Precise Refractive Index Research Articles

Refractive Index Evaluation in Active TDBC Layers for Photonics Applications

Tetrachlorodiethyl Benzimidazo Carbocyanine (TDBC) layers are very interesting for photonics applications due to their huge oscillator strength, narrow absorption and low-cost fabrication. They are mainly used in strong coupling studies but also for wavelength selective grating fabrication, light concentration, absorption enhancement and so on. However, these intrinsic properties, particularly the refractive index, require further investigation. In this work, we first reviewed the values of the refractive index of TDBC layers reported in the literature. Using fitting with the Drude–Lorentz model, differences are highlighted. We then fabricated pure TDBC layers and measured their properties using ellipsometry and absorption spectroscopy. Finally, we also evaluated the refractive index as a function of the layer bleaching. This work shows that although the precise refractive index evaluation of pure TDBC layers is dependent on the measurement method, their oscillator strength force still remains very high without bleaching.

Multilayer all-dielectric metasurfaces expanding color gamut

Abstract Structural color, arising from the interaction between nanostructures and light, has experienced rapid development in recent years. However, high-order Mie resonances in dielectric materials often induce unnecessary sub-peaks, particularly at shorter wavelengths, reducing the vibrancy of colors. To address this, we have developed a multilayer dielectric metasurface based on silicon-rich silicon nitride (SRN), achieving expanded color gamut through precise refractive index matching and suppression of high-order resonances. This strategy introduces more design dimensions and can reduce the complexity of material deposition. It enables the generation of vibrant colors in a 3 × 3 array, with a resolution of approximately 25,400 dpi, demonstrating its potential applications in displays.

Precise refractive index measurement of fused silica optics

Abstract We have developed an experimental platform that non-destructively and precisely measures the Refractive Index (RI) and dispersion of ultra-polished fused silica optics. Using Total Internal Reflection Digital Holographic Microscopy (TIRDHM), we exploit the phase change of reflected light in Total Internal Reflection (TIR) mode. This phase change depends on the incident angle at the TIR interface and the refractive indices of the involved media. We have optimized a combination of higher TIR phase sensitivity, considerable penetration depth, and minimized phase measurement inaccuracies through simulations to design our experiment. Key features include a custom-made precision Right-Angle Prism (RAP) of Astrositall material, a seamless interface with fused silica optics on TIR interface through optical contact, and single-shot measurement. We have demonstrated the accuracy of measuring fused silica optics through proof of concept and experimental results. Our measurements on two different samples show accuracy better than ±3 × 10−4 compared to those obtained using a commercially available critical angle Refractometer (Metricon). Importantly, the setup offers the advantage of spatially mapping the refractive index, unlike point measurements by available Refractometers.

Highly Sensitive On-Chip Grating-Based Optical Sensor on Glass Substrate: Cost-Effective Design

This paper presents a novel on-chip optical sensing system using ion exchange technology and phase-shifted Bragg grating design. The sensor geometry has been optimized using a finite difference time domain (FDTD) solver to achieve maximum sensitivity and figure of merit (FOM). The proposed design offers high sensitivity to changes in refractive index and low fabrication costs. The ion exchange process used to create the sensor allows for precise refractive index control, optimizing the sensor’s sensitivity and FOM. A graded-index waveguide and a phase-shifted Bragg grating structure also contribute to the sensor’s high sensitivity. The proposed sensor design was tested for water-based sensing applications, achieving a FOM of 227.63 and a 343.1 nm/RIU sensitivity. These values are significantly higher than those reported for other Bragg grating sensors, highlighting the potential of the proposed design for high-performance sensing applications. The sensor’s high sensitivity and low fabrication costs make it a promising technology for future sensing and monitoring applications.

Dual high-Q Fano resonances metasurfaces excited by asymmetric dielectric rods for refractive index sensing

Abstract The metasurface refractive index sensor has a high degree of tunability and flexibility, providing excellent performance for high precision refractive index sensing applications. The metasurface absorber with metallic structure has been hindered in further sensor applications due to the inherent Ohmic loss of the metallic material. In this study, a dual nanorod metasurface structure based on semiconductor Si was designed, introducing a symmetry-breaking structure to excite dual ultra-narrow q-BIC resonance peaks with Fano line shapes. Both peaks are located in the near-infrared region, and multipole analysis shows that this strong field enhancement effect is induced by a magnetic dipole. Experimental results demonstrate the potential of this sensor to provide dual-channel detection while achieving high sensitivity and high Q-factor. We believe that this device exhibits outstanding performance and high practicality, providing a reference for the development and application of biological and environmental sensors.

