A Novel Diamond Ring Fiber-Based Surface Plasmon Resonance Sensor

We propose a highly sensitive novel diamond ring fiber (DRF)-based surface plasmon resonance (SPR) sensor for refractive index sensing. Chemically active plasmonic material (gold) layer is coated inside the large cavity of DRF, and the analyte is infiltrated directly through the fiber instead of selective infiltration. The light guiding properties and sensing performances are numerically investigated using the finite element method (FEM). The proposed sensor shows a maximum wavelength and amplitude interrogation sensitivity of 6000 nm/RIU and 508 RIU^?1, respectively, over the refractive index range of 1.33–1.39. Additionally, it also shows a sensor resolution of 1.67 × 10^?5 and 1.97 × 10^?5 RIU by following the wavelength and amplitude interrogation methods, respectively. The proposed diamond ring fiber has been fabricated following the standard stack-and-draw method to show the feasibility of the proposed sensor. Due to fabrication feasibility and promising results, the proposed DRF SPR sensor can be an effective tool in biochemical and biological analyte detection.     

Highly Sensitive Side-Polished Birefringent PCF-Based SPR Sensor in near IR

We propose a highly sensitive side-polished birefringent photonic crystal fiber (PCF) sensor based on surface plasmon resonance (SPR). The polished surface of the proposed structure is coated with indium tin oxide (ITO) to excite plasmon and the analytes can be placed on the flat surface easily instead of filling the voids. The birefringent nature of the structure helps in coupling more fields to the ITO-dielectric interface. With the optimum thickness of 110 nm of ITO, the structure shows a maximum wavelength sensitivity of 17000 nm/RIU with a resolution of 5.8?×?10^?6 RIU. Further this also showed an amplitude sensitivity of 74 RIU^?1 along with a resolution of 1.35?×?10^?5 RIU. Moreover, the effect of bending on this low loss structure is also analyzed.     

Doped Single-Wall Carbon Nanotubes in Propagating Surface Plasmon Resonance-Based Fiber Optic Refractive Index Sensing

We propose a surface plasmon resonance (SPR)-based fiber coupled refractive index sensing probe utilizing single-wall carbon nanotubes (SWCNTs) as the upper most layer. The sensor is designed by considering indium tin oxide (ITO) film on the bare core of a multi-moded step-index fiber, followed by the deposition of silicon, and then by that of the highly doped bundled SWCNTs layers. The film thicknesses of different constituent layers are optimized with respect to the sensitivity and the detection accuracy of the sensor. The theoretical analysis results in high sensitivity of 9.78 μm per refractive index unit (μm/RIU) for the optimized probe in the infra-red (IR) region of the electromagnetic (EM) spectrum.     

Metallic Grating-Assisted Fiber Optic SPR Sensor with Extreme Sensitivity in IR Region

We theoretically propose a surface plasmon resonance (SPR)-based fiber optic refractive index (RI) sensor. A surface plasmon exciting metallic grating formed with the alternation of indium tin oxide (ITO) and silver (Ag) stripes is considered on the core of the fiber. A thin film of silicon is used as an overlay. Silicon film not only protects the metallic grating from oxidation but also enhances the field to improve the device sensitivity. The sensor is characterized in terms of sensitivity, detection accuracy (DA), figure of merit (FoM), and quality factor (QF). The maximum sensitivity in the RI range 1.33 to 1.38 refractive index unit (RIU) is reported to be?~25 µm/RIU in infra-red region of investigation.

     

Analysis of a Surface Plasmon Resonance Probe Based on Photonic Crystal Fibers for Low Refractive Index Detection

A photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) probe with gold nanowires as the plasmonic material is proposed in this work. The coupling characteristics and sensing properties of the probe are numerically investigated by the finite element method. The probe is designed to detect low refractive indices between 1.27 and 1.36. The maximum spectral sensitivity and amplitude sensitivity are 6 × 10³ nm/RIU and 600 RIU^?1, respectively, corresponding to a resolution of 2.8 × 10^?5 RIU for the overall refractive index range. Our analysis shows that the PCF-SPR probe can be used for lower refractive index detection.     

Optimization of Surface Plasmon Resonance Biosensor with Ag/Au Multilayer Structure and Fiber-Optic Miniaturization

In this paper, we report a novel wavelength interrogation-based surface plasmon resonance (SPR) system, in which a film of three Ag layers and three Au layers are alternately deposited on a Kretschmann configuration as sensing element. This multilayer film shows higher sensitivity for refractive index (RI) measurement by comparing with single Au layer structure, which is consistent with its theoretical calculation. A sensitivity range of 2056–5893 nm/RIU can be achieved, which is comparable to RI sensitivities of other wavelength-modulated SPR sensors. Compared with Ag film, this Ag/Au multilayer arrangement offers anti-oxidant protection. This SPR biosensor based on a cost-effective Ag/Au multilayer structure is applicable to the real-time detection of specific interactions and dissociation of low protein concentrations. To extend the application of this highly-sensitive metal film device, we integrated this concept on an optical fiber. The range of RI sensitivities with Ag/Au multilayer was 1847–3309 nm/RIU. This miniaturized Ag/Au multilayer-based fiber optic sensor has a broad application in chemical and biological sensing.     

