Refractive Index Sensitivity Research Articles

Ultrasensitive RI and temperature sensor based on cascaded microfiber MZIs with Vernier effect

A sensitivity amplified optical fiber sensor based on Vernier effect by cascaded microfiber Mach-Zehnder interferometers (MZIs) is proposed and demonstrated for the simultaneous measurement of refractive index (RI) and temperature. The traditional single mode fiber is tapered into micrometer near the dispersion turning point to constitute MZI, and then cascaded to generate Vernier effect for the sensitivity enhancement. A novel phenomenon that the envelope of Vernier effect shifted in opposite directions decided by the free spectral ranges of the sensing and reference arms is clarified and demonstrated for the first time. Experimental results show that the RI sensitivity can be improved from 944 nm/RIU of single MZI to 18504 nm/RIU of Vernier effect, while the temperature sensitivity can be improved from −0.53 nm/℃ of single MZI to 9.49 nm/℃ of Vernier effect. Moreover, the proposed sensor with the merits of low hysteresis effect, good repeatability, high stability and good reliability, making it of great potential for biochemical trace detections without temperature-induced cross-sensitivity.

Role of lateral Core-Offset in the sensitivity enhancement of single Mode-No core-Single mode fiber cascaded structure

In this study, an extensive examination of the influence of core-offset on different sensitivities of a single-mode-no-core-single-mode (SNS) fiber sensor probe in a variety of external disturbances, including refractive index changes, strain, and temperature, has been presented and thoroughly investigated. To achieve this, a consistent sensing length of 5 cm no-core fiber (NCF) was utilized between two single-mode fibers (SMFs) with different offset values, facilitated by the built-in options of a fusion splicer. The theoretical and experimental principles of sensitivity are discussed in detail. Key aspects such as spectral analysis and fast Fourier transform of the disclosed transmission spectrum underwent meticulous scrutiny. The outcome of the experiment indicates that the SNS fiber sensor exhibits an increasing sensitivity to refractive index (RI) over the 1.333–1.398 range when the offset value increases, with a maximum of 201.221 nm/RIU sensitivity. On the other hand, the RI sensitivity decreases with increasing offset value in the range of 1.402–1.441; the highest sensitivity attained is 656.641 nm/RIU. The strain sensitivity shows a decreasing trend with increasing offset, with the maximum sensitivity reaching −1.59 pm/µε in a wide range of 0 to 2000 µε. For temperature sensing, the sensitivity remains unaffected with a core offset; consistently, the achieved sensitivity is approximately 11.51 pm/°C over a broad range of temperature of 30 °C to 120 °C Compared with RI, it is found that the reliance of core-offset on strain sensitivity is very low whereas on temperature sensing almost insensitive, provided that the sensing length and diameter remain constant. These results provide insightful direction for the creation of extremely sensitive sensors for a variety of applications.

Optimize Nonlinear Effects on Fundamental and High-order Soliton in Photonic Crystal Fiber

Nonlinear effects in optical fibers are mainly caused by two sources: inelastic scattering behaviour or the intensity sensitivity of the medium's refractive index. The propagation process in photonic crystal fibers is more complex than the propagation process of first-order solitons, second-order solitons, and third-order solitons. This article discusses the effects of propagation on first-, second- and third-order solitons. A popular approach to supercontinuum generation through soliton fission is the higher-order soliton technique for spectral generation.

Highly sensitive refractive index sensing based on nanostructured porous silicon interferometers

In this study, we present the experimental evidence demonstrating the utility of electrical double layer (EDL)-induced ion accumulation, using sodium (Na+) ion in water as model substances, on a negatively charged nanostructured surface, specifically thermally grown silicon dioxide (SiO2). This novel approach, termed Ion Surface Accumulation (ISA), aims to enhance the performance of nanostructured porous silicon (PSi) interferometers in optical refractometric applications. The experimental results show that the electrical double layer-induced ion surface accumulation (EDL-ISA) on oxidized PSi interferometers enables remarkable amplification of the interferometer output signal (the spectral interferogram), even when the bulk refractive index variation is below 10-3 RIU. This substantial signal enhancement translates into an increase in sensitivity of up to two orders of magnitude, facilitating the reliable measurement of refractive index variations with both a detection limit (DL) and resolution (R) as low as 10-4 RIU. This achievement elevates the performance of PSi interferometers in photonics and plasmonics-based refractive index platforms.