Graphene-based phononic crystal lenses: Machine learning-assisted analysis and design

The advance of artificial intelligence and graphene-based composites brings new vitality into the conventional design of acoustic lenses which suffers from high computation cost and difficulties in achieving precise desired refractive indices. This paper presents an efficient and accurate design methodology for graphene-based gradient-index phononic crystal (GGPC) lenses by combing theoretical formulations and machine learning methods. The GGPC lenses consist of two-dimensional phononic crystals possessing square unit cells with graphene-based composite inclusions. The plane wave expansion method is exploited to obtain the dispersion relations of elastic waves in the structures and then establish the data sets of the effective refractive indices in structures with different volume fractions of graphene fillers in composite materials and filling fractions of inclusions. Based on the database established by the theoretical formulation, genetic programming, a superior machine learning algorithm, is introduced to generate explicit mathematical expressions to predict the effective refractive indices under different structural information. The design of GGPC lenses is conducted with the assistance of the machine learning prediction model, and it will be illustrated by several typical design examples. The proposed design method offers high efficiency, accuracy as well as the ability to achieve inverse design of GGPC lenses, thus significantly facilitating the development of novel phononic crystal lenses and acoustic energy focusing.

Ultra-high-Q and ultra-sensitive refractive index sensing in low refractive index dielectric gratings based on bound states in the continuum

The realization of a high quality factor (Q-factor) and strong local optical fields has long been of great interest in the field of nanophotonics. Unfortunately, it is still challenging to achieve high-Q and strong localized fields in nanostructures made of low refractive index materials. In this study, drawing upon the concept of bound states in the continuum (BICs), an asymmetric dielectric grating composed of low refractive index materials is demonstrated to generate an ultra-high-Q symmetry-protected quasi-BIC in the visible wavelength. Importantly, the design of BIC (quasi-BIC) mode enables the strong localized confinement of light in air. By leveraging this property, we realize ultra-sensitive refractive index sensing with a remarkable sensitivity of 669 nm/RIU and a high figure of merit (FOM) of 45,314RIU−1. This study offers an approach to achieve highly sensitive and high precision refractive index sensing with potential applications in the practical realization of strong light–matter interactions using low index materials.

A double-layer scheme for extrapolating terahertz complex refractivity from reflectance of aerocraft materials

The complex refractive indexes of aero craft materials are seldom investigated. In this paper, we fabricate two aero craft materials: a white painted aluminum plate and a coated window glass, and measure the reflectance spectra in the region of 3–50 THz. Different from the acquainted metal materials, the reflectance spectra of the two materials have apparent peaks and valleys, which is known as the spectral fingerprint. With the preliminary analysis by using Lorentz-Drude/Lorentz dispersion oscillator models and Kramers-Kronig (KK) algorithm, we find that the errors of the extracted complex refractivity from the dispersion oscillator models are apparent at the abrupt regions, and the accuracy of the real refractive index n and extinction coefficient κ from KK algorithm differs dramatically. To reduce the inevitable errors, we design a double-layer extraction scheme to reestablish the oscillator models based on the KK relation. Compared with the inversed data from KK relation and dispersion oscillator models, the procedure assures the accuracy of the real refractive index and extinction coefficient concurrently, the error of n/κ values at the abrupt region is also improved significantly. The method is reliable and convenient to obtain the precise broadband refractive index from spectra with apparent spectral fingerprints. And the obtained complex refractivity can support the further THz scattering analysis for aero craft targets.

DESIGN OF A PRECISE FSO NETWORK INCLUDING IMPROVED MODIFIED REFRACTIVE INDEX STRUCTURE PROFILING FOR SHIMLA CITY, INDIA

The most commonly used altitude-dependent Hufnagel-Valley (HV) model for Refractive-Index Structure Parameter (C<sub>n</sub><sup>2</sup>) calculation has limited accuracy due to the reason of not considering various meteorological parameters for mountainous locations which have negative influence on optical signals. Although altitude and turbulent winds (due to temperature and pressure variations) are significant parameters giving rise to atmospheric turbulences intensity but the presence of water droplets and snowflakes in humid climate of mountainous locations play major role in reducing optical visibility, so should not be ignored. We have included the effect of snowfall in existing HV model while calculating path loss due to atmospheric turbulences to estimate these effects in Shimla city. The computed results show considerable decrement in Q-factor due to the additional losses imposed by snowflakes to the laser beam. Enlargement in effects of absorption and scattering is also observed in bit error rate (BER) results. So, the proposed modified model presents more accurate estimation of the refractive index profile as it takes more parameters into consideration responsible for atmospheric turbulence.

