Refractive Index Sensitivity Research Articles

Realization of triple dispersion turning points in tapered seven-core fiber interferometer for sensitivity enhancement

Seven-core fibers have been widely applied in optical network, astronomy, and sensing applications. In this work, we report and demonstrate the existence of triple dispersion turning points of the super-mode interferences in a seven-core fiber spliced between two single-mode fibers by modifying the diameter of the multi-core fiber. The sensitivities of the interferometer on external variables, i.e., temperature, axial strain, and refractive index (RI), are measured, showing significant enhancement around the dispersion turning points. Compared to the regular multi-core fiber device without dimension modification, the sensitivities of temperature and axial strain are improved by about 60 times and 73 times around the dispersion turning points, respectively. While the regular device has no response to the external refractive index, the magnitude of RI sensitivity can reach as high as 33 459.0 nm/RIU in air, when the fiber diameter is modified to approach the dispersion turning point, enabling the discrimination of the refractive indices of N2 and CO2. The modification method of locating the modes at the dispersion turning points significantly enhances device sensitivity, providing an avenue for integrating ultra-sensitive sensors into commercial fiber networks.

Temperature self-compensated dual core fiber-optic sensor integrated with moisture sensitive MIL-96(Al) film for breath detection

In this paper, a dual-core fiber optic sensor has been proposed for dynamic monitoring of temperature and humidity. The side core is polished into a D-type optical fiber, and a layer of nano-silver film and a layer of polydimethylsiloxane (PDMS) are deposited to construct a surface plasmon resonance (SPR) temperature compensator. The middle core is constructed as the long-period fiber grating (LPFG) sensing channel. The mode conversion of LPFG is achieved by TiO2 film, and its refractive index (RI) sensitivity is increased by about 4 times. Meanwhile, a layer of moisture sensitive MIL-96(Al) film is grown outside the TiO2 film. Due to the particular pore structure for water molecules, it is possible to change the effective RI of MIL-96(Al) film by absorbing water molecules. Research has shown that the LPFG sensor can achieve a relative humidity sensitivity of −0.411 nm/%RH. Finally, the sensor successfully achieved real-time dynamic monitoring of respiratory rate.

GST and MXene-Based Highly Sensitive Refractive Index Sensor with Gold Gratings Resonator Operating for Infrared Region

GST and MXene-Based Highly Sensitive Refractive Index Sensor with Gold Gratings Resonator Operating for Infrared Region

Relative humidity sensor based on a nano-film deposited LPFG with hydrophilic polymer composite coating

A novel high-performance relative humidity (RH) sensor is proposed by using a hydrophilic polymer composite film-coated nano-film deposition deposited long-period fiber grating (LPFG). A high refractive index (RI) TiO2 nano-film is deposited on the LPFG as an inner layer. The film thickness is selected to maximize the surrounding refractive index (SRI) sensitivity of the grating when the SRI is approaching the cladding index. A composite film made of polyvinyl alcohol (PVA) and polyethylene glycol (PEG) is used as the an external humidity-to-RI transducer. The concentration and proportion of the PVA/PEG solutions are appropriately chosen to tune the refractive index (RI) RI of the polymer coating to the transition range during the RH varying from 60 % to 95 %. For the LPFG coated with two layers film, the a RH sensitivity up to 3.3 nm/% is achieved. The variation in RH is found to have a linear response. The proposed device offers a reliable alternative for higher RH environment sensing.

Highly sensitive plasmonic sensor with a coupled split-square-ring resonator

A novel plasmonic sensor, which is composed of a metal-insulator-metal (MIM) waveguide and a coupled split-square-ring resonator, is proposed and investigated by utilizing the two-dimensional (2D) finite-difference time-domain (FDTD) method. The optimized results demonstrate that the proposed sensor is highly sensitive to variations of refractive index (RI) and temperature, i.e. the RI and temperature sensitivities are up to 2040 nm RIU−1 and −1.2 nm/°C, respectively. Numerical simulations reveal that the sensing performance can still be further improved via optimization of the structure. This research may open up new schemes in realizing high-sensitive biosensors with compact configurations.

