Refractive Index Measurements Research Articles

Large-range refractive index sensor based on photonic jet

We report a large-range refractive index(RI) sensor based on photonic jet(PJ). It consists of a dielectric scatterer and a metallic reflection plate with a microwell containing the sample to be measured. The sensing is based on relations of two PJ characteristic parameters, focal length(FL) and intensity at focal point(Imax/I0), to sample RI. Studies show that the two parameters reveal an asynchronously periodic change with sample RI. The sensor has controllable sensitivity, small sensing limit 0.02 and extra-large RI sensing range from 1.0 to an infinity, which are better than those of other RI sensors reported previously. The RI can be overall determined from measured FL and Imax/I0, increasing measurement reliability. In addition, the sensor has scalability and generality. With scaled geometry, the sensor can operate at various wavelength regions. The sensor is applicable to RI measurement of all dielectric optical materials, especially to that of a powder or liquid material.

Refractive index in the JUNO liquid scintillator

In the field of rare event physics, it is common to have huge masses of organic liquid scintillator as detection medium. In particular, they are widely used to study neutrino properties or astrophysical neutrinos. Thanks to its safety properties (such as low toxicity and high flash point) and easy scalability, linear alkyl benzene is the most common solvent used to produce liquid scintillators for large mass experiments. The knowledge of the refractive index is a pivotal point to understand the detector response, as this quantity (and its wavelength dependence) affects the Cherenkov radiation and photon propagation in the medium. In this paper, we report the measurement of the refractive index of the JUNO liquid scintillator between 260–1064 nm performed with two different methods to cover wide range of refractive index (an ellipsometer and a refractometer), with a sub percent level precision. In addition, we used an interferometer to measure the group velocity in the JUNO liquid scintillator and verify the expected value derived from the refractive index measurements.

SPR based dual parameter wide range curling pot shaped photonic crystal fiber sensor

This article proposes a curling pot shaped photonic crystal fiber (PCF) sensor based on surface plasmon resonance (SPR), which utilizes two parallel polished surfaces in the cladding to achieve dual parameter measurements of liquid refractive index (RI) and temperature. The mode characteristics and sensing performance of the designed PCF sensor are studied using the finite element method, and the effects of changes in structural parameters such as pore radius, spacing, and gold film thickness on the resonance spectrum are analyzed. The sensing accuracy of the sensor is insensitive to the change of structural parameters, and it has the characteristics of a wide detection range, high sensitivity, and easy manufacture. When the measured RI is in the range of 1.33∼1.42, the maximum RI sensitivity is 20400 nm RIU−1, and the maximum FOM is 483.3 RIU−1. When the temperature ranges from −10 °C to 100 °C, the maximum sensitivity is 15.4 nm °C−1, and the maximum FOM is 0.43 RIU−1. The tight structure design of the sensor core close to the polishing surface and the anti-spill light design with a uniform arrangement of air holes enhance the SPR effect, which is the essential reason for achieving a wide detection range and high sensitivity.

Temperature-insensitive optical fiber sensor based on SMF-NCF-FMF-NCF-SMF spindle-shaped structure for refractive index measurement

Temperature-insensitive optical fiber sensor based on SMF-NCF-FMF-NCF-SMF spindle-shaped structure for refractive index measurement

Deep-learning approach to measuring the refractive index of transparent liquids

A deep-learning approach is introduced to determine the refractive index of transparent liquids based on variations in the displacement of ultra-smooth interference fringes. The phase characteristics of these fringe variations captured in video data were analyzed and modeled using group-phase fitting. A neural network model, integrating a dense convolutional network with a long short-term memory network, was then developed and trained for high-precision liquid refractive index measurements. Experiments demonstrated an R2 accuracy of 99.70% and a mean squared error of 0.0003. This methodology has been confirmed to be temperature-dependent, considerably stable against external disturbances, highly accurate, and capable of real-time processing.

