Average Refractive Index Research Articles

Designing a terahertz optical sensor based on helically twisted photonic crystal fiber for toxic gas sensing

A novel helically twisted photonic crystal fiber (PCF) is designed and proposed for sensing toxic gases with refractive indices ranging from 1.00 to 1.08. The PCF consists of twelve hollow pipes arranged circularly around the hollow core to support THz radiation propagation. Low-loss polymer Topas is used as the background material of cladding. The fiber is twisted 360° over 50 cm to enhance anti-resonance in the THz region. The fundamental LP01 mode is analyzed using the finite-difference eigenmode (FDE) method. The sensor operates across four frequency bands (0.2 to 3.0 THz) with minimal transmission loss (~ 10⁻⁴ 1/cm). Key parameters such as refractive index sensitivity, relative sensitivity, resolution, and figure of merit (FOM) are evaluated. The average refractive index sensitivities are 1450, 2250, 3000, and 2550 for Bands 1 to 4, respectively, with 100% relative sensitivity across all bands. The sensor detects refractive index changes as small as 10⁻⁴. The FOM, defined as the inverse of the full width at half maximum, exceeds 30 1/RIU, reaching up to 250 1/RIU due to sharp resonance peaks. Compared to other THz sensors, this design offers enhanced performance in sensing gases like SOx, NOx, and CO, while maintaining a simple structure.

The Refractive Index of Human Milk Serum: Natural Variations and Dependency on Macronutrient Concentrations.

The refractive index (RI) of human milk serum (also known as whey, milk soluble fraction or milk plasma) depends on the individual molecular species dissolved in the serum and their concentrations. Although the human milk serum RI is known to influence milk analysis methods based on light scattering, the RI dependency on human milk serum composition is currently unknown. Therefore, we systematically evaluate how the RI depends on natural variations in macronutrient concentrations in the soluble fraction. We measure RI variations in serum simulating samples with controlled macronutrient concentrations, as well as skimmed and whole fore-, bulk, and hindmilk from 19 donors. For both types of samples, we relate the measured RI to the macronutrient composition. From the serum simulating samples, we observe that the RI depends more on variations in whey protein, than carbohydrate concentrations, while minerals have negligible influence. For all donated samples, the average RI was 1.3470 (range 1.3466-1.3474). Per donor, no significant differences were observed in RI between fore-, bulk, and hindmilk. We conclude that protein and solids-not-fat (i.e., the total contribution of carbohydrates, proteins and minerals present in milk) concentrations are most predictive for human milk serum RI.

Spray pyrolysis–derived V2O5 thin films as an alternative electrochromic layer for electrochromic devices

In this study, vanadium oxide films were produced in the vanadium pentoxide (V2O5) films phase as an alternative electrochromic layer for electrochromic devices by ultrasonic spray pyrolysis technique at 300°C. The V2O5 films were subjected to heat treatment at different temperatures (400°C–450°C–500°C) and the ellipsometric, structural, morphological, optical, and electrical properties of the films were investigated. The optical parameters of the V2O5 films were calculated by different theoretical methods and compared with the ellipsometric results. The average thickness and refractive index values are 172 nm and 2.29, respectively. The V2O5 films showed high optical absorbance in the visible region and bandgap energy values ranged from 2.10 to 2.44 eV. The resistivity and roughness of the films decreased. The average roughness was determined to be 45 nm on average. These results suggest that V2O5 phase films, whose physical properties are improved by annealing without phase change, can be considered as promising electrochromic layers for electrochromic devices applications.

