Refractive Index Contrast Research Articles

Extending the diatom's color palette: non-iridescent, disorder-mediated coloration in marine diatom-inspired nanomembranes.

The intricate, siliceous exoskeleton of many marine diatoms (single-celled phytoplankton) is decorated with an array of sub-micron, quasi-ordered pores that are known to provide protective and multiple life-sustaining functions. However, the optical functionality of any given diatom valve is limited because valve geometry, composition, and ordering are genetically programmed. Nonetheless, the near- and sub-wavelength features of diatom valves provide inspiration for novel photonic surfaces and devices. Herein, we explore the optical design space for optical transmission, reflection, and scattering in diatom-like structures by computationally deconstructing the diatom frustule, assigning and nondimensionalizing Fano-resonant behavior with configurations of increasing refractive index contrast (Δn), and gauging the effects of structural disorder on the resulting optical response. Translational pore disorder, especially in higher-index materials, was found to evolve Fano resonances from near-unity reflection and transmission to modally confined, angle-independent scattering, which is key to non-iridescent coloration in the visible wavelength range. High-index, frustule-like TiO2 nanomembranes were then designed to maximize backscattering intensity and fabricated using colloidal lithography. These synthetic diatom surfaces showed saturated, non-iridescent coloration across the visible spectrum. Overall, this diatom-inspired platform could be useful in designing tailored, functional, and nanostructured surfaces for applications in optics, heterogeneous catalysis, sensing, and optoelectronics.

Structured illumination phase and fluorescence microscopy for bioimaging.

This study presents a dual-modality microscopic imaging approach that combines quantitative phase microscopy and fluorescence microscopy based on structured illumination (SI) to provide structural and functional information for the same sample. As the first imaging modality, structured illumination digital holographic microscopy (SI-DHM) is implemented along the transmission beam path. SI-DHM acts as a label-free, noninvasive approach and provides high-contrast and quantitative phase images utilizing the refractive index contrast of the inner structures of samples against the background. As the second imaging modality, structured illumination (fluorescence) microscopy (SIM) is constructed along the reflection beam path. SIM utilizes fluorescent labeling and provides super-resolution images for specific functional structures of samples. We first experimentally demonstrated phase imaging of SI-DHM on rice leaves and fluorescence (SIM) imaging on mouse kidney sections. Then, we demonstrated dual-modality imaging of biological samples, using DHM to acquire the overall cell morphology and SIM to obtain specific functional structures. These results prove that the proposed technique is of great importance in biomedical studies, such as providing insight into cell physiology by visualizing and quantifying subcellular structures.

Spectroscopic Microtomography in the Short-Wave Infrared Wavelength Range.

Spectroscopic microtomography provides the ability to perform 4D (3D structural and 1D chemical) imaging of a thick microscopic specimen. Here, we demonstrate spectroscopic microtomography in the short-wave infrared (SWIR) wavelength using digital holographic tomography, which captures both the absorption coefficient and refractive index. A broadband laser in tandem with a tunable optical filter allows us to scan the wavelength from 1100 to 1650 nm. Using the developed system, we measure human hair and sea urchin embryo samples. The resolution estimated with gold nanoparticles is 1.51 μm (transverse) and 1.57 μm (axial) for the field of view of 307 × 246 μm2. The developed technique will enable accurate and efficient analyses of microscopic specimens that have a distinctive absorption or refractive index contrast in the SWIR range.

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.

Single-scan ultrafast laser inscription of waveguides in IG2 for type-I and type-II operation in the mid-infrared.

We demonstrate, for the first time to our knowledge, single-scan ultrafast laser inscription and performance of mid-infrared waveguiding in IG2 chalcogenide glass in the type-I and type-II configurations. The waveguiding properties at 4550 nm are studied as a function of pulse energy, repetition rate, and additionally separation between the two inscribed tracks for type-II waveguides. Propagation losses of ∼1.2 dB/cm in a type-II waveguide and ∼2.1 dB/cm in a type-I waveguide have been demonstrated. For the latter type, there is an inverse relation between the refractive index contrast and the deposited surface energy density. Notably, type-I and type-II waveguiding have been observed at 4550 nm within and between the tracks of two-track structures. In addition, although type-II waveguiding has been observed in the near infrared (1064 nm) and mid infrared (4550 nm) in two-track structures, type-I waveguiding within each track has only been observed in the mid infrared.

