of integrated on-chip fluorescence spectroscopy. Specifically, we show that fluorescent light from a dye-doped aqueous solution generated in the core of a nanoprinted dual-ring light cage can be efficiently captured and guided to the waveguide ports. Notably, the fluorescence collection occurs predominantly in the fundamental mode, a property that distinguishes it from evanescent field-based waveguide detection schemes that favor collection in higher-order modes. Through exploiting the flexibility of waveguide design and 3D nanoprinting, both excitation and emission have been localized in the high transmission domains of the fundamental core mode. Fast diffusion, detection limits comparable to bulk measurements, and the potential of this approach in terms of device integration were demonstrated. Together with previous results on absorption spectroscopy, the achievements presented here suggest that the optofluidic light cage concept defines a novel photonic platform for integrated on-chip spectroscopic devices and real-time sensors compatible with both the fiber circuitry and microfluidics. Applications in areas such as bioanalytics and environmental sciences are conceivable, while more sophisticated applications such as nanoparticle tracking analysis and integrated Raman spectroscopy could be envisioned.

Fiber-integrated hollow-core light cage for gas spectroscopy

Interfacing integrated on-chip waveguides with spectroscopic approaches represents one research direction within current photonics aiming at reducing geometric footprints and increasing device densities. Particularly relevant is to connect chip-integrated waveguides with established fiber-based circuitry, opening up the possibility for a new class of devices within the field of integrated photonics. Here, one attractive waveguide is the on-chip light cage, confining and guiding light in a low-index core through the anti-resonance effect. This waveguide, implemented via 3D nanoprinting and reaching nearly 100% overlap of mode and material of interest, uniquely provides side-wise access to the core region through the open spaces between the cage strands, drastically reducing gas diffusion times. Here, we extend the capabilities of the light cage concept by interfacing light cages and optical fibers, reaching a fully fiber-integrated on-chip waveguide arrangement with its spectroscopic capabilities demonstrated here on the example of tunable diode laser absorption spectroscopy of ammonia. Controlling and optimizing the fiber circuitry integration have been achieved via automatic alignment in etched v-grooves on silicon chips. This successful device integration via 3D nanoprinting highlights the fiber-interfaced light cage to be an attractive waveguide platform for a multitude of spectroscopy-related fields, including bio-analytics, lab-on-chip photonic sensing, chemistry, and quantum metrology.

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.

Ultrahigh-aspect-ratio light cages: fabrication limits and tolerances of free-standing 3D nanoprinted waveguides

Three-dimensional laser nanoprinting represents a unique approach for implementing on-chip hollow-core waveguides. Here we discuss the fabrication characteristics of the light cage geometry arising from the used two-photon polymerization lithography. We reveal the current limits of achievable waveguide length (3 cm), single strand aspect ratio (8200) and modal attenuation. Very high reproducibility for light cages on the same chip is found, while different conditions in fabrication cycles impose chip-to-chip variations. We also highlight the relevance of including reinforcement rings to prevent structural collapse. The results presented uncover key issues that result from nanoprinting light cages and can be transferred to other nanoprinted waveguides.

What optical fiber modes reveal: group velocity and effective index for external perturbations

Precise control of fiber modes and their dispersion is essential, particularly for fields such as nonlinear frequency conversion or biosensing, both of which often require extensive and time-consuming simulations for design optimization. Here, we develop a first-order perturbation theory for predicting the effective index of bound and leaky fiber modes that is applicable for arbitrary global perturbations as long as the perturbations in the external surrounding are constantly homogeneous and isotropic deviations from the unperturbed fiber. This includes changes not only in permittivity and permeability, but also in wavelength. Thus, we are able to calculate the group velocity solely from the field distributions of the fiber modes at a single wavelength, which therefore allows for large-scale parameter sweeps for accurately managing dispersion. We demonstrate the capabilities of our theory for various trial systems such as step index fibers, photonic crystal fibers, and light cages.

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.

