Photonic crystal fiber-based surface plasmon resonance sensor with selective analyte channels and graphene-silver deposited core

Ultra-high sensitivity weak magnetic field detecting magnetic fluid surface plasmon resonance sensor based on a single-hole fiber

An ultra-high sensitivity weak magnetic field detecting magnetic fluid surface plasmon resonance (SPR) sensor based on a single-hole fiber (SHF) is proposed for detecting weak magnetic fields. The sensor is constructed with a single-hole fiber in which an exclusive air hole in the cladding is embedded with a metal wire and filled with a magnetic fluid (MF) to enhance the magnetic field sensitivity. The effects of the structural parameters, embedded metals, and refractive index difference between the core and cladding on the magnetic field sensitivity and peak loss are investigated and optimized. The sensitivity, resolution, figure of merit (FOM), and other characteristics of the sensor are analyzed systematically. The numerical results reveal a maximum magnetic field sensitivity of 451,000 pm/mT and FOM of 15.03 mT-1. The ultra-high magnetic field sensitivity renders the sensor capable of detecting weak magnetic fields at the pT level for the first time, in addition to a detection range from 3.5 mT to 17 mT. The SHF-SPR magnetic field sensor featuring high accuracy, simple structure, and ease of filling has immense potential in applications such as mineral resource exploration as well as geological and environmental assessment.

Review of Biosensors Based on Plasmonic-Enhanced Processes in the Metallic and Meta-Material-Supported Nanostructures

Surface plasmons, continuous and cumulative electron vibrations confined to metal-dielectric interfaces, play a pivotal role in aggregating optical fields and energies on nanostructures. This confinement exploits the intrinsic subwavelength nature of their spatial profile, significantly enhancing light-matter interactions. Metals, semiconductors, and 2D materials exhibit plasmonic resonances at diverse wavelengths, spanning from ultraviolet (UV) to far infrared, dictated by their unique properties and structures. Surface plasmons offer a platform for various light-matter interaction mechanisms, capitalizing on the orders-of-magnitude enhancement of the electromagnetic field within plasmonic structures. This enhancement has been substantiated through theoretical, computational, and experimental studies. In this comprehensive review, we delve into the plasmon-enhanced processes on metallic and metamaterial-based sensors, considering factors such as geometrical influences, resonating wavelengths, chemical properties, and computational methods. Our exploration extends to practical applications, encompassing localized surface plasmon resonance (LSPR)-based planar waveguides, polymer-based biochip sensors, and LSPR-based fiber sensors. Ultimately, we aim to provide insights and guidelines for the development of next-generation, high-performance plasmonic technological devices.

Silver-coated hollow fiber surface plasmon resonance sensor for glucose detection with enhanced limit of detection

A fiber-optic surface plasmon resonance (SPR) biosensor based on a silver-coated hollow fiber (HF) structure for glucose detection is presented. The sensor surface was immobilized with 4-mercaptophenylboronic acid (PMBA) acting as a glucose recognition monolayer. Then, gold nanoparticles (AuNPs) modified with 2-aminoethanethiol (2-AET) and PMBA were introduced onto the sensor surface after glucose was captured to enhance the wavelength shift of the SPR phenomenon excited by the light transmitted in the wall of the HF sensor. Instead of the conventional one-step sensitization pretreatment commonly used in the deposition process of silver films for fiber-optic SPR sensors, a sensitization-activation two-step activation method was adopted in the fabrication of the proposed sensor. Experiments for glucose detection were performed on the fabricated sensors in the concentration range of 1 nM-1 mM. Results showed that the sensor fabricated by the two-step activation method has a much larger shift of resonance wavelength than the sensor fabricated using the one-step sensitization method. The resonance wavelength shift was found to be linear to the logarithm of the concentration in the range of 1 nM-1 mM. The sensor achieved a limit of detection (LOD) of as low as 1 nM, which is at least an order of magnitude lower than that of other fiber-optic sensors for glucose detection reported previously. The presented HF glucose sensor has the potential for biosensing applications and provides a large reference value in the study of optical fiber SPR sensors for biosensing.

Recent Progress on Microfluidics Integrated with Fiber-Optic Sensors for On-Site Detection

This review introduces a micro-integrated device of microfluidics and fiber-optic sensors for on-site detection, which can detect certain or several specific components or their amounts in different samples within a relatively short time. Fiber-optics with micron core diameters can be easily coated and functionalized, thus allowing sensors to be integrated with microfluidics to separate, enrich, and measure samples in a micro-device. Compared to traditional laboratory equipment, this integrated device exhibits natural advantages in size, speed, cost, portability, and operability, making it more suitable for on-site detection. In this review, the various optical detection methods used in this integrated device are introduced, including Raman, ultraviolet-visible, fluorescence, and surface plasmon resonance detections. It also provides a detailed overview of the on-site detection applications of this integrated device for biological analysis, food safety, and environmental monitoring. Lastly, this review addresses the prospects for the future development of microfluidics integrated with fiber-optic sensors.

Cross-sensitivity immune SPR sensor based on fan-shaped microstructured optical fiber for temperature and refractive index sensing

To avoid coating and filling into the fiber holes, facilitate the phase-matching and eliminate cross-sensitivity problems, we propose a surface plasmon resonance sensor based on a fan-shaped microstructured optical fiber (MOF) for the simultaneous sensing of temperature and refractive index (RI). The fan-shaped structure is fabricated by polishing two sides of MOF with an angle of 120°. One side is coated with the gold film and polydimethylsiloxane layer for temperature sensing, and the other side is only coated with the gold film for RI sensing. The two sensing sides can support resonance peaks with two polarized directions at the angle of 120°, which are independent without cross-sensitivity. By monitoring the shifts of the two polarized peaks, our numerical results show that the temperature sensitivity is 2.932 nm/°C in the range of 30 °C to 40 °C, and RI sensitivity is 4235 nm/RIU in the range of 1.38 to 1.39, respectively.

