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In this article, the design and operation of a cylindrical capacitive sensor based on the dielectric reactance capacitance and conductance changes of the gap medium is reported. The proposed system was used to determine characteristics of different water liquids as a result of the capacitance and resistance variations. The air gap capacitance (dry signal) is measured and then by filling the gap with a liquid, the capacitance (wet signal) is monitored for different liquids. A reported sensor is used for the distilled, tap, boiled, and salt water measurements and the capacitance and resistance results are compared. A big difference of about 38.5 μF in the measured capacitance values for the salt and distilled water shows a high sensitivity, which can be used to recognize different water liquids. The experimental results are promising for water liquids and verify the successful operation of such a device as a liquid sensor, a useful method for checking the electrical quality of the water that is required for different applications. It is also possible to monitor the resistance change of the filling medium as a function of time.

A novel microwave sensor based on waveguide filled with a metamaterial particle, which is composed of meander line and split-ring-resonator (SRR), is presented and modeled using a full wave electromagnetic commercial simulator CST. Simulating results show that evanescent mode are enhanced and transmitted through the structured particle within the frequency band that exhibits negative permittivity and permeability. Evanescent mode is sensitive to permittivity of dielectric material in the detecting zone. Comparing with conventional microwave resonant sensor, the microwave sensor using evanescent mode allows for much higher detection sensitivity.

In this article, two possible applications of prism-based probes for liquid-switching, level-sensing, and as a refractometer are described. Theoretical formulation is developed for the emergent intensity for each system and possible range of operation for each system is reported. By comparing the theoretical investigations, some hints are given in optimum usage of prism in each case. Variation of the beam divergence, incident angle, and prism glass index are major parameters that are considered in this investigation. The obtained results are compared with some available experimental results and practical points concerning the effective application of each geometry is reported. Theoretical expressions developed here are in good agreement with the experimental measurements.

Based on the field enhancement principle of trapped modes, two new asymmetric metamaterial resonators are presented. Transmission response (S₂₁) of the rectangular wave-guide filled with an asymmetric metamaterial resonator is simulated. Results show that the asymmetric resonator possesses high Q-factor and improved sensitivity. The microwave sensor based on the asymmetric resonator can be flexibly tailored to design requirement by varying the asymmetry parameter or the topological structure of the resonator. The asymmetric metamaterial resonator-assisted microwave sensor will have potential applications in biosensor and chemosensor fields for sensing minute amounts of dielectric sample substance.

A Photonic Crystal Fiber based on Surface Plasmon Resonance (PCF-SPR) temperature sensor with liquid core is proposed in this paper. Glycerin liquid with a high refractive index is filled in the central air hole of the hollow core photonic bandgap (PBG) PCF, the transmission type of PCF will change to total internal reflection (TIR), which will significantly broaden its transmission bands. The refractive index of glycerin changes with temperature within a certain temperature range and can be detected by measuring the transmission spectra, thus the accurate ambient temperature can be obtained. Numerical results indicate that the plasmon on the surface of the gold-coated channels containing glycerin liquid can be intensively excited by the core-guided mode and the excitation of the plasmon mode is sensitive to the change of the temperature. Resolution of the PCF-SPR temperature sensor with liquid core is demonstrated to be as low as 4 × 10^-6 RIU, where RIU means refractive index unit.

We demonstrate numerically and experimentally chiral metamaterials (MTMs) based on gammadion-bilayer cross-wires that uniaxially create giant optical activity and tunable circular dichroism as a result of the dynamic design. In addition, the suggested structure gives high negative refractive index due to the large chirality in order to obtain an efficient polarization converter. We also present a numerical analysis in order to show the additional features of the proposed chiral MTM in detail. Therefore, a MTM sensor application of the proposed chiral MTM is introduced and discussed. The presented chiral designs offer a much simpler geometry and more efficient outlines. The experimental results are in a good agreement with the numerical simulation. It can be seen from the results that, the suggested chiral MTM can be used as a polarization converter, sensor, etc. for several frequency regimes.

