Refractive Index Unit Research Articles

The modified minimum deviation method for measuring the refractive index

We present a modified method for measuring the refractive index (RI) based on the minimum deviation technique, which allows us to determine the RI of transparent triangular prisms with unknown apex angles. In the proposed method, the angles of light deviation are measured on three faces of the prism, and RI of the material and prism angles are determined from the solution of a system of equations. A high precision dynamic goniometer was used to implement the proposed method. A set of reference prisms made of optical glass were experimentally studied and the measurement errors were estimated. It is shown that the modified method can be used for high-precision measurements of the RI in cases when the prism angles are unknown, or their measurement is associated with technical difficulties. The method of measuring RI proposed in the article was implemented using the State Primary Standard of the Refractive Index Unit GET 138–2021 of the All-Russian Research Institute for Optical and Physical Measurements. This standard is intended for storing, reproducing, and transmitting the unit size of the RI of solid and liquid substances.

Photonic Crystal Waveguides Composed of Hyperbolic Metamaterials for High-FOM Nano-Sensing

This study introduces an innovative integration of hyperbolic metamaterials (HMMs) and photonic crystals (PtCs), each possessing unique dispersion properties that effectively manipulate the propagation of light. We present a PtC waveguide consisting of arrays of HMM nanorods, denoted as HMM PtCs. This waveguide configuration enables the realization of a high figure of merit (FOM) nano-sensor. HMMs and PtCs share the same underlying physics. HMMs can generate surface plasmonics, while PtCs offer a bandgap for the waveguide. This configuration presents a novel sensing solution that directly couples surface plasmonics and waveguide modes. By modifying the refractive indices of the surrounding materials, the PtC waveguide exhibits alterations in absorption and transmission, allowing for the detection of temperature, pressure, and material variations. The refractive indices of the surrounding materials can be adjusted based on the sensor’s intended application. For instance, when the sensor is utilized for temperature sensing, thermal infrared materials can serve as the surrounding medium. As the temperature rises, the refractive index of the surrounding material changes accordingly, impacting the waveguide modes and thereby altering absorption and transmission. We utilized the finite element method to conduct numerical simulations in order to assess the absorption and transmission characteristics of the proposed system. Given that this approach involves a full electromagnetic calculation based on Maxwell’s equations, it closely approximates real-world scenarios. The employed numerical method demonstrates the remarkable performance of this proposed system, achieving a sensitivity of 324.16 nm/RIU (refractive index unit) and an impressive FOM of 469.58 RIU−1. These results signify a substantial improvement over surface plasmonic sensors, which typically exhibit limited FOMs. The direct coupling between surface plasmonics and waveguide modes provides a distinct advantage, allowing the proposed sensor to deliver a superior performance. As a consequence, the HMM PtC waveguide sensor emerges as an exceptionally appealing option for photonic sensing applications. The complexity of the proposed system presents a fabrication challenge. Nevertheless, as fabrication technology continues to advance, we anticipate that this issue can be effectively resolved. The proposed HMM PtC waveguide holds vast potential across diverse fields, including biology, medicine, and clinics, representing an exciting advancement for both industry and scientific research.

A numerical investigation of the opposing-face semi circular refractive index sensor for detection of chemical pollutants and heavy metals in water

This research presents a surface plasmon polariton-based refractive index (RI) nanosensor consisting of a metal–insulator–metal waveguide coupled with an opposing-face semicircular shaped resonator with silver nanodots that demonstrates high sensitivity with up to a standard figure of merit (FOM). The transmission characteristics are derived using the finite element method (FEM), and the transmission spectrum is analyzed to determine the resonant wavelength with the help of an optical spectrum analyzer (OSA). Furthermore, nanodots have been introduced to improve the light-matter interaction and boost the sensing performance parameters of the sensor. The maximum recorded sensitivity of the sensor design is 2975.96 nm per refractive index unit (RIU) with a corresponding FOM of 26.32. After thorough analysis, it has been confirmed that the sensor can effectively detect and identify chemical pollutants in seawater. Additionally, this innovative sensor configuration exhibits an impressive capacity to accurately quantify the concentration levels of heavy metal ions, encompassing pivotal elements such as zinc (Zn2+), lead (Pb2+), and mercury (Hg2+), within water samples. The proposed design is suitable for on-chip plasmonic nanosensors due to its high sensing characteristics and compact architecture.

