Intrinsic Refractive Index Research Articles

Fabrication of UV-Curable Polysiloxane Coating with Tunable Refractive Index Based on Controllable Hydrolysis.

In order to improve laser transmission efficiency at 1053 nm and 527 nm, a potassium deuterium phosphate (DKDP) crystal (a key component of high-power laser systems) needs a bi-layer antireflection coating system on its incident surface. UV-curable polysiloxane coatings with a refractive index varying from 1.500 to 1.485 were prepared through the polycondensation of a methacryloxy propyl trimethoxylsilane (MPS) monomer with a controllable degree of hydrolysis. Additionally, the influence rule of the coating structure on the refractive index was intensively studied, and the primary factors that dominate the hydrolysis process were discussed. Further refractive index adjustment was achieved using only a small amount of dopant based on the polysiloxane coating with refractive index of 1.485, allowing for high antireflection of the bi-layer coating system at desired wavelengths to be achieved. In addition, high laser damage resistance and remarkable mechanical properties of the coating were simultaneously realized through the incorporation of a minor quantity of dopants, which benefited from the successful modulation of the intrinsic refractive index of the polysiloxane coating.

Bronzing effect: predicting the color shine of printed surfaces by estimating the refractive index of the inks.

This paper investigates the optical phenomenon responsible for the colored shine that sometimes appears at the surface of ink layers in the specular direction, often called "bronzing" or "gloss differential." The prediction of this shine effect relies on the Fresnel formulas of the air/ink interface. The complex refractive index of the ink must therefore be determined, which is made difficult because of the roughness of inked printing supports. We propose a generic method that can be applied to any ink, without any prior knowledge of its composition or the printing substrate. In order to reduce light scattering, a solid colored area is printed with the studied ink on a glossy paper previously printed with black ink. By ellipsometry, we determine the effective refractive index of the sample. The intrinsic complex refractive index of the ink can then be extracted by modeling the optical response of the inked surface with a set of Gaussian oscillators, among which one of them approaches residual scattering. With this data, we could proceed to a fine colorimetric analysis of the bronzing color of some cyan, magenta, and yellow inks. In particular, we show that this gloss color is slightly shifted from the complementary of the ink's usual color in diffuse reflection.

Machine-learning-based diagnosis of thyroid fine-needle aspiration biopsy synergistically by Papanicolaou staining and refractive index distribution

We developed a machine learning algorithm (MLA) that can classify human thyroid cell clusters by exploiting both Papanicolaou staining and intrinsic refractive index (RI) as correlative imaging contrasts and evaluated the effects of this combination on diagnostic performance. Thyroid fine-needle aspiration biopsy (FNAB) specimens were analyzed using correlative optical diffraction tomography, which can simultaneously measure both, the color brightfield of Papanicolaou staining and three-dimensional RI distribution. The MLA was designed to classify benign and malignant cell clusters using color images, RI images, or both. We included 1535 thyroid cell clusters (benign: malignancy = 1128:407) from 124 patients. Accuracies of MLA classifiers using color images, RI images, and both were 98.0%, 98.0%, and 100%, respectively. As information for classification, the nucleus size was mainly used in the color image; however, detailed morphological information of the nucleus was also used in the RI image. We demonstrate that the present MLA and correlative FNAB imaging approach has the potential for diagnosing thyroid cancer, and complementary information from color and RI images can improve the performance of the MLA.

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.

Automatic Colorectal Cancer Screening Using Deep Learning in Spatial Light Interference Microscopy Data.

The surgical pathology workflow currently adopted by clinics uses staining to reveal tissue architecture within thin sections. A trained pathologist then conducts a visual examination of these slices and, since the investigation is based on an empirical assessment, a certain amount of subjectivity is unavoidable. Furthermore, the reliance on external contrast agents such as hematoxylin and eosin (H&E), albeit being well-established methods, makes it difficult to standardize color balance, staining strength, and imaging conditions, hindering automated computational analysis. In response to these challenges, we applied spatial light interference microscopy (SLIM), a label-free method that generates contrast based on intrinsic tissue refractive index signatures. Thus, we reduce human bias and make imaging data comparable across instruments and clinics. We applied a mask R-CNN deep learning algorithm to the SLIM data to achieve an automated colorectal cancer screening procedure, i.e., classifying normal vs. cancerous specimens. Our results, obtained on a tissue microarray consisting of specimens from 132 patients, resulted in 91% accuracy for gland detection, 99.71% accuracy in gland-level classification, and 97% accuracy in core-level classification. A SLIM tissue scanner accompanied by an application-specific deep learning algorithm may become a valuable clinical tool, enabling faster and more accurate assessments by pathologists.

