Surface Phonon Resonance Assisted Refractive Index Sensing in Mid-Infrared Wavelength Range

We propose a highly sensitive refractive index sensor based on the surface phonon resonance (SPhR) in the mid-IR spectral range. Surface phonon polaritons (SPhPs) are formed on polar dielectrics such as SiC in mid-IR wavelength range and can be excited with the help of a metallic grating at specific wavelength termed as resonance wavelength. The resonance wavelength of SPhP is significantly affected by the refractive index of the analyte medium placed over the grating. This forms the basis of a refractive index sensor. We have numerically evaluated the performance of such an SPhP-based refractive index sensor by using rigorous coupled wave analysis (RCWA) in terms of sensitivity, detection accuracy, and quality factor. The quality factor and detection accuracy of the sensor formed on SiC substrate are found to be 225.1 RIU^–1 (inverse of refractive index unit) and 6.75, respectively. We have also extended the study for other polar dielectric substrates cBN and GaN and observed considerable enhancement in the performance of the sensor for GaN. The values of quality factor and detection accuracy could be increased to 361.2 RIU^–1 and 10.84, respectively, by using GaN substrate. The proposed sensor finds applications in refractive index sensing of liquids and biomolecules having refractive index in the range 1.33–1.36.

     

Design Analysis of Refractive Index Sensor with High Quality Factor Using Au-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Grating on Aluminum

We propose a surface plasmon resonance (SPR)-based refractive index sensor using gold-alumina grating over aluminum film for biosensing. Conventional SPR sensor based on gold grating exhibits broader SPR dips whereas that based on aluminum grating exhibits narrow reflection dip. A narrow reflection dip is desirable as it provides good resolution and improves the accuracy of measurement. Aluminum is less stable and generally is not preferred for an SPR-based sensor. It is more prone to being oxidized, which reduces the sensitivity and increases the width of the reflection dip of the sensor. While gold cannot provide narrow SPR reflection dips, but is used as an SPR active metal due to its more chemical stability. In order to improve the accuracy of gold grating-based sensor while taking care of oxidation problem of aluminum, in this paper, we propose a gold grating over aluminum film for SPR-based sensor and show that this configuration improves the sensitivity and the detection accuracy of the conventional sensor. Moreover, the oxidation problem is reduced to some extent as a part of aluminum is covered with gold. In order to completely avoid the oxidation of aluminum, we further propose to cover the exposed part of the aluminum with alumina and show that this configuration further improves the accuracy by reducing the width of the SPR reflection dip without affecting the sensitivity significantly. Numerical simulations show that sensitivity of proposed sensor is 270.33°/RIU with quality factor of more than 267.65 RIU^?1.     

Dual-wavelength plasmonic perfect absorber suitable for refractive index sensing

In this paper, a plasmonic perfect absorber (PPA) based on metal-insulator-metal-insulator-metal (MIMIM) structure has been designed for refractive index sensing of glucose solutions (analyte) and then a new method has been proposed for fast, low-cost, and easy measurement of sensor’s sensitivity. Simulation results show that the absorption spectrum of the proposed sensor has two resonance peaks that with an increase in analyte refractive index, both of them are red-shifted. In our proposed measurement technique, two conventional single-wavelength lasers (with wavelengths of 1050 nm and 1750 nm) are used for vertical plane wave light illumination on the structure. Then, the absorbed powers at 1750 nm (A₂) and 1050 nm (A₁) wavelengths are calculated and variation of the absorption ratio (A₂/A₁) due to change of analyte refractive index would be introduced as the sensitivity of sensor (S = Δ(A₂/A₁)/Δn). Obtained results show that the increase of analyte refractive index from n = 1.312 to n = 1.384 will result in an increase of sensor’s sensitivity from 9.3/RIU to 33.475/RIU.

     

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.

     

Ultra-stable D-shaped Optical Fiber Refractive Index Sensor with Graphene-Gold Deposited Platform

In this paper, we demonstrate a high sensitivity refractive index (RI) sensor with D-shaped structure covered with gold and graphene film. Specifically, the effect of structural parameters on the stability of fiber sensor is analyzed. In our research, it have been found that the sensor we proposed is not very sensitive to the change of structure parameters on the premise of ensuring the sensing precision. This advantage means that the requirements for machining errors are reduced. Further probing shows that the proposed sensor shows a maximum wavelength interrogation sensitivity of 4391nm/RIU with the dynamic refractive index range from 1.33 to 1.39 and a maximum amplitude sensitivity of 1139RIU^− 1 with the analyte RI = 1.38 in the visible region. The corresponding resolution are 2.28 × 10^− 5 and 8.78 × 10^− 6 based on the methods of wavelength interrogation and amplitude-(or phase-) based method. These characteristics of compact sensing architectures, simple to fabricate, and high sensitivity open the possibility of using this type of sensor in biological applications.