Refractive index measurement for plane parallel plate using fiber point diffraction lateral interference and numerical modeling

A plane-parallel-plate (PPP) refractive index (RI) measurement method based on fiber point diffraction lateral interferometry and numerical modeling is proposed. Two fiber point light sources (PLSs) generate lateral interference through PPP, and the interferograms are collected by a linear array camera (LAC). Employing ray tracing of an ideal PLS and numerical simulation, the theoretical model of RI measurement is established by polynomial fitting with the RI of PPP as the output, the PPP thickness and the phase change at specific positions of the LAC detector as the inputs. To enhance measurement accuracy, the theoretical model is further corrected using PPP samples with known RI, measured thicknesses and phase changes. The experimental results indicate that the RI measurement accuracy can reach 10−4. Remarkably, this method features a simple structure and eliminates the intermediate optical components, except for the fiber PLSs, PPP samples, and LAC. We demonstrate how to simulate an ideal PLS with fiber PLSs and correct the numerical simulation model to improve the accuracy of RI measurement. Furthermore, this approach can be extended to other optical measurements.

State primary special standard of complex refractive index and length in the field of measuring the thickness of optical coatings GET 203-2024

In many areas of science and technology, there is a task to measure the optical and geometric characteristics of thin fi lms. The need to ensure the uniformity of measurements in this area led to the creation of the State primary standard of complex refractive index units GET 203-2012. In it, the complex refractive index was measured using spectral ellipsometry by measuring ellipsometric angles. However, this standard did not provide metrological support for coating thickness measurements when measuring their complex refractive index. In the period 2020–2023, VNIIOFI GET 203-2012 improved and expanded its functionality in terms of reproducing the unit of length in the fi eld of thickness measurements of optical coatings. The improved standard has been approved as the State primary special standard for units of complex refractive index and units of length in the fi eld of thickness measurements of optical coatings GET 203-2024. GET 203-2024 ensures the unity of measurements of complex refractive index and units of length in the fi eld of optical thickness measurements in the range from 1 nm to 50 μm. The range expansion was achieved by introducing an ellipsometer with an infrared range wavelength range equipped with an FTIR Fourier spectrometer. This range expansion is important for such industries as optics, microelectronics, optoelectronics, integrated optics and other areas of science and technology. This article presents the composition, operating principle and main metrological characteristics of GET 203-2024.

Simultaneous measurement of temperature and liquid refractive index based on fiber open Fabry-Pérot cavity and Bragg grating

Simultaneous measurement of temperature and liquid refractive index based on fiber open Fabry-Pérot cavity and Bragg grating

NaBa12(BO3)7F4 (NBBF) dichroic crystals: optical properties and dielectric permittivity.

Three NaBa12(BO3)7F4 crystals were grown in the BaO-BaF2-B2O3-Na2O system from three different compositions of high-temperature solution. With the use of optical spectroscopy, energy-dispersive X-ray spectroscopy, and the Schering bridge method it was found that the crystals revealed sharp differences in their optical and dielectric properties. The relative permittivity of the crystals in direction perpendicular to the optical axis reached 319(5). The minimum deviation technique was used for refractive index measurements.

Measurement and thermodynamic correlation of liquid–liquid equilibria of water + n-propanol + toluene at 20°C and atmospheric pressure

Although the system water + n-propanol + toluene is important for research and industry, its liquid–liquid equilibria LLE are scarcely covered in literature, with no thermodynamic modeling reported using typical equations of state or activity coefficient models. The current work addresses this gap by presenting the equilibria of the system at 20°C and atmospheric pressure. That includes the binodal curve and twelve tie lines covering the entire miscibility gap, whereby the compositions of the conjugate phases are determined using high-precision refractive index measurements. The quality of the data is verified by applying the techniques of conjugate lines and mass balances, whereas it is shown that empirical correlations cannot be used for that purpose. Three different methods to extrapolate the critical point are then employed and, using the more reliable one, that point is located and added to the dataset. Further thermodynamic correlation of the data is conducted using the activity coefficient models, UNIQUAC and NRTL. While the former did not perform well, the latter was able to model the LLE and adapt to temperature changes.

Dual parameter sensor for RI and temperature detection by cascading Ag/WO3 film PCF and Ag/MoS2/PDMS film PCF.

