Design of a plasmonic sensor based on a square array of nanorods and two slot cavities with a high figure of merit for glucose concentration monitoring

MIM waveguide system with independently tunable double resonances and its application for two-parameter detection

A metal-insulator-metal (MIM) waveguide system consisting of a MIM waveguide, a ring cavity, and a semi-ring cavity is proposed. Using the finite element method, the transmission characteristics of the MIM waveguide system are discussed under the different geometry parameters. By detecting the resonance wavelength and varying the refractive index, the sensing performance of the MIM waveguide system is analyzed. The proposed structure can be used as a refractive index sensor with the maximum sensitivity of 2412 nm/RIU. Due to isolating the ring cavity and semi-ring cavity, the independent tuning of double resonances can be realized by changing the refractive index of the insulator in the ring cavity or the semi-ring cavity. Benefiting from two independent refractive index sensing modes, the structure with two isolated resonators can realize the simultaneous measurement of glucose solution concentration and blood plasma concentration. The sensitivity of glucose solution sensing in the ring cavity is 0.13133 nm/(g/L). Meanwhile, the blood plasma concentration detection in the semi-ring cavity is realized with the sensitivity of 0.358 nm/(g/L). The system with two isolated cavities has the potential to be used as an efficient nano sensor, which can achieve simultaneous measurement of two parameters.

Multimode Fano Resonances Sensing Based on a Non-Through MIM Waveguide with a Square Split-Ring Resonance Cavity

In this article, a non-through metal–insulator–metal (MIM) waveguide that can excite fivefold Fano resonances is reported. The Fano resonances are obtained by the interaction between the modes excited by the square split-ring resonator (SSRC) and the bus waveguide. After a detailed analysis of the transmission characteristics and magnetic field strength of the structure using the finite element method (FEM), it was found that the independent tuning of Fano resonance wavelength and transmittance can be achieved by adjusting the geometric parameters of SSRC. In addition, after optimizing the geometric parameters, the refractive index sensing sensitivity (S) and figure of merit (FOM) of the structure can be optimal, which are 1290.2 nm/RIU and 3.6 × 10⁴, respectively. Additionally, the annular cavity of the MIM waveguide structure can also be filled with biomass solution to act as a biosensor. On this basis, the structure can be produced for optical refractive index sensing in the biological, micro and nano fields.

Metamaterial perfect absorber using elliptical nanoparticles in a multilayer metasurface structure with polarization independence

A metamaterial perfect absorber (MPA) using elliptical silver nanoparticles is proposed and investigated to provide 100% absorption for both transverse electric and transverse magnetic polarizations with a wide range of incident angles and polarization independence. Metamaterial absorbers with narrow absorption performance over a wide frequency range are significantly desired in sensing applications. Incident angle insensitivity and polarization angle independence are key features of MPAs. The output characteristics are examined using the three-dimensional finite difference time domain method. The effective medium theory and transmission line theory are applied to investigate the simulation results. Here, the 100% absorption occurs at resonance wavelength of λ_res= 2290 nm, and maximum sensitivity and figure of merit become 200 nm/RIU and 720 RIU^-1, respectively. The results show that an absorption spectrum is insensitive to the incident angle of 0°-60°. The proposed device can be used as a high-performance biosensor and photodetector.

Optical Transmission Plasmonic Color Filter with Wider Color Gamut Based on X-Shaped Nanostructure

Extraordinary Optical Transmission Plasmonic Color Filters (EOT-PCFs) with nanostructures have the advantages of consistent color, small size, and excellent color reproduction, making them a suitable replacement for colorant-based filters. Currently, the color gamut created by plasmonic filters is limited to the standard red, green, blue (sRGB) color space, which limits their use in the future. To address this limitation, we propose a surface plasmon resonance (SPR) color filter scheme, which may provide a RGB-wide color gamut while exceeding the sRGB color space. On the surface of the aluminum film, a unique nanopattern structure is etched. The nanohole functions as a coupled grating that matches photon momentum to plasma when exposed to natural light. Metals and surfaces create surface plasmon resonances as light passes through the metal film. The plasmon resonance wavelength can be modified by modifying the structural parameters of the nanopattern to obtain varied transmission spectra. The International Commission on Illumination (CIE 1931) chromaticity diagram can convert the transmission spectrum into color coordinates and convert the spectrum into various colors. The color range and saturation can outperform existing color filters.

Cog-shaped refractive index sensor embedded with gold nanorods for temperature sensing of multiple analytes

This article presents a refractive index (RI) nanosensor utilizing gold as the plasmonic material. The layout of the sensor includes metal-insulator-metal (MIM) waveguides coupled with a cog-shaped resonator studded with gold nanorods. At the mid-infrared (MIR) spectrum, the spectral characteristics of the sensor are numerically analyzed employing the finite element method (FEM). Moreover, the refractive index sensing property is thoroughly explored by varying the key parameters, establishing a linear correlation with the transmittance profile. After extensive simulations, the most optimum structure displays the highest sensitivity of 6227.6 nm/RIU. Furthermore, the capability of the proposed device as a temperature sensor is investigated with five different liquids (ethanol, polydimethylsiloxane, toluene, chloroform, and the mixture of toluene and chloroform); among these, chloroform exhibits maximum temperature sensitivity of 6.66 nm/°C. Due to being chemically stable and demonstrating satisfactory performance in RI and temperature sensing, the suggested schematic can be a suitable replacement for silver-based sensors.