     

Design of a High-Resolution Metal–Insulator–Metal Plasmonic Refractive Index Sensor Based on a Ring-Shaped Si Resonator

In this paper, a high-resolution refractive index sensor is proposed based on a novel metal–insulator–metal plasmonic topology. The structure is based on a Si nano-ring located inside a circular cavity. It acts as an optical notch filter with a quality factor equal to 269. The proposed filter topology is numerically simulated using the finite difference time domain method. It is shown that the proposed filter can also act as a refractive index sensor with a sensitivity of 636 nm/RIU and a fairly high figure of merit (FoM) equal to 211.3 RIU⁻¹. It is shown that the sensor can easily detect a refractive index change of ± 0.001 for dielectrics whose refractive index is between 1 and 1.2. For the refractive index range of 1.33 to 1.52, the maximum FoM of the sensor is 191 RIU⁻¹. The simplicity of the design and its high resolution are the two main features of the proposed sensor which make it a good candidate for biomedical applications.

     

Ambiguous Refractive Index Sensitivity of Fano Resonance on an Array of Gold Nanoparticles

We investigate the optical response to refractive index changes of a Fano resonance occurring in a random array of gold nanoparticles supported on a glass substrate. The Fano resonance results from the interference between localized surface plasmon on a gold nanoparticle and the light reflected at the boundary of the glass substrate. We demonstrate that the sensitivity of the resonance to the refractive index of the surrounding medium is highly dependent on the excitation geometry and can assume either positive or negative values. We furthermore present a theoretical analysis explaining this behavior based on the rigorous coupled wave analysis (RCWA) as well as the island film theory.

     

1-D Metal-Dielectric-Metal Grating Structure as an Ultra-Narrowband Perfect Plasmonic Absorber in the Visible and Its Application in Glucose Detection

The need for an easy to fabricate perfect and narrowband light absorber in the visible range of electromagnetic (EM) spectrum has always been in demand for many scientific and device applications. Here, we propose a metal-dielectric-metal (MDM) 1-D grating plasmonic structure as a perfect narrow band light absorber in the visible and its application in glucose detection. The proposed structure consists of a 1- D grating of gold on the top of a dielectric layer on a gold film. Optimization for dielectric grating index (n), grating thickness (t), grating width (W), and grating period (P) has been done to improve the performance of plasmonic structure by calculating its quality factor and figure-of-merit (FOM). The optimized plasmonic structure behaves as a perfect narrowband light absorber. The flexibility to work at a specific wavelength is also offered by the proposed structure through an appropriate selection of the geometrical parameters and refractive index of the dielectric grating. The equivalent RC model is used to understand different components of the proposed structure on the optical response. The absorption response of the structure is invariant to the incident angle. Moreover, the calculated absorbance of the proposed plasmonic structure is ~ 100% with a narrow full-width half maxima (FWHM) of ~ 2.8 nm. We have numerically demonstrated a potential application of the proposed MDM absorber as a plasmonic glucose sensor in the visible range with detection sensitivity in the range of 140 to 195 nm/RIU.

     

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.

     

Highly Sensitive SPR Sensor Based on D-shaped Photonic Crystal Fiber Coated with Indium Tin Oxide at Near-Infrared Wavelength

A surface plasmon resonance (SPR) sensor based on D-shaped photonic crystal fiber (PCF) coated with indium tin oxide (ITO) film is proposed and numerically investigated. Thanks to the adjustable complex refractive index of ITO, the sensor can be operated in the near-infrared (NIR) region. The wavelength sensitivity, amplitude sensitivity, and phase sensitivity are investigated with different fiber structure parameters. Simulation results show that ～6000 nm/refractive index unit (RIU), ～148/RIU, and ～1.2?×?10⁶ deg/RIU/cm sensitivity can be achieved for wavelength interrogation, amplitude interrogation, and phase interrogation, respectively, when the environment refractive index varies between 1.30 and 1.31. It is noted that the wavelength sensitivity and phase sensitivity are more pronounced with larger refractive index. The proposed SPR sensor can be used in various applications, including medicine, environment, and large-scale targets detection.     

