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In this work, we present wavelength splitting characteristics of whispering gallery modes in a system where two disks with different refractive indices are coupled together. This study utilizes finite difference time domain based simulations. The spectral changes caused by the presence of nanoparticles are analyzed, taking factors such as the number of nanoparticles, their size and distance from the surface of the disks into account. The investigation also encompasses the interaction of a thin nanolayer. Our findings demonstrate that the wavelength splitting is highly influenced by the specific disk where the nanoparticle or nanolayer is located. This distinct property sets it apart from conventional coupled disks with identical features. A perturbation theory of coupled structures has also been applied to gain insights into the simulation results. • Exploring how different sized nano scatterers near the disk surface impact wavelength splitting in coupled dissimilar disks. • Observation of position-dependent wavelength splitting that changes based on the attachment of particles to a specific disk. • Employed perturbation theory to gain insights into the Finite-Difference Time-Domain (FDTD) simulation results. • A new refractive index sensor is suggested to utilize a coupled dissimilar disk because of its unique spectral features. [ABSTRACT FROM AUTHOR]

• A dielectric metamaterial is theoretically proposed to simultaneously achieve ultra-narrow linewidth of 0.35 nm, a Q -factor of 4223.43 and a strong field enhancement up to 300 times. • These excellent spectral characteristics can be primarily attributed to the mode coupling between the anapole mode and the gap mode. • It exhibits an impressive sensitivity of 307.69 nm RIU−1 and achieves a figure of merit of 879.11 RIU−1. All-dielectric metamaterials have recently received significant attention in the field of nanophotonics, due to their negligible dissipative losses and strong polariton response compared with metallic metamaterials. However, simultaneously achieving an ultra-narrow resonance linewidth and a significant field enhancement in the all-dielectric metamaterials is still a great challenge, limiting their further practical application. In this work, a novel nanostructure composed of silicon ring and nanodimer arrays is theoretically proposed and numerically simulated. Through introducing the coupling between the anapole mode originating from the ring arrays and the gap mode deriving from the nanodimer arrays, the as-designed dielectric metamaterial manifests dual-band high-quality resonance characteristics with strong electric field enhancement reaching up to 300 times, an ultra-narrow linewidth as narrow as 0.35 nm, and a large Q -factor of 4223.43 synchronously. Benefiting from the exceptional optical spectral characteristics, the proposed dielectric nanodimer-ring metamaterial can work as a high-available refractive index sensor, whose sensitivity can reach 307.69 nm RIU−1 with its figure of merit attaining 879.11 RIU−1, efficiently distinguishing an index change of less than 0.01. This research opens up a promising path for achieving extremely narrow spectral linewidth and significant enhancement of the electric field, exhibiting immense possibilities in applications such as biochemical sensing, nonlinear optics, and surface enhanced spectroscopy. [ABSTRACT FROM AUTHOR]

Sensors are important tools in the fields of biology and chemistry. Highly sensitive, robust and low-cost sensors and their preparation methods have always constituted a research topic. In this paper, an all-dielectric metasurface with a double-opening resonant ring is proposed. The metasurfaces can support MD and ED modes to excite three q-BIC modes. And the Q-factor can reach 4. 97 × 1 0 4 , 2. 02 × 1 0 5 and 1. 39 × 1 0 6 respectively. For the asymmetric structure, the three excited peaks have certain polarization-insensitive characteristics. With the introduction of graphene, the dynamic regulation of transmission peak can be realized, specifically the regulation of resonance wavelength, and the intensity of transmittance. Since the all-dielectric metasurfaces assist in enhancing the interaction of the light field with dielectric materials, and the field enhancement effect is obvious, so we analyze the sensing properties of the metasurface. It has a sensitivity of up to 1340 nm/RIU and FOM can reach 1. 17 × 1 0 6 . Finally, we additionally analyze another asymmetric form, which enables the excitation of the q-BIC by changing the polarization angle, making the structure potentially valuable for optical switching as well. • An all-dielectric metasurface producing multi-BIC in the infrared band was designed. • As sensors, the sensitivity can reach 1340 nm/RIU and the FOM can reach 1. 17 × 1 0 6 . • The resonance type of the symmetry-protected BICs was studied by multipole analysis. • The Fano peak can be switched between BIC and q-BIC by polarizing angle. • The effect of different graphene layups on the resonance peaks was investigated. [ABSTRACT FROM AUTHOR]

Plasmonics is gaining prominence in the area of optical sensing due to the unique way that noble metals and light interact to produce subwavelength confinement. A Metal Insulator Metal waveguide based plasmonic nanosensor exhibiting multi Fano resonance is proposed. The characteristics of transmittance of the proposed sensor are investigated using the Finite Difference Time Domain methodology. Three Fano resonances can be seen in the transmission characteristic with different sensitivities of 992.4 nm/RIU, 1294.8 nm/RIU and 2065.5 nm/RIU at 1.0257 μ m, 1.3239 μ m and 2.0798 μ m respectively. Furthermore, the sensor performance is investigated for potential fabrication issues arising out of variation in structural parameters such as the coupling distance and the radius (both inner and outer) of the semi-ring arc resonator. The performance of the sensor is also assessed for performance metrics like the Figure of Merit (FOM), Q factor, and Detection Limit, which are obtained as 39.7 RIU−1, 39.9 and 0.025 respectively. The characteristics of the Fano resonances obtained through simulation is also validated by matching it with the theoretical Fano line shape function. The proposed sensor can find its use in biosensing applications. [Display omitted] • The proposed sensor has a lesser complex structure as compared to the existing reported sensors. • Multiple FRs obtained which enhances the light energy utilisation of the device. • The sensitivity obtained i.e. 2065.5 nm/RIU is also quite high compared to the recently reported sensors. • The simulated values have been validated by matching it with the theoretical values with good accuracy. [ABSTRACT FROM AUTHOR]