SPR Sensor Based on a Concave Photonic Crystal Fiber Structure with MoS2/Au Layers

We propose a surface plasmon resonance (SPR) sensor based on the concave photonic crystal fiber (PCF) coated with molybdenum disulfide (MoS2) and Au layers, which can detect the refractive index (RI) of the analyte. The finite element method (FEM) was used to verify our design, and the loss spectra of the fundamental mode are calculated. Compared with the SPR sensor with only a Au layer, the wavelength sensitivity can be improved by from 3700 to 4400 nm/RIU. Our proposed sensor works in near-infrared band and has a wide RI range from 1.19 to 1.40. The influences of the geometrical parameters of PCF and the thicknesses of Au and MoS2 layers on the loss spectra are discussed in detail, and the maximum wavelength sensitivity of 5100 nm/RIU can be achieved. Meanwhile, a high resolution of 1.96 × 10-5 RIU and the largest FOM of 29.143 can be obtained. It is believed that our findings show the sensor's excellent potential in medical testing, unknown biological detection, environmental monitoring and organic chemical detection.

Numerical Analysis of Highly Sensitive Twin-Core, Gold-Coated, D-Shaped Photonic Crystal Fiber Based on Surface Plasmon Resonance Sensor

This research article proposes and numerically investigates a photonic crystal fiber (PCF) based on a surface plasmon resonance (SPR) sensor for the detecting refractive index (RI) of unknown analytes. The plasmonic material (gold) layer is placed outside of the PCF by removing two air holes from the main structure, and a D-shaped PCF-SPR sensor is formed. The purpose of using a plasmonic material (gold) layer in a PCF structure is to introduce an SPR phenomenon. The structure of the PCF is likely enclosed by the analyte to be detected, and an external sensing system is used to measure changes in the SPR signal. Moreover, a perfectly matched layer (PML) is also placed outside of the PCF to absorb unwanted light signals towards the surface. The numerical investigation of all guiding properties of the PCF-SPR sensor is completed using a fully vectorial-based finite element method (FEM) to achieve the finest sensing performance. The design of the PCF-SPR sensor is completed using COMSOL Multiphysics software, version 1.4.50. According to the simulation results, the proposed PCF-SPR sensor has a maximum wavelength sensitivity of 9000 nm/RIU, an amplitude sensitivity of 3746 RIU^-1, a sensor resolution of 1 × 10^-5 RIU, and a figure of merit (FOM) of 900 RIU^-1 in the x-polarized direction light signal. The miniaturized structure and high sensitivity of the proposed PCF-SPR sensor make it a promising candidate for detecting RI of analytes ranging from 1.28 to 1.42.

High-performance surface plasmon resonance fiber sensor based on cylindrical vector modes

Cylindrical vector modes with azimuthal polarization and low transmission loss are proposed for the first time to be utilized in a novel design of a surface plasmon resonance (SPR) sensor based on a circular photonic crystal fiber (C-PCF). A C-PCF with a ring of air holes in the cladding is designed where a gold layer with a thickness of 44 nm is coated on the outer cladding surface. The optimal geometric parameters are determined using the finite-element method (FEM) for a high-quality TE₀₁ mode and high sensitivity of the sensor. The proposed SPR sensor shows high sensitivity for analyte refractive index (RI) ranging from n_a = 1.29 to 1.34 over the wavelength range of 1400-2000 nm. It is expected that the proposed sensor can sense low concentrations of hemoglobin, lymphocytes and monocytes of red and white blood cells which are effective in diagnosing the progress of cancer tumors. The maximum sensitivity of 13,800 nm/RIU is obtained in the refractive index environment of 1.33-1.34. The sensor resolution is of the order of 10^-6 and the amplitude sensitivity reaches its maximum of 2380 RIU^-1 at n_a = 1.30 which is the highest value ever reported. Our proposed sensor shows high sensitivity and simultaneously simple design with high performance.

HE1,1 mode excited surface plasmon resonance for high-sensitivity sensing by photonic crystal fibers

Surface plasmon resonance (SPR) is widely used in photonic crystal fiber sensors. In this work, a photonic crystal fiber sensor based on HE_1,1 mode excited SPR is designed and analyzed by the finite element method. The maximum wavelength sensitivity, optimal resolution, and amplitude sensitivity of the optical fiber sensor are 24,600 nm/RIU, 4.07×10^-6RIU, and 1164.13RIU^-1, respectively, for the refractive index range between 1.29 and 1.39. The sensor has excellent properties and wide application prospects in bimolecular and biochemical sensing, environmental monitoring, food safety, and other fields.

A Surface Plasmon Resonance-Based Photonic Crystal Fiber Sensor for Simultaneously Measuring the Refractive Index and Temperature

In this paper, a surface plasmon resonance (SPR)-based photonic crystal fiber (PCF) sensor is proposed for simultaneously measuring the refractive index (RI) and temperature. In the design, the central air hole and external surface of the proposed PCF are coated with gold films, and an air hole is filled with the temperature-sensitive material (TSM). By introducing the inner and outer gold films and TSM, the RI and temperature can be measured simultaneously at different wavelength regions. The simulation results show that the average wavelength sensitivities of the proposed SPR-based PCF sensor can reach 4520 nm/RIU and 4.83 nm/°C in the RI range of 1.35~1.40 and a temperature range of 20~60 °C, respectively. Moreover, because of using the different wavelength regions for sensing, the RI and temperature detections of the proposed SPR-based PCF sensor can be achieved independently. It is believed that the proposed SPR-based PCF RI and temperature sensor has important applications in biomedicine and in environmental science.