Integrated Gallium Nitride Nonlinear Photonics

AbstractGallium nitride (GaN) as a wide bandgap material is widely used in solid‐state lighting. Thanks to its high nonlinearity and high refractive index contrast, GaN‐on‐insulator (GaNOI) is also a promising platform for nonlinear optical applications. Despite its intriguing optical proprieties, nonlinear applications of GaN are rarely studied owing to the relatively high optical loss of GaN waveguides (typically ≈2 dB cm−1). In this paper, GaNOI microresonators with intrinsic quality factor over 2.5 million are reported, corresponding to an optical loss of 0.17 dB cm−1. Parametric oscillation threshold power as low as 6.2 mW is demonstrated, and the experimentally extracted nonlinear index of GaN at telecom wavelengths is estimated to be n2 = 1.4 × 10−18 m2 W−1, which is several times larger than that of commonly used platform such as Si3N4, LiNbO3, and AlN. Single soliton generation in GaN is implemented by an auxiliary laser pumping scheme, so as to mitigate the high thermorefractive effect in GaN. The large intrinsic nonlinear refractive index, together with its broadband transparency window and high refractive index contrast, make GaNOI a promising platform for chip‐scale nonlinear applications.

Multiscale label-free volumetric holographic histopathology of thick-tissue slides with subcellular resolution

Histopathology relies upon the staining and sectioning of biological tissues, which can be laborious and may cause artifacts and distort tissues. We develop label-free volumetric imaging of thick-tissue slides, exploiting refractive index distributions as intrinsic imaging contrast. The present method systematically exploits label-free quantitative phase imaging techniques, volumetric reconstruction of intrinsic refractive index distributions in tissues, and numerical algorithms for the seamless stitching of multiple three-dimensional tomograms and for reducing scattering-induced image distortion. We demonstrated label-free volumetric imaging of thick tissues with the field of view of 2 mm × 1.75 mm × 0.2 mm with a spatial resolution of 170 nm × 170 nm × 1400 nm. The number of optical modes, calculated as the reconstructed volume divided by the size of the point spread function, was ∼20 giga voxels. We have also demonstrated that different tumor types and a variety of precursor lesions and pathologies can be visualized with the present method.

Doubly-Resonant Photonic Crystal Cavities for Efficient Second-Harmonic Generation in III-V Semiconductors.

Second-order nonlinear effects, such as second-harmonic generation, can be strongly enhanced in nanofabricated photonic materials when both fundamental and harmonic frequencies are spatially and temporally confined. Practically designing low-volume and doubly-resonant nanoresonators in conventional semiconductor compounds is challenging owing to their intrinsic refractive index dispersion. In this work we review a recently developed strategy to design doubly-resonant nanocavities with low mode volume and large quality factor via localized defects in a photonic crystal structure. We built on this approach by applying an evolutionary optimization algorithm in connection with Maxwell equations solvers; the proposed design recipe can be applied to any material platform. We explicitly calculated the second-harmonic generation efficiency for doubly-resonant photonic crystal cavity designs in typical III–V semiconductor materials, such as GaN and AlGaAs, while targeting a fundamental harmonic at telecom wavelengths and fully accounting for the tensor nature of the respective nonlinear susceptibilities. These results may stimulate the realization of small footprint photonic nanostructures in leading semiconductor material platforms to achieve unprecedented nonlinear efficiencies.

Development of a Label-Free LSPR-Apta Sensor for Staphylococcus aureus Detection.