Multi-parameter surface plasmon resonance (SPR) sensors generally have low detection sensitivity due to detection wavelength limitations. We developed a two-parameter SPR sensor for refractive index (RI) and temperature detection by cascading Ag/WO3 film photonic crystal fiber (PCF) and Ag/MoS2 film PCF together. By using WO3 film with a low effective refractive index to prevent oxidation of silver-based PCF and maintain a wider RI sensing channel detection band; at the same time, MoS2 film with a high effective refractive index is used to modulate the detection range of temperature sensing channel. The detection performance and stability of the two-parameter SPR sensor were verified by software simulation and experiments. The data results indicate that the sensor can perform stable and independent tests during RI and temperature detection. The maximum sensitivity of the sensor within the RI detection range of 1.333 to 1.395 is as high as 6443 nm/RIU; the maximum temperature sensitivity within the temperature detection range of 40 °C∼90 °C is 8.72 nm/°C. This sensor can achieve high-sensitivity RI measurement in a high-temperature environment. This will have broad application prospects in the field of biochemistry.

Whispering gallery mode sensing through the lens of quantum optics, artificial intelligence, and nanoscale catalysis

Ultra-sensitive sensors based on the resonant properties of whispering gallery modes (WGMs) can detect fractional changes in nanoscale environments down to the length and time scales of single molecules. However, it is challenging to isolate single-molecule signals from competing noise sources in experiments, such as thermal and mechanical sources of noise, and—at the most fundamental level—the shot noise limit of classical light. Additionally, in contrast to traditional bulk refractive index measurements, analyzing single-molecule signals is complicated by the localized nature of their interactions with nanoscale field gradients. This perspective discusses multifaceted solutions to these challenges, including the use of quantum light sources to boost the signal-to-noise ratio in experiments and leveraging the power of supercomputers to predict the electronic response of molecules to WGM optoplasmonic fields. We further discuss the role of machine learning in WGM sensing, including several advanced models that can predict molecular polarizability and solvent effects. These advancements in WGM spectroscopy and computational modeling can help to decipher the molecular mechanics of enzymes, enable studies of catalysis on the nanoscale, and probe the quantum nature of molecules.

A cascade SPR sensor based on Ag/Au coated coreless optical fiber for RI and pH measurement

Due to the restriction of resonant wavelength, the detection range of traditional two-parameter sensors is greatly limited. To solve this problem, a cascade SPR sensor with Ag/Au coating is proposed to measure refractive index (RI) and pH value. In this paper, coreless optical fiber is used as the sensor probe, and Ag/Au are coated on its surface by a magnetron sputtering method. The two sensing channels of this cascade sensor are independent of each other and have a wide parameter detection range. The effects of sensor length and coating time on the performance of the sensor were investigated, and the optimal sensor length and coating time were determined. The experimental results show that the maximum refractive index sensitivity is 3888.6 nm /RIU in the RI range of 1.333–1.385, and the maximum pH sensitivity is 38.01 nm/pH in the pH range of 3.15–8.86. The sensor has the advantages of strong stability and high integration and has a good application prospect in the fields of biosensing and environmental detection.

Measurement Error Analysis of Seawater Refractive Index: A Measurement Sensor Based on a Position-Sensitive Detector.

The seawater refractive index is an essential parameter in ocean observation, making its high-precision measurement necessary. This can be effectively achieved using a position-sensitive detector-based measurement system. However, in the actual measurement process, the impact of the jitter signal measurement error on the results cannot be ignored. In this study, we theoretically analysed the causes of long jitter signals during seawater refractive index measurements and quantified the influencing factors. Through this analysis, it can be seen that the angle between the two windows in the seawater refractive index measurement area caused a large error in the results, which could be effectively reduced by controlling the angle to within 2.06°. At the same time, the factors affecting the position-sensitive detector's measurement accuracy were analysed, with changes to the background light, the photosensitive surface's size, and the working environment's temperature leading to its reduction. To address the above factors, we first added a 0.9 nm bandwidth, narrow-band filter in front of the detector's photosensitive surface during system construction to filter out any light other than that from the signal light source. To ensure the seawater refractive index's measuring range, a position-sensitive detector with a photosensitive surface size of 4 mm × 4 mm was selected; whereas, to reduce the working environment's temperature variation, we partitioned the measurement system. To validate the testing error range of the optimised test system, standard seawater samples were measured under the same conditions, showing a reduction in the measurement system's jitter signal from 0.0022 mm to 0.0011 mm, before and after optimisation, respectively, as well as a reduction in the refractive index's deviation. The experimental results show that the refractive index of seawater was effectively reduced by adjusting the measurement system's optical path and structure.