Multiband highly sensitive metamaterial-based THz absorber for edible oil adulteration sensing

A terahertz (THz) wave technology is very appealing for applications in detection in biomedicine, biological spectroscopy, material characterization, detection, imaging, sensing, and other convenient applications. Metamaterials (MMs) are artificial structures that have structures in a regular periodic arrangement and don’t exist in nature. By utilizing a proposed terahertz metamaterial absorber, a broad spectrum of adulterated lubricants can be detected. It is an efficient detection process that enables the development of a THz detection approach by making use of the metamaterial as a sensor. The proposed terahertz absorber is analyzed and optimized using the CST Microwave Studio 2019 software. The article proposed a multiband THz metamaterial absorber with a unit size of 70 µm. According to the demonstrated results, we can achieve five narrow absorption peaks with perfect absorption within the frequency range between (0.2–4.2) Terahertz. Also, the result shows that, the average absorbance of 99.07% at the resonance frequencies of 0.56, 1.42, 2.676, 3.14, and 3.89 THz. The E (electric) and H (magnetic)-field distributions on the resonator at different resonance frequencies were also analyzed as well as. The recommended metamaterial absorber has a good sensing capability at different resonant frequencies, with refractive index sensitivities of 80 GHz/RIU, 240 GHz/RIU, 480 GHz/RIU, 480 GHz/RIU, and 520 GHz/RIU obtained by modifying the refractive index of the sample holder or the analytic layer, respectively. As a result, the proposed detector has a high refractive index sensitivity, high Q-factor, and high FOM, making it suitable for detecting oil adulteration.

Engineering gold nanoworms with tunable longitudinal plasmon peak in the near infrared and their refractive index sensing properties.

The plasmonic properties of rod-shaped Au nanoparticles make them promising for numerous applications. The synthesis recipes for Au nanorods are well established and their longitudinal plasmon peak can be tuned over a wide wavelength range. Herein, we demonstrate that the longitudinal plasmon peak of gold NWs (NWs), which are bent nanorods, can be finely tuned in the near-infra-red region. The NWs were synthesized using a one-step reaction method. We have seen that the length and aspect ratio of NWs can be tuned by simply changing the pH of the reaction medium. Under higher pH reaction conditions, NWs with relatively smaller sizes were obtained. Similar to nanorods, NWs have a well-defined longitudinal plasmon peak, which scales linearly with their aspect ratio. Finite element analysis was used to model the optical properties of Au NWs. The simulated results matched well with the experimental spectra. The synthesized NWs have shown good refractive index sensitivities (RIS). The RIS of NWs increased with an increase in their aspect ratio. A maximum sensitivity value of 542 nm per RIU, was obtained for NWs with the plasmon peak at 1033 nm. The RIS values are comparable to that of Au nanorods and bipyramids.

Multifunctional topological photonic crystal device for multichannel frequency routing and highly sensitive refractive index sensing

We propose a novel photonic crystal (PC) structure with C-4 symmetry of the square lattice that utilizes topological edge states (TESs) to achieve multichannel frequency routing and high-sensitivity refractive index (RI) sensing. The proposed system combines trivial and nontrivial unit cells to form a unique lattice arrangement that supports multiple TESs. These states enable the realization of seven distinct output ports, each capable of transmitting a specific frequency, significantly enhancing optical communication systems’ bandwidth and data transfer capabilities. This feature makes the proposed PC structure an ideal candidate for multichannel frequency routing applications. Additionally, the structure incorporates a central cavity that exhibits robust light localization, making it highly sensitive to changes in the surrounding medium’s refractive index. Simulations demonstrate that the proposed structure can achieve RI sensing sensitivity of up to 721 nm/RIU, coupled with a high-quality factor exceeding 105. With its exceptional RI sensing capabilities, the proposed multichannel PC device holds immense potential for various technological advancements in frequency routing.