Nanostructure shape effects on response of plasmonic aptamer sensors

A localized surface plasmon resonance (LSPR) sensor surface was fabricated by the deposition of gold nanorods on a glass substrate and subsequent immobilization of the DNA aptamer, which specifically bind to thrombin. This LSPR aptamer sensor showed a response of 6‐nm λ_max shift for protein binding with the detection limit of at least 10 pM, indicating one of the highest sensitivities achieved for thrombin detection by optical extinction LSPR. We also tested the LSPR sensor fabricated using gold bipyramid, which showed higher refractive index sensitivity than the gold nanorods, but the overall response of gold bipyramid sensor appears to be 25% less than that of the gold nanorod substrate, despite the approximately twofold higher refractive index sensitivity. XPS analysis showed that this is due to the low surface density of aptamers on the gold bipyramid compared with gold nanorods. The low surface density of the aptamers on the gold bipyramid surface may be due to the effect of shape of the nanostructure on the kinetics of aptamer monolayer formation. The small size of aptamers relative to other bioreceptors is the key to achieving high sensitivity by biosensors on the basis of LSPR, demonstrated here for protein binding. The generality of aptamer sensors for protein detection using gold nanorod and gold nanobipyramid substrates is anticipated to have a large impact in the important development of sensors toward biomarkers, environmental toxins, and warfare agents. Copyright © 2013 John Wiley & Sons, Ltd.     

Extremely High Sensitive Plasmonic Refractive Index Sensors Based on Metallic Grating

This work shows that a grating-based surface plasmon (SP) resonance sensing system can exhibit extremely high sensitivity to detect a small change of refractive index in an analyte. The corresponding sensitivity can be much higher than that of the prism-based systems. Both analytical calculation and rigorous coupled-wave analysis are used to study the angular sensitivity of the system. It is found that the system’s sensitivity can be over 600° per unit index change if (1) first-order diffracted wave is chosen to excite SP mode, (2) large SP resonant angles are used in the operation, and (3) grating filling factor is selected to be varied between 0.3 and 0.7. Furthermore, the sensing system has the best performance for detecting low-index analyte with a small change of refractive index.     

A Broadband Nanosensor based on Multi-Interference of Surface Plasmon Polaritons

A novel broadband refractive index nanosensor based on multi-interference of surface plasmon polaritons is reported. It is composed of a metallic nanoslit flanked by periodical grooves on its two sides. Extraordinary high-throughput, high-resolution, and high-sensitivity detections can be realized by observing the shift of the resonant wavelength. The sensor covers a large range of the refractive index change due to both the narrow linewidth of the single resonant peak in the broadband spectrum and the sensitive shift of the peak position withthe refractive index change. A theoretical model is developed to well predict the optical response of the sensor. An excellent linearity between the resonant wavelength and the refractive index can be achieved. The sensitivity, which is 620 nm/refractive index unit, can be further increased by tuning the period of the grooves and the high throughput; high resolution can be simultaneously achieved by adding the number of grooves.     

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.     

Compact and low-cost biosensor based on novel approach to spectroscopy of surface plasmons

A high-performance surface plasmon resonance (SPR) sensor based on a novel approach to spectroscopy of surface plasmons is reported. This approach employs a special diffraction grating structure (referred to as surface plasmon resonance coupler and disperser, SPRCD) which simultaneously couples light into a surface plasmon and disperses the diffracted light for spectral readout of SPR signal. The developed SPRCD sensor consists of a miniature cartridge integrating the diffraction grating and microfluidics and a compact optical system which simultaneously acquires data from four independent sensing channels in the cartridge. It is demonstrated that the SPRCD sensor is able to measure bulk refractive index changes as small as 3 × 10⁻⁷ RIU (refractive index units) and to detect short oligonucleotides in concentrations down to 200 pM.     

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.     