Novel composites of nano-metal–organic frameworks (IRMOF-3) and silver nanoparticles for the ultra-sensitive performance of SERS sensing and optical fiber modes

Rapid chemical detection of drugs of abuse in biological fluids such as blood and saliva is a concern within medicine and law enforcement. As an outcome, a label-free detection platform takes biological fluid samples with concentrations ranging from extremely high to very low. We obtained experimentally the limit of detection (LOD) of glycerol concentration, corresponding to a minimum resolvable refractive index of 3.4 × 10−7 RIU (refractive index unit). We applied the microfluidic device sensors to the quantitating concentration dopamine solutions. The experiment was recorded in real-time as the concentration of analyte injected exposed to the fiber core surface resulted in a sensitive change in optical power transmission. The presented sensors also exhibited reasonably good reproducibility and higher sensitivity, providing the LOD as 1.02 × 10−11 M. A relatively simple scheme procedure is proposed for the simultaneous detection and quantitative assessment of rhodamine B using surface-enhanced Raman spectroscopy with a limit of detection of 10−12 M and a relative standard deviation of 5.78% across a detection concentration range mainly from 10−5 M to 10−12 M. The obtained results demonstrate the potential of nano-metal–organic framework materials as large-area, label-free optical fiber sensors and SERS-based platforms for biomedical sensing and environmental applications.

Photonic crystal concentric dual-microring resonator for refractive index sensing

A high-performance photonic crystal (PhC) concentric dual-microring resonator (PhCCDMRR) for refractive index sensing is proposed in this paper. It confines the energy into PhC air holes to enhance the light–matter interaction, and the increased modal area of the concentric rings improves the sensing sensitivity. The slow-light effect near the photonic bandgap results in a PhC waveguide with a maximum group index of 18.2. In the transmission spectrum, a high extinction ratio of 22 dB is achieved. This sensor obtains a refractive index (RI) sensitivity of 265 nm/RIU (RI unit) using sodium chloride solution as samples, which has promising applications in RI sensing.

Multiparametric, Label-Free Analysis of Microfluidic Droplets via a Dynamic Phase Grating Approach.

Droplet-based microfluidic systems possess many fundamental advantages as a platform for the analysis of chemical and biological species. However, whereas on-chip operations have rapidly developed over the past decades, approaches for analyzing target molecules within droplets have largely remained limited to methods requiring bulky and expensive instrumentation. In this work, we describe a droplet analysis approach whereby the droplet train itself is the sensing construct. Specifically, the droplet train is interrogated as a transmission phase grating, allowing high-throughput, label-free, solution-phase, and multi-parametric analysis of droplet contents. Importantly, three distinct properties of generated droplets can be simultaneously extracted using this conceptually simple and experimentally straightforward measurement approach. Under constant droplet generation conditions, measurement of droplet viscosity is achieved by monitoring changes in zero order to first order peak separation in the far-field diffraction pattern, with a sensitivity of 2.28 × 10-4 cSt per μm change in peak separation. In parallel, measurement of droplet refractive index (RI) is achieved by measuring changes in the ratio of the zero order to first order peak intensity, with a sensitivity of 2.14 × 10-4 RI units per unit change in a diffracted peak intensity ratio. Finally, droplet generation frequency is determined from the time-varying oscillation of the peak height ratio, yielding comparable results to an expensive high-speed camera commonly used for droplet imaging. Importantly, the experimental strategy for this approach is straightforward and does not require expensive instrumentation; therefore, it may find utility in affordable and portable analysis approaches applied to diverse droplet microfluidic assays.

A Liquid Crystal-Modulated Metastructure Sensor for Biosensing.

In this paper, a liquid crystal-modulated metastructure sensor (MS) is proposed that can detect the refractive index (RI) of a liquid and change the detection range under different applied voltages. The regulation of the detection range is based on the different bias states of the liquid crystal at different voltages. By changing the sample in the cavity that is to be detected, the overall electromagnetic characteristics of the device in the resonant state are modified, thus changing the position of the absorption peaks so that different RI correspond to different absorption peaks, and finally realizing the sensing detection. The refractive index unit is denoted as RIU. The range of the refractive index detection is 1.414-2.828 and 2.121-3.464, and the corresponding absorption peak variation range is 0.8485-1.028 THz and 0.7295-0.8328 THz, with a sensitivity of 123.8 GHz/RIU and 75.6 GHz/RIU, respectively. In addition, an approach to optimizing resonant absorption peaks is explored, which can suppress unwanted absorption generated during the design process by analyzing the energy distribution and directing the current flow on the substrate. Four variables that have a more obvious impact on performance are listed, and the selection and change trend of the numerical values are focused on, fully considering the errors that may be caused by manufacturing and actual use. At the same time, the incident angle and polarization angle are also included in the considered range, and the device shows good stability at these angles. Finally, the influence of the number of resonant rings on the sensing performance is also discussed, and its conclusion has guiding value for optimizing the sensing demand. This new liquid crystal-modulated MS has the advantages of a small size and high sensitivity and is expected to be used for bio-detection, sensing, and so on. All results in this work were obtained with the aid of simulations based on the finite element method.