Ultrasensitive refractive index sensing of optical microfiber couplers coated with Ti3C2 MXene

Two-dimensional (2D) MXene has recently been found to play a significant role in constructing biosensing platforms due to its excellent biocompatibility, large specific surface area, and broad optical absorption. In this article, an ultra-sensitive refractive index (RI) sensor based on an optical microfiber coupler (OMC) coated with Ti3C2 MXene is proposed and experimentally demonstrated. Numerical results reveal that surface modification of MXene enhances the RI sensitivity of OMC and creates a new artificial dispersion turning point (ADTP). The OMC with a dispersion turning point (DTP) at 1300 nm is fabricated using a hybrid fusion elongation-wet etching method. It has a waist width of approximately 2.46 µm. We experimentally achieved the RI sensitivity of 9.1 × 104 ± 0.42 × 104 nm/RIU using the OMC. MXene improves the sensitivity of the OMC because Ti3C2 MXene can change the effective refractive index (ERI) for both the even and odd modes, leading to the interference dips shifting closer to the DTP. The experimental results demonstrate that the RI sensitivity of the OMC coated with Ti3C2 MXene can reach -1.7 × 105 ± 0.2 × 105 nm/RIU, which shows a ∼87% sensitivity boost compared with bare OMC. The proposed sensor is expected to be used for high-sensitivity biochemical molecule detection.

Plasmonic Sensor System Embedded with Orthogonal Mode Couplers for Simultaneous Monitoring of Temperature and Refractive Index

AbstractThe significance of plasmonic sensors lies in their ability to enhance sensitivity and precision in detecting minute variations in the refractive index of surrounding media. In this work, a novel plasmonic sensor design utilizing a metal-insulator-metal (MIM) waveguide and two circular cavities is presented, specifically tailored for refractive index and temperature sensing applications. Each cavity serves as an individual and autonomous sensing unit. Tailored for temperature sensing, a designated cavity is filled with polydimethylsiloxanes (PDMS), while a separate cavity is exclusively allocated for biosensing and contains a liquid with distinct refractive indices. This dual-cavity system allows for precise and specialized sensing functionalities, ensuring accurate measurements and diverse applications. Furthermore, this work integrates orthogonal mode couplers into the plasmonic device, providing an essential capability to seamlessly transform the dielectric mode into a plasmonic mode and vice versa. The device exhibits a refractive index sensitivity of 737.71 nm/RIU and a temperature sensitivity of − 0.336 nm/°C. Additionally, its Q-factor is determined at 20.5 for the refractive index sensing module and 16.5 for the temperature sensing module.

Multi-mode resonance of bound states in the continuum in dielectric metasurfaces

Bound states in the continuum (BIC) represent distinct non-radiative states endowed with infinite lifetime and vanishing resonance linewidth. Introducing asymmetric perturbation to the system can convert true BICs into high quality leaky modes which is useful in many photonic applications. Previously, such perturbation and resonance of interest is only limited to a single factor. However, different perturbations by unit cell gap, geometry and rotation angle result distinctive resonance modes. The combination of two perturbation factors can excite multi-mode resonance contributed from each asymmetric factor which coexist simultaneously; thus, the number of reflectance peaks can be controlled. In addition, we have carefully analyzed the electric field variations under different perturbation factors, followed by a multipolar decomposition of resonances to reveal underlying mechanisms of distinct resonance modes. Through simulations, we find that the introduction of multiple asymmetric perturbations also influences the metasurface sensitivity in refractive index sensing and compare the performance of different resonance modes. These observations provide structural design insights for achieving high quality resonance with multiple modes and ultra-sensitive sensing.

Echelon grating refractive index sensor.