Thermodynamic and volumetric properties of hydroxamic acids in dimethylsulfoxide at T = (298.15 to 313.15) K

Physical properties, such as density, (ρ) and refractive index, (n) of three hydroxamic acids (N-phenylbenzo-, N-phenyl-4-methyl-3-nitrobenzo-, and N-phenyl-4-nitrobenzo-) were measured in dimethyl sulfoxide (DMSO) as a function of concentrations at T = (298.15, 303.15, 308.15, and 313.15) K . The apparent molar volumes (Vϕ0), limiting apparent molar volumes (Vϕ0) at infinite dilution, slope (SV*) and Hepler's constant (∂2Vϕ0∂T2) are calculated from the experimental values of densities (ρ) by applying the Masson's equation. The apparent molar expansibilities at infinite dilution (ϕ0E), molar volumes (V), thermal expansion coefficient (α2) and the excess molar volumes (VE) are also computed. The precise refractive indices (n) data have been used to evaluate the steric parameters viz. molar refractions (RM), polarizability (α) and excess molar refraction (RME) of these molecules. It is inferred from these results that the above mentioned drugs act as structure-making compound due to hydrophobic hydration of the molecules in the drug.

Controllable photonic spin hall effect of bilayer graphene

Bilayer graphene, composed of two layers of monolayer graphene in AB stacking order, has emerged as an alternative platform for atomically thin plasmonic and optoelectronic devices. However, its behavior of photonic spin hall effect remains largely unexplored. In this work, we have theoretically observed that bilayer graphene has two obvious discontinuities but monolayer graphene only has a single step in the spectra of the spin shifts as a function of wavelength at the Brewster angle over the midinfrared frequency range, which enables a possible route of distinguishing monolayer graphene and bilayer graphene. Additionally, the magnitudes and positions of the peak and valley values in the spectrum of spin shifts of bilayer graphene can be tuned by its Fermi energy. We also achieved the enhanced out-of-pane spin shift of the glass-AB stacking bilayer graphene-air structure at both the Brewster angle () and the critical angle () with the aid of the high order of Laguerre–Gaussian beam. The realization of large and controlled spin shift in bilayer graphene indicates its promising applications in precision measurements and refractive index sensors at the midinfrared frequency region.

Precision size and refractive index analysis of weakly scattering nanoparticles in polydispersions

Characterization of the size and material properties of particles in liquid suspensions is in very high demand, for example, in the analysis of colloidal samples or of bodily fluids such as urine or blood plasma. However, existing methods are limited in their ability to decipher the constituents of realistic samples. Here we introduce iNTA as a new method that combines interferometric detection of scattering with nanoparticle tracking analysis to reach unprecedented sensitivity and precision in determining the size and refractive index distributions of nanoparticles in suspensions. After benchmarking iNTA with samples of colloidal gold, we present its remarkable ability to resolve the constituents of various multicomponent and polydisperse samples of known origin. Furthermore, we showcase the method by elucidating the refractive index and size distributions of extracellular vesicles from Leishmania parasites and human urine. The current performance of iNTA already enables advances in several important applications, but we also discuss possible improvements.

Wideband tunable microwave signal generation in a silicon-micro-ring-based optoelectronic oscillator

Si photonics has an immense potential for the development of compact and low-loss opto-electronic oscillators (OEO), with applications in radar and wireless communications. However, current Si OEO have shown a limited performance. Si OEO relying on direct conversion of intensity modulated signals into the microwave domain yield a limited tunability. Wider tunability has been shown by indirect phase-modulation to intensity-modulation conversion. However, the reported tuning range is lower than 4 GHz. Here, we propose a new approach enabling Si OEOs with wide tunability and direct intensity-modulation to microwave conversion. The microwave signal is created by the beating between an optical source and single sideband modulation signal, selected by an add-drop ring resonator working as an optical bandpass filter. The tunability is achieved by changing the wavelength spacing between the optical source and a resonance peak of the resonator. Based on this concept, we experimentally demonstrate microwave signal generation between 6 GHz and 18 GHz, the widest range for a Si-micro-ring-based OEO. Moreover, preliminary results indicate that the proposed Si OEO provides precise refractive index monitoring, with a sensitivity of 94350 GHz/RIU and a potential limit of detection of only 10−8 RIU, opening a new route for the implementation of high-performance Si photonic sensors.