Tunning the omnidirectional bandgap of nanoporous silicon using a semi-sinusoidal refractive index profile

We report the theoretical comparison of the omnidirectional bandgap in a 1-D photonic crystal using sinusoidal and semi-sinusoidal refractive index profiles. It is found that the corresponding omnidirectional bandgap of the semi-sinusoidal widens and shifts to a higher wavelength range as a function of the asymmetric ratio of semi-sinusoidal profile. The asymmetric ratio plays an essential role in tunning the width of the omnidirectional bandgap due to the changed average refractive index and optical thickness. The semi-sinusoidal refractive index is experimentally achieved by changing the current waveform. Novel omnidirectional nanoporous silicon mirrors with an omnidirectional bandgap covering optical communication wavelength (1550 nm) were fabricated using a semi-sinusoidal current waveform. The experimental analogy was carried out by building up the multilayered dielectric structures of omnidirectional mirrors by anodic etching using a semi-sinusoidal current waveform. The experimental results were compared with the theoretical results investigated by the Transfer matrix method. It was shown that the distorted current profile impacts the quality of the omnidirectional bandgap although it does not affect the porous size range.

Improving the label-free rapid semi-quantification of E.coli by AgNPs-decorated hydrogel inverse opal photonic crystals

Using photonic crystal materials for colorimetric sensing, the influences of structure and composition on the photonic bandgap (PBG) of materials are always vital to detection. In this work, the effects of pores in the structure of polyethylene diacrylate-based inverse opal photonic crystal (PEGDA-based IOPC) and the concentration of silver nanoparticles (AgNPs) used for decoration of the IOPC by chemical reaction on the shift of PBG center (λrp) have been studied for rapid semi-quantification of Escherichia coli (E. coli). We found that pores in the structure of PEGDA-based IOPC significantly increased the amount of AgNPs attached to this material compared with the structure created by the ordered self-assembly of spherical SiO2 particles of the parent template, leading to a pronounced red-shift of λrp. At the concentration of 20 µg/L AgNPs (CAgNPs) used for decoration, the shift of λrp reached 80 nm due to the increase in the average refractive index (naver.) of the material. It thus enabled us to observe changes in the color of the reflected light with the naked eye. In addition, since the increase in the concentration of E.coli (CE.coli) also caused a red-shift of the λrp, the CAgNPs, thus, could be reduced but still give a sufficiently strong shift of λrp to detect E.coli at a high level. The spectral position of the reflectance peak changed from green to red with increasing the CE.coli from 50 cfu/mL to 106 cfu/mL at the CAgNPs = 10 µg/L. These results indicated the potential application of AgNPs decorated PEGDA-based IOPC in rapid semi-quantification of E.coli by the naked eye.

Enzyme-Free Identification of Monosaccharide Enantiomers on TiO2 Nanotube Array-Based Fabry-Pérot Interferometer.

Chirality is a vital property across various domains, especially for biological activity. Herein, an enzyme-free sensing platform for monosaccharide enantiomer identification was developed by utilizing the Fabry-Pérot interferometer feature of TiO2 nanotube arrays modified with enantioselective metal-organic framework and glucose oxidase-mimicking Au NPs. In this design, optical property is monitored by reflective interferometric Fourier transform spectroscopy (RIFTS), a highly sensitive technique for detecting changes in the average refractive index within nanotubular structures. Using glucose (Glu) enantiomers as the model targets, after the recognition of L-/d-Glu on mesoporous homochiral MIL-101 (Fe), Au NPs anchored in MIL-101(Fe) catalyze the oxidation of Glu molecules to produce hydrogen peroxide (H2O2). Benefiting from the confinement effect of frameworks, MIL-101(Fe), as an artificial enzyme with excellent peroxidase-like activity, catalyzes the conversion of 4-chloro-1-naphthol (4-CN) into insoluble precipitates. These gathered precipitates effectively change the average refractive index of the interferometric substrate. Based on the variation of effective optical thickness (ΔEOT) values, the enantioselective determination of l-Glu and d-Glu can be achieved. Moreover, the proposed RIFTS sensor also presents broad applicability for the identification of other monosaccharide enantiomers. As the enzyme-free homochiral interferometer is directly constructed on a Ti-metal sheet, the RIFTS platform offers a robust, sensitive, and low-cost device for monosaccharide enantiomer recognition.

