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 Highly Sensitive Dual-Core Photonic Crystal Fiber Based on a Surface Plasmon Resonance Biosensor with Silver-Graphene Layer

A highly sensitive dual-core photonic crystal fiber based on a surface plasmon resonance (PCF-SPR) biosensor with a silver-graphene layer is described. The silver layer with a graphene coating not only prevents oxidation of the silver layer but also can improve the silver sensing performance due to the large surface-to-volume ratio of graphene. The dual-core PCF-SPR biosensor is numerically analyzed by the finite-element method (FEM). An average spectral sensitivity of 4350 nm/refractive index unit (RIU) in the sensing range between 1.39 and 1.42 and maximum spectral sensitivity of 10,000 nm/RIU in the sensing range between 1.43 and 1.46 are obtained, corresponding to a high resolution of 1 × 10⁻⁶ RIU as a biosensor. Our analysis shows that the optical spectra of the PCF-SPR biosensor can be optimized by varying the structural parameters of the structure, suggesting promising applications in biological and biochemical detection.

     

Dual-Band Infrared Perfect Absorber Based on a Ag-Dielectric-Ag Multilayer Films with Nanoring Grooves Arrays

In this paper, we demonstrate a dual-band metamaterial perfect absorber based on a Ag-dielectric-Ag multilayer nanostructure. The structure of top metal film covers nanoring grooves array. A dielectric layer has a function of confining electromagnetic fields. Theoretical analysis shows that two absorption peaks (1059 nm and 1304 nm) with the absorption of 99.2% and 99.9% have been achieved, respectively. The physical origin of perfect absorption peaks are related to the Fabry-Perot resonance effect and localized surface plasmon resonance (LSPR) of the nanoring grooves. Its perfect absorption and resonance wavelength can be well regulated by adjusting the relevant structural parameters. Additionally, the absorber demonstrates good operation angle-polarization-tolerance at wide incident angles (0–60°). We believe that our design has a promising application in plasmon-enhanced photovoltaic, optical absorption switching, and modulator optical communications in the infrared regime.

     

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.     

Long-Range Surface Plasmon Resonance–Based Sensitivity Study on D-shaped Ag-MgF2-Coated Models with Analyte Variations

In this report, a novel D-shaped long-range surface plasmon resonance (LRSPR) fiber base sensor has been introduced. The demonstration of proposed sensor involves two D-shaped silver-coated models to study the sensitivity responses. The entire study with the constructed models is based on a single-mode fiber. The models are multilayered consisting of metal, dielectric, and analyte as separate layers. Silver (Ag) and magnesium fluoride (MgF2) strips are used as metal and dielectric layers respectively. The constituency of analyte as an interface excellently standardized the models for sensitivity detection. In this report, a large range of analyte refractive indices (RI) which varies from 1.33 to 1.38 is appraised for the proposed models to characterize the sensitivity. The entire context is encompassed by the wavelength region from 450 to 850 nm with an interval of 20 nm. Sensitivities in this report are measured based on the analyte position from the core and metal for both models. For each of the two models, the analyte is placed as the top layer. RIs of the applied metal (Ag) are measured using the Drude-Lorentz formula. The simulated sensitivities for model-1 and model-2 vary from 6.3?×?10³ nm/RIU to 8.7?×?10³ nm/RIU.

     

Ultra-narrow-band Infrared Absorbers Based on Surface Plasmon Resonance

Due to the advantage of improving the sensing performance, narrow-band metamaterial perfect absorbers (MPAs) have attracted much attention in the sensor field. Here, we propose an ultra-narrow-band infrared absorber (UNBIRA) based on localized surface plasmon resonance. The peak absorption of the UNBIRA exceeds 99% with the full width at half maximum (FWHM) of 1.94 nm and 6.32 nm for transverse electric (TE) wave and transverse magnetic (TM) wave in 1.5–1.8 μm. The corresponding Q-factors for TE wave and TM wave are 817 and 266, respectively. When used as an infrared refractive index sensor, the sensitivity of UNBIRA is as high as 1632.5 nm/RIU for TE wave and 1647.5 nm/RIU for TM wave. Accordingly, the figure of merits (FOMs) of 816.2/RIU for TE wave and 260.7/RIU for TM wave are achieved. This UNBIRA possesses a simple geometry structure and an excellent sensing performance, implying a great potential for application of ultra-narrow infrared sensing or detecting.