ETPTA Inverse Photonic Crystals for the Detection of Alcohols

We developed a comparatively simple and inexpensive approach for the determination of the concentration of alcohols in water. The method is based on the study of the optical properties of ethoxylate trimethylolpropane triacrylate (ETPTA) inverse photonic crystals (IPhCs). The position of the transmission minimum associated with the first photonic stop band (PSB) is used as the analytical signal. The PSB position depends on the swelling degree of ETPTA photoresist and the refractive index of the tested alcohols and their mixtures with water. The signal increases linearly with increasing concentration of ethylene glycol and increases nonlinearly but monotonically with the concentration of methanol and ethanol in water. Sensitivity to alcohols, in the case of the ethylene glycol–water mixtures, reached about 0.55 nm/v.% or 560 nm/RIU (refractive index unit), which is sufficient for various applications in bio/chemical detection and environmental monitoring.

Reflector-less refractive index sensors based on etched-tapered high-scattering single-mode optical fibers

Reflector-less refractive index (RI) sensors employ simple optical fiber strictures for the accurate detection of RI changes. They operate in absence of any reflective element such as gratings, filters, interferometers while maintaining the form factor of a reflective probe; the RI dependence is encoded in the Rayleigh backscattering signatures, and is interrogated using optical backscatter reflectometry. In this work, we present a new reflector-less fiber-optic sensor based on two-steps: (1) tapering an optical fiber, followed by (2) rapid etching of the fiber. The combination of the two steps allows obtaining significant advantages in terms of manufacturing, making the process simpler and increasing the distribution of the refractive index detection. The sensitivity obtained is up to 110.9 GHz/RIU (refractive index units), with the possibility of detection over a length of 2.1 mm, with 0.035 mm average spatial resolution over distributed RI sensing.

Mirrored nano-structures plasmonic biosensor using two-dimensional materials for detection of PSA

Surface plasmon resonance methods are a very useful option to provide optical and label-free detection of target biological analytes with many advantages. In this work, a highly sensitive plasmonic biosensor is designed to diagnose prostate cancer using Prostate-Specific Antigen (PSA). To detect PSA, an optical sensor with a mirrored structure based on the Kretschmann configuration is investigated. A numerical calculation finite-difference time-domain (FDTD) method for validation and sensitivity measurement is used. The contact surface of graphene with a sensing layer is very important. Our designed structure benefits from the large surface area and significant field enhancement and this help us to design a biosensor with appropriate performance. The sensitivity of 19.68° per refractive index units (°/RIU) is obtained for mirrored structure while this value was 14.48°/RIU for basic structure. So, the use of a mirror structure increases the sensitivity (nearly 36%) compared to the basic structure. Here, this increase is in compared with the basic structure and the mirrored technique can be applied to other multi-layers structures to further increase their sensitivities.

Intracavity dynamics-based gain-assisted sensing with microtubule Raman microlaser

Microcavity lasers show excellent performance as a miniaturized microsensor in various applications. However, their relatively weak power may be easily submerged in system noises and disturbed by environmental fluctuations, rendering them ineffective at detecting small signals for precise sensing. To solve this problem, the laser differential frequency-shift feedback technique is demonstrated in a microtubule Raman laser to achieve the optical gain assistance. When the microlaser is frequency-shift-modulated and returns back to the resonator, the measurement signal can resonate with the laser relaxation oscillation and be significantly enhanced. The intracavity dynamics-based enhancement makes it effective for increasing intensity changes caused by analytes. Small signals that would otherwise be buried in system noises and go undetected can be more easily resolved. In addition, the microsensor reduces the spectral measurement range and offers a way to observe the fast dynamic response. Based on that, a measurement resolution of 50 nm nanoparticle detection limit and a refractive index noise-limited resolution of 8.18 × 10−7 refractive index unit (RIU) are demonstrated. The dynamic phase transition of thermosensitive hydrogel is further investigated as a validation of its rapid detection capability. Integrated with an inherent microfluidic channel, the proposed microsensor provides a direct interaction between analytes and probe light with ultrasmall sample consumption down to 50 pl. It is expected to boost the detection of weak signals in microlasers and enlighten the development of optofluidic microsensors in exploring diverse biochemical processes.