Highly sensitive dual-core photonic quasicrystal fiber methane sensor based on surface plasmon resonance

A highly sensitive dual-core photonic quasicrystal fiber methane sensor based on surface plasmon resonance is designed and analyzed. In this sensor, cryptophane E is doped with polysiloxane and Ag and used as the sensitive film and plasma medium, respectively, for sensitive detection of methane. The influence of the structural parameters on the sensor properties is analyzed by the finite element method. The optimized dual-quasi-D-shape structure has excellent methane-sensing properties such as maximum and average wavelength sensitivities of 14 and 10.98 nm/%, respectively, in the methane concentration range of 0%-3.5%. The sensitivity is better than that of similar sensors reported previously.

Tunable surface plasmon resonance sensor based on graphene-coated photonic crystal fiber in terahertz

A terahertz surface plasmon resonance (SPR) sensor is designed based on photonic crystal fiber (PCF). Graphene is selectively coated in the cladding hole of the PCF and used as plasmonic material. The coupling mechanism, loss properties, tunability, and refractive index sensing performance of the designed SPR sensor are investigated using the finite element method. The peak of the loss spectrum corresponding to the SPR frequency can be dynamically tuned by adjusting graphene's chemical potential, and a tuning sensitivity of 767.5 GHz/eV is obtained. The SPR frequency red shifts linearly with an increase in the refractive index of analyte from 1.0 to 1.5. An average frequency sensitivity of 208.14 GHz/RIU is obtained. This research provides theoretical guidance for the design of terahertz in-fiber SPR sensors and filters.

High index core flat fiber surface plasmon resonance bio-sensor

Due to tremendous design flexibility and ease of light control capability, the photonic crystal fiber offers efficient, flexible, and miniaturized plasmonic biosensors with attractive features. In this work, a high index (GeO₂ doped silica) core flat fiber is proposed and analyzed for RI sensing ranging from 1.53 to 1.60. A rectangular analyte channel is created on top of a flat fiber to better handle the liquid analyte. To introduce the plasmonic effect, TiO₂ and gold are deposited to the analyte channel. The sensing performance is carried out for two operating wavelengths, as two peaks are obtained for each analyte. The second operating wavelength shows better sensing performance than the first one. However, the proposed sensor offers average wavelength sensitivity of 5000 nm/RIU with a sensor resolution of 2×10^-05 RIU. In addition, the proposed sensor shows identical linearity, which is quite rare in prior sensors. Moreover, the proposed flat sensor provides outstanding detection accuracy of 0.01nm^-1, detection limit of 79.28 nm, signal to noise ratio of -4.1497dB, and figure of merit of 50RIU^-1. Owing to outstanding sensing performance and a unique detection range, this sensor can be effectively used in biological and organic analyte sensing applications.

A Reconfigurable Surface-Plasmon-Based Filter/Sensor Using D-Shaped Photonic Crystal Fiber

A reconfigurable surface-plasmon-based filter/sensor using D-shaped photonic crystal fiber is proposed. Initially a D-shaped PCF is designed and optimized to realize the highly birefringence and by ensuring the single polarization filter. A tiny layer of silver is placed on the flat surface of the D-shaped fiber with a small half-circular opening to activate the plasmon modes. By the surface plasmon effect a maximum confinement loss of about 713 dB/cm is realized at the operating wavelength of 1.98 µm in X-polarized mode. At this wavelength the proposed fiber only allows Y-polarization and filters the X-polarization using surface plasmon resonance. It is also exhibiting maximum confinement loss of about 426 dB/cm at wavelength 1.92 µm wavelength for Y-polarization. At this 1.92 µm wavelength the proposed structure attenuated the Y-polarization completely and allowed X-polarization alone. The proposed PCF polarization filter can be extended as a sensor by adding an analyte outside this filter structure. The proposed sensor can detect even a small refractive index (RI) variation of analytes ranging from 1.34–1.37. This sensor provides the maximum sensitivity of about 5000 nm/RIU; it enables this sensor to be ideally suited for various biosensing and industrial applications.

Numerical Analysis of Multifunctional Biosensor with Dual-Channel Photonic Crystal Fibers Based on Localized Surface Plasmon Resonance

A multifunctional biosensor composed of a dual-channel photonic crystal fiber (PCF) based on localized surface plasmon resonance (LSPR) is presented to measure dynamic changes in the magnetic field, temperature, and analyte refractive index at mid-infrared wavelengths. The finite-element method (FEM) is used to model and determine the sensing properties of the sensor. The flat dual-channel surface is coated with a gold film, and two nanowires are put on the fan-shaped openings to create directional resonance coupling to detect the analyte refractive index and temperature. By utilizing that the refractive index (RI) of the filled magnetic fluid (MF) is sensitive to the external magnetic field and temperature, a sensor with multi-physical detection functions is obtained. For refractive indexes ranging from 1.47 to 1.52, the maximum sensitivity is as high as 31,000 nm/RIU, with a resolution of 3.22 × 10−6 RIU. The maximum sensitivities for the magnetic field and temperature are 1970 pm/Oe and −5500 pm/°C, respectively.

Green syntheses of graphene and its applications in internet of things (IoT)-a status review

Internet of Things (IoT) is a trending technological field that converts any physical object into a communicable smarter one by converging the physical world with the digital world. This innovative technology connects the device to the internet and provides a platform to collect real-time data, cloud storage, and analyze the collected data to trigger smart actions from a remote location via remote notifications, etc. Because of its wide-ranging applications, this technology can be integrated into almost all the industries. Another trending field with tremendous opportunities is Nanotechnology, which provides many benefits in several areas of life, and helps to improve many technological and industrial sectors. So, integration of IoT and Nanotechnology can bring about the very important field of Internet of Nanothings (IoNT), which can re-shape the communication industry. For that, data (collected from trillions of nanosensors, connected to billions of devices) would be the 'ultimate truth', which could be generated from highly efficient nanosensors, fabricated from various novel nanomaterials, one of which is graphene, the so-called 'wonder material' of the 21st century. Therefore, graphene-assisted IoT/IoNT platforms may revolutionize the communication technologies around the globe. In this article, a status review of the smart applications of graphene in the IoT sector is presented. Firstly, various green synthesis of graphene for sustainable development is elucidated, followed by its applications in various nanosensors, detectors, actuators, memory, and nano-communication devices. Also, the future market prospects are discussed to converge various emerging concepts like machine learning, fog/edge computing, artificial intelligence, big data, and blockchain, with the graphene-assisted IoT field to bring about the concept of 'all-round connectivity in every sphere possible'.