     

Highly Absorptive Chiral L-Shape MDM Plasmonic Metasurface as Multifunction Device: Design and Computational Studies

A multifunction plasmonic metasurface made of metal-dielectric-metal (MDM) layers is designed, and its chiral, absorption, and refractive index sensing properties are studied numerically using finite difference time domain (FDTD) computation. Top layer of the proposed novel metasurface consists of four L-shape gold strips arranged in a specific orientational sequence into a square unit cell whose period (along X direction and Y direction) is varied from 800 to 1400 nm in a step of 200 nm. The proposed super-structure shows highly chiral behaviour with multi bands circular dichroism (CD) between ~ 600 and 1200 nm with highest CD value of about 0.4. The CD spectral response is seen to be tunable with the structural parameters such as periods and appropriate L-strip length. True chiral nature of the proposed structure is cross-checked by computing its enantiomer that shows a mirror reflection of CD response of the original structure. Multi-work functionalities are investigated by studying perfect absorption and refractive index sensing properties of the metasurface. The study shows polarization independent multi-resonance spectral absorption that reaches to ~ 100% in some cases. On the other hand, refractive index sensing study shows high sensitivity (S) of 700–750 nm/RIU (per refractive index unit) with figure of merit (FOM) of 5–10. Owing to its exotic optical properties, the novel metasurface may be considered for chip level integration for multi-purpose work functionalities.

     

Broadband Absorption Enhancement of Refractory Plasmonic Material with Random Structure

We demonstrate a broadband absorber using random structures on refractory plasmonic material. The random microstructure is fabricated by femtosecond laser on tungsten and characterized with surface roughness which described by root mean square (RMS) and correlation length. Results show that the absorption efficiency of random microstructure with RMS of 0.8 μm and correlation length of 0.55 μm is over 90 % in the wavelength range from 200 to 1100 nm. However, the sample with surface structure RMS of 0.08 μm has much lower absorption (less than 70 % for λ > 600 nm). Numerical simulations agree well with the experimental results and illustrate that the structure with 0.8-μm RMS and 0.55-μm correlation length has the cut-off wavelength of 2400 nm which prevents mid-infrared emission. The possibility of realizing broadband absorption by using random structures presents a flexible and efficient way for solar cell, thermophotovoltaics, and energy harvesting.

     

Individual Split Au Square Nanorings for Surface-Enhanced Raman and Hyper-Raman Scattering

In this paper, individual split Au square nanorings were numerically proposed as novel substrates for surface-enhanced Raman and hyper-Raman scattering (SERS and SEHRS) simultaneously. The peak wavelengths of their localized surface plasmon resonance (LSPR) fall in the near-infrared and visible light regions, respectively, which are able to be finely tuned to match well with the wavelengths of the incident laser and hyper-Raman scattered light beams. Their SEHRS and SERS performances along with electromagnetic (EM) field distributions are numerically investigated by finite element method. With the enhancement of near electric-fields generated by LSPRs, the maximum SEHRS and SERS enhancement factors are demonstrated to reach 1.22?×?10¹² and 10⁸, respectively. Meanwhile, the corresponding SERS-based refractive index (RI) sensitivity factor reaches as high as 258 nm/RIU and 893 nm/RIU, at visible and near-infrared wavelengths, respectively. The proposed structure holds great promise both for developing SEHRS- and SERS-based RI sensing substrates, which shows strong potential applications in nanosensing and enhanced Raman scattering.

