Low Index Contrast Research Articles

3D positional metrology of a virus-like nanoparticle with topologically structured light

Locating and identifying viruses in in vitro optical measurements is desirable for disease control; however, the sub-diffraction-limit dimensions and low refractive index contrast of viruses make this challenging. Here, we introduce a 3D positional nanometrology for a virus-like 100 nm polystyrene sphere that can reveal the position of the nanoparticle with deeply sub-wavelength accuracy using topologically structured light illumination and machine learning. Scattering of topologically structured light is highly sensitive to the position of a nano-object. Exploiting this, our experiments demonstrate deeply sub-wavelength (λ) precision reaching 5 nm (λ/91). Our findings indicate a route toward label-free in vitro nanometrology of viruses and similar nano-objects with precision far beyond the Abbe–Rayleigh diffraction limit.

Solution-Processed One-Dimensional Photonic Crystals Based on Hollow Silica Exhibiting High Refractive Index Contrast.

Poor interfacial quality and low refractive index contrast (Δn) are critical challenges for the development of high-performance one-dimensional photonic crystals (1DPhCs) via solution methods that impede their optical efficiency. Herein, we introduce an innovative approach by hybridizing hollow SiO2 with poly(vinyl alcohol), referred to as PHS, followed by alternate assembly with TiO2 via spin-coating, achieving a 1DPhC with Δn = 0.76 at the wavelength of 550 nm. This method circumvents the need for high-temperature treatment and complex curing conditions, resulting in a 1DPhC with superior interfacial and optical characteristics. By adjusting the thickness of the PHS layers, we can finely tune the reflectance spectrum, attaining over 99% reflectance at the photonic band gap. Furthermore, 1DPhC demonstrates excellent adhesion to polycarbonate substrates and retains its optimal optical performance even after rigorous environmental testing, including hygrothermal cycles, exposure to hot water, friction, and solvent sonication. This research paves the way for the facile fabrication of high-performance 1DPhCs under mild conditions, offering new perspectives for photonic material processing.

Bright and Switchable Whiteness in Macro‐Crosslinked Hydrogels

AbstractBright white color is often achieved in nature by the combination of polydisperse scattering structures and high refractive index contrast between the scatterer and the surrounding medium. Similarly, synthetic systems have commonly utilized inorganic materials as the scattering centers to achieve white color, which, however, lacks the ability to switch the optical properties. While hydrogels capable of scattering light are utilized in applications such as smart windows, their reflection properties have remained limited due to the low refractive index contrast between the polymer and water. As a result, thick layers in the millimeter range are often required to achieve reasonable whiteness. Here a hydrogel consisting of a temperature‐responsive poly(N‐isopropylacrylamide) (PNIPAm) and chemically modified agarose used as a chemical macro‐crosslinker is presented. The hydrogel exhibits high whiteness at temperatures above the phase transition (≈31 °C). The reflectance at 800 nm is four times as high as for standard PNIPAm, and a change in transmittance can be induced by laser pulses as short as 30 ms. The macro‐crosslinked structure of this hydrogel provides superior reflectance at a lower thickness compared to reported hydrogel systems, enabling a variety of potential applications including smart windows, responsive displays, optical switches, and camouflage.

Optimization and comprehensive comparison of thermo-optic phase shifter with folded waveguide on SiN and SOI platforms

The thermo-optic phase shifter (TOPS) using metal heaters is widely used in various integrated photonics devices for its advantages of simple fabrication, low loss, and low cost. While folded waveguides are commonly employed to enhance the performance of these phase shifters, a comprehensive understanding of how varying the number of folds and the dimensions of heaters impact the device's performance is still lacking. In this paper, we conducted a comprehensive comparison and optimization into the impact of the number of folded waveguides and the width of heater on the performance of folded waveguide thermo-optic phase shifter (FW-TOPS) on silicon nitride (SiN) and silicon-on-insulator (SOI) platforms using the finite element method (FEM). The research has revealed that both the number of waveguides and the width of the heater exert a substantial influence on the performance of FW-TOPS, particularly on platforms with low refractive index contrast. In cases where the number of waveguides is the same, and the width of the heater electrode is identical, the comparison indicates that on the SiN platform, the FW-TOPS in waveguide Combination 2 (waveguide 1 is a single-mode waveguide, while waveguide 2 is a multi-mode waveguide) exhibits superior overall performance, compared to waveguide Combination 1 (both waveguide 1 and waveguide 2 are single-mode waveguides). However, on the SOI platform, FW-TOPS with waveguide Combination 1 demonstrates better overall performance compared to waveguide Combination 2. Furthermore, the simulation results indicate that although air trenches can significantly reduce the power consumption of the FW-TOPS, they also lead to a substantial decrease in the response speed. Therefore, on the 300 nm-thick SiN and 220 nm-thick SOI platforms, air trenches do not effectively enhance the overall performance of the FW-TOPS.