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Abstract A refractive index (RI) sensor which contains a dielectric cavity composed of two parallel planar waveguide gratings (WGGs) is proposed. Reflection and transmittance through stacked gratings are studied after converting the diffraction problem into the two coupled resonances. The method of analysis is applied to estimate the performance when used as a biosensor. The sensor achieves its high-sensitive detection by tracking a narrow resonant reflection peak via a tunable wavelength light source, and the desired RI can be determined from the resonant wavelength shift. It will be shown that, in comparison with conventional single grating guided mode resonance (GMR) sensor with equal available area, extremely narrow resonance linewidth can be obtained by using stacked gratings. The sensitivity of the sensor can reach to 497.83 nm/RIU, and the value of the figure of merit (FOM) can reach up to 551. Because of its advantages such as high-sensitivity, label-free, small size and non-destructive, the sensor has an extremely broad application prospect in the field of biomedicine detection. [ABSTRACT FROM AUTHOR]

A dual-core photonic crystal fiber sensor based on surface plasmon resonance is theoretically proposed for the high-sensitive detection of high refractive index liquid analytes. Dual-core construction can effectively enhance the coupling effect between the fiber-core mode and the surface plasmon polariton modes, leading to sharp loss peaks at the resonance wavelengths. As the refractive index of the targeted analyte varies, the resonance condition will change as well and cause a certain shift of loss peak. Numeric results show that this dual-core fiber sensor exhibits an average linear sensitivity 9538 nm/RIU and a maximum sensitivity is 11400 nm/RIU with a resolution 8.77 × 10−6 RIU. The detected range is broad and covers the high refractive index range from 1.45 to 1.58 with an average figure of merit 284.5 RIU−1. The dependence of structure parameters and the thickness of coated-metal thin-film on sensing performance is performed systemically and suggests different responses. The proposed sensor is highly promising in detecting high refractive index liquid analytes in the fields of biological detection, environmental monitoring and chemical analysis. • A high-sensitive PCF-SPR sensor for high refractive index liquid is numerically proposed. • The design realizes a wide detection range of 1.45 − 1.58 and a large FOM. • The effects of structural parameters and gold film thickness on the sensing performance are investigated. [ABSTRACT FROM AUTHOR]

• The first introduction of the concept of long-range and short-range Tamm plasmon polariton. • The potential of LRTPP for sensing applications is investigated for the first time. • The structural parameter optimization of LRTPP-based hollow-core fiber sensor utilizing the ray transmission model. • Ultra-wide refractive index detection range and enhanced figure of merit. The potential of long-rang Tamm plasmon polariton (LRTPP) for sensing applications is theoretically investigated in a planar distributed Bragg reflector (DBR) structure containing a thin metal film. Results show that both sensitivity and figure of merit (FOM) increase with increasing incidence angle under transverse electric polarization. Due to the difficulty of achieving very large incidence angles in planar structures in practice, a LRTPP based hollow-core fiber (HF) sensor is proposed, where all transmitted lights in the fiber core have large incidence angle close to 90° on the inner surface of the HF. The ray transmission model is adopted to calculate the transmission spectra of the proposed HF LRTPP sensor. A comprehensive theoretical analysis about the influence of the structural parameters on the sensor performance is carried out, including the thickness of the metal layer and the number of periods of the DBRs. The HF LRTPP sensor achieves a sensitivity of 774 nm/RIU and FOM of 227.6 RIU−1 at the refractive index (RI) around 1.00 for gas sensing. In the RI range of 1.33–1.53, a sensitivity of 705 nm/RIU and FOM of 201.4 RIU−1 are achieved. The FOM of the HF LRTPP sensor is 2–7 times higher than that of the HF Tamm plasmon polariton (TPP) sensor, while the sensitivities are comparable. Benefiting from the lower attenuation and narrower resonance dip, the idea of utilizing LRTPP mode in DBR-metal-DBR structure to improve the sensor performance will provide a new way for the design of TPP sensors and promote the related researches. [ABSTRACT FROM AUTHOR]

Numerous studies have used the Drude model to optimize silver-based Metal-Insulator-Metal plasmonic sensors for some limited sensing applications; however, Lab-on-a-chip sensing for cell protein concentration measurement still needs to be accomplished. Furthermore, little study has been conducted on sensing for the salinity measurement of seawater, Bovine serum albumin concentration measurement, human tissue categorization, mitigating the oxidation issue of these sensors, and nanofabrication challenges. In this article, a Metal-Insulator-Metal based oxidation free material (gold modeled with Drude-Lorentz model) built round-edged hexagonal plasmonic refractive index sensor with nanorods embedded both in the straight waveguide and resonator has been investigated using the Finite element method (FEM) for all of the above sensing applications. This sensor achieves 19.05 nm/g/100mL and 1476.6 nm/ppm sensitivity for cell protein concentration and salinity measurement. The sensor's max sensitivity and Sensing Resolution are 9231.7 nm/RIU and 1. 083 ∗ 1 0 − 7 . This sensor has also been employed as an electromagnetic interference-free temperature sensor using toluene, achieving 5.16 nm/°C sensitivity. Thus, this label-free and low-footprint sensor can be employed to study the salinity of seawater, which is linked with global warming and the earth's hydrologic cycle, study protein concentration for nanomedicine applications, and Lab-on-chip sensing as an oxidation-free alternative to silver-based sensors. • Highly sensitive oxidation-resistant nanosensor with a max sensitivity of 9231.7 nm /RIU. • Increased electromagnetic confinement of surface plasmon polaritons using nanorods. • This sensor has a sensitivity of 19.05 nm/g/100 mL and 1476.6 nm/ppm for measuring cell protein content and salinity. • Toluene-based electromagnetic interference-free optical temperature sensor achieving 5.16 nm/°C sensitivity. • The effect of geometric parameters on coupling has been studied extensively and compared with theoretical analysis. [ABSTRACT FROM AUTHOR]