Semi-distributed interferometers fiber-optic sensors for high-sensitivity refractive index detection: Design and sensitivity analysis

In this work, we present the design and analysis of fiber-optic refractive index (RI) sensors based on a simple semi-distributed interferometer (SDI). The SDI is a cavity formed between a cleaved fiber tip and the reflective interface between a single-mode fiber and a high-scattering fiber with core doped by Mg-silicate nanoparticles. The fabrication is rapid (∼1 min) and straightforward, as only a splice and cleave of the fibers are required; the resulting spectral fingerprint is an interferometer-shaped envelope, with a stochastic distribution due to the random scattering centers of the in-cavity fiber. We fabricated 18 sensors with 6 different enhanced-scattering fibers, having different drawing temperature and speed; by analyzing the RI sensitivity of each unit, we obtain sensitivity values up to 787.0 dB/RIU (refractive index units), whereas the optimal fiber allows achieving average sensitivity of 457.9 dB/RIU.

Plasmonic sensor for rapid detection of water adulteration in honey and quantitative measurement of lactose concentration in solution

In this article, a novel surface plasmon polariton-based plasmonic refractive index sensor is proposed and numerically investigated. It consists of a structure of an octagonal cavity resonator inside a square disk along with silver nanodots and two slits which are coupled to a straight metal–insulator–metal (MIM) waveguide arrangement. The finite element method (FEM) has been employed to perform a numerical analysis of the proposed structure in the near-IR (Infrared) and mid-IR spectrum. The sensor detects changes in refractive index in terms of changes in resonant wavelength which can be measured using an optical spectrum analyzer. The recorded maximum sensitivity is 2527.6 nm per refractive index unit (RIU) with a sensing resolution of 3.956×10−7 RIU. The proposed sensor has been further numerically analyzed, and its ability to measure the quality of honey samples and detect lactose concentration in a solution has been verified.

Optical fiber in-line Mach-Zehnder interferometer based on down-up tapers for refractive index measurement

An optical fiber in-line Mach-Zehnder interferometer based on down-up tapers is proposed and demonstrated for refractive index measurement. The device is fabricated by splicing two segments of multimode fibers in a heating and pushing process to form an up taper and followed by heating and pulling of the fiber adjacent to the up taper area to form a down taper. The down-up fiber taper structure constitutes a Mach-Zehnder interferometer and the whole device length is only 3 mm. The experimental results show that the refractive index sensitivity is ∼109.71 nm/RIU (refractive index unit) in the refractive index range of 1.340 – 1.370. Such a device is compact in size, robust in structure, good in sensitivity, convenient in operation and low in cost, and has high potential for chemical and biomedical sensing applications.

Common-mode plasmon sensing scheme as a high-sensitivity compact SPR sensor.

A deep metal grating enables quasi-phase-matched simultaneous excitation of two counterpropagating surface plasmon modes by means of its +1st and -2nd diffraction orders. The resulting angular reflection spectra of the scattered -1st and zeroth orders exhibit three interleaved zeros and maxima in a range centered around the Littrow angle. The spectra differ thoroughly from the usual reflection dip resulting from single-order plasmon coupling that produces strong absorption. The zeroth and -1st orders exhibit two crossing angles enabling high-sensitivity common-mode detection schemes designed to reject variations in source power and environmental noise. The proof of concept and experimental assessment of this new surface plasmon resonance (SPR) sensing scheme are demonstrated by monitoring gases in a pressure-controlled chamber. A limit of detection (LOD) of 2 × 10-7 refractive index unit (RIU) was achieved.