Parity-time symmetry in a single-loop photonic system

Non-Hermitian photonics based on parity-time (PT) symmetry has been considered an effective solution to achieve mode selection for optical or microwave single-mode oscillation. A PT-symmetric system is usually implemented using two mutually coupled elements with identical geometry or a single element based on precise longitudinal refractive index modulation. In this study, we propose a novel scheme to achieve PT symmetry based on polarization mode manipulation in a single optical loop. By controlling the polarization states of two light waves that are propagating along the clockwise (CW) and counter-clockwise (CCW) directions in the single optical loop, two equivalent mutually-coupled loops with identical geometry and a balanced gain and loss are formed. Based on this concept, a wavelength-tunable single-mode fiber laser with a sub-kHz linewidth is realized. The tunability is achieved by thermally tuning the wavelength of an integrated microdisk resonator (MDR) incorporated in the optical loop. Experimental results show that a continuously tunable single-mode laser with a sub-kHz linewidth of 640 Hz and a wavelength tuning range from 1552.953 to 1554.147 nm is realized. The key advantages of using a single optical loop to implement PT symmetry include greatly simplified implementation and highly improved stability. The demonstration opens new avenues to implement high-performance PT-symmetric systems for applications in photonic and microwave photonic systems.

Design a Stratiform Metamaterial with Precise Optical Property

In this work, a stratiform metamaterial is arranged as multiple periods of metal-dielectric symmetrical film stack to provide precise equivalent refractive index and admittance. There are multiple solutions of equivalent refractive index retrieved from the characteristic matrix of the film stack. The correct refractive index is derived by connecting different branches of solution at different ranges of wavelength or thickness of the dielectric layer. The refractive index of an Ag-TiO2 five-layered symmetrical film stack shown in previous work is demonstrated to be positive real instead of negative real. The associated type I iso-frequency curve supports negative refraction. In order to extend the operating wavelength of type I metamaterial, the number of the metal-dielectric symmetrical film stack is increased to reduce the thickness of the dielectric film to approach subwavelength requirement.

Comparative study of quantitative phase imaging techniques for refractometry of optical waveguides.

A comparative study of quantitative phase imaging techniques for refractometry of optical waveguides is presented. Three techniques were examined: a method based on the transport-of-intensity equation, quadri-wave lateral shearing interferometry and digital holographic microscopy. The refractive index profile of a SMF-28 optical fiber was thoroughly characterized and served as a gold standard to assess the accuracy and precision of the phase imaging methods. Optical waveguides were inscribed in an Eagle2000 glass chip using a femtosecond laser and used to evaluate the sensitivity limit of these phase imaging approaches. It is shown that all three techniques provide accurate, repeatable and sensitive refractive index measurements. Using these phase imaging methods, we report a comprehensive map of the photosensitivity to femtosecond pulses of Eagle2000 glass. Finally, the reported data suggests that the phase imaging techniques are suited to be used as precise and non-destructive refractive index shift measuring tools to study and control the inscription process of optical waveguides.

Robustness of Light-Transport Processes to Bending Deformations in Graded-Index Multimode Waveguides.

Light transport through a multimode optical waveguide undergoes changes when subjected to bending deformations. We show that optical waveguides with a perfectly parabolic refractive index profile are almost immune to bending, conserving the structure of propagation-invariant modes. Moreover, we show that changes to the transmission matrix of parabolic-index fibers due to bending can be expressed with only two free parameters, regardless of how complex a particular deformation is. We provide detailed analysis of experimentally measured transmission matrices of a commercially available graded-index fiber as well as a gradient-index rod lens featuring a very faithful parabolic refractive index profile. Although parabolic-index fibers with a sufficiently precise refractive index profile are not within our reach, we show that imaging performance with standard commercially available graded-index fibers is significantly less influenced by bending deformations than step-index types under the same conditions. Our work thus predicts that the availability of ultraprecise parabolic-index fibers will make endoscopic applications with flexible probes feasible and free from extremely elaborate computational challenges.