Highlights • An integrated plasmonic sensor, using an MIM racetrack resonator structure, is designed and analyzed. • Refractive index and temperature sensitivity can be obtained as high as 4650 nm/RIU and 0.69 nm/°C, respectively. • The sensing performances are investigated numerically by the FDTD method. • It can potentially be applicable for biosensing applications such as hemoglobin concentration detection. Abstract In this paper, we designed a high-sensitivity plasmonic sensor using three-dimensional plasmonic metal–insulator–metal (MIM) waveguides and a racetrack resonator. By detecting the resonance wavelength, changes in the refractive index of the resonator can be sensed, on the basis of the linear relationship between these two parameters. The structure was numerically simulated by the finite-difference time-domain method (FDTD), and the results show that the refractive index and temperature sensitivity values can be obtained as high as 4650 nm per refractive index unit (RIU) and 0.33 nm /° C , respectively. We show that such improved sensitivity can be obtained by using a long lateral interaction length along the entire flat resonator sidewalls. The effects of radius and refractive index of racetrack resonator are studied from the sensing spectra to evaluate the sensitivity performance, as well. The proposed structure with such high sensitivity will be useful in bio-sensing that can provide a new possibility for the detection of biological parameters such as hemoglobin concentration. Furthermore, the applicability of our structure in the detection of hemoglobin concentration in three different blood groups is presented. [ABSTRACT FROM AUTHOR]

Two photonic crystal optical refractive index sensors have been proposed in this paper. These sensors can be used for detection of basal cell cancer. A rod-type square lattice of GaAs is used for this purpose. The sensor topologies are based on a photonic crystal ring-shaped resonator coupled to two and waveguides respectively. Instead of using ultra-high quality factor cavities for improving the sensitivity, the resonance profile is localized and concentrated on the analyte to achieve good distinction. A 13 nm cavity resonance frequency shift for normal and cancer cells is obtained for detection of basal cell carcinoma. Finite difference time domain method and plane wave expansion methods are used to simulate and analyze the structures. The proposed biosensors have a sensitivity equal to 720 and 638 nm/RIU. The simplicity of the design and its high sensitivity make it a suitable choice for bio-sensing applications. [ABSTRACT FROM AUTHOR]

Slotted photonic crystal waveguides (SPCWs) were designed to act as refractive index sensing devices at mid-infrared (IR) wavelengths around λ = 3.6 μm. In particular, effort was made to engineer the input and output slot waveguide interfaces in order to increase the effective sensitivity through resonant tapering. A slotted PhC waveguide immersed in air and liquid cladding layers was considered. To determine the performance of the sensor, the sensitivity of the device was estimated by calculating the shift in the upper band edge of the output transmission spectrum. The results showed that the sensitivity of a conventionally designed SPCW followed by modifications in the structure parameter yielded a 510 nm shift in the wavelength position of the upper band edge, indicating a sensitivity of more than 1150 nm per refractive index unit (RIU) with an insertion loss level of −0.3 dB. This work demonstrates the viability of photonic crystal waveguide high sensitivity devices in the Mid-IR, following a transposition of the concepts inherited from the telecom band and an optimization of the design, in particular a minimization of photonic device insertion losses. [ABSTRACT FROM AUTHOR]

• Gold is less corrosive than silver counterparts. • Sensor is designed for Mid Infrared Region (MIR). • Fabrication is comparatively easier due to simplicity of structure. • High performance indicated by Sensitivity and FOM. • Applicable as an organic gas and temperature sensor. In this article, an optical refractive index (RI) nanosensor has been presented. The layout is arranged in a metal–insulator-metal (MIM) waveguide configuration with gold being the plasmonic metal. The waveguides are coupled with quintuple grids resonant cavity where the grids are uneven in length. The transmission spectral profile of the schematic is numerically analyzed employing the two-dimensional Finite Element Method (FEM) within the mid-infrared region (MIR). Moreover, the key structural parameters are optimized individually to realize the maximum performance of the schematic. The most optimal structure displays a high sensitivity of 4737.87 nm/RIU and a FOM of 9.37. Furthermore, the structure can be suitably used in applications such as sensing organic gas vapors and temperature. Due to the practicality of the applications and the simplicity of the schematic along with the higher longevity provided by gold, this device can replace complex structures effectively. [ABSTRACT FROM AUTHOR]