Research on optimization of magnetic field sensing characteristics of PCF sensor based on SPR

A photonic crystal fiber utilizing surface plasmon resonance (PCF-SPR) sensor based on refractive index (RI) control of magnetic fluid (MF) is designed. The air holes of the sensor are arranged in a hexagonal shape, and the optical field transmission channels on both sides of the central air hole can effectively confine the energy of the optical field. We use MF as the sensing medium, and coat the inner wall of the central air hole with gold. It can effectively stimulate the SPR effect to achieve the purpose of magneto-refractive modulation. We study the sensing characteristics of the proposed sensor by finite element analysis. The results show that the highest refractive index sensitivity reaches 19520 nm/RIU in the RI range of 1.42-1.435 and the maximum figure of merit (FOM) is 374.3 RIU^-1. In addition, the magnetic field and the temperature response characteristics of the designed sensor are also investigated. In the magnetic field range of 50-130 Oe, the magnetic field sensitivity is 590 pm/Oe. In the temperature range of 24.3-144.3 °C, the temperature sensitivity is only -29.7 pm/℃. The proposed sensor has significant advantages such as stable structure, high sensitivity, easy integration, resistance to electromagnetic interference and can be used for weak magnetic magnitude detection. It has wide application prospects in industrial production, military, and medical equipment.

Sensitive Detection of SARS-CoV-2 Using a Novel Plasmonic Fiber Optic Biosensor Design

The coronavirus (COVID-19) pandemic has put the entire world at risk and caused an economic downturn in most countries. This work provided theoretical insight into a novel fiber optic-based plasmonic biosensor that can be used for sensitive detection of SARS-CoV-2. The aim was always to achieve reliable, sensitive, and reproducible detection. The proposed configuration is based on Ag-Au alloy nanoparticle films covered with a layer of graphene which promotes the molecular adsorption and a thiol-tethered DNA layer as a ligand. Here, the combination of two recent approaches in a single configuration is very promising and can only lead to considerable improvement. We have theoretically analyzed the sensor performance in terms of sensitivity and resolution. To highlight the importance of the new configuration, a comparison was made with two other sensors. One is based on gold nanoparticles incorporated into a host medium; the other is composed of a bimetallic Ag-Au layer in the massive state. The numerical results obtained have been validated and show that the proposed configuration offers better sensitivity (7100 nm\RIU) and good resolution (figure of merit; FOM = 38.88RIU - 1 and signal-to-noise ratio; SNR = 0.388). In addition, a parametric study was performed such as the graphene layers' number and the size of the nanoparticles.

Sensitive Near-Infrared Refractive Index Sensors Based on D-Shaped Photonic Crystal Fibers

We report a numerical study of D-shaped photonic crystal fiber based plasmonic refractive index sensor with high resolution and sensitivity in the near-infrared region. D-shaped photonic crystal fiber is formed by side polishing one part of photonic crystal fiber. It has a polishing surface where plasmonic gold layer is coated to modulate the resonant wavelength and enhance the refractive index sensitivity. Several D-shaped photonic crystal fiber plasmonic sensors with various distances from the photonic crystal fiber's core to the polishing surface and gold thicknesses are designed and their characteristics are analyzed by the finite element method. The simulation results indicate that distance from the photonic crystal fiber's core to the polishing surface causes modifications in the loss intensity, the resonant wavelength, and the refractive index sensitivity of D-shaped photonic crystal fiber plasmonic sensor. Mass production of refractive index sensors were achieved using a simple fabrication process, whereby the D-shaped photonic crystal fiber is grinded where distance from the photonic crystal fiber's core to the polishing surface is less than one layer thickness and then coated with the gold layer. For the refractive index sensing applications, the maxima theoretical resolution and sensitivity of D-shaped photonic crystal fiber plasmonic sensor reach 2.98 × 10 ⁶refractive index unit and 6,140 nm/refractive index unit in range of 1.30-1.37, respectively. We also report an initial fabrication of the D-shaped photonic crystal fiber following the standard stack-and- draw method to demonstrate the feasibility of the proposed device by using our in-house equipments. The proposed D-shaped photonic crystal fiber plasmonic sensor design in this work would be useful for the development of cheap refractive index sensors with high sensitivity and resolution.

Statistical Modelling of Photonic Crystal Fibre Based Surface Plasmon Resonance Sensors Resonant Peak Wavelength for Tolerance Studies

We report a statistical approach to model the resonant peak wavelength (RPW) equation(s) of a photonic crystal fibre (PCF)-based surface plasmon resonance (SPR) sensors in terms of the PCF structural parameters (air-hole diameter, pitch, core diameter and gold layer thickness) at various tolerance levels. Design of experiments (statistical tool) is used to investigate the role played by the PCF structural parameters for sensing performance evaluation-RPW, across three tolerance levels (±2%, ±5% and ±10%). Pitch of the hollow-core PCF was discovered to be the major influencing parameter for the sensing performance (RPW) of the PCF-based SPR sensor while the inner metal (gold) layer thickness and core diameter are the least contributing parameters. This novel statistical method to derive the sensing performance parameter(s) of the PCF-based SPR sensors can be applied effectively and efficiently in the designing, characterisation, tolerance analysis not only at the research level, but also in optical fibre sensor fabrication industry to improve efficiency and lower cost.