Measuring Concentration of Nanoparticles in Polydisperse Mixtures Using Interferometric Nanoparticle Tracking Analysis.

Quantitative measurements of nanoparticle concentration in liquid suspensions are in high demand, for example, in the medical and food industries. Conventional methods remain unsatisfactory, especially for polydisperse samples with overlapping size ranges. Recently, we introduced interferometric nanoparticle tracking analysis (iNTA) for high-precision measurement of nanoparticle size and refractive index. Here, we show that by counting the number of trajectories that cross the focal plane, iNTA can measure concentrations of subpopulations in a polydisperse mixture in a quantitative manner and without the need for a calibration sample. We evaluate our method on both monodisperse samples and mixtures of known concentrations. Furthermore, we assess the concentration of SARS-CoV-2 in supernatant samples obtained from infected cells.

Ultra-high-sensitive plasmonic sensor based on asymmetric hexagonal nano-ring resonator for cancer detection

A highly sensitive sensor based on two metal-insulator-metal waveguides coupled to an asymmetric hexagonal nano-ring resonator detecting cancerous cells is proposed. This novel design is utilized to facilitate the sensing of human cells. The sensing mechanism of the presented optical structure can act as a refractive index measurement in biological, chemical, biomedical diagnosis, and bacteria detection, which leads to achieving high sensitivity in the structure. The main goal is to achieve the highest sensitivity concerning the optimum design. As a result, the sensitivity of the designed topology reaches a maximum value of about 1800 nm/RIU (nm/refractive index unit) by controlling the angle of the resonator. It is evident that the sensitivity parameter is improved, and the reason for the increase in sensitivity is due to the asymmetry of the resonator, which has an 81 % increase in sensitivity compared to the symmetrical resonator, especially for blood cancer cells. The maximum quality factor obtains 131.65 with a FOM of 90.4 (RIU−1). The sensing performance of this proposed structure is numerically investigated using the finite difference time domain (FDTD) method with the perfectly matched layer (PML). Accordingly, the suggested high sensitivity sensor makes this structure a promising therapeutic candidate for sensing applications that can be used in on-chip optical devices to produce highly complex integrated circuits.

Modal group refractive index measurement of few-mode fibers based on time-domain cross-correlation

We propose a measurement method based on the time-domain cross-correlation technique, combined with the cut-back method, enabling the measurement of group refractive indices (n g ) in few-mode fibers (FMF). A Mach–Zehnder interferometric system, equipped with high-precision and extensive range delay devices, is established. The system records off-axis holograms of spatial reference light at various delays interfering with the emitted light from the fiber under test. The interference energy is extracted from these holograms, and a time-domain mode energy curve is developed utilizing the principle of cross correlation. Optimal holograms at each of the curve peaks are used to reconstruct the modal field distribution, effectively separating and accurately identifying each mode within the FMF. By integrating the cut-back method, the n g corresponding to each mode is calculated based on the changes in group delay before and after fiber cutting. The n g of modes in the two-mode fibers was measured and the differential group delay calculated from the measurement agrees with the manufacturer’s specifications. The measured n g of a standard single-mode fiber aligns with the manufacturer’s specifications. Furthermore, the n g of the higher-order modes in four-mode fibers were measured by exciting them at different angles and validating the wave optics theory that the n g of the fiber modes is independent of the excitation angle. This method can simultaneously measure the n g of several modes in a fiber, providing support for the development and application of FMFs.