Significantly enhanced coupling effect and gap plasmon resonance in a MIM-cavity based sensing structure

Herein, we design a high sensitivity with a multi-mode plasmonic sensor based on the square ring-shaped resonators containing silver nanorods together with a metal–insulator-metal bus waveguide. The finite element method can analyze the structure's transmittance properties and electromagnetic field distributions in detail. Results show that the coupling effect between the bus waveguide and the side-coupled resonator can enhance by generating gap plasmon resonance among the silver nanorods, increasing the cavity plasmon mode in the resonator. The suggested structure obtained a relatively high sensitivity and acceptable figure of merit and quality factor of about 2473 nm/RIU (refractive index unit), 34.18 1/RIU, and 56.35, respectively. Thus, the plasmonic sensor is ideal for lab-on-chip in gas and biochemical analysis and can significantly enhance the sensitivity by 177% compared to the regular one. Furthermore, the designed structure can apply in nanophotonic devices, and the range of the detected refractive index is suitable for gases and fluids (e.g., gas, isopropanol, optical oil, and glucose solution).

Independently tunable triple Fano resonances based on MIM waveguide structure with a semi-ring cavity and its sensing characteristics

In this paper, a metal-insulator-metal (MIM) waveguide structure consisting of a side-coupled rectangular cavity (SCRC), a rightward opening semi-ring cavity (ROSRC), and a bus waveguide is reported. The finite element method is used to analyze the transmission characteristics and magnetic-field distributions of the structure in detail. The structure can support triple Fano resonances, and the Fano resonances can be tuned independently by altering the geometric parameters of the structure. Moreover, the structure can be applied in refractive index sensing and biosensing. The maximum sensitivity of refractive index sensing is up to 1550.38 nm/RIU, and there is a good linear relationship between resonance wavelength and refractive index. The MIM waveguide structure has potential applications in optical on-chip nano-sensing.

Blood Glucose Level Regression for Smartphone PPG Signals Using Machine Learning

Diabetes is a chronic illness that affects millions of people worldwide and requires regular monitoring of a patient’s blood glucose level. Currently, blood glucose is monitored by a minimally invasive process where a small droplet of blood is extracted and passed to a glucometer—however, this process is uncomfortable for the patient. In this paper, a smartphone video-based noninvasive technique is proposed for the quantitative estimation of glucose levels in the blood. The videos are collected steadily from the tip of the subject’s finger using smartphone cameras and subsequently converted into a Photoplethysmography (PPG) signal. A Gaussian filter is applied on top of the Asymmetric Least Square (ALS) method to remove high-frequency noise, optical noise, and motion interference from the raw PPG signal. These preprocessed signals are then used for extracting signal features such as systolic and diastolic peaks, the time differences between consecutive peaks (DelT), first derivative, and second derivative peaks. Finally, the features are fed into Principal Component Regression (PCR), Partial Least Square Regression (PLS), Support Vector Regression (SVR) and Random Forest Regression (RFR) models for the prediction of glucose level. Out of the four statistical learning techniques used, the PLS model, when applied to an unbiased dataset, has the lowest standard error of prediction (SEP) at 17.02 mg/dL.

MIM waveguide structure consisting of a semicircular resonant cavity coupled with a key-shaped resonant cavity

We describe the optical transmission properties of a surface plasmon polariton waveguide structure consisting of a metal-insulator-metal (MIM) waveguide and a semicircular resonant cavity coupled with a key-shaped resonant cavity. Finite element algorithm simulated the optical response of a MIM waveguide structure. The influence of coupling length, geometrical size, and asymmetry of the key-shaped cavity and the radius of the semicircular resonant cavity on the Fano resonance line was investigated. Results demonstrate that variation of the key-shaped cavity asymmetry leads to the appearance of dual Fano resonances. When materials with different refractive index fill in the key-shaped cavity, the MIM waveguide structure achieves a sensitivity of 1261.67 nm/RIU. This performance allows the waveguide to be used for nanoscale biosensor applications such as measuring glucose concentrations. We simulated various spiked glucose concentrations by calculating the frequency shift as the second Fano resonance line moves towards longer wavelength. Glucose concentrations were calculated from variations of the Fano resonant wavelength. The waveguide structure proposed in this paper shows impressive practical prospects for many applications in the chemical sensing and biomedical fields.

Multi-band MIM refractive index biosensor based on Ag-air grating with equivalent circuit and T-matrix methods in near-infrared region

In this paper, a multi-band metal-insulator-metal (MIM) perfect absorber with refractive index sensing capability has been investigated in near-infrared region. The proposed structure has been studied for biomedical applications such as detection of solution of glucose in water, diagnosis of different stages of malaria infection, bacillus bacteria and cancer cells. The MIM configuration improves the sensing parameters of the biosensor due to the good interaction with the analyte. The high sensitivity and figure of merit of 2000 nm/RIU and 100 RIU⁻¹ have been achieved, respectively. Also, the Ag-air grating in the suggested plasmonic sensor helps the localized surface plasmons excitation and makes the structure sensitive to the incident lightwave polarization. Therefore, the presented biosensor behaves like a polarization switch with the high extinction ratio and fast response time of 25.15 dB and 100 fs, respectively. The methods of equivalent circuit model and transmission matrix have been utilized to verify the simulation results, as a new challenge in near-infrared region. The new idea of multi-application plasmonic devices, the feasibility of fabrication for the presented structure and utilizing mentioned analytical methods in near-infrared region could pave the way for the future of plasmonic structures.