A simplified laser-to-chip edge coupling scheme using 3D SU-8 taper

In this work, we design and simulate a three-dimensional (3D) SU-8 tapered edge coupler for effectively guiding light from an edge-emitting semiconductor laser diode directly to a silicon waveguide to realize a hybrid on-chip silicon light source. A series of coupling efficiencies for the commonly used top silicon thicknesses of the silicon-on-insulator wafers from 220 to 3000 nm are obtained, showing that this polymer taper can largely improve the coupling efficiency when the silicon waveguide thickness is moderate, for instance, from 41% (with no taper) to 67% for the 700 nm- thick silicon waveguide under 1 μm gap offset and relax the lateral misalignment tolerance to above one micron by suppressing the butt refractive index contrast and the mode mismatch between the laser diode and the silicon waveguide. Wave transformation inside the tapered coupler and the input silicon waveguide has also been revealed by 3D finite-difference time-domain simulations. This 3D tapered coupler is cost effective and easily fabricated which can have wide practical applications in massive production of various silicon photonic chips based on the present complementary metal-oxide-semiconductor foundry.

The Interplay of Processing-Related Influences on the Formation of Volume Holographic Gratings in a Free-Surface Epoxy-Based Recording Material

Understanding the formation processes of holographic gratings in polymers as a function of material composition and processing is important for the development of new materials for holography and its associated applications. Among the processing-related factors that affect grating formation in volume holographic recording material, pre-exposure, prebaking and dark storage, as well as the associated variations in layer thickness and composition, are usually underestimated. This study highlights the influence and interaction of these factors and shows that they should not be neglected. This is of particular importance for samples with a free surface. Here, one such epoxy-based free-surface material is investigated. To determine the influence of prebaking on the holographic grating formation, as well as on the achieved refractive index contrast, angular resolved analysis of volume holographic phase gratings is applied through point-by-point scanning of the local material response. Grating characteristics are determined by comparison with simulations based on rigorous coupled wave theory. Thus, the optimal dose for prebaking can be determined, as well as the optimal exposure time, depending on the dose. The influence of dark storage on the material response is investigated over a period of 12 weeks and shows a strong dependence on the deposited energy density.

Van der Waals Materials for Applications in Nanophotonics

AbstractNumerous optical phenomena and applications have been enabled by nanophotonic structures. Their current fabrication from high refractive index dielectrics, such as silicon (Si) or gallium phosphide (GaP), pose restricting fabrication challenges while metals, relying on plasmons and thus exhibiting high ohmic losses, limit the achievable applications. An emerging class of layered, so‐called van der Waals (vdW), crystals is presented as a viable nanophotonics platform in this work. The dielectric response of 11 mechanically exfoliated thin‐film (20–200 nm) vdW crystals is extracted, revealing high refractive indices up to n = 5, pronounced birefringence up to Δn = 3, sharp absorption resonances, and a range of transparency windows from ultraviolet to near‐infrared. Nanoantennas are subsequently fabricated on silicon dioxide (SiO2) and gold, utilizing the compatibility of vdW thin films with a variety of substrates. Pronounced Mie resonances are observed due to the high refractive index contrast on SiO2, leading to a strong exciton‐photon coupling regime as well as largely unexplored high‐quality‐factor, hybrid Mie‐plasmon modes on gold. Additional vdW‐material‐specific degrees of freedom in fabrication are further demonstrated by realizing nanoantennas from stacked twisted crystalline thin‐films, enabling control of nonlinear optical properties, and post‐fabrication nanostructure transfer, important for nano‐optics with sensitive materials.