     

Wideband Graphene-Based Fractal Absorber and its Applications as Switch and Inverter

In this paper, the idea of square fractal geometry has been utilized to introduce a tunable wideband graphene-based perfect plasmonic absorber in the near-infrared region. It consists of a MgF₂ layer and an array of gold squares fractal loaded on a graphene layer. In the designed absorber a single layer of graphene has been used instead of multilayered graphene structures. The structure is polarization-insensitive under normal incidence due to the geometric symmetry. The absorption and bandwidth of the structure are almost insensitive to the incident angle up to 15° and 45° for TE and TM polarizations, respectively. Moreover, by choosing appropriate structural parameters, the resonance wavelength of the desired plasmonic absorber can be controlled. The absorption of the introduced structure can be tuned by changing the chemical potential of the graphene. Therefore, the proposed fractal absorber can act as switch and inverter at λ = 1995 nm. Furthermore, the equivalent circuit model of the absorber has been derived to confirm the validity of the simulation results. The superiorities of our fractal absorber are wide full-width at half-maximum of 406 nm, multi-applicant, perfect absorption, and fabrication feasibility due to the simple structure with the maximum absorption tolerance error of 5.12%.

     

Tuning Plasmonic Properties of Truncated Gold Nanospheres by Coating

The influence of TiO₂ coating on resonant properties of gold nanoisland films deposited on silica substrates was studied numerically and in experiments. The model describing plasmonic properties of a metal truncated nanosphere placed on a substrate and covered by a thin dielectric layer has been developed. The model allows calculating a particle polarizability spectrum and, respectively, its surface plasmon resonance (SPR) wavelength for any given cover thickness, particle radius and truncation parameter, and dielectric functions of the particle, the substrate, the coating layer, and the surrounding medium. Dependence of the SPR position calculated for truncated gold nanospheres has coincided with the measured one for the gold nanoisland films covered with titania of different thicknesses. In the experiments, gold films with thickness of 5 nm were deposited on a silica glass substrate, annealed at 500 °C to form nanoislands of 20 nm in diameter, and covered with amorphous titania layers using atomic layer deposition technique. The resulting structures were characterized with scanning electron microscopy and optical absorption spectroscopy. The measured dependence of the SPR position on titania film thickness corresponded to the one calculated for truncated sphere-shaped nanoparticles with the truncation angle of ~50°. We demonstrated the possibility of tuning the SPR position within ~100 nm range by depositing to 30 nm thick titania layer.

     

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.

     

Analysis and Simulation of a Novel Localized Surface Plasmonic Highly Sensitive Refractive Index Sensor

Dividing a metal nanoparticle into smaller components and the occurrence of the plasmonic phenomenon in the gap between these components can improve the sensitivity of the detector to variation of the refraction coefficient of liquid. In this paper, in a constant volume of metal, a golden disk is divided into two rings and one smaller disk. With a proper arrangement of these components, the surface plasmon resonance phenomenon takes place at the wavelength of 945.7 nm. The occurrence of this phenomenon increases the field in the distance between nanoparticles surrounded by liquid. The sensitivity of the detector that designed using nanodisks is 300 nm/RIU while it increases to 500 nm/RIU for the new structure. The increase of LSPR displacement, for a variation of 0.01 in the liquid refraction coefficient, from 3 nm for a disk to 5 nm for a proposed structure verifies a 67% improvement in the sensitivity of the sensor.

     

Analytical Methods of Equivalent Circuit Model and Transmission Matrix for a Plasmonic Switch with Sensing Capability in Near-Infrared Region

In this paper, the simultaneous switching and sensing capabilities of a compact plasmonic structure based on a conventional rectangular hole in a silver film are proposed and investigated. The proposed structure has ultrahigh sensitivity up to 3000 nm/RIU and high figure of merit of 170 RIU⁻¹. Also, the simulation results show the potential of the presented refractive index sensor to detect malaria infection, cancer cells, bacillus bacteria, and solution of glucose in water. Simultaneously, by changing the incident lightwave polarization, the structure behaves like a plasmonic switch, which has high extinction ratios of 15.81, 31.20, and 25.03 dB at three telecommunication wavelengths of 850, 1310, and 1550 nm, respectively. The ultrafast response time of 20 fs is achieved for the wideband application of the switching capability at the wavelength range of 1056 to 1765 nm. Moreover, the equivalent circuit model and transmission (ABCD) matrix methods are derived to validate the simulated results. Simple design, good agreement between the numerical and analytical results, biomedical applications, ultrahigh sensitivity, and ultrafast performance of the proposed structure help this idea to open up paths for design and implementation of other multi-application plasmonic devices in near-infrared region. To the best of our knowledge, the mentioned analytical methods have not been studied former at near-infrared wavelengths. Therefore, the achievements could pave the way for verifying the simulation results of plasmonic nanostructures in future investigations.