The risk of foodborne diseases has increased over the last years. We have developed a simple, portable, and label-free optical sensor via aptamer recognition of Staphylococcus aureus at nanostructured plasmonic elements. The developed aptamers conjugated to a localized surface plasmon resonance (LSPR) sensing device were applied in both pure culture and artificially contaminated milk samples enabling a limit of detection of 103 CFU/mL for S. aureus in milk. There was no need for a pre-enrichment step, and the total analysis time decreased from 30 min to 120 s. Finite-difference time-domain was used to simulate the experimentally measured optical responses for a range of different sensor designs (100 and 200 nm disks), addressing the role of the near field and intrinsic refractive index sensitivity. A comparison of the aptamer to antibody-based recognition approaches showed that the thickness of the sensing layer was critical with a significantly larger response for the thinner aptamer layer. Comparison of differently sized metal nanostructures showed a significantly higher sensitivity for 200 nm diameter compared to 100 nm diameter disk structures resulting from both increases in bulk refractive index sensitivity and the extent to which the local field extends out from the metal surface. These findings confirmed that the developed gold nanodisk-based LSPR sensing chips could facilitate sensitive detection of S. aureus in food samples.

Combining Harmonic Laser Beams by Fiber Components for Refractivity–Compensating Two-Color Interferometry

A perfect spatial overlap with multiple beams of different wavelengths is a prerequisite for multi–wavelength interferometry. Beam combination with the help of fibers seems to be an interesting method for this. We investigated three different types of fiber components, multi–mode wavelength division multiplexers (MM–WDMs), single–mode wavelength division multiplexers (SM–WDMs), and endlessly single–mode polarization maintaining photonic crystal fibers (PM–PCFs). All three seem potential candidates for a perfect spatial overlap of laser beams separated by an octave, i.e. fundamental and harmonic beams. We performed an experimental study on the impact of these components on polarization, wavefront, and coherence. These properties are essential for high–accuracy interferometry. As a test system, we used the fundamental and second harmonic beam of a Nd:YAG laser system at 1064 and 532 nm as a popular system in two–color interferometry for intrinsic refractive index compensation. The experimental results show the potential of PM–PCFs for this challenging application.

Shine Bright Like a Diamond: New Light on an Old Polymeric Semiconductor.

Brilliance usually refers to the light reflected by the facets of a gemstone such as diamond due to its high refractive index. Nowadays, high-refractive-index materials find application in many optical and photonic devices and are mostly of inorganic nature. However, these materials are usually obtained by toxic or expensive production processes. Herein, the synthesis of a thin-film organic semiconductor, namely, polymeric carbon nitride, by thermal chemical vapor deposition is presented. Among polymers, this organic material combines the highest intrinsic refractive index reported so far with high transparency in the visible spectrum, even reaching the range of diamond. Eventually, the herein presented deposition of high quality thin films and their optical characteristics open the way for numerous new applications and devices in optics, photonics, and beyond based on organic materials.

Refractive index of sputtered germanium films in the 25-13 µm infrared spectral region

The objective of this work is to quantify the refractive index of germanium films relative to film thickness. The films are fabricated using radio-frequency sputter deposition with film thickness spanning 820 nm to 3950 nm. The dispersive refractive index of the films is measured by Fabry-Perot transmission measurements. Using this data, it is then verified by full-spectral matching of the resonant response of guided-mode resonance grating structures fabricated into the films. This is a new method to quantify the dispersion response of materials that can be deposited to form thin films. At a wavelength of 10 µm, the refractive index is found to vary from 3.84 for an 820 nm thick film to a value of 4.6 for a 3950 nm thick film. This, respectively, represents a ∼4% decrease and a ∼15% increase from the intrinsic crystalline refractive index. Chemical composition of the films is verified using energy dispersive spectroscopy with the film structure analyzed using X-ray diffraction.

Intrinsic High Refractive Index Siloxane–Sulfide Polymer Networks Having High Thermostability and Transmittance via Thiol–Ene Cross-Linking Reaction

Siloxane-based polymer films obtained by the sol–gel process are often porous and exhibit low refractive indices. To address this issue, we have prepared two intrinsic high refractive index siloxane–sulfide polymer networks via the Piers–Rubinsztajn reaction and thiol–ene click chemistry based on readily available tetraethoxysilane (TEOS). Our siloxane–sulfide polymers consisting of aromatic groups and sulfur display enhanced thermostability with high Tg (>120 °C) and T5d (>320 °C) as compared to related materials. Such good thermal properties significantly expand their application as high thermostable optical materials. Besides, both flexible and colorless polymer films P1 and P2 exhibit high refractive indices of 1.625 and 1.665 at 546 nm, respectively. P2 also shows high refractive index of 1.648 at 1000 nm. Moreover, both siloxane–sulfide polymer films are uniform and homogeneous with high optical transmittance of 90% for wavelength ranging from 400 to 2000 nm. These data indicate that the new siloxan...