A study of multiple solid-state dewetting of sputtered Au ultra-thin films for chip-based LSPR sensor applications

This article proposes a lithography-free technique to fabricate Au NPs on glass substrates using multiple solid-state dewetting (SSD) of sputtered Au ultra-thin films for LSPR sensor chip applications. We studied the influence of initial film thickness and the number of repeated process cycles on the morphology and LSPR sensing performance. This fabrication process allowed control over particle size, gap spacing, and density of the Au NPs, which influenced the LSPR peak position as observed using field emission scanning electron microscopy (FE-SEM) and UV–Vis–NIR spectrophotometry. To demonstrate LSPR sensing performance, the refractive index (RI) sensitivity was evaluated by measuring the wavelength shift of the LSPR peak in a series of glycerol/phosphate-buffered saline (PBS) mixtures, varying the refractive index from 1.33909 to 1.37409. The results showed that RI sensitivity and the figure of merit (FOM) for all prepared samples ranged from 37.191 ± 12.26–73.592 ± 9.70 and 0.35 ± 0.12–0.75 ± 0.02, respectively. An increase in repeated process cycles tended to decrease RI sensitivity and FOM. The best LSPR performance was achieved with an 8 nm initial film thickness after the first cycle, with an RI sensitivity of 70.937 ± 2.60 and an FOM of 0.75 ± 0.02, attributed to optimal Au size and density. Additionally, the binding efficiency response to human IgG with high regeneration cycles was demonstrated, highlighting the potential for biosensor applications.

Operando monitoring of gelation kinetics of polyacrylamide hydrogel using in-fiber dual-MZI

Operando monitoring of gelation kinetics and exploring the gelation law are crucial for optimizing the performance of hydrogels. In this paper, we propose a novel superposition sensing structure based on an integrated dual-Mach–Zehnder interferometer (MZI), which can simultaneously sense temperature and refractive index (RI), with an RI sensitivity of −10,173 nm/RIU and a temperature sensitivity of 8.14 nm/ °C. For the first time, its application for the operando monitoring of gelation kinetics of polyacrylamide hydrogel (PAH) is demonstrated. By establishing the kinetic curves of temperature and RI information in the gelation process, it is confirmed that the gelation process can be divided into three stages: initial lag, rapid gelation, and final saturation. In addition, two gelation laws are verified through experiments: (1) As the concentration of monomers and crosslinkers increases, RI changes, and the amount of heat released gradually increases. (2) As the concentration of catalysts increases, the gelation time decreases significantly. This superposition sensing technique based on dual-MZI can be further applied to monitor other complex dynamic biochemical processes, thereby providing insights into the laws and mechanisms of reaction processes.

A three-band narrow-band terahertz perfect absorber for patch antennas and other sensors

We have studied a multi-mode resonance and actively tunable high-sensitivity terahertz perfect absorber using a microstructured Dirac semimetal. The absorber is composed of gold-dielectric-Dirac semimetal three-layer structure, which has good optical sensing characteristics. The absorber produces three perfect absorption modes (>95%) in the range of 2–6 THz, with a minimum bandwidth (FWHM) of 0.18 THz. We studied the adaptability of the device response mode to the actual complex electromagnetic environment by simulating the deflection incident angle. In addition, we also analyzed the influence of various parameter variables on the absorption performance, which provides a clearer idea and more possibilities for the wider application of the device. In terms of sensing detection, the absorber has a maximum refractive index sensitivity S of 610 GHz/RIU and a maximum figure of merit FOM of 3.4 in the ambient refractive index range of 1 to 1.8. Finally, we also realized the tunable absorption frequency of the absorber in 60–80 meV. We believe that based on the good optical properties of the absorber, the device has potential application value in communication technology, space exploration and so on.