Nanowire integration in silica based integrated optical circuits: Limitations and challenges towards quantum computing

Optical integrated circuits suggest a very promising platform for the development of robust and efficient quantum computers. A critical issue to their development and scalability is the integration of multiple single photon sources in the circuits. One major category of single photon sources is based on quantum dots that are embedded in semiconductor optical nanowires (NWQD) that allow their accurate handing and deterministic integration. Successful integrations of such nanowires have been reported in high index platforms like silicon or silicon nitride, with adequate coupling efficiency due the modal characteristics compatibility. On the other hand, Silica-on-Silicon is a major integration platform, which combined with new fabrication approaches like direct laser writing, for the definition of optical structures with refractive index modification, can provide the fabrication of highly optimized and tailor-made circuits by rapid prototyping. Considering for the first time the integration scenario of NWQD in laser written silica-based waveguides it is shown that the low (∼10-3) achievable refractive index contrast imposes strict limitations on the compatibility of such waveguides with NWQD resulting in general in low coupling efficiency. By considering several design and fabrication issues, suitable integration approaches with adequate efficiency are demonstrated, while also the limitations and challenges are revealed thus triggering new research directions.

Compact and Efficient Thin‐Film Lithium Niobate Modulators

Thin‐film lithium niobate (TFLN) modulators have garnered significant attention in the field of integrated photonics due to their ability to manipulate light. Numerous TFLN modulators have been demonstrated over the past few years, with speed records consistently being broken. However, due to the low refractive index contrast and anisotropic properties of TFLN, modulators based on this material feature larger device sizes and lower efficiency. A more compact and efficient modulator is important, as it is required for future dense integration and low power consumption applications. There have been some reports on how to reduce device size, and research is ongoing. A comprehensive summary can help individuals understand the current state of research and existing problems. In this review, we provide an overview of recent advancements in TFLN modulator technology, focusing on compactness and efficiency. Herein, various types of modulators, such as the Mach–Zehnder interferometer, Michelson interferometer, resonator cavity, and Z‐cut type modulators, are discussed. Moreover, potential improvement strategies and applications for compact and efficient TFLN modulators in advanced photonic systems are explored. In conclusion, in this review, the recent achievements in compact and efficient TFLN modulators are summarized and their significance in various optoelectronic applications is emphasized.

Light wave propagation and diffraction inhibition in three-dimensional photonic quasicrystal lattices

We have investigated the linear propagation of light wave in three-dimensional photonic quasicrystal lattices by numerical simulation. The diffraction characteristics of the light waves in the longitudinal non-modulated, in-phase modulated and out-of-phase modulated quasicrystal lattices have been compared and discussed. We have demonstrated the diffraction inhibition of light wave in three-dimensional photonic quasicrystals. Linear transverse light localization has been realized in the longitudinal out-of-phase modulated three-dimensional quasicrystal lattices by adjusting the structural parameters appropriately. Furthermore, the results also show that the longitudinal out-of-phase modulated photonic quasicrystals are easier to produce diffraction inhibition than the longitudinal out-of-phase modulated periodic lattices under low transverse refractive index contrast. Distortionless dot image transmission has numerically demonstrated in out-of-phase modulated photonic quasicrystals array. It provides a new way for distortion-free image transmission, which has promising applications in optical information transmission and all-optical signal processing.

Morphogenetic Design of Self-Organized Correlated Disordered Electromagnetic Media

The last decades witnessed the emergence of the field of correlated disordered media, a great challenge offering a large panel of new perspectives for applications in theoretical modeling and material fabrication. The efficient design of structures with a controlled level of spatial correlation is a central challenge in this field, in a context where existing techniques generally rely on gradient descent on nonconvex functions and on the use of stochastic methods to explore vast design spaces more efficiently. In this work, we propose a new generative technique based on Alan Turing’s morphogenesis theory for designing correlated disordered materials. Inspired by the structuring of living organisms, this technique relies on the definition of simple local interactions guiding the self-organization of a generated medium. The decentralization of design constraints and the elimination of cost function minimization make this approach natively scalable to the design of large domains with controlled levels of disorder. As a validation, the morphogenetic generation of stealthy hyperuniform disordered structures is exploited to reproduce an experiment of isotropic electromagnetic bandgap synthesis in the microwave range using a low refractive index contrast.