In this article, a novel surface plasmon polariton-based plasmonic refractive index sensor is proposed and numerically investigated. It consists of a structure of an octagonal cavity resonator inside a square disk along with silver nanodots and two slits which are coupled to a straight metal–insulator–metal (MIM) waveguide arrangement. The finite element method (FEM) has been employed to perform a numerical analysis of the proposed structure in the near-IR (Infrared) and mid-IR spectrum. The sensor detects changes in refractive index in terms of changes in resonant wavelength which can be measured using an optical spectrum analyzer. The recorded maximum sensitivity is 2527.6 nm per refractive index unit (RIU) with a sensing resolution of 3. 956 × 1 0 − 7 RIU. The proposed sensor has been further numerically analyzed, and its ability to measure the quality of honey samples and detect lactose concentration in a solution has been verified. • A novel surface plasmon polariton-based plasmonic refractive index sensor is proposed and thoroughly investigated through numerical analysis. • Numerical analysis using the finite element method (FEM) is employed to explore the sensor's performance in the near-IR and mid-IR spectrum. • The proposed sensor is analyzed to assess its effectiveness in measuring the quality of honey samples. • The sensor demonstrates its versatility by successfully detecting lactose concentration in solution, expanding its potential applications. [ABSTRACT FROM AUTHOR]

• The sensor can realize double resonant peak sensing in the same spectrum, significantly improving detection accuracy. • By adjusting the thickness of the perovskite, it can be used in different working environments. • The fiber base of the sensor is a simple PCF, and it is insensitive to changes in structural parameters. • The maximum sensitivity of the sensor for RI measurement is 36400 nm/RIU, and the corresponding FOM is 1170.04 RIU−1. Two-dimensional polymer perovskite (CH 3 NH 3 PbI 3) has very potential properties in sensing applications. Hence, for high sensitivity, a lossy mode resonance refractive index sensor based on perovskite coating is proposed. The sensor uses a photonic crystal fiber (PCF) with three air holes and a polished surface. Perovskite is deposited on the PCF plane to form the Kretschmann structure. We analyzed its sensing characteristics by using the finite element method. A significant advantage of the sensor is its strong tunability, allowing it to be used in various operating environments by controlling the thickness of the film. In addition, two sharp resonant peaks in the loss spectrum can realize double resonant peak sensing and increase measurement accuracy. Numerical results show that the maximum sensitivity of the two resonant peaks is 36400 nm/RIU and 15400 nm/RIU, respectively, and the corresponding refractive index range is 1.39–1.43. The results also show that small structural changes do not cause a change in performance, meaning that the sensor has a high fault tolerance rate. The high sensitivity response with adjustable detection range makes the proposed sensor distinct. It has a good application prospect in water quality monitoring and biomolecular detection. [ABSTRACT FROM AUTHOR]

• A novel sensor with simple structure consisting of two asymmetric distributed Bragg reflectors attached to graphene in the cavity is proposed. • The sensor can realize tri-band sensing at the frequencies of 1.04 THz, 3.00 THz and 3.13 THz. • The corresponding sensitivities and the figure of merit can achieve 1.02, 2.58, 3.00 THz/RIU, and 559.34, 652.15, 776.17 RIU−1, respectively. In this paper, a novel terahertz refractive index sensor consisting of two asymmetric distributed Bragg reflectors (DBRs) attached to graphene in the cavity, is proposed. Based on finite difference time domain simulation, Tamm plasmon polariton mode excited at the interface between the DBR and the graphene is coupled with the resonance modes of the cavity, which helps to improve the sensing performance. In its reflection spectra, there are three dips with the bandwidths of 1.82, 3.95 and 3.86 GHz at 1.04 THz, 3.00 THz and 3.13 THz after simulation optimization. And the corresponding sensitivities and the figure of merit can achieve 1.02, 2.58, 3.00 THz/RIU, and 559.34, 652.15, 776.17 RIU−1, respectively. Due to its high sensing performance and simple structure, the tri-band sensor may pave the way for the research on the detection of multimodal microscale analytes. [ABSTRACT FROM AUTHOR]

• The deposition of sodium and silver on PCF broadens the sensing bandwidth, thus enlarging the measurement range of the two parameters. • The two parameters to be measured can be represented in the same spectrum without secondary measurement; the number of light sources and spectrometers can be reduced. • The two channels for measuring RI and temperature have no mutual interference. • The sensor has a simple structure and high tolerance. When temperature and refractive index are measured in same spectrum, the measurement range of parameters is generally narrow due to bandwidth limitation. We proposed a sensor for simultaneous measurement of them in a wide wavelength range for this problem. The proposed sensor is a dual-channel structure based on the photonic crystal fiber (PCF) with three air holes and a polished plane. The silver film deposited on the polished plane is responsible for refractive index measurement, and the sodium film and polydimethylsiloxane (PDMS) filled in an air hole for temperature measurement. According to research findings, the two kinds of metal films work in different wavebands, which increases the sensing bandwidth and the measurement range of the parameters to be measured. Numerical results show that when the refractive index of analyte is 1.35 ∼ 1.41 and the temperature is 10 ∼ 60℃, the maximum spectral sensitivity of the sensor is 8400 nm/RIU and 10.2 nm/℃, respectively. The results show that the performance of the sensor is not sensitive to structural parameters. It makes the sensor easy to manufacture. The excellent performances and wide detection range make the proposed sensor have a good application prospect in biochemical detection, environmental monitoring, medical sensing, and other fields. [ABSTRACT FROM AUTHOR]