Cadmium-doped zinc oxide nanoparticles were derived by simple chemical co-precipitation route using zinc acetate dihydrate and cadmium acetate dihydrate as precursor materials. The thick films were casted from chemical co-precipitation route prepared nanoparticles by economic facile screen printing method. The structural, morphological, optical and electrical properties of the film were characterized relevant to alcohol vapor sensing application by powder XRD, SEM, UV-VIS and DC conductivity techniques. The response and sensitivity of alcohol (ethanol) vapor sensor are obtained from the recovery curves at optimum working temperature range from 20${}^{\circ }$C to 50${}^{\circ }$C. The result shows that maximum sensitivity of the sensor is observed at 25${}^{\circ }$C operating temperature. On varying alcohol vapor concentration, minor variation in resistance has been observed. The sensing mechanism of sensor has been described in terms of physical adsorption and chemical absorption of alcohol vapors on cadmium-doped zinc oxide film surface and inside film lattice network through weak hydrogen bonding, respectively.

We propose a metal–insulator–metal (MIM) structure which consists of a $\pi$-shaped resonator and a surface plasmon polariton (SPP) waveguide. The finite element method (FEM) is employed in the simulation. The results show that this structure forms an optical pressure sensor. The transmission spectra have a redshift with increasing pressure, and the relation between the wavelength shift and the pressure is linear. The nanoscale pressure sensor shows a high sensitivity and may have potential applications in biological and biomedical engineering.

We investigate the characteristics of resonant modes in the side-coupled rectangular-ring resonator (SRR). The results show we can manipulate the resonant wavelengths of TM_a mode and TM_s mode by adjusting the outer wall width (Lx1) or the inner wall width (Lx2) of the ring resonators, and the effects of coupling distance on the full-width at half-maximum (FWHM) of resonant spectra are discussed. In sensing application, the proposed structure can work as a highly sensitive plasmonic nanosensor with a sensitivity of 1000 nm/RIU and a figure of merit (FOM) of 67. The values are comparable to periodic structures and the structures based on Fano resonance.

Perfect metamaterial absorber (MA)-based sensor applications are presented and investigated in the microwave frequency range. It is also experimentally analyzed and tested to verify the behavior of the MA. Suggested perfect MA-based sensor has a simple configuration which introduces flexibility to sense the dielectric properties of a material and the pressure of the medium. The investigated applications include pressure and density sensing. Besides, numerical simulations verify that the suggested sensor achieves good sensing capabilities for both applications. The proposed perfect MA-based sensor variations enable many potential applications in medical or food technologies.

A high-sensitivity sensor based on the resonant transmission characteristics of terahertz (THz) metamaterials was investigated, with the proposal and fabrication of rectangular bar arrays of THz metamaterials exhibiting a period of 180 $\mu \mathrm{\text{m}}$ on a 25 $\mu \mathrm{\text{m}}$ thick flexible polyimide. Varying the size of the metamaterial structure revealed that the length of the rectangular unit modulated the resonant frequency, which was verified by both experiment and simulation. The sensing characteristics upon varying the surrounding media in the sample were tested by simulation and experiment. Changing the surrounding medium from that of air to that of alcohol or oil produced resonant frequency redshifts of 80 GHz or 150 GHz, respectively, which indicates that the sensor possessed a high sensitivity of 667 GHz per unit of refractive index. Finally, the influence of the sample substrate thickness on the sensor sensitivity was investigated by simulation. It may be a reference for future sensor design.

Based on surface plasmon resonance (SPR), a novel refractive index (RI) sensor comprising a square photonic crystal fiber (PCF) is proposed to realize the detection of the annular analyte. Instead of hexagon structure, four large air-holes in a square array are introduced to enhance the sensitivity by allowing two polarization directions of the core mode to be more sensitive. The gold is used as the only plasmonic material. The design purpose is to reduce the difficulty in gold deposition and enhance the RI sensitivity. The guiding properties and the effects of the parameters on the performance of the sensor are numerically investigated by the Finite Element Method (FEM). By optimizing the structure, the sensor can exhibit remarkable sensitivity up to 7250 nm/RIU and resolution of $1.0638\times 10^{-5}$ RIU with only one plasmonic material, which is very competitive compared with the other reported externally coated and single-layer coated PCF-based SPR (PCF–SPR) sensors, to our best knowledge.

This paper studies transverse vibration of nanoscale cantilevered and bridged sensors carrying a nanoparticle. The nanoscale sensors are modelled as Euler–Bernoulli beams with surface effect and nanoparticle as a concentrated mass. Frequency equations of cantilevered and bridged beam-mass system are derived and exact resonance frequencies are calculated. An alternative Fredholm integral equation method is used to obtain an approximate explicit expression for the fundamental frequency for both cases. A comparison between the approximate and analytical results is made and the approximation accuracy is satisfactory. The influences of the residual surface stress, surface elasticity, and attached mass on the resonance frequencies and mode shapes are discussed. These results are useful to illustrate the surface phenomena and are helpful to design micro-/nano-mechanical sensors.