There are few reports on optical refractive index sensors that have both high resonant-wavelength resolution (RWR) and high refractive index sensitivity (RIS). Herein, based on an echelon grating, we design a refractive index sensor that combines the two advantages together. The principal fringe of echelon grating has a small full width at half maximum and a good signal-to-noise ratio, leading to a high RWR. The wavefront splitting interference makes the sensor have high RIS. The large free spectral range (FSR) of the principal fringes expands the dynamic range of the sensor. The experimentally realized RWR, RIS, and FSR are 2 × 10-2 nm, 1.14 × 104 nm/RIU (RIU: refractive index unit), and 130 nm, respectively. The detection limit of refractive index is 1.59 × 10-6 RIU. The dynamic range of the sensor is 1.14 × 10-2 RIU. In addition, there are schemes to improve RWR and RIS, which can further reduce the detection limit of refractive index. The echelon grating refractive index sensor features low detection limit, low cost, high stability, and good robustness.

A high sensitivity refractive index sensor based on microstructure fiber with simple structure and SPR effect

A novel dual-side polished photonic crystal fiber (PCF) based surface plasmon resonance (SPR) polarization filtering and refractive index (RI) sensor is presented in this paper. The cladding of PCF was composed of eight air holes and the outer surface was dual-sided polished and coated with gold film. Finite element method (FEM) was used for the simulation and design of the proposed sensor. Simulation results showed that sensor performance was better in analytes ranging from 1.33 to 1.42 RI. The wavelength and amplitude sensitivities at resonance wavelengths of 1269 and 995 nm were 27400 nm/RIU and −100.43 RIU−1 for an analyte of 1.42 and 1.41 RI, respectively, along with a spectral resolution of 3.64×10−6 RIU. Maximum loss of 52806 dB/m was obtained for an analyte of 1.41 RI towards Y-Polarization (Y-P). An extinction ratio of −105.374 dB was obtained using a fiber transferring length of 2 mm to achieve better polarization filtering effects. The proposed sensor is made of same size air holes which is easy to fabricate, cost effective, and shows better sensitivity and polarization filtering effects than several other sensors. It is expected that the sensor is suitable for wide RI detection range and capable of providing excellent polarization effect. The proposed sensor can be used for various applications including environmental detection, chemical sensing, biosensors, and pharmaceutical inspection.

Investigation of the effect of amine and thiol as functional groups on gold Nanobipyramids properties

Metal nanoparticles are increasingly popular due to their unique properties, such as their vast surface area, tiny size, strong reactivity in live cells, and high-temperature stability. Gold nanoparticles, as one of the metal nanoparticle types, are frequently utilized in biosensor applications because of their adjustable Localized Surface Plasmon Resonance (LSPR) characteristics, less cytotoxic nature, and increased sensitivity to changes in the surrounding medium. The anisotropic metal nanoparticle of gold nanobipyramids (GNBPs) provides for higher Refractive Index Sensitivity (RIS) and Figure of Merit (FOM) value than others, allowing for higher sensitivity. This work lies in the comprehensive investigation of GNBPs functionalized with amine and thiol groups over varying shorter time durations (0.5 – 12 h) through the immediate change in optical, structural, morphological, electrical surfaces, and molecular properties. The findings show amine groups enable the red-shifted spectrum, whereas thiol groups impact the blue-shifted spectrum. By employing amine groups, it is able to produce side peaks in the XRD pattern made up of amine atoms. The use of thiol groups, meanwhile, has an impact on the growth increase in the (111) direction. Then, the highest surface density is obtained in 3 h, 59.78 % for amine-functionalized GNBPs (a-GNBPs) and 75.49 % for thiol-functionalized GNBPs (t-GNBPs). Next, a-GNBPs generate the highest angle slope compared to t-GNBPs, i.e., 140.422⁰. In addition, for the electrical surface effect, amine functionalization increases the zeta potential value from 0.3 to 2.5 mV, indicating the inhibition of nanoparticle agglomeration. On the other hand, thiol functionalization affects the decreasing of zeta potential value from 0.3 to -0.5 mV, allowing for nanoparticle agglomeration. Subsequent FTIR characterization analysis obtained shifts in the wave numbers, with the -OH bond exhibiting changes within the range of 3306.75 – 3269.95 cm-1, C=-CH bond providing for 2138.48 – 2136.56 cm-1 range, and the C = O bond showing alterations between 1637.59 – 1637.33 cm-1 following amine and thiol functionalization. Extended observation reveals that both a-GNBPs and t-GNBPs exhibit significant signal responses in glucose detection applications utilizing LSPR-based sensors, suggesting the feasibility of functionalizing GNBPs with amine and thiol groups for potential biosensor development.