Graphene-Coated Highly Sensitive Photonic Crystal Fiber Surface Plasmon Resonance Sensor for Aqueous Solution: Design and Numerical Analysis

This paper presents the design and analysis of a surface plasmon resonance (SPR) sensor in a photonic crystal fiber (PCF) platform, where graphene is used externally to attain improved sensing performance for an aqueous solution. The performance of the proposed sensor was analyzed using the finite element method-based simulation tool COMSOL Multiphysics. According to the simulation results, the proposed sensor exhibits identical linear characteristics as well as a very high figure of merit (FOM) of 2310.11 RIU−1 in the very low detection limit of 10−3. The analysis also reveals the maximum amplitude sensitivity of 14,847.03 RIU−1 and 7351.82 RIU−1 for the x and y polarized modes, respectively, which are high compared to several previously reported configurations. In addition, the average wavelength sensitivity is 2000 nm/RIU which is comparatively high for the analyte refractive index (RI) ranging from 1.331 to 1.339. Hence, it is highly expected that the proposed PCF-based SPR sensor can be a suitable candidate in different sensing applications, especially for aqueous solutions.

Photonic Crystal Fiber SPR Liquid Sensor Based on Elliptical Detective Channel

This paper proposes a Photonic Crystal Fiber (PCF) refractive index sensor model based on the surface plasmon resonance effect. The proposed PCF model also uses the full vector finite element method to transfer the structure under the anisotropic Perfect Matching Layer (PML) boundary condition. Numerical calculations were carried out on the sensor characteristics. The calculation results show that the elliptical air hole on the left side of the PCF core is coated with a gold-nano film which serves as a Surface Plasmon Resonance (SPR) sensing channel to detect the refractive index of liquid materials. Compared with other structures, the resonant peak generated by the excited SPR effect from the elliptical sensing channel has a high sensitivity to the change of the refractive index of the liquid to be measured. With the help of this attribute, it is relatively easy to adjust the sensitivity. The refractive index range of this structure is within 1.43–1.49 and the sensitivity is up to 12,719.97 nm·RIU⁻¹. The linearity is good; R² = 0.99927, which is very suitable for liquid sensing.

A D-Shaped Photonic Crystal Fiber Refractive Index Sensor Coated with Graphene and Zinc Oxide

A surface plasmon resonance (SPR) sensor based on a D-shaped photonic crystal fiber (PCF) with an uncomplicated structure is proposed to detect the change of refractive index of liquid analytes, and numerical simulation is carried out by the finite element method (FEM). Using silver as the plasmonic metal, the performances of the SPR-PCF sensor coated with a graphene layer and zinc oxide (ZnO) layer were assessed. The sensor designed is only coated with material on the polished surface, which makes the sensor production uncomplicated and solves the problems of filling material in the hole and coating on the hole wall. The effects of structural parameters such as graphene layer thickness, silver layer thickness, ZnO thickness, lattice spacing and manufacturing tolerance of blowhole diameter on the sensor performance were numerically simulated. The numerical results show that the sensitivity of the SPR-PCF sensor coated with 25 nm ZnO is highest in the ZnO thickness range from 10 to 25 nm. In the refractive index range of 1.37-1.41 for liquid analyte, the maximum sensitivity and corresponding resolution reach 6000 nm/RIU and 1.667 × 10-5, respectively. In addition, the sensor has good stability and high structural tolerance under the tolerance of ±5% of blowhole diameter. This work has wide application value in the detection of biochemical analytes, water pollution monitoring, food quality, and medical diagnosis.

Sensitivity Enhancement of Modified D-Shaped Microchannel PCF-Based Surface Plasmon Resonance Sensor

In this work, a highly sensitive dual-core configured microchannel-based plasmonic refractive index (RI) sensor was investigated, which can be used for low RI detection. Both the sensing layer and the plasmonic material layer were built outside of the fiber design to detect the surrounding medium’s RI changes. Additionally, the effects of different plasmonic materials gold (Au), silver (Ag), and copper (Cu) toward sensitivity were investigated for the same structure. An adhesive agent was used in this work, titanium dioxide (TiO₂), and was coated on top of the plasmonic material to prevent the oxidation of Ag and Cu. The coupling strength between the fundamental mode and the surface plasmon polariton (SPP) mode was observed to be very strong due to the TiO₂ adhesive agent. With a resolution of 7.41 × 10⁻⁷ RIU, maximum wavelength sensitivity (WS) of 135,000 nm/RIU and amplitude sensitivity (AS) of 3239 RIU⁻¹ were achieved using the proposed sensor while using Au as a plasmonic material for an analyte RI range of 1.29–1.39. A detailed study of relevant literature revealed that the achieved wavelength sensitivity for plasmonic material gold (Au) is the highest among reported photonic crystal fiber (PCF)-surface plasmon resonance (SPR) sensors to date.

A Highly Magnetic Field Sensitive Photonic Crystal Fiber Based on Surface Plasmon Resonance

A novel magnetic field sensor comprising a photonic crystal fiber (PCF) is designed and investigated based on surface plasmon resonance (SPR). We use finite element analysis in order to analyze the sensing characteristics of the magnetic field sensor. The simulation results show that the sensor is very sensitive to the change of refractive index and has good linearity in the refractive index range from 1.43–1.45. The thickness of the metal film and the metal material has great influence on the resonance wavelength and the peak of the loss spectrum, the diameter of the central air hole will affect SPP excitation. When the thickness of gold layer is 50 nm, the refractive index sensitivity is 4125 nm/RIU in the refractive index range from 1.43–1.45. Using the designed sensor for magnetic field sensing, the loss spectrum is red-shifted with the increase of the magnetic field, the highest magnetic field sensitivity can reach 61.25 pm/Oe in the range from 50 Oe to 130 Oe. The sensor not only has high sensitivity of refractive index, but it can also realize accurate measurement of magnetic field. It has huge application potential in complex environment, remote sensing, real-time monitoring, and other fields.