Machine learning ellipsometry as a sensitive diagnostic tool to study reproductive biology in Zika virus infected murine models

Machine-learning spectroscopy is a recent scientific and fast-growing area where specific machine-learning solutions are used to process spectroscopic data beyond traditional physical modeling. Here, several machine learning models are optimized and trained for processing highly sensitive broadband variable angle spectroscopic ellipsometry measurements as a new tool in reproductive biology. This new method is applied to classify semen samples obtained from male offspring from ZIKV-infected and uninfected murine models. The method is label-free, cost-effective, simple, and reaches excellent performance. The combination of spectroscopic depolarization degree data with a well-optimized and trained SVM model resulted in the excellent performance of AUROC = 0.99, accuracy = 92%, specificity = 87% and sensibility = 97%. Optical modeling of ellipsometry spectra with Kramers–Kronigconsistent B-splines that takes care of incoherent reflections in the samples allowed for the extraction of the average semen sample refraction index for each class, revealing small differences that were attributed to the observed semen head defects, protamination failure, and DNA fragmentation.

Microstructure formation mechanisms and property regulation methods during ceramic additive manufacturing

Anisotropic surface lamellar defects, dimensional shrinkage, and bending strength are important scientific issues that constrain vat photopolymerization additive manufacturing of biological, electronic, and core ceramics with high precision and complex structures. Through the single-factor experiment of photoinitiator concentration, average particle size, volume fraction, and refractive index constant in ceramic-resin slurry, this work studies the photopolymerization characteristics and profile curves, surface lamellar defects, and crosslinking density, verifies the reliability of the photopolymerization profile equations affected by the slurry composition, and acquires the anisotropic property formation mechanisms and control methods. The single-point scanning photopolymerization profile shape is parabolic, resulting in x-, y-, and z-direction lamellar defects before and after sintering (1550 °C), with no obvious weakening. After the quantitative description of the lamellar defects, the average surface roughnesses before and after sintering of the green body are 6.82 and 4.92 μm, respectively. Additionally, the laser energy attenuation along the penetration direction induces a gradient distribution of crosslinking density in the photopolymerization profile region. The crosslinking density gradient distribution of the decay from the slurry surface toward the depth direction is influenced by the slurry component concentration, leading to a gradient of cured powder concentration, which induces anisotropic behaviors. Through the crosslinking density controlling, the average bending strength in x(y) and z directions is increased by 63.19 % (46 %), the difference in anisotropic strength is reduced by 22.55 %, and the dimensional shrinkage difference of x and z directions is reduced by 48.32 %. This provides a theoretical and experimental research basis for ceramic vat photopolymerization with high dimensional accuracy, excellent performance, and morphology control.

Combining Infrared Refraction and Attenuated Total Reflection Spectroscopy.

We have specified and obtained a ZnSe prism with an unconventional face angle cut to 30°. This prism, with internal incidence angles ranging from 30° to 48°, allows users to record internal reflection spectra below the critical angle and attenuated total reflection (ATR) spectra above the critical angle without the need to change optics or move or replace the sample. We demonstrate its capabilities using 102 spectra of benzyl benzoate taken with s- and p-polarization at different angles of incidence. The subcritical spectra were analyzed to obtain n∞, a key parameter for correcting the ATR spectra. These corrected spectra were subsequently used to determine the complex refractive index for all ATR measurements. The averaged complex refractive index function shows excellent agreement with that obtained through ATR spectroscopic ellipsometry.

Refractometry of β-modification LiNH4SO4 crystals with an admixture of Mn2+ and Cu2+

Crystals of β-LiNH4SO4 doped with 2 and 5 wt% of Cu2+ and Mn2+ were synthesized. The crystal structure was determined using X-ray diffraction, and the dispersion of refractive indices and birefringence was investigated. The study revealed that adding impurities increased the absolute values of the refractive indices for all crystal physical directions. As the concentration of Mn2+ and Cu2+ impurities increased to 5%, the average refractive index showed an almost linear increase, attributed to the increase in crystal density due to a decrease in the volume of the unit cell. Furthermore, the impurities of Mn2+ and Cu2+ ions significantly altered the absolute values of birefringence. The refractions, electronic polarizabilities, and dispersion parameters were calculated using the Sellmeier formula, showing that adding impurities increased the values of molar refractions and electronic polarizabilities. The study also found that introducing Mn2+ and Cu2+ impurities shifted the optical isotropic point to the long-wavelength part of the spectrum by approximately 26 and 31 nm, respectively.