In this paper, a novel refractive index sensor based on liquid infiltrated dual-core photonic crystal fibers (PCFs) is proposed. The two fiber cores, separated by an air hole inside the cross-section of the designed PCFs, are filled with analyte liquids which form two independent waveguides. A novel and simple refractive index sensing system is proposed using mode coupling theory and its sensing performances have been studied numerically. The results show that the refractive index sensor with 1 cm length PCF has a large sensing range and high sensitivity of −65,166.10 nm/RIU at wavelength 1.25 μm. [ABSTRACT FROM AUTHOR]

In this paper, an optical refractive index (RI) sensor based on a photonic crystal and plasmonic (PhC-P) hybrid topology is designed. The sensor's structure is composed of nested circular metallic split-ring resonators (NCMSRR), which are embedded inside a hexagonal cavity in a rod-type photonic crystal (PhC) structure. The finite-difference time-domain (FDTD) method is used for the numerical simulation of the sensor. The obtained results show that by the cooperative coupling between the surface plasmon polaritons (SPP) induced by the NCMSRR and resonant PhC cavity modes, a hybrid PhC-P mode is created. The hybrid mode provides unique opportunities, not only for incrementing the sensitivity but also for obtaining a much higher figure of merit (FoM) compared to modes belonging to purely plasmonic or purely PhC sensors. The simulation results demonstrate a fairly high sensitivity of 1250 nm/RIU and an excellent FoM of 2083 RIU−1 when the proposed sensor is exposed to hazardous gases. These results show considerable improvement compared to the works published in the literature. Accordingly, the sensor proposed in this paper has tremendous potential to be used for high-sensitivity and high-resolution sensing applications at optical communication wavelengths. • An optical gas sensor based on a photonic crystal and plasmonic (PhC-P) hybrid topology is designed. • By the cooperative coupling between the SPP and PhC modes, a hybrid PhC-P mode is created. • The hybrid mode shows a sensitivity of 1250 nm/RIU and an excellent FoM of 2083 RIU−1. [ABSTRACT FROM AUTHOR]

We present the design and analysis of the Refractive Index sensor having the discontinuity between sensing region and cladding area by large core diameter. If sensing region is placed in different refractive index medium then the cladding, the modal field distribution changes. The overlap of modal fields in the cladded region and the sensing region gives an estimate of fractional power transfer from the input end to output end. This fractional power transfer gives us the refractive index of the medium. [ABSTRACT FROM AUTHOR]

We present the design and analysis of the Refractive Index sensor having the discontinuity between sensing region and cladding area by large core diameter. If sensing region is placed in different refractive index medium then the cladding, the modal field distribution changes. The overlap of modal fields in the cladded region and the sensing region gives an estimate of fractional power transfer from the input end to output end. This fractional power transfer gives us the refractive index of the medium. [ABSTRACT FROM AUTHOR]

• This paper is investigated a silicon-dielectric-metal nanostructure, it supports the optional anapole mode due to the destructive interference between electric and toroidal dipoles. • Such the proposed nanostructure exhibits a near-perfect ultra-narrow absorption spectrum in the near infrared region with the absorption bandwidth of 0.48 nm and a high-quality factor up to 1763. In addition, the structure we designed has both excellent sensing performance and field enhancement performance, which can be used to design multi-function chips. • Our design is a simple flat structure, which is easy to process and manufacture in actual production. This paper is investigated a silicon-dielectric-metal nanostructure, it supports the optional anapole mode due to the destructive interference between electric and toroidal dipoles. Such the proposed nanostructure exhibits a near-perfect ultra-narrow absorption spectrum in the near infrared region with the absorption bandwidth (FWHM) of 0.48 nm and a high-quality (Q) factor up to 1763. Besides inherently low dissipative losses and strong anapole response, this optical nanostructure can demonstrate subtle refractive index sensing properties. Our work shed new light on active ultra-narrow band perfect absorption based on the optical anapole mode, and provides potential applications such as optical sensors, narrow-band filter, non-radiative data transmission and biomolecules manipulation. [ABSTRACT FROM AUTHOR]

• We present a theoretical model of lossy mode resonance (LMR) prismatic refractive index sensor based on the SBS (sensing medium/black phosphorus/sensing medium) structure. • The sensor's performance under angular interrogation is effectively improved. • The effect of changes in sensor structure on reflectivity is investigated in detail and the principles of device design are summarised. • The device has a refractive index detection range of 1.33–1.49, with different sensing properties under TE and TM polarized light. This LMR sensor has a high sensitivity under TE polarized light with a sensitivity range of 83–420 °/RIU and a maximum quality factor of 9.9×105 RIU-1 under TM polarized light. This paper presents a theoretical model of lossy mode resonance (LMR) prism refractive index sensor based on the SBS (sensing medium/black phosphorus/sensing medium) structure. By using the sensing medium as the matching layer and the black phosphorus (BP) as the lossy layer, the sensor's performance under angular interrogation is effectively improved. In addition, the implications on the reflectance of changes in the sensor structure are investigated in detail and the principles for the design of the device are summarized in this paper. Results show that the device has a refractive index detection range of 1.33–1.49, with different sensing properties under TE and TM polarized light. This LMR sensor has a high sensitivity under TE polarized light with a sensitivity range of 83–420 °/RIU and a maximum quality factor of 9.9×105 RIU−1 under TM polarized light. We believe that the proposed structure can be broadly applied to chemical biomolecular detection, environmental monitoring, medical diagnosis, and other fields. [ABSTRACT FROM AUTHOR]