Surface Plasmon Resonance Sensor Based on Dual-Side Polished Microstructured Optical Fiber with Dual-Core

A surface plasmon resonance (SPR) sensor based on a dual-side polished microstructured optical fiber (MOF) with a dual core is proposed for a large analyte refractive index (RI; n_a) detection range. Gold is used as a plasmonic material coated on the polished surface, and analytes can be directly contacted with the gold film. The special structure not only facilitates the fabrication of the sensor, but also can work in the n_a range of 1.42–1.46 when the background material RI is 1.45, which is beyond the reach of other traditional MOF-SPR sensors. The sensing performance of the sensor was investigated by the wavelength and amplitude interrogation methods. The detailed numerical results showed that the proposed sensor can work effectively in the n_a range of 1.35–1.47 and exhibits higher sensitivity in the n_a range of 1.42–1.43.

Dual-polarized highly sensitive surface-plasmon-resonance-based chemical and biomolecular sensor

As the research work in surface plasmon resonance (SPR)-based photonic crystal fiber (PCF) is getting tighter, a perfectly circular-shaped PCF with elliptical air holes is proposed where the performance parameters are improved significantly. The performances among our designed elliptical, circular, and rectangular air holes are compared, and the best result is achieved with the elliptical air holes. The technique used for the investigation is the finite element method, and for the simulation of data COMSOL Multiphysics 5.3a software is used. The method covers a wider range of the optical spectrum from 0.59 to 1.05 µm. The highest confinement loss achieved through our design is 340 dB/cm. The wavelength sensitivity and amplitude sensitivity are 13,000 nm/RIU and ${1189.46}\;{{\rm RIU}^{ - 1}}$1189.46RIU^-1, respectively. The sensor resolution is ${7.69} \times {{10}^{ - 6}}$7.69×10^-6 for our proposed design. The proposed sensor also achieved a maximum birefringence of ${2.8} \times {{10}^{ - 3}}$2.8×10^-3, which is, to our knowledge, the highest birefringence reported so far for a PCF-SPR sensor. This enables the fiber to be operated in a dual-polarized mode. The RI for the analyte ranges from 1.33 to 1.40. Based on all the characteristics, the proposed PCF structure can be used effectively for chemical and biomolecular sensing.

Wood Protection through Plasma Powder Deposition—An Alternative Coating Process

In contrast to conventional coating processes such as varnishing, plasma powder deposition by means of an atmospheric pressure plasma jet on wood is not yet widely used. A key advantage of this process is that volatile organic compounds and organic solvents are avoided. In the present work, European beech (Fagus sylvatica L.) and pine sapwood (Pinus sylvestris L.) were coated with polymer (polyester), metal (aluminum coated silver) or metal oxide (bismuth oxide) particles. Furthermore, a layer system consisting of polyester and metal or metal oxide was investigated. The layer thickness and topography were analyzed with a laser scanning microscope and scanning electron microscope, revealing thicknesses of 2–22 µm depending on the coating material. In general, the chemical composition of the layers was determined using X-ray photoelectron spectroscopy and Fourier-transform infrared spectroscopy measurements. The coatings consisting of metal and metal oxide showed a band gap and plasmon resonance in the range of 540 and 450 nm. Through this absorption, the wood may be protected against ultraviolet (UV) radiation. In the water uptake and release tests, the polyester layers exhibited a reduction of water vapor absorption after 24 h in 100% relative humidity (RH) by 53%–66%, whereas the pure metal oxide layers indicated the best desorption performance. The combination of metal oxide and polyester in the one-layer system combines the protection properties of the single coatings against water vapor and UV radiation.

Graphene Optical Biosensors

Graphene shows great potential in biosensing owing to its extraordinary optical, electrical and physical properties. In particular, graphene possesses unique optical properties, such as broadband and tunable absorption, and strong polarization-dependent effects. This lays a foundation for building graphene-based optical sensors. This paper selectively reviews recent advances in graphene-based optical sensors and biosensors. Graphene-based optical biosensors can be used for single cell detection, cell line, and anticancer drug detection, protein and antigen-antibody detection. These new high-performance graphene-based optical sensors are able to detect surface structural changes and biomolecular interactions. In all these cases, the optical biosensors perform well with ultra-fast detection, high sensitivities, unmarked, and are able to respond in real time. The future of the field of graphene applications is also discussed.

Infiltrated Photonic Crystal Fibers for Sensing Applications

Photonic crystal fibers (PCFs) are a special class of optical fibers with a periodic arrangement of microstructured holes located in the fiber's cladding. Light confinement is achieved by means of either index-guiding, or the photonic bandgap effect in a low-index core. Ever since PCFs were first demonstrated in 1995, their special characteristics, such as potentially high birefringence, very small or high nonlinearity, low propagation losses, and controllable dispersion parameters, have rendered them unique for many applications, such as sensors, high-power pulse transmission, and biomedical studies. When the holes of PCFs are filled with solids, liquids or gases, unprecedented opportunities for applications emerge. These include, but are not limited in, supercontinuum generation, propulsion of atoms through a hollow fiber core, fiber-loaded Bose⁻Einstein condensates, as well as enhanced sensing and measurement devices. For this reason, infiltrated PCF have been the focus of intensive research in recent years. In this review, the fundamentals and fabrication of PCF infiltrated with different materials are discussed. In addition, potential applications of infiltrated PCF sensors are reviewed, identifying the challenges and limitations to scale up and commercialize this novel technology.

High Sensitivity Photonic Crystal Fiber Refractive Index Sensor with Gold Coated Externally Based on Surface Plasmon Resonance

In this paper we propose a gold-plated photonic crystal fiber (PCF) refractive index sensor based on surface plasmon resonance (SPR), in which gold is coated on the external surface of PCF for easy fabrication and practical detection. The finite element method (FEM) is used for the performance analysis, and the numerical results show that the thickness of the gold film, the refractive index of the analyte, the radius of the air hole in the first layer, the second layer, and the central air hole can affect the sensing properties of the sensor. By optimizing the sensor structure, the maximum wavelength sensitivity can reach 11000 nm/RIU and the maximum amplitude sensitivity can reach 641 RIU⁻¹. Due to its high sensitivity, the proposed sensor can be used for practical biological and chemical sensing.