     

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.

     

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.

     

Glucose Level Measurement Using Photonic Crystal Fiber–based Plasmonic Sensor

In this study, we demonstrate the design of a photonic crystal fiber (PCF)-based plasmonic sensor to measure the glucose level of urine. The sensor is designed by placing a small segment of PCF between a lead-in and a lead-out single-mode fiber. We utilize the finite element method to simulate the proposed plasmonic sensor for the measurement of glucose level in urine. To offer external sensing, the cladding layer of the PCF was coated by a thin layer of gold where the gold-coated PCF was immersed in the urine sample. As a result, the urine can easily interact with the plasmonic layer of the sensor. In the outermost laser of the PCF, we considered a perfectly matched layer as a boundary condition. The simulation results confirm excellent wavelength and amplitude sensitivities where the maximum wavelength sensitivity was 2500 nm/RIU and amplitude sensitivity was 152 RIU^?1 with a sensing resolution of 4?×?10^?6. For optimization of the plasmonic sensor, we varied the physical parameters of the cladding air holes and the thickness of the gold layer during the simulation. We strongly believe that the proposed plasmonic sensor will play a significant role to pave the way for achieving a simple but effective PCF-based glucose sensor.

     

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.

     

Resonance of Different Waveguide Structures with Various Vertical Indirect Coupled Cavities

In the paper, resonances of different waveguide structures with various vertical indirect coupled cavities were investigated by FDTD (finite difference-time domain). In the silicon cavity, Fano resonance could be observed at about 1430 nm. The coupling distance for the gold cavity/air cavity had less effect on the transmittance of the main waveguide but had a great influence on the transmission for water cavity in the visible region, which showed that water cavity could adjust resonance of waveguide structures. In addition, with the increment of refractive index n, the resonance peak at about 850 nm moved to the long wavelength (redshift). Dispersion rate about 2 × 10^–3/nm indicated that the transparent dielectric selectively absorbed the surface plasmon polariton wave and the sensitivity of the waveguide structure designed in this paper has high stability for the refractive index of the main waveguide cavity. Obvious Fano resonance could be observed with the increase of refractive index for silicon cavity. Among the four dielectrics, silicon and water are suitable for studying Fano resonance and filter dielectrics.

     

Coexistence of Plasmonic and Magnetic Properties in Bimetallic Fe/Ag Nanoparticles Synthesized by Pulsed Laser Ablation

Fe20:Ag80, Fe50:Ag50, and Fe80:Ag20 bimetallic nanoparticles were prepared with laser ablation method applied on Fe/Ag/polyvinyl alcohol (PVA) composite, at two different ablation times (20 and 50 min). The ratio of Fe with respect to Ag atoms as well as ablation time affect wavelength, intensity, and width of plasmon absorption peak. Increase in Fe content leaded to a redshift and a decrease in intensity and an increase in the width of plasmon absorption peaks. The transmission electron microscopy (TEM) analysis showed three kinds of grains which are Ag nanoparticles, Ag/Fe core/shells and Fe nanoparticles. Due to the concentration of Fe nanoparticles with the size of less than 5 nm around Ag ones with the size of 20 to 30 nm, a core–shell structure of these two metals forms and this decreases Ag plasmon resonance frequency, and as a result, its absorption peak has a redshift. According to the results of vibrating sample magnetometer (VSM), the Fe/Ag bimetallic nanoparticles were superparamagnetic.