2.7 μm mid-infrared Er-doped thin-film LiNbO3-on-insulator photonic wire laser

The 2.7 μm continuous-wave laser was demonstrated theoretically on the basis of a 1480-nm-pumped Er-doped thin-film LiNbO3-on-insulator photonic wire (Er:LNOI PW) and a three-level rate equation model of Er3+. Under 1480 nm pumping, the excited state absorption (4I13/2 → 4I9/2) and cooperative upconversion process (4I13/2 → 4I15/2: 4I13/2 → 4I9/2) populate the 4I11/2 level meanwhile bypass significantly the 4I13/2 level, enabling a population inversion between these two levels. The cavity length and output coupler's reflectivity at laser wavelength were optimized for maximizing laser output. Under the optimum configuration, dependences of laser power on both pump power and Er3+ concentration were investigated. Simulation results show that lower threshold pump power, higher laser output and slope efficiency are obtained for a higher Er3+ concentration. The laser output reveals a linear relationship to the Er3+ concentration and the relationship is strong at higher pump level. Simulation results show that the threshold pump power, laser output and slope efficiency are on orders of several tens of mW, sub-mW and 3.1%, respectively, for an Er3+ concentration of 1.5 mol%. In addition, there is no need to account for photorefractive effect on the laser performance as it is far smaller than the refractive index contrast of the LNOI PW, ∼0.7. Present work opens up the possibility of implementing an on-chip 2.7 μm mid-infrared laser based on an Er:LNOI.

Weak optical modes for high-density and low-loss photonic circuits

Dielectric optical waveguides constitute the main building blocks of photonic integrated circuits (PICs). Channels with high refractive index contrast can provide very compact PIC components, whereas structures with lower index exhibit less propagation loss. A hybrid concept that can combine the best of high- and low-index materials is highly required. Here, we devise a new approach to realize compact and low-loss hybrid optical waveguides based on the interaction of weak optical modes. This is a rather universal approach that can be applied to a wide range of optical materials. To prove the principle, the hybrid waveguide structure is formed by combining a low-index polymer and a thin layer of silicon nitride. For this material combination, a minimum bending radius of 90 µm (for a bending loss of 0.005 dB/90°) and a propagation loss of 0.7 dB/cm are achieved. The viability of this platform is demonstrated through a series of high-performance novel PIC components. This hybrid waveguide platform enabled by a powerful and simple design concept holds great promise for high-density and low-loss PICs.

Two-beam phase correlation spectroscopy: a label-free holographic method to quantify particle flow in biofluids

We introduce two-beam phase correlation spectroscopy (2B-ΦCS) as a label-free technique to measure the dynamics of flowing particles; e.g., in vitro or in vivo blood flow. 2B-ΦCS combines phase imaging with correlation spectroscopy, using the intrinsic refractive index contrast of particles against the fluid background in correlation analysis. This method starts with the acquisition of a time series of phase images of flowing particles using partially coherent point-diffraction digital holographic microscopy. Then, phase fluctuations from two selected circular regions in the image series are correlated to determine the concentration and flow velocity of the particles by fitting pair correlation curves with a physical model. 2B-ΦCS is a facile procedure when using a microfluidic channel, as shown by the measurements on flowing yeast microparticles, polymethyl methacrylate microparticles, and diluted rat blood. In the latter experiment, the concentration and average diameter of rat blood cells were determined to be (4.7±1.9)×106 μL−1 and 4.6±0.4 μm, respectively. We further analyzed the flow of mainly red blood cells in the tail vessels of live zebrafish embryos. Arterial and venous flow velocities were measured as 290±110 μm s−1 and 120±50 μm s−1, respectively. We envision that our technique will find applications in imaging transparent organisms and other areas of the life sciences and biomedicine.

Topological band gaps and edge modes materialized by symmetric silicon-on-insulator photonic crystal slabs in the mid-IR range

We successfully fabricated silicon-on-insulator (SOI) photonic crystal (PhC) slabs in which electromagnetic topological band gaps and edge modes were materialized for symmetric transverse-electric-like modes. Because the structure of our specimens can be regarded as symmetric about the horizontal middle plane of the PhC slab, their symmetric and antisymmetric eigenmodes were rigorously separated, so we could achieve genuine photonic band gaps for the former. We fabricated those specimens by electron beam lithography of the top Si layer of SOI wafers and successive plasma-enhanced chemical vapor deposition of a ${\mathrm{SiO}}_{2}$ capping layer. We confirmed the complete common band gap of topologically trivial and nontrivial PhCs and the topological edge modes on the boundary between them by angle-resolved reflection spectroscopy in the mid-infrared range. This is an observation of the common band gap and edge modes that materialized in symmetric PhC slabs without a membrane structure, that is, without the etching of a sacrificial layer under the PhC to increase the refractive-index contrast and the width of the common band gap.