Tubular optical microcavities based on rolled-up photonic crystals

The self-rolling of micro-structured membranes via the stress-engineering method opens new ways to create 3D photonic micro-objects with original designs and optical properties. This article validates this approach by producing 3D hollow micro-resonators based on rolled-up 2D photonic crystal membrane mirrors, capable of trapping light in 3D and in air. We fabricated the 3D tubular microresonators with 10 μm–20 μm diameters by rolling photonic crystal membranes using stress-engineering technique on the prestressed InGaP/InP bilayer. We also added a design feature to lift the microtubes vertically and facilitate optical measurements, but also to attach the structures to the substrate. The dispersion of the planar 2D photonic crystal membrane was optimized to exhibit high reflectivity (>95%) at normal incidence over a large spectral band (100 nm) in the near-infrared domain (1.5 μm–1.6 μm). The cylindrical cavity model and numerical simulations predicted the presence of quasi-pure radial cavity modes with a strong concentration of light over nearly 3% of the photonic microtubes’ cross section. We demonstrated experimentally the presence of those modes through scanning near-field optical microscopy measurements. Using a bowtie nanoantenna, we selectively detected and mapped transverse electric modes in the hollow core of photonic microtubes. Spatially resolved cartographies allowed for the identification of the modes in good agreement with theoretical predictions. This work brings theoretical and experimental proof of concept of light cages based on rolled-up photonic crystal membranes. It also opens the path to the realization of original photonic microstructures as combinations of a specific photonic crystal design and a targeted 3D form.

Fine-tuning of the optical properties of hollow-core light cages using dielectric nanofilms

Here, we show that the optical properties of direct-laser-written on-chip hollow-core waveguides—so-called light cages—can be controlled to a very high degree by dielectric nanofilms. Using low-temperature atomic layer deposition (ALD), alumina nanofilms are concentrically deposited on the high-aspect strands that surround the central air core and confine the light via the anti-resonant effect. In accordance with modal cutoff simulations without any free parameters, a linear spectral shift of the resonances with increasing film thickness is experimentally observed. The phenomenon is explained by a shift in the dispersions of cladding supermodes. As neither cage geometry nor polymer is affected by the film deposition, our results suggest ALD to be an essential tool for fine-tuning the properties of hollow-core light cages and to protect them from aggressive substances, being relevant for, e.g., bioanalytics or quantum technology.

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

Efficient waveguiding inside low refractive index media is of key importance for a great variety of applications that demand strong light-matter interaction on small geometric footprints. Here, we demonstrate efficient light guidance in single-defect dual-ring light cages over millimeter distances that are integrated on silicon chips via direct laser writing. The cages consist of two rings of high aspect-ratio polymer strands (length 5mm, aspect ratio >1000) hexagonally arranged around a hollow core. Clear-core mode formation via the photonic band gap effect is observed, with the experiments showing pronounced transmission bands with fringe and polarization contrasts of >20 dB and >15 dB, respectively. Numerical simulations confirm our experiments and reveal the dual-ring arrangement to be the optimal geometry within the light cage concept. Particularly, the side-wise access to the core regions and the chip integration makes the light cage concept attractive for a great number of fields such as bioanalytics or quantum technology.

Hollow Core Light Cage: Trapping Light Behind Bars

Optical waveguides represent the key element of integrated planar photonic circuitry having revolutionized many fields of photonics ranging from telecommunications, medicine, environmental science, and light generation. However, the use of solid cores imposes limitations on applications demanding strong light–matter interaction within low permittivity media such as gases or liquids, which has triggered substantial interest toward hollow core waveguides. Here, we introduce the concept of an on-chip hollow core light cage that consists of free-standing arrays of cylindrical dielectric strands around a central hollow core implemented using 3D nanoprinting. The cage operates by an antiresonant guidance mechanism and exhibits extraordinary properties such as (1) diffractionless propagation in “quasi-air” over more than a centimeter distance within the ultraviolet, visible and near-infrared spectral domains, (2) unique side-wise direct access to the hollow core via open spaces between the strands speeding up ga...

An analysis of action of intercostal muscles in human upper rib cage

The actions of the intercostal and paraspinal muscles in stabilizing the human upper rib cage have been analyzed using a geometrically realistic mathematical model of the first six ribs, vertebrae, and associated musculature. The model suggests roles of the deep layers of erector spinae in stabilizing the vertebral column so that it can support the loads placed upon it by the ribs under physiological load. If we assume that the tension exerted by an intercostal muscle is proportional to its local thickness, the model predicts that the observed distribution of intercostal thickness is close to that which minimizes the stresses in ribs when the model is subjected to peak physiological load. The observed shape of the ribs are optimal to withstand the calculated pattern of loading along their length. These calculations raise the hypothesis that the arrangement of intercostal musculature and rib geometry result in an optimally light rib cage, which is capable of withstanding the loads placed upon it. The analysis of the mechanics of the entire model indicates that the geometrical simplifications made in Hamberger's model are not valid when applied to the rib cage.