Determining the Complex Refractive Index of Neat CdSe/CdS Quantum Dot Films

Understanding how structural and synthetic factors influence the complex refractive index of quantum dot (QD) solids is crucial to tailoring the light–matter interactions of QD-containing photonic and optoelectronic devices. However, neat QD films are challenging to accurately model as they are a mixture of inorganic core/shell materials and surrounding organic ligands. Furthermore, both the QD film morphology and the complex refractive index vary due to particle size, ligand chain length, and the deposition process. Here, we study the complex refractive index of neat CdSe/CdS core/shell QD films by using variable-angle spectroscopic ellipsometry to derive the effective complex refractive index using Kramers–Kronig consistent dispersion models. We use this information in conjunction with intrinsic refractive index data of CdSe and CdSe/CdS QDs extracted from solution-state absorption data and effective medium approximations (EMA) to describe neat QD films. We find that EMAs can successfully be used as a t...

Intrinsic refractive index matched 3D dSTORM with two objectives: Comparison of detection techniques

We have built a setup for 3D single molecule localisation microscopy (SMLM) where a very high resolution is achieved by, firstly, the use of two objectives instead of one and, secondly, minimizing optical aberrations by refractive index matching with a glycerol-water mixture as immersion medium in conjunction with glycerol-immersion objectives. Multiple optical paths of the microscope allow to switch between astigmatic and interferometric localisation along the optical axis, thus enabling a direct comparison of the performance of these localisation methods.

Selenide-Containing Polyimides with an Ultrahigh Intrinsic Refractive Index.

This work developed novel selenium-containing polyimides with a high intrinsic refractive index. Four polyimides with different selenium contents and repeat unit structures were designed and synthesized via amine-dianhydride polycondensation of one of two diamines, i.e., 4,4′-oxydianiline or bis(4-aminophenyl)selanide, with one of two dianhydrides, i.e., bis(4-(3,4-dicarboxylbenzoyloxy)phenyl) ester dianhydride or 1,1′-bis(4-(3,4-dicarboxylbenzoyloxy)phenyl) selenide dianhydride. Various techniques, e.g., nuclear magnetic resonance, Fourier transformed infrared spectroscopy, and wide-angle X-ray diffraction, were used to characterize the polymers’ structures. Differential scanning calorimetry, thermogravimetric analysis, ultraviolet-visible spectroscopy, and spectroscopic ellipsometry were used to characterize the properties of the polymers. The selenium contents showed a positive effect on the refractive index of the final polymer. In addition, the refractive index can reach up to 1.968 at 633 nm, which was the highest intrinsic refractive index of a polyimide ever reported. Because of the high intrinsic refractive index, the reflective ratio of visible light on the surface of a silicon wafer was significantly reduced, indicating the potentially utility of the polymer in an anti-reflection coating.

Temperature Dependence Characteristics of Biodegradable Polycaprolactone Grafted Propargyl Dehydroabietic Ester (PCL-g-DAPE)

ABSTRACTA sample of propargyl dehydroabietic ester grafted to α-azide substituted poly(ϵ-caprolactone) (PCL-g-DAPE) in THF was characterized at 24°C, 30°C, 40°C, 50°C by a combination of on-line measurement of intrinsic viscosity, refractive index and two-angle light scattering in a size exclusion chromatograph. The weight average molecular weight (w), intrinsic viscosity ([η]), hydrodynamic radius (RH), and concentration of the polymer fractions were obtained from corresponding traces of the chromatograms. The data were used to construct double logarithmic plots of w versus [η], and w versus RH, as well as Stockmayer-Fixman plots, to scale the intrinsic viscosity, hydrodynamic radii and dimensions of the PCL-g-DAPE chains to molar mass. The results indicated that PCL-g-DAPE was more rigid than the parent PCL; however, PCL-g-DAPE can be classified as a flexible-coil polymer, similar to poly(methyl acrylate). The PCL-g-DAPE stiffness was related to the chemical structure of the bulky side chains that induced preferential conformation correlations of the polymer backbones in solution. The PCL-g-DAPE –THF system behaved as a lower critical consolute solution system with highest miscibility around 30°C, judged based on the slope of double logarithmic plots of molecular weight and intrinsic viscosity. Moreover, a transition in the conformation of the PCL-g-DAPE in THF at 30°C was observed.