Structural, optical, and shielding investigation of PVA/Te composite for technological applications

Using the casting process, polymeric films made of polyvinyl alcohol (PVA) including varying percentages of Te were created to form PVA/Te composite at doping concentrations of Te (0, 4.76, 13.04, 20 wt %), which denoted as PT1, PT2, PT3 and PT4 respectively. XRD results show semicrystalline nature in PT1 and PT2 however with further increasing of Te contents, a nano-crystallite Te appeared in PT3 and PT4. The absorption spectrum confirms that PT3 film is a good option to be used in UV-shielding. PT4 film shows an abrupt rise in absorption coefficient at 0.5 eV, 1.5 eV and UV regions, hence this qualifies this sample for various optical applications. The optical energy gap of the films varies between 4.94 eV and 3.32 eV for the indirect transitions and between 5.3 eV and 4.5 eV for the direct transitions, the PT4 film has the lowest values of both direct and indirect energy gaps. Several methods were used to measure the average refractive index nav value. Furthermore, the nonlinear refractive index (n2) was estimated. PT4 possesses the highest value of both nav and n2. Furthermore, the radiation shielding coefficients have been estimated according to the Phy-X/PSD online software. The sample with the highest Te concentration, PT4, has the best radiation shielding between the films under investigation. The investigated polymer with high Te weight percentage is an optical system that can be used for solar cells, optoelectronics, electronics, nonlinear optics, optical filters and for radiation shielding.

Age dependence of the average refractive index of the isolated human crystalline lens.

We measured the average group refractive index (RI) of 120 isolated lenses from 120 human donors (age: 0.03 to 61 years). The average group RI was calculated from a measurement of the optical thickness of the lens using optical coherence tomography and the apparent window shift of the test chamber caused by the lens. The estimated measurement uncertainty was ±0.004. The group RI at 880 nm was converted to phase RI at 589 nm using the dispersion equation of water and protein. From 2 to 61 years, the mean value of the RI was 1.415 ± 0.002 (group index at 880 nm) and 1.406 ± 0.002 (phase index at 589 nm) independent of age (p = 0.774). Two lenses from donors of age 0.33 and 3 months had significantly lower RI (group index: 1.405 and 1.403; phase index: 1.396 and 1.394). From age 2 to 61, the average lens RI is constant with age within the measurement uncertainty (±0.004).

Research Progress on the Impact of Outdoor Activities on Myopia in Children and Adolescents

The issue of myopia among children and adolescents has become a global public health concern. Existing research indicates a close correlation between outdoor activities and myopia in children and adolescents, with increased outdoor activity effectively preventing the rise in myopia incidence. This study reviews the impact of outdoor activities on myopia in children and adolescents by searching databases such as CNKI and Web of Science, aiming to explore effective strategies for myopia prevention and control, thereby providing theoretical references for myopia prevention efforts. Currently, scholars have conducted cross-sectional studies, cohort studies, and intervention studies across various countries, confirming that long-term engagement in outdoor activities can improve the average refractive index of children and adolescents, reducing the risk of myopia. However, it has also been found that the protective effect of outdoor activity duration on myopia in children and adolescents has a certain threshold. Additionally, numerous animal and human experiments have investigated the mechanisms by which outdoor activities influence myopia, revealing that outdoor activities can reduce the incidence of myopia by promoting ocular refractive development, stimulating dopamine release, and alleviating eye fatigue. Based on the literature review, this study proposes targeted suggestions for myopia prevention and control from the perspectives of the government, schools, and families.