Uniform all-solid chalcogenide Bragg fiber with wide photonic bandgap via extrusion

As the Bragg fiber can guide light by tuning the structure parameters of claddings, it is possible to improve the ability of laser-power transmission in the mid-infrared with effective omnidirectional reflection, but it usually suffers from the disturbance of the air core and functional bandgap. Here, the structural parameters of three pairs of periodic cladding layers were optimized by the plane wave expansion method, and the thickness of each layer is 3.36 µm, consisting of Ge20As20Se15Te45 and As2S3 glasses with a refractive index contrast of Δn = 0.8. The simulation results showed that a wide bandgap of 1.2 µm can be realized in the fiber after structural optimization. Then, a fiber preform was prepared via an improved stacked extrusion based on seven thickness-compensated glass plates. The experimental results show that the all-solid Bragg fiber has three pairs of uniform periodic cladding and an extra-large core. The superior optical fiber structure can also be well maintained in the whole fiber length, and the average ratio of each cladding thickness to the fiber diameter is kept nearly at 3:100. Finally, the fiber loss at 1.55 µm and 2.94 µm is 12 dB/m and 18 dB/m, respectively. In all, such a well-structured all-solid chalcogenide Bragg fiber would pave a new way to develop high-quality laser transmission or optical sensors in mid-infrared.

Sb2S3-Based Dynamically Tuned Color Filter Array via Genetic Algorithm.

Color displays have become increasingly attractive, with dielectric optical nanoantennas demonstrating especially promising applications due to the high refractive index of the material, enabling devices to support geometry-dependent Mie resonance in the visible band. Although many structural color designs based on dielectric nanoantennas employ the method of artificial positive adjustment, the design cycle is too lengthy and the approach is non-intelligent. The commonly used phase change material Ge2Sb2Te5 (GST) is characterized by high absorption and a small contrast to the real part of the refractive index in the visible light band, thereby restricting its application in this range. The Sb2S3 phase change material is endowed with a wide band gap of 1.7 to 2 eV, demonstrating two orders of magnitude lower propagation loss compared to GST, when integrated onto a silicon waveguide, and exhibiting a maximum refractive index contrast close to 1 at 614 nm. Thus, Sb2S3 is a more suitable phase change material than GST for tuning visible light. In this paper, genetic algorithms and finite-difference time-domain (FDTD) solutions are combined and introduced as Sb2S3 phase change material to design nanoantennas. Structural color is generated in the reflection mode through the Mie resonance inside the structure, and the properties of Sb2S3 in different phase states are utilized to achieve tunability. Compared to traditional methods, genetic algorithms are superior-optimization algorithms that require low computational effort and a high population performance. Furthermore, Sb2S3 material can be laser-induced to switch the transitions of the crystallized and amorphous states, achieving reversible color. The large chromatic aberration ∆E modulation of 64.8, 28.1, and 44.1 was, respectively, achieved by the Sb2S3 phase transition in this paper. Moreover, based on the sensitivity of the structure to the incident angle, it can also be used in fields such as angle-sensitive detectors.

Development of a 3D ultrafast laser written near-infrared spectro-interferometer.

Direct ultrafast laser photoinscription of transparent materials is a powerful technique for the development of embedded 3D photonics. This is particularly adaptable for astrophotonic devices when a number of inputs are required. The process relies essentially on volume fabrication of waveguiding structures in flexible 3D designs and refractive index contrast parameters adjustable for specific spectral ranges. This enables 3D geometry and thus avoids in-plane crossings of waveguides that can induce losses and cross talk in multi-telescope beam combiners. The additional novel capability of the technique allows for the fabrication of high aspect ratio nanostructures nonperturbatively sampling the optical field. Combining ultrafast laser micro- and nanoprocessing with engineered beams, we present here results for the development of chip-sized silica glass integrated robust 3D three-telescope beam combiners in the near-IR range, as well as embedded diffraction gratings, for phase closure analysis and spectro-interferometry applications in astronomy.

Enhancing Evanescent Wave Coupling of Near-Surface Waveguides with Plasmonic Nanoparticles.