Abstract: A high precision and compact refractive index sensor is proposed. The combination of coarse measurement utilizing the change of the angle of refraction and fine measurement utilizing the phase change is newly proposed to measure absolute refractive index precisely. The proposed method does not need expensive optical measurement equipment such as an optical spectrum analyzer. The integrated planar lightwave circuit (PLC) technology enables us to obtain a compact sensor that is preferable for the practical use. The principle, design, and some configurations for precise refractive index measurement are described. [Copyright &y& Elsevier]

This article presents an ultra-compact refractive index sensor based on titanium nitride (TiN), a well-known alternative plasmonic material. The designed sensor consists of a straight waveguide, coupled with a groove and a Square-Ring Resonator (SRR). In addition, nanosized TiN dots are embedded in the waveguide and groove to enhance sensing performance. The spectral characteristics of the proposed sensor are numerically analyzed using the Finite Element Method (FEM). After extensive simulations, the most optimal structure provides a maximum sensitivity of 1074.88 nm/RIU and a Figure of Merit (FOM) of 32.4 and proves to be tolerant against probable real-life manufacturing defects. Furthermore, the incorporation of TiN equips the illustrated sensor with numerous advantages, for example, immunity to oxidation, optical tunability, capability to endure high-temperature, and compatibility with the standard Complementary Metal Oxide Semiconductor (CMOS) fabrication process. The major pitfall of traditional plasmonic metals is that the metals lack all these qualities, limiting real-life implementations of plasmonic devices. Therefore, the proposed model is superior to the conventional Metal–Insulator–Metal (MIM) setup and is expected to facilitate the development of practical, robust sensors. • A refractive index sensor based on TiN, a well-known alternative material, is proposed. • Nanosized TiN dots are loaded within the schematic to enhance sensing performance. • After extensive simulations, maximum sensitivity and FOM of 1074.88 nm/RIU, and 32.4 are obtained, respectively. • The proposed sensor is CMOS compatible, immune to oxidation, and optically tunable. • Therefore, the suggested schematic is superior to the conventional MIM sensors. [ABSTRACT FROM AUTHOR]

• We report a metasurface device with plasmon cavity arrays as a refractive index sensor. • The proposed device possesses independently tunable double Fano resonances. • The proposed device provides the sensitivity of 667 nm RIU−1. We report a metasurface device based on nano metal-dielectric-metal plasmon cavity arrays. Investigation of the proposed device indicates the sharp absorption peak of double Fano resonance and multiple periods of time-domain oscillations caused by the three-dimensional double resonance. By adjusting the structure parameters, different absorption patterns can be regulated independently with high degree of freedom. The metasurface exhibits sensitive behavior to the environmental refractive index and possesses the sensitivity of 667 nm RIU−1 as a refractive index sensor in the mid-near infrared band, which can be further considered for glucose or temperature sensing. [ABSTRACT FROM AUTHOR]

• We design a polarization-insensitive and incident angle-independent Fano resonance device based on all-dielectric "lucky knot" shaped nanostructure in the mid-infrared region. • We proposed a mid-infrared refractive index sensor with a sensitivity of 986 nm/RIU and a figure of merit of 32.7 simultaneously. • The proposed device can be applied in biosensing, environmental monitoring and other fields. We design and numerically analyze a Fano resonance device based on periodical all-dielectric "lucky knot" shaped nanostructure in the mid-infrared region with high sensitivity, which is insensitive to polarization and incident angle. The proposed nanostructure is sensitive to the change of the refractive index of the substance to be tested. We established the correspondence between the change of the refractive index and the peak position of the reflection spectrum. And then we propose an optical refractive index sensor with a sensitivity of 986 nm/RIU, and a maximum figure of merit of 32.7, which is much higher compared to the sensor based on metallic materials. The maximum Q-factor can reach 520. This study may provide a further step in optical sensing, biosensing and environmental monitoring. [ABSTRACT FROM AUTHOR]

In this paper, a highly sensitive plasmonic refractive index (RI) sensor based on photonic crystal fiber (PCF) is proposed. Silver (Ag) is used as plasmonic material and tantalum pentoxide (Ta 2 O 5), high RI material, is used as an overlayer over the silver thin film. The sensor performance is realized using surface plasmon resonance (SPR) phenomenon. The effect of tantalum pentoxide overlayer thickness on sensing parameter as well as on sensitivity is analyzed in detail using finite element method (FEM). The detailed study shows an interesting property that the increasing Ta 2 O 5 layer thickness can tune the operating analyte RI from high RI region to low RI region i.e. towards the aqueous medium. The structural parameters of the proposed sensor are being optimized and its sensitivity is realized using wavelength interrogation technique. A maximum sensitivity of 50000 nm/RIU is reported. This study, no doubt, provides a new direction of designing RI sensor that could tune RI range by varying the only the Ta 2 O 5 layer thickness to the desired operating analyte region. Besides, the simple design feasibility, low fabrication cost and portable nature of the proposed sensor make it suitable for industrial and chemical sensing applications. • Performance of tantalum pentoxide, high RI material, as an overlayer on a plasmonic PCF RI sensor is analyzed in detail. • Increasing Ta 2 O 5 layer thickness tunes the operating analyte RI from high to low RI region i.e. towards the aqueous medium. • The sensor works in NIR wavelength region which is favorable for sensing. • A maximum sensitivity of 50000 nm/RIU is reported. • A advance designing direction to operate sensor in the desired analyte region only by varying Ta 2 O 5 thickness. [ABSTRACT FROM AUTHOR]