The tunable absorber films of grating structure of AlCuFe quasicrystal with high Q and refractive index sensitivity

Due to the continuous development of terahertz field and material science, terahertz absorbers have received extensive attention. The paper proposes a tunable absorber film of grating structure of AlCuFe quasicrystal. The absorber film composed of a AlCuFe quasicrystal grating structure and a gold substrate, making it easy to process. The absorber film has three perfect absorption bands in its working frequency band. When the polarization direction of the incident wave changes, it becomes perfect absorber film in a single band. Through the analysis of the relative impedance of the absorber film, the relative impedance of the absorber film in the absorption band is close to the impedance of the free space, which conforms to the classical theory. By analyzing its electric field, the absorber film absorbs the electromagnetic wave mainly by the guided film resonance and LSPR effect, and the absorber film has excellent tuning, high Q value and sensitivity of the absorber film. Among them, the highest Q value of the mode is 242, the refractive index sensitivity of each mode is more than 2700 and the highest reaches 4032. In general, the absorber film has great application prospects in fields such as biomedicine and communication science.

High-Sensitivity Seawater Refraction Index Optical Measurement Sensor Based on a Position-Sensitive Detector.

The refractive index of seawater is one of the essential parameters in ocean observation, so it is necessary to achieve high-precision seawater refractive index measurements. In this paper, we propose a method for measuring the refractive index of seawater, based on a position-sensitive detector (PSD). A theoretical model was established to depict the correlation between laser spot displacement and refractive index change, utilizing a combination of a position-sensitive detector and laser beam deflection principles. Based on this optical measurement method, a seawater refractive index measurement system was established. To effectively enhance the sensitivity of refractive index detection, a focusing lens was incorporated into the optical path of the measuring system, and simulations were conducted to investigate the impact of focal length on refractive index sensitivity. The calibration experiment of the measuring system was performed based on the relationship between the refractive index of seawater and underwater pressure (depth). By measuring laser spot displacement at different depths, changes in displacement, with respect to both refractive index and depth, were determined. The experimental results demonstrate that the system exhibits a sensitivity of 9.93×10-9 RIU (refractive index unit), and the refractive index deviation due to stability is calculated as ±7.54×10-9 RIU. Therefore, the feasibility of this highly sensitive measurement of seawater refractive index is verified. Since the sensitivity of the refractive index measurement of this measurement system is higher than the refractive index change caused by the wake of underwater vehicles, it can also be used in various applications for underwater vehicle wake measurement, as well as seawater refractive index measurement, such as the motion state monitoring of underwater navigation targets such as AUVs and ROVs.

High sensitive optical fiber SPR sensor for label-free detection of Staphylococcus aureus

Optical fiber bacterial sensors have attracted attention in many fields due to their unique advantages. However, it is still difficult to promote its practicality which is mainly subject to their low sensitivity, inferior stability, and reliability. This work reported here an optical fiber surface plasmon resonance (SPR) biosensor enhanced by gold nanoparticles (AuNPs)/Au film coupling for label-free detection the Staphylococcus aureus (S. aureus), while both the gold substrates were successfully biofunctionalized with goat anti-human immunoglobulin G (IgG) as the probe to capture and detect the bacteria. In the experiment, the refractive index（RI） sensitivity of the Multimode-Singlemode-Multimode optical fiber (MSMOF) coated with Au film was improved by 19 % after the AuNPs were homogeneously decorated on the Au film surface. To detect S. aureus by the sensor, the SPR wavelength variation in the transmission spectrum was recorded, and its limit of detection (LOD) was demonstrated experimentally to be about 50 CFU/mL and was obtained theoretically to be 1.18 CFU/mL during its concentration range of 50–108 CFU/mL. Also, it shows great linearity in the range of 102-108 CFU/mL and the sensor exhibits excellent specificity and stability. This kind of SPR-enhanced optical fiber biosensor is fully promising for its applications in biomedicine, food safety, pharmaceuticals, etc.