Unveiling the characteristics of MgAl0.5Fe1.5O4 spinel ferrite: A study of structural, optical, and dielectric properties

Aluminium-doped magnesium ferrite (MAFO) has garnered significant interest due to its recently observed low microwave loss properties, making it invaluable for applications at high-frequency operations, including fields like spintronics and magnonics. This study delves into the optical and dielectric properties of MgAl0.5Fe1.5O4 synthesized via a conventional solid-state method followed by sintering at 1350 °C for 6 hrs. The X-ray diffraction analysis confirmed the formation of pure, single-phase MAFO spinel ferrite with the lattice constant of 8.313 Å. The Williamson–Hall (W-H) method was employed to determine the crystallite size and strain from X-ray diffraction data, resulting in values of 115.4 nm and 9.37 × 10–4, respectively. UV–visible spectroscopy was used to determine the optical properties, revealing a band gap of 2.38 eV and an average refractive index of 2.09 for the investigated spinel ferrite. The dielectric properties including capacitance, dielectric constant (ε), dielectric loss, ac conductivity, complex impedance, and modulus of MAFO sample were studied as a function of frequency for various temperatures. This comprehensive analysis sheds light on the previously undiscovered traits in the material's transport phenomena and optical attributes, opening an exciting and active field of research into its other physical properties.

Soft x-ray high diffraction efficiency and spectral flux laminar-type W/B4C multilayer diffraction grating for 300-1000 eV.

Multilayer diffraction gratings are designed to improve the detection limit and sensitivity of soft x-ray flat-field spectrographs in the region of 300-1000eV, placing emphasis on Fe-L (705eV), Cu-L (930eV), and Zn-L (1012eV) emissions. For this purpose, spectral flux was used as the performance index, which is proportional to the amount of optical flux incident into a detector and correlated with detection sensitivity. A super-mirror-type W/B4C multilayer coating [Koike et al., Rev.Sci. Instrum. 94, 045109 (2023)] was employed to improve diffraction efficiency in a wide energy region. The unique feature of the multilayer structure is that the average refractive index and the period length of W/B4C layer pairs are increased from the bottom to top layers. In addition, the incidence angle was reduced to 86.03° from 88.65° and the nominal groove density was increased to 3200 lines/mm from 2400 lines/mm of our previous design, to improve spectral flux while maintaining dispersion and spectral resolution. A holographic varied-line-spacing spherical grating and a soft x-ray flat-field spectrograph were designed, using the aspherical-wavefront-recording method, assuming the nominal grating constant and incident angle described above. The numerical simulation results showed that the spectrograph employing the newly designed grating with the W/B4C multilayer indicated 3.2-8.2 times higher spectral flux compared with those using the previously designed grating while keeping the same spectral resolution.

Enhanced optical performance of solar cell using hydrophobic SnO2/TEOS/MTMS antireflection coating

AbstractAnti‐reflection coatings have potential usage in photovoltaic solar cells, sensors, and display devices to reduce reflectance, glare and enhance light transmission. ARC was developed to increase more efficiency of solar cell cover glasses, by depositing SnO2/TEOS/MTMS coating using the sol–gel spin coating technique. The coating showed a maximum transmittance of 92.70% at 399 nm wavelength with an average refractive index of 1.42. The transmittance of the antireflective film was increased by 2.29% than the bare glass substrate. The surface morphology of the coatings was investigated using FESEM analysis. The mechanical stability of the coating was evaluated using ASTM standard D 3363–05 pencil scratch test, and it demonstrated good performance against 3H hardness pencil. The efficiency of solar cells has been increased by 1.56% after depositing with single layer SnO2/TEOS/MTMS film. Moreover, the coating maintains the solar cell's performance even during dust exposure, because of its self‐cleaning ability with water contact angle of 94°. Thus, the Anti‐reflection coating can be applied to enhance the efficiency of photovoltaic system.

Line-field dynamic optical coherence tomography platform for volumetric assessment of biological tissues.