Dual-polarized highly sensitive plasmonic sensor in the visible to near-IR spectrum

We propose and numerically characterize the optical characteristics of a novel photonic crystal fiber (PCF) based surface plasmon resonance (SPR) sensor in the visible to near infrared (500-2000 nm) region for refractive index (RI) sensing. The finite element method (FEM) is used to design and study the influence of different geometric parameters on the sensing performance of the sensor. The chemically stable plasmonic material gold (Au) is used to produce excitation between the core and plasmonic mode. On a pure silica (SiO₂) substrate, a rectangular structured core is used to facilitate the coupling strength between the core and the surface plasmon polariton (SPP) mode and thus improves the sensing performance. By tuning the geometric parameters, simulation results show a maximum wavelength sensitivity of 58000 nm/RIU (Refractive Index Unit) for the x polarization and 62000 nm/RIU for the y polarization for analyte refractive indices ranging from 1.33 to 1.43. Moreover, we characterize the amplitude sensitivity of the sensor that shows a maximum sensitivity of 1415 RIU^-1 and 1293 RIU^-1 for the x and y polarizations, respectively. To our knowledge, this is the highest sensitivity for an SPR in published literature, and facilitates future development of sensors for accurate and precise analyte measurement. The sensor also attains a maximum figure of merit (FOM) of 1140 and fine RI resolution of 1.6 × 10^-6. Owing to strong coupling strength, high sensitivity, high FOM and improved sensing resolution, the proposed sensor is suited for real-time, inexpensive and accurate detection of biomedical and biological analytes, biomolecules, and organic chemicals.

High-performance sensor achieved by hybrid guide-mode resonance/surface plasmon resonance platform

We perform a comprehensive analysis of multiband absorption properties in a metal-dielectric-metal-dielectric (MDMD) nanostructure under TM wave illumination. The multiband absorption can be attributed to the hybridization of the surface plasmon resonance (SPR) and the guide-mode resonance (GMR), and we identify the hybrid GMR/SPR by the dispersion relation equations of the SPR and GMR, respectively. More importantly, the MDMD nanostructure is very sensitive to the change of the dielectric environment for the special hybrid structure; thus, it can function as a good candidate for ultrasensitive biochemical sensing. The highest sensitivity of the MDMD nanostructure reaches 1087 nm/RIU with the figure of merit (FoM) of 23 and the new figure of merit (FoM^*) of 483; it is performed by the absorption peak at 1796.1 nm of the double surface plasmon polariton with the strongest field enhancement at the surface.

Hybrid Metasurface Based Tunable Near-Perfect Absorber and Plasmonic Sensor

We propose a hybrid metasurface-based perfect absorber which shows the near-unity absorbance and facilities to work as a refractive index sensor. We have used the gold mirror to prevent the transmission and used the amorphous silicon (a-Si) nanodisk arrays on top of the gold mirror which helps to excite the surface plasmon by scattering light through it at the normal incident. We numerically investigated the guiding performance. The proposed absorber is polarization independent and shows a maximum absorption of 99.8% at a 932 nm wavelength in the air medium. Considering the real applications, by varying the environments refractive indices from 1.33 to 1.41, the proposed absorber can maintain absorption at more than 99.7%, with a red shift of the resonant wavelength. Due to impedance matching of the electric and magnetic dipoles, the proposed absorber shows near-unity absorbance over the refractive indices range of 1.33 to 1.41, with a zero-reflectance property at a certain wavelength. This feature could be utilized as a plasmonic sensor in detecting the refractive index of the surrounding medium. The proposed plasmonic sensor shows an average sensitivity of 325 nm/RIU and a maximum sensitivity of 350 nm/RIU over the sensing range of 1.33 to 1.41. The proposed metadevice possesses potential applications in solar photovoltaic and photodetectors, as well as in organic and bio-chemical detection.

Ultrahigh sensitivity refractive index sensor of a D-shaped PCF based on surface plasmon resonance

We propose a D-shaped photonic crystal fiber (PCF) refractive index sensor with ultrahigh sensitivity and a wide detection range. The gold layer is deposited on the polished surface, avoiding filling or coating inside the air holes of the PCF. The influences of the gold layer thickness and the diameter of the larger air holes are investigated. The sensing characteristics of the proposed sensor are analyzed by the finite element method. The maximum sensitivity can reach 31,000  nm/RIU, and the refractive index detection range is from 1.32 to 1.40. Our proposed PCF has excellent sensing characteristics and is competitive in sensing devices.

Symmetrical dual D-shape photonic crystal fibers for surface plasmon resonance sensing

Symmetrical dual D-shape photonic crystal fibers (PCFs) for surface plasmon resonance (SPR) sensing are designed and analyzed by the finite element method (FEM). The performance of the sensor is remarkably enhanced by the directional power coupling between the two fibers. We study the influence of the structural parameters on the performance of the sensor as well as the relationship between the resonance wavelengths and analyze refractive indexes between 1.36 and 1.41. An average spectral sensitivity of 14660 nm/RIU can be achieved in this sensing range and the corresponding refractive index resolution is 6.82 × 10^-6 RIU. The characteristics of a single D-shape PCF-SPR sensor with the same structural parameters are compared with those of the dual PCFs sensor and the latter has distinct advantages concerning the spectral sensitivity, resolution, amplitude sensitivity, and figure of merits (FOM).