Evanescent field excitation is a powerful means to achieve a high surface-to-bulk signal ratio for bioimaging and sensing applications. However, standard evanescent wave techniques such as TIRF and SNOM require complex microscopy setups. Additionally, the precise positioning of the source relative to the analytes of interest is required, as the evanescent wave is critically distance-dependent. In this work, we present a detailed investigation of evanescent field excitation of near-surface waveguides written using femtosecond laser in glass. We studied the waveguide-to-surface distance and refractive index change to attain a high coupling efficiency between evanescent waves and organic fluorophores. First, our study demonstrated a reduction in sensing efficiency for waveguides written at their minimum distance to the surface without ablation as the refractive index contrast of the waveguide increased. While this result was anticipated, it had not been previously demonstrated in the literature. Moreover, we found that fluorescence excitation by waveguides can be enhanced using plasmonic silver nanoparticles. The nanoparticles were also organized in linear assemblies, perpendicular to the waveguide, with a wrinkled PDMS stamp technique, which resulted in an excitation enhancement of over 20 times compared to the setup without nanoparticles.

Inverse design of dynamically tunable phase-change material based metamaterial absorber induced structural color

Structural colors using phase change material based metasurfaces/metamaterials have enabled high saturation, high resolution, and wide-gamut color printing. The wide color gamut with high resolution is achieved by changing the geometry of the metamaterials. However, finding the design parameter for desired color is computationally costly. Designing such parameters to precisely show the required color is therefore essential for producing metamaterials for use in real-world applications. In this work, we report a tunable reflective metamaterial using Ge2Sb2Te5 (GST) for the generation of structural colors. Exploiting the large contrast in refractive index of GST with the change in phase we can dynamically tune between two different colors. Further, we report an inverse design of tunable reflective metamaterial structure using deep neural network. The inverse design based on bidirectional artificial neural network can accurately predict the design parameter for a desired color. Our results show that tunable color filters using GST nanostructures can be designed accurately and efficiently using deep neural networks and could find potential applications in color printing and display technologies.

Optical Manipulation Heats up: Present and Future of Optothermal Manipulation.

Optothermal manipulation is a versatile technique that combines optical and thermal forces to control synthetic micro-/nanoparticles and biological entities. This emerging technique overcomes the limitations of traditional optical tweezers, including high laser power, photon and thermal damage to fragile objects, and the requirement of refractive-index contrast between target objects and the surrounding solvents. In this perspective, we discuss how the rich opto-thermo-fluidic multiphysics leads to a variety of working mechanisms and modes of optothermal manipulation in both liquid and solid media, underpinning a broad range of applications in biology, nanotechnology, and robotics. Moreover, we highlight current experimental and modeling challenges in the pursuit of optothermal manipulation and propose future directions and solutions to the challenges.

The Formation of Volume Transmission Gratings in Acrylamide-Based Photopolymers Using Curcumin as a Long-Wavelength Photosensitizer.

Curcumin, a natural dye found in the Curcuma longa rhizome, commonly called turmeric, is used as a photosensitizer in acrylamide-based photopolymers for holographic data storage. We studied the absorbance of photopolymer films that show two absorption bands due to curcumin, acrylamide monomer (AA), and the crosslinking agent N,N'-methylenebisacrylamide (MBA). Analysis of the real-time diffraction efficiency of these films shows a maximum of 16% for the sample with the highest curcumin concentration. Moreover, increasing the curcumin load enhanced the refractive index contrast from 7.8 × 10-4 for the photopolymer with the lowest curcumin load to 1.1 × 10-3 for the photopolymer with the largest load. The sensitivity and diffraction efficiency of the recorded gratings also increased from 7.0 to 9.8 cm·J-1 and from 7.9 to 16% with the increase in curcumin load, respectively. Finally, the influence of NaOH on the photopolymerization of the AA-curcumin-based sample shows a diffraction efficiency increase with the NaOH content, revealing that the curcumin enol form is more efficient as a photosensitizer.

10 dB emission suppression in a structured low index medium

Significant suppression of radiation in 3D structured media with small refractive indices of 1.4–1.6, such as glass or polymers, is a desirable feature yet to be obtained. We present here quasiperiodic 3D structures with a homogeneous distribution of Bragg peaks on a sphere in reciprocal space obtained by an overlap of multiple gratings. Simulations on 3D structures revealed a limitation of emission suppression contrasting from theoretical predictions. The effect of number of gratings and refractive index contrast on the obtained suppression is investigated. We demonstrate an almost refractive index independent −10dB suppression of emission for a refractive index as low as 1.30.