Novel optical fiber Vernier immunosensor based on cascading Sagnac loops embedded with excessively tilted fiber grating for specific detection of canine distemper virus

A novel optical fiber Vernier effect (VE) biosensor based on cascading Sagnac loops embedded with excessively tilted fiber grating (ExTFG) is proposed for the label free and specific detection of canine distemper virus (CDV). The VE was realized by cascading two different Sagnac loops with similar free spectrum range (FSR), one of which was integrated with panda-type polarization maintaining fiber (PMF) as the reference loop, and the other was embedded with ExTFG as the sensing loop. Owning to the amplified function of the VE, the refractive index (RI) sensitivity of the proposed sensing structure reached −1914.89 nm/RIU, which is approximately 12 times higher than that of the single ExTFG based RI sensor. Furthermore, the ExTFG in sensing loop was modified by graphene oxide (GO) and bio-functionalized by the CDV monoclonal antibodies (anti-CDV MAbs) for the specific detection of the CDV. Experimental results show that the proposed optical fiber Vernier sensor could detect the CDV in buffer solution with concentration as low as 1 pg/mL, and the sensitivity was about −1.18 nm/[log(mg/ml)] in the concentration range of 1 pg/mL ~ 50 ng/mL. The excellent specific and clinical properties of the biosensor were verified by immunoassays for fetal bovine serum, Toxoplasma gondii, rabies virus and CDV serum in sequence. Due to the sensitivity amplification function of VE, dense comb spectrum of the Sagnac loop and the stable interference spectra maintained by the polarized light, the proposed biosensor possesses the combined advantages of high sensitivity, high Q-factor and high stability, which may have potential applications in biosensing fields.     

Optical Magnetism in Surface Plasmon Resonance–Based Sensors for Enhanced Performance

Surface plasmon resonance (SPR)–based structures are finding important applications in sensing biological as well as inorganic samples. In SPR techniques, an angle-resolved reflection (R) profile of the incident light from a metal-dielectric interface is measured and the resonance characteristics are extracted for the identification of the target sample. However, the performance, and hence the applicability of these structures, suffers when the weight and concentration of the target samples are small. Here, we show that SPR-based sensors can create strong magnetism at optical frequency, which can be used for the detection of target samples instead of using the conventional R profiles, as the magnetic resonance varies depending on the refractive index of the target sample. Using scattering parameters retrieval method, we computationally find out the effective permeability (μ_eff) of a SPR sensor with a structure based on Kretschmann configuration, and use it to calculate the performance of the sensor. A comparison with the conventional technique that uses R profile to detect a target sample shows a significant increase in the sensor performance when μ_eff is used instead.

     

Highly Sensitive Plasmonic Sensor Based on Fano Resonance from Silver Nanoparticle Heterodimer Array on a Thin Silver Film

We investigate the optical spectrum of a multilayer metallic slab using multiple-scattering formalism. A thin silver film is attached to a periodic array of heterodimers consisting of two vertically spaced silver nanoparticles of different radii. Depending on the radius of nanoparticles, heterodimer array presents a simple nanoscale geometry which gives rise to remarkable plasmonic properties of multipolar resonances. Due to the coherent interference of the localized nanoparticle plasmons (discrete mode) and surface plasmon polaritons of metallic film (continuous mode), the reflection spectrum represents a sharp asymmetric Fano resonance dip, which is strongly sensitive to the refractive index of the surrounding embedded dielectric host. The physical features contribute to a highly efficient plasmonic sensor for refractive index sensing with sensitivity of ～1.5?×?10^?3 RIU/nm.     

Short order nanohole arrays in metals for highly sensitive probing of local indices of refraction as the basis for a highly multiplexed biosensor technology

A small array of subwavelength apertures patterned in a gold film on glass was characterized for use as a biosensor. It is widely believed that such arrays allow the resonance of photons with surface plasmons in the metallic film. Surface plasmon methods (and other evanescent wave methods) are extremely well suited for the measure of real time biospecific interactions. An extremely high sensitivity of 88,000%/refractive index unit was measured on an array with theoretical active area of .09 microm2. The formation of a biological monolayer was monitored. Both sensitivity and resolution were determined through measurement. The measured resolution, for a sensor with an active area of less than 1.5 microm2, is 9.4 x 10(-8) refractive index units which leads to a calculated sensitivity of 3.45E6%/refractive index unit. These values far exceed theoretical and calculated values of other grating coupled surface plasmon resonance (SPR) detectors and prism based SPR detectors. Because the active sensing area can be quite small (.025 microm2) single molecule studies are possible as well as massive multiplexing on a single chip format.