Options for refractive index and viscosity matching to study variable density flows

Variable density flows are often studied by mixing two miscible aqueous solutions of different densities. To perform optical diagnostics in such environments, the refractive index of the fluids must be matched, which can be achieved by carefully choosing the two solutes and the concentration of the solutions. To separate the effects of buoyancy forces and viscosity variations, it is desirable to match the viscosity of the two solutions in addition to their refractive index. In this manuscript, several pairs of index matched fluids are compared in terms of viscosity matching, monetary cost, and practical use. Two fluid pairs are studied in detail, with two aqueous solutions (binary solutions of water and a salt or alcohol) mixed into a ternary solution. In each case: an aqueous solution of isopropanol mixed with an aqueous solution of sodium chloride (NaCl) and an aqueous solution of glycerol mixed with an aqueous solution of sodium sulfate ( $$\hbox {Na}_2\hbox {SO}_4$$ ). The first fluid pair allows reaching high-density differences at low cost, but brings a large difference in dynamic viscosity. The second allows matching dynamic viscosity and refractive index simultaneously, at reasonable cost. For each of these four solutes, the density, kinematic viscosity, and refractive index are measured versus concentration and temperature, as well as wavelength for the refractive index. To investigate non-linear effects when two index-matched, binary solutions are mixed, the ternary solutions formed are also analyzed. Results show that density and refractive index follow a linear variation with concentration. However, the viscosity of the isopropanol and NaCl pair deviates from the linear law and has to be considered. Empirical correlations and their coefficients are given to create index-matched fluids at a chosen temperature and wavelength. Finally, the effectiveness of the refractive index matching is illustrated with particle image velocimetry measurements performed for a buoyant jet in a linearly stratified environment. The creation of the index-matched solutions and linear stratification in a large-scale experimental facility are detailed, as well as the practical challenges to obtain precise refractive index matching.

Microwave analog experiments on optically soft spheroidal scatterers with weak electromagnetic signature

Light scattering by optically soft particles is being theoretically investigated in many radiative studies. An interest is growing up to develop approximate methods when the resolution of Maxwell's equations is impractical due to time and/or memory size problems with objects of complex geometries. The participation of experimental studies is important to assess novel approximations when no reference solution is available. The microwave analogy represents an efficient solution to perform such electromagnetic measurements in controlled conditions. In this paper, we take advantage of the particular features of our microwave device to present an extensive experimental study on the electromagnetic scattering by spheroidal particles analogs with low refractive indices, as a first step toward the assessment of micro-organisms with low refractive index and heterogeneities. The spheroidal analogs are machined from a low density material and they mimic soft particles of interest to the light scattering community. The measurements are confronted to simulations obtained with Finite Element Method and T-Matrix method. A good agreement is obtained even with refractive index as low as 1.13. Scattered signals of low intensities are correctly measured and the position of the targets is precisely controlled. The forward scattering measurements show high sensitivity to noise and require careful extraction. The configuration of the measurement device reveals different technical requirements between forward and backward scattering directions. The results open interesting perspectives about novel measurement procedures as well as about the use of high prototyping technologies to manufacture analogs of precise refractive indices and shapes.

Application of strongly overcoupled resonant modes of long-period fiber gratings to measure the adulteration of olive oil.

We have shown that strongly overcoupled resonant modes of long-period fiber gratings (LPFGs) can be used to assess the adulteration of olive oil. In this background, we investigate the response characteristics of strongly overcoupled resonant modes of different orders to a surrounding medium, the refractive index (RI) of which is greater than that of the fiber cladding, and in the range where a precise refractive index measurement is immensely useful for inspecting the quality of olive oils and other edible oils. A theoretical simulation that would help in designing a sensor with suitable sensitivity and range of measurement has been presented in detail and also validated with experimental results. It was interesting to observe that in a high RI surrounding, a lower order overcoupled resonant mode is much more sensitive as compared to a higher-order one having a similar coupling coefficient. A quantitative analysis demonstrates that for a particular LPFG, the sensitivity of a strongly overcoupled LP06 mode was found to be ∼2000 dB/RIU, while that of the LP07 mode having similar coupling strength was ∼550 dB/RIU in the surrounding refractive index range from 1.458 to 1.520. The results have been validated experimentally.