Comparison of Gold and Silver/Gold Bimetallic Surface for Highly Sensitive Near-infrared SPR Sensor at 1550 nm

A large majority of surface plasmon resonance (SPR) sensors reported in the literature are designed to operate in the visible electromagnetic spectrum. However, the near-infrared, particularly at the telecommunications wavelength of 1550 nm, is also especially attractive for SPR sensing applications. In fact, SPR sensors operating in this region benefit from narrower resonance and deeper field penetration. In this paper, we report a theoretical and experimental study of an SPR sensor operating at a fixed wavelength of 1550 nm. The influence of the choice of metals and the interrogation methods on the sensitivity of the resulting SPR sensor is investigated. Two types of sensor chips (simple gold (Au) and bimetallic silver/Au structure) and three interrogation methods (monitoring of the position of the reflectivity minimum, the position of the centroid, and the intensity evolution of the reflectivity) are examined. We show that a refractive index resolution of 2.7?×?10^?6 refractive index unit can be easily obtained, and with further optimization of the measurement system, the ultimate limit of detection is expected to be even lowered. Therefore, the approach discussed here already shows a promising potential for highly sensitive SPR sensors.     

Plasmonic Biosensor Based on Triangular Au/Ag and Au/Ag/Au Core/Shell Nanoprisms onto Indium Tin Oxide Glass

Gold–silver core–shell triangular nanoprisms (Au/AgTNPs) were grown onto transparent indium tin oxide (ITO) thin film-coated glass substrate through a seed-mediated growth method without using peculiar binder molecules. The resulting Au/AgTNPs were characterized by scanning electron microscopy, atomic force microscopy, X-ray diffraction, UV–vis spectroscopy, and cyclic voltammograms. The peak of dipolar plasmonic resonance was located at near infrared region of ～700 nm, which showed the refractive index (RI) sensitivity of 248 nm/RIU. Moreover, thin gold shells were electrodeposited onto the surface of Au/AgTNPs in order to stabilize nanoparticles. Compared with the Au/AgTNPs, this peak of localized surface plasmon resonance (LSPR) was a little red-shift and decreased slightly in intensity. The refractive index sensitivity was estimated to be 287 nm/RIU, which showed high sensitivity as a LSPR sensing platform. Those triangular nanoprisms deposited on the ITO substrate could be further functionalized to fabricate LSPR biosensors. Results of this research show a possibility of improving LSPR sensor by using core–shell nanostructures.     

High-Sensitivity Refractive Index Sensor Based on D-Shaped Photonic Crystal Fiber with Rectangular Lattice and Nanoscale Gold Film

We propose and investigate a D-shaped photonic fiber refractive index sensor with rectangular lattice based on surface plasmon resonance. In such sensor, the nanoscale gold metal film is deposited on the flat surface where it is side polished. Numerical results show that the average sensitivity of Au-metalized surface plasmon resonance (SPR) sensor could reach as high as 8,129 nm/refractive index unit (RIU) in the dynamic index range from 1.35 to 1.41 as well as 2,000 nm/RIU from 1.33 to 1.35. Compared to conventional Au-metalized SPR sensors, the performance of our device is obviously better, and the production process is greatly simplified.     

Integrated Terahertz Surface Plasmon Resonance on Polyvinylidene Fluoride Layer for the Profiling of Fluid Reflectance Spectra

We design terahertz (THz) surface-plasmon-resonance (SPR) sensors using a ferroelectric polyvinylidene fluoride (PVDF) thin layer for biological sensing. The reflectivity properties based on SPR are described using transfer matrix method (TMM) and numerically simulated using finite-difference time domain (FDTD) method. The sensing characteristics of the structure are systematically analyzed through the examination of the reflectivity spectrum. The results reveal that the pronounced SPR resonance peak has quasi-linear relationship with the refractive index variation of the material under investigation. Through analyzing and optimizing the structural parameters of the THz SPR sensor, we achieved the theoretical value of the refractive index detection sensitivity as high as 0.393 THz/unit change of refractive index (RIU) for a 20-μm-thick liquid sample with a 10-μm PVDF layer. This work shows great promise toward realizing a THz SPR sensor with high sensitivity for identifying the signatures of biological fluid sample.     