Highly sensitive refractive index based biofuel adulteration sensor using multiband absorber

A multi-band electromagnetic absorber is proposed for sensing the adulterant concentration in biofuels based on refractive index variations at room temperature (300 K). The proposed absorber structure consists of a gold based metallic resonating geometry and metallic ground plane on the top and bottom layer of the cavity loaded dielectric material. The unit cell has volume of 24μm×24μm×3.57μm (0.1088λL×0.1088λL×0.01618λL where λL is wavelength at lowest operating frequency), five absorption peaks at 1.36, 3.349, 5.338, 5.887 & 9.307 THz with peak absorptivity of 96.15%, 97.55%, 95.63%, 83.60% & 96.13% and Full Width at Half Maxima (FWHM) of 0.098, 0.275, 1.01 & 0.417 THz respectively. For solid substrate, the absorption performance is polarization insensitive and with cavity inside the substrate, the absorption performance is polarization sensitive in normal incidence. The absorber’s Refractive Index (RI) and biofuel adulteration sensing performance is also analyzed by loading analyte on its top surface and inside the substrate cavity. Peak sensitivity of ∼3 THz/RIU with a maximum Figure of merit (FOM) of ∼5.69RIU−1 and maximum ∼0.16% error in ethanol based biofuel adulteration estimation are achieved. The proposed absorber has advantages of ultracompact dimensions, very high sensitivity and very less estimation error in adulteration estimation over other absorber based sensors.

Elucidating refractive index sensitivity on subradiant, superradiant, and fano resonance modes in single palladium-coated gold nanorods

Herein, we investigated the distinctive scattering properties exhibited by single gold nanorods coated with palladium (AuNRs@Pd), with variations in the Pd shell thicknesses and morphologies. AuNRs@Pd were synthesized through bottom-up epitaxial Pd growth using two different concentrations of Pd precursor. These single AuNRs@Pd displayed the characteristic of subradiant and superradiant localized surface plasmon resonance peaks, characterized by a noticeable gap marked by a Fano dip. We revealed the effect of local refractive index (RI) on the subradiant and superradiant peak energies, as well as the Fano dip in the scattering spectra of AuNRs@Pd with different Pd shell thicknesses. We demonstrated the applicability of the inflection points (IFs) method on detecting peaks and dip changes across different RIs. Thin AuNRs@Pd1mM displayed more pronounced sensitivity to peak shifts in response to variations in local RIs compared to thick AuNRs@Pd2mM. In contrast, thick AuNRs@Pd2mM exhibited greater sensitivity to changes in curvature near the subradiant and superradiant peak energies rather than peak shift sensitivity across different local RIs. Moreover, the Fano dip shift was more noticeable in thick AuNRs@Pd2mM compared to thin AuNRs@Pd1mM across different local RIs. Therefore, we provided new insight into the RI sensitivity on subradiant, superradiant, and Fano resonance modes in single AuNRs@Pd.

Diameter Optimization of No-Core Fiber for High Refractive Index Sensing

This study presents a simulation-based modal analysis to optimize the no-core fiber (NCF) sensor design for high refractive index (HRI) applications. The goal is to improve the sensitivity of the sensor towards HRI by solving problems in conventional optical fiber sensors (OFS), such as sophisticated fabrication methods, high fragility, and potential effects on long-term reliability. Furthermore, in previous work, most OFS applications focused on detecting mediums with a low refractive index (LRI). Hence, the goal of this study is to overcome these constraints. For this research, the Wave Optics module in COMSOL Multiphysics® software was used to optimize the NCF diameter, which ranged from 100 μm to 150 μm. Simulations include changes in the refractive index (RI) of the analyte in the surrounding medium of the sensor to analyse the efficiency of the NCF in HRI sensing completely, ranging from 1.46 RIU to 1.56 RIU. The performance is then assessed by monitoring the intensity change in the power spectrum, which reflects the reaction of leaky modes to various HRI conditions. The findings indicate that decreasing the size of the NCF sensor enhances sensitivity in detecting HRI mediums by amplifying the leakage loss of each order leaky mode, as opposed to a larger NCF diameter. This effect is due to the reduced confinement of modes in a smaller diameter NCF. The 100 μm diameter NCF has exceptional refractive index (RI) sensitivity and linearity, measuring 88.996 dB/RIU and 0.7783, respectively. Accordingly, NCF proves to be a promising candidate for utilization in HRI sensing applications, given its high sensitivity and straightforward fabrication, making it easily implementable and practical for real-world use.