High operational functionality nanofluids from choline chloride, ethylene glycol, and multi-walled carbon nanotubes – Economical substitutes for ionanofluids

Deep eutectic solvents (DESs) are an emerging class of green, easy-to-synthesize, and inexpensive solvents with designable properties widely used in nanotechnology applications. The optimization of such working fluids for energy transfer purposes requires the simultaneous maximization of their thermal/electrical conductivities and thermal/shelf-life stabilities, as well as minimization of their viscosity and overall material/maintenance costs. In this work, there is provided a comprehensive analysis of physicochemical properties, including, first-ever measurements of electrical conductivity and refractive index, of new highly-concentrated DES-based nanofluids composed of choline chloride/ethylene glycol (ChCl/EG) with molar ratios of 1:15, 1:5, 1:3, and 0.25–1 wt% of originally 860-µm-long multi-walled carbon nanotubes (MWCNTs), cut upon sonication to 14.0–16.9 µm. The study deepens the understanding of the mechanisms occurring at the molecular level during heat and electricity transfer in DES-based nanofluids and provides new empirical transport-property correlations. The analyzed nanodispersions are highly viscous (150–840 mPa·s at 25 °C, 9.3 s–‍1) non-Newtonian fluids (flow behavior index 0.49 ≤ m ≤ 0.66), though their viscosity can be notably reduced by elevated temperature and shear rate (28.3–120 mPa·s at 40 °C, 186 s−1). Importantly, DES-based nanofluids exhibit significant improvement in thermal conductivity (λ = 0.307 W·m−1·K−1), AC electrical conductivity (σ = 42.6 mS·cm−1), and decomposition temperature (T50%=255 °C) for ChCl/EG 1:3 + 1 wt% MWCNTs at 25 °C, thus, they can potentially become an interesting alternative to ionanofluids with up to 83 times lower raw material costs and at least 30-month shelf-life stability.

S-shaped tellurite optical fiber surface plasmon resonance sensor for temperature and refractive index measurement

In this paper, an all-fiber S-shaped structure optical fiber surface plasmon resonance (SPR) sensor is proposed for temperature and refractive index measurement. The sensor is made of a single-mode tellurite optical fiber (TOF), and the middle part of the single-mode TOF is heated by a ceramic heating rod to form an S-shaped sensing region, and a 50 nm gold film is deposited in the S-shape sensing area to excite SPR. Based on the excellent properties of tellurite optical fiber, the prepared optical fiber sensor can be used for high sensitivity temperature and refractive index detection. The highest refractive index sensitivity is up to 1068.59 nm/RIU in the range of 1.333–1.380, and the temperature sensitivity is up to −0.713 nm/°C in the range of 30–150 °C. The proposed sensor is simple to fabricate, low-cost, and compact, and can be widely used in biochemical analysis, complex environmental temperature monitoring and other fields.

Applicability of non-invasive and live-cell holotomographic imaging on fungi

The ability to acquire three-dimensional (3D) information of cellular structures without the need for fluorescent tags or staining makes holotomographic imaging a powerful tool in cellular biology. It provides valuable insights by measuring the refractive index (RI), an optical parameter describing the phase delay of light that passes through the living cell.Here, we demonstrate holotomographic imaging on industrial relevant ascomycete fungi and study their development and morphogenesis. This includes conidial germination, subcellular dynamics, and cytoplasmic flow during hyphal growth in Aspergillus niger. In addition, growth and budding of Aureobasidium pullulans cells are captured using holotomographic microscopy. Coupled to fluorescence imaging, lipid droplets, vacuoles, the mitochondrial network, and nuclei are targeted and analyzed in the 3D RI reconstructed images. While lipid droplets and vacuoles can be assigned to a specific RI pattern, mitochondria and nuclei were not pronounced. We show, that the lower sensitivity of RI measurements derives from the fungal cell wall that acts as an additional barrier for the illumination light of the microscope. After cell wall digest of hyphae and protoplast formation of A. niger expressing GFP-tagged histone H2A, location of nuclei could be determined by non-invasive RI measurements. Furthermore, we used coupled fluorescence microscopy to observe migration of nuclei in unperturbed hyphal segments and duplication during growth on a single-cell level. Detailed micromorphological studies in Saccharomyces cerevisiae and Trichoderma reesei are challenging due to cell size restrictions.Overall, holotomography opens up new avenues for exploring dynamic cellular processes in real time and enables the visualization of fungi from a new perspective.