Highly Sensitive Surface Plasmon Resonance Based D-Shaped Photonic Crystal Fiber Refractive Index Sensor

In this article, a D-shaped photonic crystal fiber based surface plasmon resonance sensor is proposed for refractive index sensing. Surface plasmon resonance effect between surface plasmon polariton modes and fiber core modes of the designed D-shaped photonic crystal fiber is used to measure the refractive index of the analyte. By using finite element method, the sensing properties of the proposed sensor are investigated, and a very high average sensitivity of 7700 nm/RIU with the resolution of 1.30 × 10^?5 RIU is obtained for the analyte of different refractive indices varies from 1.43 to 1.46. In the proposed sensor, the analyte and coating of gold are placed on the plane surface of the photonic crystal fiber, hence there is no necessity of the filling of voids, thus it is gentle to apply and easy to use.     

Tunable Surface Plasmon Resonance Sensor Based on Photonic Crystal Fiber Filled with Gold Nanoshells

We present a photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) sensor, whose operating wavelength range is tunable. Gold nanoshells, consisting of silica cores coated with thin gold shells, are designed to be the functional material of the sensor because of their attractive optical properties. It is demonstrated that the resonant wavelength of the sensor can be precisely tuned in a broad range, 660 nm to 3.1 μm, across the visible and near-infrared regions of the spectrum by varying the diameter of the core and the thickness of the shell. Furthermore, the effects of structural parameters of the sensor on the sensing properties are systematically analyzed and discussed based on the numerical simulations. It is observed that a high spectral sensitivity of 4111.4 nm/RIU with the resolution of 2.45 × 10^?5 RIU can be achieved in the sensing range of 1.33–1.38. These features make the sensor of great importance for a wide range of applications, especially in biosensing.     

Highly Sensitive Plasmonic Refractive Index Sensor Based on Dual D-Shaped Photonic Crystal Fiber with Aluminum Nitride-Silver Films

A dual-core and dual D-shaped photonic crystal fiber (PCF)-based surface plasmon resonance (SPR) sensor with silver and aluminum nitride (AlN) films is designed. The distribution characteristics of the electromagnetic fields of core and plasmon modes, as well as the sensing properties, are numerically studied by finite element method (FEM). The structure parameters of the designed sensor are optimized by the optical loss spectrum. The results show the resonance wavelength variation of 489 nm for the refractive index (RI) range of 1.36?~?1.42. In addition, a maximum wavelength sensitivity of 13,400 nm/RIU with the corresponding RI resolution of 7.46?×?10^?6 RIU is obtained in the RI range of 1.41?~?1.42. The proposed sensor with the merits of high sensitivity, low cost, and simple structure has a wide application in the fields of RI sensing, such as hazardous gas detection, environmental monitoring, and biochemical analysis.

     

A Speckle-Free Angular Interrogation SPR Imaging Sensor Based on Galvanometer Scan and Laser Excitation

A speckle-free fast angular interrogation surface plasmon resonance imaging (SPRi) sensor based on a diode-pumped all-solid-state laser and galvanometer is reported in this work. A bidirectional scan using a galvanometer realizes the fast scanning of the incidence angle. The experimental results showed that the time needed for completing an SPR dip measurement was decreased to 0.5 s. And through cascading an immovable diffuser and two diffusers rotating in opposite directions, laser speckle was eliminated. The dynamic detection range and the sensitivity reached 4.6 × 10⁻² and 1.52 × 10⁻⁶ refractive index unit (RIU), respectively, in a 2D array sensor when the angle scanning range was set as 7.5°. More importantly, the results demonstrated that the angular interrogation SPR imaging sensor scheme had the capability to perform fast and high-throughput detection of biomolecular interactions at 2D sensor arrays.

     

One Dimensional Plasmonic Grating: High Sensitive Biosensor

We report a simple 1D grating device fabrication on ～50 nm gold (Au) film deposited on glass, which is employed as a high performance refractive index (RI) sensor by exploiting the surface plasmon polaritons (SPP) excited by the grating device along the Au/analyte interface. A finite element analysis (FEA) method is employed to maximize the sensitivity of the sensor for a fixed period and thickness of a gold film and its close correspondence with experiment has given the insight for high sensitivity and enhanced transmission. Significantly, in the context of economic design and performance, it is shown that an optimally designed and fabricated 1D grating can be as sensitive as 524 nm/RIU (linearity RI?=?1.33303 to 1.47399), which is remarkably higher than existing reports operating in a similar wavelength region.     