Sensitivity detection of imidacloprid pesticide using a metasurface sensor in THz spectrum regime

Imidacloprid (C9H10ClN5O2) pesticide is frequently used in agriculture as it effectively controls various insect pests. Accurate detection of imidacloprid concentrations is crucial for food safety and other fields. This work proposes a novel metasurface (MS) sensor with efficient sensitivity in the THz frequency regime to detect imidacloprid pesticides. The MS sensor has an electrical size of 7.17λ × 7.17λ with the smallest wavelength. The transmission coefficient remains stable for the metasurface sensor’s TE and TM polarization modes. Due to the symmetrical design, it exhibits strong electric and magnetic field enhancement with polarization insensitivity for different incident angles. The sensing capability is tested by implementing different sample analyte thicknesses of imidacloprid in the top surface of the MS sensor. Imidacloprid analyte thickness increases cause a red and blue change in the metasurface’s transmission frequency. The simulation results show that the suggested MS sensor has a refractive index (RI) sensitivity of 0.125 THz/RIU, and a Q factor of 5.262 and 4.084. This is a simple, cost-effective, sufficiently sensitive, and efficient THz MS sensor with a wide range of applications in pesticide concentration measurement, including imidacloprid.

Ultra-sensitive DNA detection with single-base mismatch resolution using a tapered thin-diameter photonic crystal fiber interferometer

A tapered thin-diameter photonic crystal fiber interferometer (TTD-PCFI) biosensor is proposed and demonstrated for DNA hybridization detection with an ultra-low limit of detection (LoD) and high specificity. The TTD-PCFI is fabricated by arc-discharged melting and adiabatic tapering, with a lower loss and stronger evanescent optical field. The fabricated TTD-PCFI with a diameter of 8.63 µm achieves a high refractive index sensitivity of 3842.5 nm/RIU. DNA hybridization detection of concentration ranging from 1 fM to 1 pM is achieved, with a sensitivity of 632 pm/lg(fM). The measured LoD is as low as 410 aM. Single-base mismatch detection of 25-base target single-stranded DNA is also achieved. The proposed label-free DNA biosensor has the advantages of the ultra-low LoD, high specificity, and real-time detection, which has great application prospects in the fields of disease diagnosis, microbial detection, and environmental science.

Citrate polymer optical fiber for measuring refractive index based on LSPR sensor

Fiber optic localized surface plasmon resonance (LSPR) sensors have become an effective tool in refractive index (RI) detection for biomedical applications because of their high sensitivity. However, using conventional optical fiber has caused limitations in implanting the sensor in the body. This research presents the design and construction of a new type of polymer-based LSPR sensors to address this issue. Also, finite element method (FEM) is used to design the sensor and test it theoretically. The proposed polymer optical fiber (POF) based on citrate is biocompatible, flexible, and degradable, with a rate of 22% and 27 over 12 days. The step RI structure utilizes two polymers for light transmission: poly (octamethylene maleate citrate) (POMC) as the core and poly (octamethylene citrate) (POC) as the cladding. The POF core and cladding diameters and lengths are 700 µm, 1400 µm, and 7 cm, respectively. The coupling efficiency of light to the POF was enhanced using a microsphere fiber optic tip. The obtained results show that the light coupling efficiency increased to 77.8%. Plasma surface treatment was used to immobilize gold nanoparticles (AuNPs) on the tip of the POF, as a LSPR-POF sensor. Adsorption kinetics was measured based on the pseudo-first-order model to determine the efficiency of immobilizing AuNPs, in which the adsorption rate constant (k) was obtained be 8.6 × 10–3 min−1. The RI sensitivity of the sensor in the range from 1.3332 to 1.3604 RIU was obtained as 7778%/RIU, and the sensitivity was enhanced ~ 5 times to the previous RI POF sensors. These results are in good agreement with theory and computer simulation. It promises a highly sensitive and label-free detection biosensor for point-of-care applications such as neurosciences.