Compact refractive index sensor based on offset splicing long-period fiber grating

Long-period fiber grating (LPFG) is one of the most important passive fiber devices. They work by coupling light from the fundamental core mode to the forward-propagating cladding mode, creating a series of resonant peaks in the transmission spectrum. In this paper, a series of no-core fiber (NCF) sections were periodically misaligned and embed in the single-mode fiber (SMF), to form a LPFG through the periodic refractive index (RI) modulation along the fiber axis. This fabricating method enhances the RI modulation to shorten the length of the proposed sensor to 2.82 mm. Experimental result shows that the highest RI sensitivity for the sensor reaches −4460.83 nm/RIU in a measuring range of 1.33–1.40 RIU at room temperature. Hopefully, the proposed study can lay some implementation methods for the design of the all-fiber compact RI sensor.

Ultra-sensitive fiber optic sensor for glucose concentration measurement based on Vernier effect in cascaded Mach–Zehnder interferometers

In this paper, a fiber optic glucose concentration sensor based on cascaded Mach–Zehnder interferometers (CMZIs) was proposed and experimentally verified. Two single-mode fibers (SMFs) with slightly different tapering lengths and as well free spectral ranges (FSRs) were obtained through the melt-drawn cones technique and then were cascaded together to achieve the Vernier effect (VE). Experimental results showed that the measurement sensitivity of glucose concentration based on a single tapered SMF was 0.0494 nm (mmol l−1)−1. Furthermore, the measurement sensitivity based on the cascaded tapered SMFs was enhanced by 4.86 times to an ultra-high level of 0.2402 nm (mmol l−l)−1. The resolution reached 0.083 mmol l−1. The refractive index sensitivity reached 9066 nm/RIU. The resolution reached 2.21 × 10−6 RIU. In addition, the proposed glucose concentration sensor based on CMZIs shows good hysteresis. In conclusion, the designed sensor, showing the merits of easy to fabricate, high sensitivity, and high detection resolution, could be used to monitor the tiny variation in glucose concentration.

Photonic crystal fiber sensors to excite surface plasmon resonance based on elliptical detection channels are used for highly sensitive magnetic sensing

To improve the detection performance of fiber optic magnetic field sensors a new photonic crystal fiber (PCF) based on surface plasmon resonance (SPR) was designed and investigated. The designed sensor uses an elliptical detection channel, and the modal transmission characteristics and magnetic field sensing characteristics of this fiber optic sensor are analyzed using the full vector finite element method (FVFEM). In addition, the effect of the detection channel on the detection accuracy at different curvatures was investigated. Compared with previous optical fiber magnetic field (MF) sensors, the designed sensor meets the requirements of both refractive index (RI) and MF measurements, and the MF sensitivity, RI sensitivity, and amplitude sensitivity (AS) of the sensor reach 0.739 nm/Oe, 12043.8 nm/RIU, and 754.88RIU−1, respectively. The designed sensor expands the application range of optical fiber sensors and reduces the cost. It has great potential for application in complex environments.

Research on sensing characteristics of microfluidic sensor based on photonic crystal fiber

To address the challenges associated with sample injection into the air hole of photonic crystal fiber (PCF) and collimation, in this paper, we assemble a single-mode photonic crystal single-mode fiber structure sensor chip based on the Mach–Zehnder interference principle using microfluidic chip processing technology. The sensing principle is analyzed mathematically and the sensing characteristics are verified theoretically and experimentally. The temperature sensitivity of the sensor is −1.3325 nm °C−1, and the refractive index sensitivity is 1666.2 nm RIU−1. This structure solves the difficulty of filling and coupling PCFs. Furthermore, it introduces a novel research methodology for the design and assembly of high-performance biosensors based on PCFs.