A Localized Surface Plasmon Resonance Sensor Using Double-Metal-Complex Nanostructures and a Review of Recent Approaches

From active developments and applications of various devices to acquire outside and inside information and to operate based on feedback from that information, the sensor market is growing rapidly. In accordance to this trend, the surface plasmon resonance (SPR) sensor, an optical sensor, has been actively developed for high-sensitivity real-time detection. In this study, the fundamentals of SPR sensors and recent approaches for enhancing sensing performance are reported. In the section on the fundamentals of SPR sensors, a brief description of surface plasmon phenomena, SPR, SPR-based sensing applications, and several configuration types of SPR sensors are introduced. In addition, advanced nanotechnology- and nanofabrication-based techniques for improving the sensing performance of SPR sensors are proposed: (1) localized SPR (LSPR) using nanostructures or nanoparticles; (2) long-range SPR (LRSPR); and (3) double-metal-layer SPR sensors for additional performance improvements. Consequently, a high-sensitivity, high-biocompatibility SPR sensor method is suggested. Moreover, we briefly describe issues (miniaturization and communication technology integration) for future SPR sensors.

Optical Fibre Sensors Using Graphene-Based Materials: A Review

Graphene and its derivatives have become the most explored materials since Novoselov and Geim (Nobel Prize winners for Physics in 2010) achieved its isolation in 2004. The exceptional properties of graphene have attracted the attention of the scientific community from different research fields, generating high impact not only in scientific journals, but also in general-interest newspapers. Optical fibre sensing is one of the many fields that can benefit from the use of these new materials, combining the amazing morphological, chemical, optical and electrical features of graphene with the advantages that optical fibre offers over other sensing strategies. In this document, a review of the current state of the art for optical fibre sensors based on graphene materials is presented.

Plasmonic Fiber Optic Refractometric Sensors: From Conventional Architectures to Recent Design Trends

Surface Plasmon Resonance (SPR) fiber sensor research has grown since the first demonstration over 20 year ago into a rich and diverse field with a wide range of optical fiber architectures, plasmonic coatings, and excitation and interrogation methods. Yet, the large diversity of SPR fiber sensor designs has made it difficult to understand the advantages of each approach. Here, we review SPR fiber sensor architectures, covering the latest developments from optical fiber geometries to plasmonic coatings. By developing a systematic approach to fiber-based SPR designs, we identify and discuss future research opportunities based on a performance comparison of the different approaches for sensing applications.

Multi-element hollow-core anti-resonant fiber for infrared thermal imaging

A bundle composed of 245 anti-resonant glass hollow optical fibers with a total diameter of 1 mm and fiber core diameter of 60 μm is fabricated for endoscopic infrared-thermal imaging. The bundle fiber shows low losses in the wavelength range of 3 to 4 μm owing to the anti-resonant effect of the thin glass wall. An image resolution of around 420 μm with a field-of-view of 3-mm diameter is obtained although crosstalk between adjacent fibers is observed. The experimental results of an imaging system using the fiber bundle with a half-ball lens at the distal end, which can be inserted into a working channel of endoscopes, are also shown.

Molecular Spectrum Capture by Tuning the Chemical Potential of Graphene

Due to its adjustable electronic properties and effective excitation of surface plasmons in the infrared and terahertz frequency range, research on graphene has attracted a great deal of attention. Here, we demonstrate that plasmon modes in graphene-coated dielectric nanowire (GNW) waveguides can be excited by a monolayer graphene ribbon. What is more the transverse resonant frequency spectrum of the GNW can be flexibly tuned by adjusting the chemical potential of graphene, and amplitude of the resonance peak varies linearly with the imaginary part of the analyte permittivity. As a consequence, the GNW works as a probe for capturing the molecular spectrum. Broadband sensing of toluene, ethanol and sulfurous anhydride thin layers is demonstrated by calculating the changes in spectral intensity of the propagating mode and the results show that the intensity spectra correspond exactly to the infrared spectra of these molecules. This may open an effective avenue to design sensors for detecting nanometric-size molecules in the terahertz and infrared regimes.

Highly sensitive multi-core flat fiber surface plasmon resonance refractive index sensor

A simple multi-core flat fiber (MCFF) based surface plasmon resonance (SPR) sensor operating in telecommunication wavelengths is proposed for refractive index sensing. Chemically stable gold (Au) and titanium dioxide (TiO(2)) layers are used outside the fiber structure to realize a simple detection mechanism. The modeled sensor shows average wavelength interrogation sensitivity of 9,600 nm/RIU (Refractive Index Unit) and maximum sensitivity of 23,000 nm/RIU in the sensing range of 1.46-1.485 and 1.47-1.475, respectively. Moreover, the refractive index resolution of 4.35 × 10(-6) is demonstrated. Additionally, proposed sensor had shown the maximum amplitude interrogation sensitivity of 820 RIU(-1), with the sensor resolution of 1.22 × 10(-5) RIU. To the best of our knowledge, the proposed sensor achieved the highest wavelength interrogation sensitivity among the reported fiber based SPR sensors. Finally we anticipate that, this novel and highly sensitive MCFF SPR sensor will find the potential applications in real time remote sensing and monitoring, ultimately enabling inexpensive and accurate chemical and biochemical analytes detection.

Surface Plasmon Scattering in Exposed Core Optical Fiber for Enhanced Resolution Refractive Index Sensing

Refractometric sensors based on optical excitation of surface plasmons on the side of an optical fiber is an established sensing architecture that has enabled laboratory demonstrations of cost effective portable devices for biological and chemical applications. Here we report a Surface Plasmon Resonance (SPR) configuration realized in an Exposed Core Microstructured Optical Fiber (ECF) capable of optimizing both sensitivity and resolution. To the best of our knowledge, this is the first demonstration of fabrication of a rough metal coating suitable for spectral interrogation of scattered plasmonic wave using chemical electroless plating technique on a 10 μm diameter exposed core of the ECF. Performance of the sensor in terms of its refractive index sensitivity and full width at half maximum (FWHM) of SPR response is compared to that achieved with an unstructured bare core fiber with 140 μm core diameter. The experimental improvement in FWHM, and therefore the detection limit, is found to be a factor of two (75 nm for ECF in comparison to 150 nm for the large core fiber). Refractive index sensitivity of 1800 nm/RIU was achieved for both fibers in the sensing range of aqueous environment (1.33-1.37) suitable for biosensing applications.