Dealing with plasmonic crystal biosensors: Sensitivity assessment of nanodisks/nanoholes arrayed plasmonic system for label-free DNA detection

Label-free optical deoxyribonucleic acid (DNA) sensing with arrayed plasmonic nanostructures (plasmonic crystals) is a promising technology for biomedical diagnosis and bioanalytical science. Plasmonic biosensors can detect target biomolecules by utilizing the shift in plasmonic resonance caused by changes in the surrounding refractive index (RI) attributed to the capture of target biomolecules using a recognizer. Conventional explanations for the sensitivity of plasmonic crystals are based on bulk (BRIS) and surface RI sensitivities (SRIS) for basic plasmonic nanoparticles despite their unique properties such as surface lattice resonances (SLRs), wherein localized surface plasmons (LSPs) cooperatively oscillate with their pitch. Therefore, investigating the sensitivity of SLRs is imperative for improving sensing performance. In this study, the sensitivity of adenomatous polyposis coli (APC) gene-related DNA hybridization detection of complementary plasmonic crystals composed of nanodisks (PNDs) on or under plasmonic nanoholes (PNHs) was investigated considering the SLR properties. The BRIS was measured using the conventional definition of the peak wavelength shift per unit RI increment (nm/RIU) followed by the SRIS measurement using the layer-by-layer method. The BRIS and SRIS measurements reflect the practical sensitivity for DNA detection. PNHs had higher sensitivity than PNDs, with a limit of detection of 0.30 nM. Further, only the SLR-based mode responded to localized RI changes because of DNA hybridization, whereas both the LSPs- and SLR-based modes responded to uniform RI changes caused by layer-by-layer coating. Our investigation will open up possibilities and opportunities for plasmonic crystal biosensors.

High-Sensitivity Refractive Index Sensor with Dual-Channel Based on Surface Plasmon Resonance Photonic Crystal Fiber.

In order to achieve a high-precision synchronous detection of two different refractive index (RI) analytes, a D-type surface plasmon resonance (SPR) photonic crystal fiber (PCF) RI sensor based on two channels is designed in this paper. The sensor uses a D-shaped planar region of the PCF and a large circular air hole below the core as the sensing channels. Surface plasmon resonance is induced by applying a coating of gold film on the surface. The full-vector finite-element method (FEM) is used to optimize the structural parameters of the optical fiber, and the sensing characteristics are studied, including wavelength sensitivity, RI resolution, full width at half maximum (FWHM), figure of merit (FOM), and signal-to-noise ratio (SNR). The results show that the channel 1 (Ch 1) can achieve RI detection of 1.36-1.39 in the wavelength range of 1500-2600 nm, and the channel 2 (Ch 2) can achieve RI detection of 1.46-1.57 in the wavelength range of 2100-3000 nm. The two sensing channels can detect independently or simultaneously measure two analytes with different RIs. The maximum wavelength sensitivity of the sensor can reach 30,000 nm/RIU in Channel 1 and 9900 nm/RIU in Channel 2. The RI resolutions of the two channels are 3.54 × 10-6 RIU and 10.88 × 10-6 RIU, respectively. Therefore, the sensor realizes dual-channel high- and low-RI synchronous detection in the ultra-long wavelength band from near-infrared to mid-infrared and achieves an ultra-wide RI detection range and ultra-high wavelength sensitivity. The sensor has a wide application prospect in the fields of chemical detection, biomedical sensing, and water environment monitoring.

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.

Subwavelength Grating Cascaded Microring Resonator Biochemical Sensors with Record-High Sensitivity.