• A multi-band absorber based on a hybrid metal-graphene metasurface is proposed in theoretically. • Three absorption peaks with the absorption of 96.4%, 99.4% and 99.9% have been achieved. • The sensitivities and maximum figure of merit are 3.98, 4.13 and 5.06 µm/RIU, 16.6, 20.7 and 18.1. A multi-band absorber based on a hybrid metal-graphene metasurface, which is also suitable for detecting surrounding refractive index, is proposed and studied by numerical simulation. The structure combines a metal disc with a graphene plasmon, which greatly enhances the coupling of light and graphene, and realizes multi-band resonance absorption. The simulated results show that three absorption peaks at 23.5 µm, 24.3 µm and 27.8 µm with the maximal absorption of 96.4%, 99.4% and 99.9% have been achieved, respectively. At the same time, under normal incidence, the absorber is not restricted by the polarization angle of the incident light source. Moreover, the light source can be incident obliquely under two polarization conditions, and the absorber of this structure maintains approximately stable absorption within the range of oblique incident 50°. The dynamic adjustment of the absorption peak can be achieved by flexibly changing the Fermi level of graphene. Furthermore, the absorber we proposed can also be used as a refractive index sensor to detect the surrounding refractive index. The sensitivities are 3.98 µm/RIU, 4.13 µm/RIU and 5.06 µm/RIU, and the maximum figure of merit (FOM) is 16.6, 20.7 and 18.1, respectively. [ABSTRACT FROM AUTHOR]

• A compact optical gas sensor using graphene-assisted metamaterial has been proposed. • Graphene coating of gold nano-disks can significantly improve the sensitivity. • Sensor detection limit is approximately 1 × 10−5 RIU with an average sensitivity of 720 nm/RIU. • Sensor has a linear response even for gas RI variations in the order of 10−5 RIU. • The sensor can be used to monitor low concentration of various gases. In this paper, we propose a graphene-assisted plasmonic metamaterial absorber to operate as an ultra-compact optical gas sensor in the infrared (IR) region. The metamaterial absorber is comprised of a thin silicon-dioxide layer that is middling between a gold film and an array of graphene-coated gold nano-disks. Using the finite element method, we investigate the resonance characteristics of the proposed plasmonic metamaterial and show that the metamaterial has three distinct resonances in visible and infrared regions. We demonstrate that the strongest absorption peak of the proposed metamaterial is attributed to a fundamental localized surface plasmon resonance in IR, which has also the highest spectral sensitivity to the refractive index (RI) variations of the surrounding medium. The sensing mechanism is based on evaluation of gaseous medium RI surrounding the nano-disks, by illuminating the metamaterial with IR radiation and subsequently measuring the reflection or absorption spectra of the structure. We show that graphene coating of the gold nano-disks improves the sensitivity of the metamaterial to the gas RI by more than two times. Using the optimized design parameters, an average linear spectral sensitivity of 720 nm/RIU and an approximate detection limit of 1 × 10−5 RIU has been obtained for gas RI variations between 1 and 1.05. We also show that the proposed sensor has a linear response even for extremely small variations in gas RI in the order of 10−5 RIU. The proposed metamaterial configuration is relatively easy to fabricate and can be used to monitor low concentrations of various gases in different applications ranging from environmental monitoring to home safety monitoring systems. [ABSTRACT FROM AUTHOR]

• The U-shaped cavity and semi-annular straight waveguide are superimposed on and coupled to each other to form the Fano resonance transmittance spectrum. • The refractive index of the optimised waveguide structure can achieve up to 825 nm/RIU. • The structure could accurately identify the five proposed pure edible vegetable oils at room temperature. In recent years, optical refractive index sensors have received significant attention in the field of chemical and biological sensing owing to the advantages of small size. In order to achieve the requirements of small structure and high sensitivity of nano refractive index sensing, this paper proposes a Fano resonance–based waveguide structure in which a metal-insulator-metal (MIM) structure using surface plasmons and U-shaped cavities are coupled unilaterally. The transmittance spectrum of the waveguide-coupled resonator with different incident light wavelengths can be obtained. The U-shaped cavity and semi-annular straight waveguide are superimposed on and coupled to each other to form the Fano resonance transmittance spectrum. An extremely narrow impedance is formed in the transmittance spectrum. The proposed structure can obtain superior sensing performance at the nanoscale after the optimisation. The results confirm that the sensing sensitivity of the refractive index of the optimised waveguide structure can achieve up to 825 nm/RIU. The results of experiments show that this structure could accurately identify the five proposed pure edible vegetable oils at room temperature. But experiments have also shown that it could not effectively distinguish between the five proposed mixed edible vegetable oils. This structure provides an effective reference for the design and development of refractive index sensors for application to pure edible vegetable oils. [ABSTRACT FROM AUTHOR]