Plasmon-induced transparency based on a triangle cavity coupled with an ellipse-ring resonator.

In this paper, a novel compact plasmonic system is introduced to realize the phenomenon of plasmon-induced transparency. The proposed device consists of a triangle defect coupled with an ellipse-ring resonator based on a metal-insulator-metal platform. By the finite-difference time-domain method, the transmission characteristics are numerically studied in detail. In order to verify the simulation results, the coupled mode theory is utilized. In the following, the effect of geometrical parameters, namely, the major and minor radii of the ellipse-ring and the gap between cavities, are investigated. Moreover, the fundamental factors of transmission spectra including intrinsic Drude loss and refractive index of dielectric region are studied. As a result, the transmission peak is obtained near 70% and the full width at half-maximum is close to 28nm. The sensitivity and figure of merit of the proposed structure are 860 nm/RIU and 31.6 RIU-1, respectively. The mentioned compact structure has the ability and potential to be used in integrated optical circuits like slow light devices, nanoscale filters and nanosensors.

Tuning optical properties of MOF-based thin films by changing the ligands of MOFs

The preparation and development of novel optical thin films are of great importance to functional optical and opto-electric components requiring a low refractive index. In this study, a typical metal-organic framework (MOF), MIL-101(Cr), is selected as the research model. The corresponding MOF nanoparticles are prepared by a hydrothermal method and the optical thin films are successfully prepared by spin-coating. The optical properties of the corresponding MOF thin films are controlled by changing the type of functional groups on the benzene ring of the ligand (terephthalic acid) on MOFs. The functional groups are hydrogen atoms (H), electron donating groups (–NH2, –OH) and electron withdrawing groups (–NO2, –(NO2)2 or F4), respectively. It is found that the effective refractive index ( n eff) of MOF thin films decreases along with the increasing voids among MOF nanoparticles. In addition, the extinction coefficient ( k ) increases with the addition of electron donating groups, and decreases with the addition of electron withdrawing groups. Among the MOFs used in this study, the n eff of NO2-MIL-101(Cr) containing electron withdrawing groups is as low as ~1.2, and value of k is particularly low, which suggests its potential application in antireflective devices. In addition, the intrinsic refractive index ( n dense) of the dense MOF materials evaluated according to their porosity increases with the number of the functional groups, and the n dense of the two nitro-substituted MOFs is greater than that of the single nitro-substituted one, and the latter is bigger than that of hydroxyl-substituted one, which is close to that of amino-functionalized one. The diversity of ligands in MOFs makes them a promising new generation of optical materials.

Investigation of spectral properties of the glass matrix of the optical color glass for1 interference filter

The current state of colorless and colored optical glasses production for optoelectronic systems is considered. The actuality of creating domestic infrared glass for use as an interference filter in optoelectronic systems of defense detachments is shown. The aim of the work is to create a glass matrix of infrared optical glass with an intrinsic refractive index τ = 90% at a wavelength λ = 1060 ± 20 nm. In work the standard methods of spectral analysis, the determination of the number and size of bubbles and fibrils in glass are used. The experimental glass compositions were melted in an electric furnace with silicon carbide heaters. The factors responsible for providing full absorption in the UV and visible parts of the spectrum and maximum transmission at a wavelength of 1060 nm are established. The results of studies on the development of infrared glasses with these characteristics and high quality of the products from them are worked out. The possibility of creating a boron-containing and aluminium- containing technological and cost-effective glass matrices with a refractive index of τ = 91.5% and τ = 92.8%, respectively, at a wavelength λ = 1060 ± 20 nm is established. The technological parameters of their production have been worked out. All this is the basis for the development of domestic optical colored infrared glass of a new generation, which will act as an interference filter in optoelectronic systems.