Photonic integrated circuit biochemical and biomedical sensors show promising applications in medical diagnosis, food security, healthcare, and environmental monitoring. Silicon-on-insulator subwavelength grating waveguides and cascaded microring resonator structures enhance photon-analyte interaction, offering superior sensing performance (higher sensitivity with lower limit of detection and larger free spectral range) compared to traditional strip and slot waveguide microring resonator structures. In this study, we design, simulate, and experimentally demonstrate a novel and compact biochemical sensor integrating subwavelength grating cascaded microring resonators and multibox subwavelength grating straight waveguides on a silicon-on-insulator platform. We achieve a record-high refractive index sensitivity of 810 nm/RIU with a limit of detection value of 2.04 × 10-5 RIU. The measured concentration sensitivity for sodium chloride solutions is 1430 pm/% with a limit of detection of 0.04%. The free spectral range is 35.8 nm, and the measured Q factor is 1.9 × 103. By combining the advantages of cascaded microring resonators with those subwavelength gratings, our sensor offers unprecedented sensitivity for biochemical sensing applications, promising significant enhancements in healthcare diagnostics and environmental monitoring systems.

Multi-resonance plasmonic refractive index sensor based on maze-shaped resonators for biological applications

Herein, a plasmonic refractive index (RI) sensor based on a metal–insulator–metal waveguide coupled with maze-shaped resonators is proposed and numerically investigated using finite element method. Various geometrical parameter impacts on the transmission spectrum are examined to optimize the sensor’s performance. Additionally, the effect of using SiO2 as a dielectric material instead of air has been investigated. The proposed sensor can achieve maximum RI sensitivity, figure of merit, and sensing resolution of 3340 nm RIU−1, 143.33 RIU−1, and 2.99 × 10−6 RIU, respectively, in the 500–3500 nm wavelength range. The designed structure is investigated for potential applications in different biological fields, including detecting cancer cells, determining blood hemoglobin (HB) levels, and glucose concentrations. This sensor can detect MCF-7 cancer cells with a maximum sensitivity of 3543 nm RIU−1 and can achieve the sensitivity of 0.407 nm·l g−1 for glucose concentration and 3329.41 nm RIU−1 for blood HB level. The structure presented in this study has promising specifications, making it suitable for use in optical integrated circuits, particularly in highly sensitive sensors.

Highly sensitive near-field leakage-enhanced optical fiber SPR sensor based on gold nanoarrays modulation

A novel near-field leakage-enhanced optical fiber surface plasmon resonance (NLF-SPR) sensor based on the synergistic sensitization of cuboid gold nanoarrays and WSe2 film is proposed for the first time to our knowledge. The sensor is composed of a side-polished multimode fiber, a continuous gold film, a continuous tungsten selenide (WSe2) film, and well-arranged cuboid gold nanoarrays. The sensor leverages the high carrier mobility characteristic of the WSe2 film, the near-field leakage enhancement arising from the nanoarrays structure, and the localized electric field enhancement property introduced by nanotips to enhance the refractive index sensitivity. In order to optimize the spectral characteristic, the modulation of electric field distribution and spectral characteristic by the structural parameters of gold nanoarrays is investigated through three-dimensional (3D) finite element simulation. Results show that in the refractive index range of 1.3332–1.3432 refractive index unit (RIU), the average sensitivity of the NLF-SPR sensor can reach up to 10108 nm/RIU, and the highest sensitivity can reach up to 14692.46 nm/RIU. The average sensitivity and the highest sensitivity are 1.87 times and 2.56 times higher, respectively, than those of the side-polished fiber SPR sensor with a monolayer gold film. Furthermore, the experimental results demonstrate that the near-field leakage enhancement introduced by gold nanoarrays improves the performance of the fiber SPR sensor, resulting in a 49.3 % increment in sensitivity. The experimental results are in agreement with the trend of the simulation results. Even better performance improvements have been achieved compared to existing methods and a more flexible approach to performance tuning is provided. With the maturity and development of various nanoarray processing techniques such as focused ion beam, electron beam lithography, and nanoimprint lithography, the proposed NLF-SPR sensor has the potential to be fabricated on a large scale and is expected to be widely employed in biomedical, clinical applications, and other fields.