Ultracompact LiNbO3 ridge waveguide multi-mode interferometer based on low-effective-index mode interference

Waveguides fabricated in crystals, created by utilizing techniques such as ion implantation, femtosecond laser inscription, and proton exchange, have low refractive index contrast with the crystal substrate, which limits their potential development to create compact passive devices, such as waveguide interferometers. In this work, a new waveguide multi-mode interferometer fabrication strategy based on low-effective-index mode interference is presented. Numerical tools have been used for the analysis of this new guided-wave approach used for the device design. The research has demonstrated that a series of high-order modes with an effective refractive index much lower than the substrate can exist in trench-cladded ridge crystal waveguides that have a tiny index contrast in the vertical direction. Simple trench-cladded tapered waveguide configurations to excite such modes and to realize compact waveguide multi-mode interferometers with lengths of several tens and up to hundreds of micrometers are presented. The waveguide multi-mode interferometer design is compact in size, easy to modulate, and with low insertion loss. Furthermore, refractive index sensing is realized, with a sensitivity of ∼490 nm/RIU for aqueous solution samples. The novel multi-mode interference phenomenon present here offers new possibilities and significant opportunities for waveguide modulation and, thus, the development of compact waveguide refractometers.

Polymeric Photonic Crystal Fibers for Textile Tracing and Sorting

AbstractCircular supply chains require more accurate product labeling and traceability. In the apparel industry, product life cycle management is hampered in part by inaccurate, poorly readable, and detachable standard care labels. Instead, this article seeks to enable a labeling system capable of being integrated into the fabric itself, intrinsically recyclable, low‐cost, encodes information, and allows rapid readout after years of normal use. In this work, all‐polymer photonic crystals are designed and then fabricated by thermal drawing with >100 layers having sub‐micrometer individual thickness and low refractive index contrast (Δn = 0.1). The fibers exhibit reflectance features in the 1–5.5 µm wavelength range, characterized using insitu Fourier transform infrared spectroscopy. Drawn photonic fibers are then woven into fabrics, characterized by near‐infrared spectroscopy and short‐wave infrared imaging, techniques commonly used in industrial facilities for sorting materials. The fibers’ optical design also enables the use of overtone peaks to avoid overlap with parasitic molecular absorption, substantially improving the signal‐to‐noise ratio (and therefore ease and speed) of readout. The ability to produce kilometers of fiber that are compatible with existing textile manufacturing processes, coupled with low input material cost, make these a potential market‐viable improvement over the standard care label.

Towards complete photonic band gap in a high refractive index nanoparticle-doped blue phase liquid crystal.

Three-dimensional (3D) photonic crystals with complete photonic band gap (PBG) are fascinating due to the possibility of controlling light in all directions. Realizing such photonic crystals is nontrivial due to symmetry requirements and associated fabrication challenges. Liquid crystalline cubic blue phases (BPs) are soft 3D photonic crystals with an incomplete PBG due to the low refractive index contrast (<0.1). The present work attempts to drive a cubic BP towards a complete PBG via a simple approach of high refractive index nanoparticle-doping. The photonic band diagrams and reflection spectra of the nanoparticle-doped BP simulated using the finite element method show an increased PBG width, a parameter that quantifies the complete PBG. The reflection spectra obtained from UV-Vis-NIR spectroscopy show an increase (by a factor of >2) in PBG width for the nanoparticle-doped BP, validating the simulations. The findings are explained based on increased refractive index contrast (∼1.4) due to the nanoparticles getting trapped in the cores of disclination lines that make up the BP lattice. The simulations also indicate effective confinement of electric field eigenmodes in the nanoparticle-doped BP leading to high attenuation of the incident light. Further, the iso-frequency contours extracted from the band diagrams exhibit self-collimation and negative refraction of light.

Enhancing light emission near low-refractive-index contrast nanostructures via mirror-symmetry breaking

A platform of recent interest and large application potential is one in which the light emitters are directly integrated with an optical metasurface. Plasmonic metals and high-refractive-index, dielectric Mie metasurfaces have been explored in this connection but have their challenges. We propose low-refractive-index, contrast nanostructured thin films for studying metasurface-emitter systems, specifically focusing on two configurations of practical importance: LED white light generation using color converting polymers and fluorescence-based sensing in aqueous media. To achieve light emission enhancement in a low-index contrast, low optical absorption setting, we exploit the excitation of quasi-bound states in the continuum modes in a mirror-symmetry broken grating. Our numerical study predicts widely tuneable sharp-linewidth emission enhancements, near-zero quenching, and large and controllable active volume in the grating vicinity, which are significant improvements in comparison with both plasmonic and high-index contrast, all-dielectric platforms. When compared with simple gratings, mirror-symmetry broken gratings give four times larger radiative enhancement. Our results are of interest in furthering experimental activity and realizing applications such as light converter for efficient white LEDs and smart detection electronics-integrated substrates for sensing.