Dynamic optical coherence tomography (dOCT) utilizes time-dependent signal intensity fluctuations to enhance contrast in OCT images and indirectly probe physiological processes in cells. Majority of the dOCT studies published so far are based on acquisition of 2D images (B-scans or C-scans) by utilizing point-scanning Fourier domain (spectral or swept-source) OCT or full-field OCT respectively, primarily due to limitations in the image acquisition rate. Here we introduce a novel, high-speed spectral domain line-field dOCT (SD-LF-dOCT) system and image acquisition protocols designed for fast, volumetric dOCT imaging of biological tissues. The imaging probe is based on an exchangeable afocal lens pair that enables selection of combinations of transverse resolution (from 1.1 µm to 6.4 µm) and FOV (from 250 × 250 µm2 to 1.4 × 1.4 mm2), suitable for different biomedical applications. The system offers axial resolution of ∼ 1.9 µm in biological tissue, assuming an average refractive index of 1.38. Maximum sensitivity of 90.5 dB is achieved for 3.5 mW optical imaging power at the tissue surface and maximum camera acquisition rate of 2,000 fps. Volumetric dOCT images acquired with the SD-LF-dOCT system from plant tissue (cucumber), animal tissue (mouse liver) and human prostate carcinoma spheroids allow for volumetric visualization of the tissues' cellular and sub-cellular structures and assessment of cellular motility.

Nonlinear metasurface engineering with disordered gold nanorods on ITO: a cost-effective approach to broadband response, polarization-independence, and weak nonlinear index dispersion.

The strong coupling of epsilon-near-zero materials with nanoantennas has demonstrated enhanced nonlinear optical responses, yet practical challenges persist. Here, we propose an alternative: an ultrathin metasurface featuring broadband response with a weakly dispersive nonlinear index, achieved through a simple implementation. Our metasurface, comprising a disordered gold nanorod array on indium tin oxide, exhibits polarization-independent behavior and a large average nonlinear refractive index of 5 cm2/GW across a broad wavelength range (1000-1300 nm). Enhanced performance is attributed to the weak coupling between gold nanorods and indium tin oxide, offering a cost-effective method for nonlinear optical metasurfaces and a flexible design in nanophotonic applications.

Controlling the Optical Properties of Transparent Auxetic Liquid Crystal Elastomers.

Determining the tunability of the optical coefficients, order parameter, and transition temperatures in optically transparent auxetic liquid crystal elastomers (LCEs) is vital for applications, including impact-resistant glass laminates. Here, we report measurements of the refractive indices, order parameters, and transition temperatures in a family of acrylate-based LCEs in which the mesogenic content varies from ∼50 to ∼85%. Modifications in the precursor mixture allow the order parameter, ⟨P2⟩, of the LCE to be adjusted from 0.46 to 0.73. The extraordinary refractive index changes most significantly with composition, from ∼1.66 to ∼1.69, in moving from a low to high mesogenic content. We demonstrate that all LCE refractive indices decrease with increasing temperature, with temperature coefficients of ∼10-4 K-1, comparable to optical plastics. In these LCEs, the average refractive index and the refractive index anisotropy are tunable via both chemical composition and order parameter control; we report design rules for both.

Pigments, minerals, and copper-corrosion products: Terahertz continuous wave (THz-CW) spectroscopic characterization of antlerite and atacamite

In this work, the optical properties of atacamite, antlerite and azurite are characterized for the first time by exploiting a high-resolution coherent THz-CW spectrometer in the spectral range 0.1–2.8 THz adopting transmission configuration; the spectral response of atacamite and antlerite is reported for the first time in literature in this frequency range. The absorption spectra of the two copper-based pigments showed the presence of distinct absorption peaks defining their fingerprints. In particular, atacamite presents a peak centered at 2.45 THz and antlerite presents two spectral fingerprints centered at 1.77 and 2.52 THz; the retrieved average refractive index is 1.19 and 1.29, respectively. The absorption spectrum of azurite is in good agreement with previous works in the far infrared region and presents two sharp peaks centered at 1.83 and 2.23 THz with an average value of the refractive index of 1.65. Furthermore, by means of the same technique, it is possible to discriminate and quantify pure components in binary mixtures. This result is of great interest for the identification of pigments and metallic corrosion products, and it demonstrates that THz-CW spectroscopy can open the way to an innovative approach for the Cultural Heritage field.