Analytic model for the complex effective index dispersion of metamaterial-cladding large-area hollow core fibers

Optical meta-waveguides for integrated photonics and beyond

The growing maturity of nanofabrication has ushered massive sophisticated optical structures available on a photonic chip. The integration of subwavelength-structured metasurfaces and metamaterials on the canonical building block of optical waveguides is gradually reshaping the landscape of photonic integrated circuits, giving rise to numerous meta-waveguides with unprecedented strength in controlling guided electromagnetic waves. Here, we review recent advances in meta-structured waveguides that synergize various functional subwavelength photonic architectures with diverse waveguide platforms, such as dielectric or plasmonic waveguides and optical fibers. Foundational results and representative applications are comprehensively summarized. Brief physical models with explicit design tutorials, either physical intuition-based design methods or computer algorithms-based inverse designs, are cataloged as well. We highlight how meta-optics can infuse new degrees of freedom to waveguide-based devices and systems, by enhancing light-matter interaction strength to drastically boost device performance, or offering a versatile designer media for manipulating light in nanoscale to enable novel functionalities. We further discuss current challenges and outline emerging opportunities of this vibrant field for various applications in photonic integrated circuits, biomedical sensing, artificial intelligence and beyond.

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 5 mm, 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.

Combining Hollow Core Photonic Crystal Fibers with Multimode, Solid Core Fiber Couplers through Arc Fusion Splicing for the Miniaturization of Nonlinear Spectroscopy Sensing Devices

The presence of fiber optic devices, such as couplers or wavelength division multiplexers, based on hollow-core fibers (HCFs) is still rather uncommon, while such devices can be imagined to greatly increase the potential of HCFs for different applications, such as sensing, nonlinear optics, etc. In this paper, we present a combination of a standard, multimode fiber (MMF) optic coupler with a hollow core photonic bandgap fiber through arc fusion splicing and its application for the purpose of multiphoton spectroscopy. The presented splicing method is of high affordability due to the low cost of arc fusion splicers, and the measured splicing loss (SL) of the HCF-MMF splice is as low as (0.32 ± 0.1) dB, while the splice itself is durable enough to withstand a bending radius (rbend) of 1.8 cm. This resulted in a hybrid between the hollow core photonic bandgap fiber (HCPBF) and MMF coupler, delivering 20 mW of average power and 250-fs short laser pulses to the sample, which was good enough to test the proposed sensor setup in a simple, proof-of-concept multiphoton fluorescence excitation-detection experiment, allowing the successful measurement of the fluorescence emission spectrum of 10−5 M fluorescein solution. In our opinion, the presented results indicate the possibility of creating multi-purpose HCF setups, which would excel in various types of sensing applications.

UV Absorption Spectroscopy in Water-Filled Antiresonant Hollow Core Fibers for Pharmaceutical Detection

Due to a worldwide increased use of pharmaceuticals and, in particular, antibiotics, a growing number of these substance residues now contaminate natural water resources and drinking supplies. This triggers a considerable demand for low-cost, high-sensitivity methods for monitoring water quality. Since many biological substances exhibit strong and characteristic absorption features at wavelengths shorter than 300 nm, UV spectroscopy presents a suitable approach for the quantitative identification of such water-contaminating species. However, current UV spectroscopic devices often show limited light-matter interaction lengths, demand sophisticated and bulky experimental infrastructure which is not compatible with microfluidics, and leave large fractions of the sample analyte unused. Here, we introduce the concept of UV spectroscopy in liquid-filled anti-resonant hollow core fibers, with large core diameters and lengths of approximately 1 m, as a means to overcome such limitations. This extended light-matter interaction length principally improves the concentration detection limit by two orders of magnitude while using almost the entire sample volume-that is three orders of magnitude smaller compared to cuvette based approaches. By integrating the fibers into an optofluidic chip environment and operating within the lowest experimentally feasible transmission band, concentrations of the application-relevant pharmaceutical substances, sulfamethoxazole (SMX) and sodium salicylate (SS), were detectable down to 0.1 µM (26 ppb) and 0.4 µM (64 ppb), respectively, with the potential to reach significantly lower detection limits for further device integration.

Resonance-enhanced multi-octave supercontinuum generation in antiresonant hollow-core fibers

Ultrafast supercontinuum generation in gas-filled waveguides is an enabling technology for many intriguing applications ranging from attosecond metrology towards biophotonics, with the amount of spectral broadening crucially depending on the pulse dispersion of the propagating mode. In this study, we show that structural resonances in a gas-filled antiresonant hollow core optical fiber provide an additional degree of freedom in dispersion engineering, which enables the generation of more than three octaves of broadband light that ranges from deep UV wavelengths to near infrared. Our observation relies on the introduction of a geometric-induced resonance in the spectral vicinity of the ultrafast pump laser, outperforming gas dispersion and yielding a unique dispersion profile independent of core size, which is highly relevant for scaling input powers. Using a krypton-filled fiber, we observe spectral broadening from 200 nm to 1.7 μm at an output energy of ∼ 23 μJ within a single optical mode across the entire spectral bandwidth. Simulations show that the frequency generation results from an accelerated fission process of soliton-like waveforms in a non-adiabatic dispersion regime associated with the emission of multiple phase-matched Cherenkov radiations on both sides of the resonance. This effect, along with the dispersion tuning and scaling capabilities of the fiber geometry, enables coherent ultra-broadband and high-energy sources, which range from the UV to the mid-infrared spectral range.

Analytic model for the complex effective index of the leaky modes of tube-type anti-resonant hollow core fibers

Due to their promising applications, hollow-core fibers, in particular, their anti-resonant versions, have recently attracted the attention of the photonics community. Here, we introduce a model that approximates, using the reflection of a wave on a single planar film, modal guidance in tube-type anti-resonant waveguides whose core diameters are large compared to the wavelength. The model yields analytic expressions for the real and imaginary parts of the complex effective index of the leaky modes supported, and is valid in all practically relevant situations, excellently matching all the important dispersion and loss parameters. Essential principles such as the fourth power dependence of the modal loss on the core radius at all wavelengths and the geometry-independent transition refractive index, below which modal discrimination favors the fundamental mode are discussed. As application examples, we use our model for understanding higher-order mode suppression in revolver-type fibers and for uncovering the tuning capabilities associated with nonlinear pulse propagation.

Guiding light in a water core all-solid cladding photonic band gap fiber - an innovative platform for fiber-based optofluidics

We present a single-channel photonic band gap fiber design allowing for guiding light inside a water core, which is surrounded by solid microstructured cladding, consisting of an array of high refractive index strands in silica. We address all relevant properties and show that the microstructure substantially reduces loss. We also introduce a ray reflection model, matching numerical modelling and allowing for time-effective large-scale parameter sweeps. Our single channel fiber concept is particularly valuable for applications demanding fast and reliable injection of liquids into the core, with potential impact in fields such as optofluidics, spectroscopy or bioanalytics.