SPR Label-Free Biosensor with Oxide-Metal-Oxide-Coated D-Typed Optical Fiber: a Theoretical Study

In this article, a surface plasmon resonance (SPR) biosensor based on D-typed optical fiber coated by Al₂O₃/Ag/Al₂O₃ film is investigated numerically. Resonance in near infrared with an optimized architecture is achieved. Refractive index sensitivity of 6558 nm/RIU (refractive index unit) and detection limit of 1.5 × 10⁻⁶ RIU, corresponding to 0.4357 nm/μM and detection limit of 23 nM in BSA (bovine serum albumin) concentration sensing, are obtained. The analysis of the performance of the sensor in gaseous sensing indicates that this proposed SPR sensor is much suitable for label-free biosensing in aqueous media.

     

Utilizing the Metallic Nano-Rods in Hexagonal Configuration to Enhance Sensitivity of the Plasmonic Racetrack Resonator in Sensing Application

A high sensitive plasmonic refractive index sensor based on metal-insulator-metal (MIM) waveguides with embedding metallic nano-rods in racetrack resonator has been proposed. The refractive index changes of the dielectric material inside the resonator together with temperature changes can be acquired from the detection of the resonance wavelength, based on their linear relationship. With optimum design and considering a tradeoff among detected power, structure size, and sensitivity, the finite difference time domain simulations show that the refractive index and temperature sensitivity values can be obtained as high as 2610 nm per refractive index unit (RIU) and 1.03 nm/°C, respectively. In addition, resonance wavelengths of resonator are obtained experimentally by using the resonant conditions. The effects of nano-rods radius and refractive index of racetrack resonator are studied on the sensing spectra, as well. The proposed structure with such high sensitivity will be useful in optical communications that can provide a new possibility for designing compact and high-performance plasmonic devices.     

Alphabetic-Core Assisted Microstructure Fiber Based Plasmonic Biosensor

Light control capability of photonic crystal fiber (PCF) is a unique feature which can be applied to improve biosensing and plasmonic performance. Here, we reported alphabetic-core microstructure fiber-based plasmonic biosensor. Three different alphabetic R-, M-, and S-shaped cores of PCF-based plasmonic microstructures show controllable light propagation to enhance biosensor sensitivity and resolution. The light-guiding properties and sensing performance are investigated numerically using the finite element method (FEM). The proposed R-shaped core (RSC), M-shaped core (MSC), and S-shaped core (SSC) PCF-based plasmonic sensors show the maximum wavelength and amplitude sensitivities of 12,000, 11,000, 10,000 nm/RIU and 478, 533, and 933 RIU⁻¹, respectively, in the refractive index (RI) range of 1.33 to 1.40. The sensors also exhibit promising wavelength resolution of 8.33 × 10⁻⁶, 9.09 × 10⁻⁶, and 1.0 × 10⁻⁶ RIU, with figure of merit (FOM) of 108, 143, and 217 RIU⁻¹ for RSC, MSC, and SSC PCFs, respectively. The tunable sensing performance is also observed in design structures due to controllable light traveling path and their interaction with analytes. The proposed alphabetic-core PCF SPR sensors would be a promising candidate for the application of light controlling, trapping in microscale environment, and biosensing.

     

Self-Referencing Plasmonic Array Sensors

A self-referencing plasmonic platform is proposed and analyzed. By introducing a thin gold layer below a periodic two-dimensional nano-grating, the structure supports multiple modes including localized surface plasmon resonance (LSPR), surface plasmon resonance (SPR), and Fabry-Perot resonances. These modes get coupled to each other creating multiple Fano resonances. A coupled mode between the LSPR and SPR responses is spatially separated from the sensor surface and is not sensitive to refractive index changes in the surrounding materials or surface attachments. This mode can be used for self-referencing the measurements. In contrast, the LSPR dominant mode shifts in wavelength when the refractive index of the surrounding medium is changed. The proposed structure is easy to fabricate using conventional lithography and electron beam deposition methods. A bulk sensitivity of 429 nm/RIU is achieved. The sensor also has the ability to detect nanometer thick surface attachments on the top of the grating.

     

Platinum Layers Sandwiched Between Black Phosphorous and Graphene for Enhanced SPR Sensor Performance

Highly sensitive surface plasmon resonance (SPR) sensor consisting of Ag-Pt bimetallic films sandwiched with 2D materials black phosphorus (BP) and graphene over Pt layer in Kretschmann configuration is analyzed theoretically using the transfer matrix method. Numerical results show that upon suitable optimization of thickness of Ag-Pt layers and the number of layers of BP and graphene, sensitivity as high as 412°/RIU (degree/refractive index unit) can be achieved for p-polarized light of wavelength 633 nm. This performance can be tuned and controlled by changing the number of layers of BP and graphene. Furthermore, the addition of graphene and heterostructures of black phosphorus not only improved the sensitivity of the sensor but also kept the FWHM of the resonance curve much smaller than the conventional sensor utilizing Au as plasmonic metal and hence improved the resolution to a significant extent. We expect that this new proposed design will be useful for medical diagnosis, biomolecular detection, and chemical examination.