Inverse design of cavities for Bloch Surface Waves interfaced to integrated waveguides

The design of functional elements for Bloch Surface Waves (BSW) is challenging because of the relatively low index contrast offered by the respective platforms. Here, we design a supporting photonic nanostructure that extracts as much light as possible from a quantum emitter into a waveguide in an integrated BSW architecture. The inverse problem is solved using topology optimization. Emphasis is put on discussing the algorithm’s emerging strategies for the design to enhance the Purcell factor, the coupling efficiency, or both for different index contrasts. Fully three-dimensional simulations of an explicit device show the benefits of our devices and pave the way for integrating such unconventional photonic elements into future fully-integrated BSW devices.

Boosting second-harmonic generation in the LiNbO3 metasurface using high- Q guided resonances and bound states in the continuum

To date, second-harmonic generation (SHG) at nanoscale has been concentrated on employing high-refractive-index nanostructures, owing to the strong field confinement at deep subwavelength scales based on optically resonant effects. However, low-index nanostructures generally exhibit weaker resonant effects and lower field confinement. To address this issue, by harnessing the large nonlinearity of LiNbO$\mathrm{_3}$, we propose a novel approach to employ guided resonances and bound states in the continuum (BICs) with a LiNbO$\mathrm{_3}$ metasurface consisting of a LiNbO$\mathrm{_3}$ disk array sitting on a LiNbO3 thin film. Such a system can transform the guided modes supported by LiNbO$\mathrm{_3}$ thin film into high-quality guided resonances which can be excited directly under plane-wave illumination. Importantly, we further demonstrate strong field confinement inside LiNbO3 thin film with tailorable Q-factor by realising a Friedrich-Wintgen BIC. Such a unique mode engineering enables a record-high SHG efficiency of 5\% under a pump intensity as low as 0.4 $\mathrm{MW/cm^{2}}$. Moreover, we reveal the influence of nonlinear resonances and cross-coupling on the SHG by showing the anomalous SHG and efficiency tuning with the rotation of the crystal axis. Our work offers a new route to constructing enhanced SHG based on high-Q guided resonances and BICs, including low-index and high-index nonlinear materials.

A Novel Design of Optical Switch Based on Guided Mode Resonances in Dielectric Photonic Crystal Structures

In this work, a novel idea of optical switch design based on guided mode resonance in the photonic crystal structure is numerically investigated. The designed switching device work on the principle of optical amplification and wavelength shift of data signal with the help of a control signal. The data signal can be coupled into the waveguide using guided-mode resonance, whereas, a control signal is index-coupled into the waveguide to influence the data signal. The optical switching action is optimized by introducing a photonic crystal cavity and varying the number of photonic crystal elements, where the resonant wavelength, reflection peaks, linewidth, and quality factor of the data signal can be adjusted. The device is based on low refractive index contrast dielectric materials compatible with fiber optic communication and can operate in a near-infrared range of around 1.55 μm. The numerical simulations are carried out in an open source finite-difference time-domain-based software. An optical switching action is achieved with 7% amplification in the data signal at a central wavelength of 1.55 µm with a maximum shift of the wavelength of 0.001 µm. The proposed device can be easily implemented in cascade designs of programmable photonic and optical switching circuits.

Weak Localization Enhanced Ultrathin Scattering Media

AbstractThe brilliant white appearance of ultrathin scattering media with low refractive index contrast and the underlying radiative transport phenomena fascinate scientists for more than a decade. Examples of such systems are the scales of beetles of the genus Cyphochilus, photonic network structures or disordered Bragg stacks (DBS). While previous studies relate the highly efficient scattering in the scales to the anisotropy of the intra‐scale network and diffusive light transport, the coherent radiation propagation dynamics remains unaccounted for. Here, the identification of different coherent light transport regimes using time and spatially resolved coherent light scattering spectroscopy is shown. At least 20% of the collected scattered light originates from weakly localized random photonic modes, in contrast to solely diffusive light transport assumed to date. The identification of this significant role of weak localization in ultrathin brilliant scattering media establishes a new design paradigm for efficient scattering optical materials.