All-magneto-optical grating bio-sensors based on enhanced transverse magneto-optical Kerr effect

Herein, a novel structure composed of an all-magneto-optical periodic grating, without using noble metals, is presented to enhance the transverse magneto-optical Kerr effect (TMOKE). This is done by calculating the relative changes in the reflection amplitudes for the p-polarized wave using the finite element method (FEM) with COMSOL Multiphysics. Bio-sensing applications of the proposed structure can be possible by detecting alternation in the refractive index of the analyte medium through monitoring of the TMOKE. The simultaneous operation of the magneto-optic layer as a sensing layer is the advantage of our idea, where direct interaction with the analyte happens, leading to the suppression of the wave reflection amplitudes, and finally leading to extreme TMOKE in a magneto-optical (MO) grating (the TMOKE amplitudes are near 0.8 in this work). And since TMOKE shows a Fano-like shape, we calculate sensitivity and figure of merit (FoM) by fitting them together. This results in high sensitivity, and the value of the FoM is remarkable (between 2×103 and 3×103 refractive index unit) at resonance angles for different refractive indices, which makes our proposed structure able to be utilized for bio-sensing applications.

A refractive index sensor based on the MIM waveguide with a semi-elliptical and a rectangular ring resonant cavity

In this paper, a novel nanosensor comprising the metal–insulator–metal (MIM) plasmonics waveguide with a semi-elliptical and rectangular ring resonant cavity is designed. In near-infrared waveband, the propagation properties of electromagnetic waves in the structure are studied using the finite-difference time-domain (FDTD) method. The results show that, based on the coupling between the semi-elliptical and the rectangular ring resonant cavity, the transmission spectrum of the structure exhibits a sharp Fano resonance shape. Next, the influence of the refractive index and sensor structure parameters on performance is systematically investigated. The simulation results show that the sensor structure has the best sensitivity of 1384[Formula: see text]nm/RIU (refractive index unit), and the figure of merit (FOM) is 28.4. The simple MIM structure could be applied to sensitive plasmonic sensors.

Design of a nanoscale gold-coated photonic crystal fiber biosensor

A simple two-layer refractive index sensor based on surface plasmon resonance has been proposed in this work, which evinces high sensitivity for detecting unknown analytes. This sensor design includes hexagonal- and octagonal-shaped air-hole rings, which have been introduced in the first and second layers, respectively. By performing this amalgamation of different types of air holes, this gold-coated plasmonic sensing scheme provides the broad sensing limit of the analyte, which is 1.32–1.40. The sensing performance has been examined by applying the finite element method, which is being used for modeling complex and irregular-shaped geometry. The wavelength interrogation method has demonstrated a maximal wavelength sensitivity of 9,000 nm/RIU (refractive index unit). The maximal amplitude sensitivity of 1902 RIU−1 has been exhibited in the y-polarized mode using the amplitude interrogation method. Moreover, the maximal sensor resolution of 1.11 × 10−5 RIU covers the entire sensing range. Furthermore, by adjusting the air-hole diameter, pitch, and depth of the gold layer coating, many more investigations have been demonstrated in this design. This highly sensitive sensor is very convincing to be used in many applications like chemical, biological, and also in physical sensing analyses.

Ferromagnetic Subwavelength Periodic Nanogroove Structure with High Magneto-Optical Kerr Effect for Sensing Applications

The transverse magneto-optical Kerr effect (TMOKE) with gas sensing ability was comprehensively investigated in this study by directly patterning a subwavelength periodic nanogroove on a cobalt film. High-amplitude TMOKE was observed for the proposed structure, which was 243 times as intense as that of a smooth film. Further, the physical mechanism responsible for this significant improvement is elucidated by the effective activation of surface plasmon resonance at the gas–cobalt interface. The mechanism was established by investigating the electric field distributions at a resonant angle of incidence and the reflectance spectra associated with the metallic nanogroove grating structure. Moreover, we demonstrate that this scheme has a high detection sensitivity of up to 112.2° per refractive index unit and a large figure of merit, allowing the system to be integrated with microfluidics for sensing applications.

Graphene metasurface for broadband absorption and terahertz sensing

A plasmon-induced terahertz metamaterial absorber is proposed. The bottom ground layer uses gold to block the transmission of terahertz incident waves, and the pattern layer and the ground layer are separated by topas. The 90% absorption bandwidth frequency range of the absorber is 4.6–7.7 THz, and the average absorption rate in this range is 97.6%. Due to the geometric symmetry of the absorber, it has polarization insensitivity. In addition, when the incidence Angle reaches 60 degrees, more than 80% absorption can still be maintained. The numerical simulation shows that the frequency shift of the absorber has a good linear relationship with the change of the refractive index of the proposed absorber, and the sensitivity is 1100 GHz per refractive index unit (GHz/RIU). Therefore, the absorber can be used in more complex electromagnetic scenarios, or as a sensing element.