• A high-performance plasmonic sensor based on metal-insulator-metal waveguides is offered. • Deploying structural optimization technique refractive index sensitivity and figure of merit (FOM) for different sensor configurations are systematically investigated. • Simple structural shape, sharp Fanopeak profile, high sensitivity and FOM make the proposed system suitable for on-chip sensors. • Capability of detecting different blood groups (A, B, and O) utilizing a developed mathematical model is also demonstrated. A highly sensitive sensor focused on two Metal-Insulator-Metal (MIM) waveguides and three quadrilateral cavities sandwiched perpendicularly in between the MIM waveguides is proposed. Fano resonance induced by the coherent superposition of the narrow band spectral response and broadband spectral response, excites the structural transmission characteristics. The Finite Element Method (FEM) is used to numerically investigate the transmission characteristics and sensitivity of the refractive index sensor for different configurations. The linear relationship between resonant wavelength and refractive index is used to sense the materials. By optimizing the structural parameters, a numerical evaluation of the refractive index sensitivity (S) up to 1556 nm/RIU, and associate figure of merit (FOM) of 14.83 is recorded. The device is also explored as a temperature sensor with 0.61 nm/°C sensitivity. Since ethanol is used as a sensing medium, the operating range of the sensor is between −114.3 °C and 78 °C, melting and boiling temperature of ethanol. To detect human blood groups using the proposed sensor, a mathematical model is developed, which shows impressive performance for the detection of human blood groups very precisely. Compact size, ultrafine sensitivity, and sharp Fano peak profile make the proposed sensor an ideal candidate for on-chip sensors. [ABSTRACT FROM AUTHOR]

• The structure can excite resonances with a absorption bandwidth of 0.45 nm and a Q factor of 1499. • The S of 108.32 nm/RIU and FOM reaching 240.7 are obtained. • The device exhibits angle and polarization independence. Metamaterial absorbers have attracted more and more attention because they can achieve perfect absorption in some specific structures. At present, although most absorbers can achieve perfect absorption in one or more frequencies of the visible or near infrared regions, its cannot achieve ultra-narrow band absorption. Ultra-narrow band absorption means that devices have the opportunity to improve the effectiveness and accuracy of detection in many applications. Here, a novel design of narrowband perfect absorber based on a dielectric-dielectric-metal structure is studied, which comprises of an Al 2 O 3 nanodisk arrays, a SiO 2 dielectric layer, an Au film, and a substrate layer. The simulated results reveal that the optimized nanoabsorber can excite resonances with a narrow absorption bandwidth (FWHM) of 0.45 nm and a high-quality (Q) factor up to 1499 in the visible regime. Moreover, when it as a refractive index (RI) sensor, the maximum sensitivity(S) of 108.32 nm/RIU and figure of merit (FOM) reaching 240.7 are obtained. The device exhibits angle and polarization independence, which will eliminate the polarization and angle requirement of the equipment. As a consequence, we believe our narrowband absorber could be used in many potential applications such as the detection of photodetectors, biological, chemical and so on. [ABSTRACT FROM AUTHOR]

• Study of a plasmonic refractive index sensor based on the double concentric square ring resonator and stubs. • Analysis of the proposed structure by the transmission-line model. • Maximum sensitivities of 1250 and 1270 nm/RIU and figures of merit of 32.8 and 58 for the structure with 1 and 3 stubs. A plasmonic refractive index sensor based on a metal-insulator-metal waveguide coupled with a double concentric square ring resonator is proposed and analyzed both theoretically and numerically. The theoretical and the numerical results are based on the transmission-line model and the finite difference time domain method, respectively. The theoretical results agree well with the numerical ones. A maximum sensitivity of 1250 nm per refractive index unit with a high figure of merit of 32.8 for the double concentric square ring resonator with one stub is obtained in the near-infrared region. Also, the double concentric square ring resonator with three stubs is simulated, and has a maximum sensitivity of 1270 nm per refractive index unit with a high figure of merit of 58. Our proposed plasmonic structures could find useful applications in sensing systems and integrated circuits in the near-infrared region. [ABSTRACT FROM AUTHOR]

• All-dielectric metasurface composed of a silicon elliptical disk and a silicon elliptical ring has been proposed. • Four sharp Fano resonances can be observed in the transmission spectrum. The maximum quality factor reached 7421. • The contributions of the electromagnetic fields and multipoles of the four modes have been analyzed. • The sensitivity and the figure of merit could reach up to 392 nm/RIU and 3001, respectively. We believe that the proposed structure exhibits broad application prospects in refractive index sensors. In this study, an all-dielectric metasurface is proposed, whose unit cell is composed of a silicon elliptical disk and a silicon elliptical ring. The transmission of the structure were analyzed by using three-dimensional finite-difference time-domain method. In the wavelength range of 800 to 1200 nm, four sharp Fano resonances appeared in the transmission spectrum. The maximum quality factor reached 7421. Furthermore, we analyzed the electromagnetic field distributions corresponding to the four peaks, and evaluated the contributions of multipoles in forming the resonant responses. Besides, we investigated the influence of different structural parameters, including the length of the semi-major axis, the height of the elliptical disk and ring, and the period length, on the four resonance peaks. Finally, based on the high quality factor, we studied the refractive index sensing characteristics of the structure. By optimizing the structural parameters, the sensitivity and the figure of merit could reach up to 392 nm/RIU and 3001, respectively. Therefore, we believe that the proposed structure exhibits broad application prospects in refractive index sensors. [ABSTRACT FROM AUTHOR]

Abstract The article reports the numerical analysis of refractive index sensor using dual core photonic crystal fiber (PCF) which is aided for the detection of glucose concentration contains in the blood samples of humans. The sensing mechanism is facilitated by tuning the coupling between the silica mode and analyte mode. The change in concentration of glucose is reflected as shifts in the transmission spectrum. The sensitivity is reported as 8333 n m / [ R I U ] for the samples variation as 10 g / l to 20 g / l with the function of resonance wavelength. [ABSTRACT FROM AUTHOR]