Transparent Polymer Opal Thin Films with Intense UV Structural Color.

We report on shear-ordered polymer photonic crystals demonstrating intense structural color with a photonic bandgap at 270 nm. Our work examines this UV structural color, originating from a low refractive index contrast polymer composite system as a function of the viewing angle. We report extensive characterization of the angle-dependent nature of this color in the form of ‘scattering cones’, which showed strong reflectivity in the 275–315 nm range. The viewing range of the scattering was fully quantified for a number of planes and angles, and we additionally discuss the unique spectral anisotropy observed in these structures. Such films could serve as low-cost UV reflection coatings with applications in photovoltaics due to the fact of their non-photobleaching and robust mechanical behavior in addition to their favorable optical properties.

Phase Matching via Plasmonic Modal Dispersion for Third Harmonic Generation.

The overall effectiveness of nonlinear optical processes along extended nonlinear media highly depends on the fulfillment of the phase‐matching condition for pump and generated fields. This is traditionally accomplished by exploiting the birefringence of nonlinear crystals requiring long interaction lengths (cm‐scale). For nonbirefringent media and integrated photonic devices, modal phase matching can compensate the index mismatch. Here, the various interacting waves propagate in transverse modes with appropriate phase velocities, but they suffer from a low refractive index contrast and cm‐scale interaction lengths. This work harnesses modal phase matching for third‐harmonic generation (THG) in plasmonic waveguides using an organic polymer (poly[3‐hexylthiophene‐2,5‐diyl]) as the nonlinear medium. One demonstrates experimentally an effective interaction area as small as ≈ 0.11 µm2 and the phase‐matched modal dispersion results in THG efficiency as high as ≈ 10–3 W‐2 within an effective length scale of ≈ 4.3 µm. THG also shows a strong correlation with the polarization of the incident laser beam, corresponding to the excitation of the antisymmetric plasmonic modes, corroborating that plasmonic modal phase matching is achieved. This large reduction in device area of orders of magnitude is interesting for various applications where space is critical (e.g., device integration or on‐chip applications).

Combining Machine Learning and Backgrounded Membrane Imaging: A Case Study in Comparing and Classifying Different Types of Biopharmaceutically Relevant Particles

This study investigates how backgrounded membrane imaging (BMI) can be used in combination with convolutional neural networks (CNNs) in order to quantitatively and qualitatively study subvisible particles in both protein biopharmaceuticals and samples containing synthetic model particles. BMI requires low sample volumes and avoids many technical complications associated with imaging particles in solution, e.g., air bubble interference, low refractive index contrast between solution and particles of interest, etc. Hence, BMI is an attractive technique for characterizing particles at various stages of drug product development. However, to date, the morphological information encoded in brightfield BMI images has scarcely been utilized. Here we show that CNN based methods can be useful in extracting morphological information from (label-free) brightfield BMI particle images. Images of particles from biopharmaceutical products and from laboratory prepared samples were analyzed with two types of CNN based approaches: traditional supervised classifiers and a recently proposed fingerprinting analysis method. We demonstrate that the CNN based methods are able to efficiently leverage BMI data to distinguish between particles comprised of different proteins, various fatty acids (representing polysorbate degradation related particles), and protein surrogates (NIST ETFE reference material) only based on BMI images. The utility of using the fingerprinting method for comparing morphological differences and similarities of particles formed in distinct drug products and/or laboratory prepared samples is further demonstrated and discussed through three case studies.

Enhanced Light Absorption in All‐Polymer Biomimetic Photonic Structures by Near‐Zero‐Index Organic Matter

AbstractNatural photosynthetic photonic nanostructures can show sophisticated light–matter interactions including enhanced light absorption by slow light even for highly pigmented systems. Beyond fundamental biology aspects, these natural nanostructures are very attractive as blueprints for advanced photonic devices. But the soft‐matter biomimetic implementations of such nanostructures is challenging due to the low refractive index contrast of most organic photonic structures. Excitonic organic materials with near‐zero index (NZI) optical properties allow overcoming these bottlenecks. Here, it is demonstrated that the combination of NZI thin films with photonic multilayers like the ones found in nature enables broadband tunable strong reflectance as well as slow light absorption enhancement and tailored photoluminescence properties in the full VIS spectrum. Moreover, it is shown that this complex optical response is tunable, paving the way toward the development of active devices based on all‐polymer and near‐zero index materials photonic structures.