A novel micro-structured fiber magnetic field sensor based on magnetic fluid (MF) filling is proposed. The air hole radius in the cladding of fiber is reduced from inner layer to outer layer, and the numerical analysis is performed by the finite element method (FEM). For the $x$-pol mode, the proposed sensor has an average sensitivity of 960.61 pm/Oe, and for the $y$-pol mode, the average sensitivity can reach 884.85 pm/Oe. The sensor has the advantages of small size and high sensitivity and is competitive in magnetic field sensors.

Chiral metamaterial-based sensor is designed for sensing the change in substance ratio of chemical substances when combined with distilled water. These samples have been prepared with 10%, 50% and 90% volume fraction of methanol, ethanol, acetone, ammonia and isopropyl alcohol. The complex permittivity of the prepared samples has been measured by Agilent 85070E dielectric probe kit and compared with the simulations. In the proposed design, linear shifts with the frequency bands of more than 100 MHz are observed in different measurements. For different material sensing, pure samples have been used and their reflection coefficient measurement results have been presented. Unique side of this study is that the structure provides very clear and sensitive results and presents a new approach to the microfluidic sensor applications by using the sample holder.

In this paper, a rectangular split ring resonator-type metamaterial (MTM) sensor application is demonstrated in order to examine concrete materials. First, different types of concrete materials including carbon steel fiber with the purity ratio of 0.5%, 1% and 1.5% were prepared by using cement and water and their electrical properties were determined via Nicolson–Ross–Weir tehcnique. After that, compatible MTM-based sensor structure was designed and proposed. In order to find out the effect of the humidity on the concrete material, all samples were kept in the water pool for 28 days, the samples were then taken out and the temperature effect on the concrete materials was observed by increasing the heat up to 30${}^{\circ }$ and 70${}^{\circ }$ in the oven. The simulated resonance frequency shifts were observed at 910 MHz for concrete material sensor, 120 MHz for humidity sensor validation and 400 MHz for temperature sensor applications. By having large bandwidth in different fiber contents and validating the physical sensor applications as temperature and humidity, it was shown that the proposed study has novelty in the area of MTM-based sensor applications when it is compared with current state-of-the-art.

One of the most remarkable features of surface plasmon polaritons (SPPs) is their usefulness for sensing. In this paper, we introduce a spoof SPP sensor consisting of a transmission line with a rectangular groove array. We investigated its dispersion relation and verified the broadband transmission characteristics of its SPP mode. A sensing model with spoof SPPs has been developed and a prototype fabricated. Three analytes with different permittivity were introduced into the model and prototype — edible oil, glycerol, and water — and the results compared to those obtained with air. The results of simulation and measurement show that the sensor is extremely sensitive to the permittivity of a liquid. The results indicate that spoof SPP sensors have important potential applications in industrial processing, food safety, and environmental monitoring.

High sensitivity and response ratio of magnetoimpedance (MI) sensors have raised interest for using them in different environments for detection of weak magnetic fields of magnetic elements even though the high dependence of the MI response to the surface condition of the MI sensor has limited its application in some environments. In this study, we investigate the effects originating from the MI measurement in moisturized air. Using scanning electron microscopy (SEM) images, it is observed that the surface of an Fe-based MI sensor has become rough and granular after the presence of moisture on its surface. Results can be useful for developing MI sensors for use in different environmental conditions.

We propose a novel bio-sensor structure composed of double sided-wall Bragg gratings and dual-slot-micro-ring waveguides. The slot waveguide is a better choice to interact the bio-material under investigation with the propagating light with in the slot region. The incident light field propagates clockwise through the slot micro-ring resonator, the reflection light field propagates counterclockwise in the slot Bragg grating. By optimizing the geometric parameters of the device, the spectral response is tailored to obtain a sharp resonant peak simulated by the finite- difference time-domain (FDTD) method. The spectrum can be tuned not only by geometrically changing the couple distance in slot Bragg grating resonator, but also by dynamically altering the depth and number of the Bragg grating. Furthermore, the device is easy to yield an extinction ratio of 11 dB, a FWHM of 1.1 nm and a quality factor of $1.422\times 10^3$. The device with a small footprint can enable integration with some photonic devices on a chip and have great promising for applications including tunable sensors, slow-light devices and optical communication.

This study presented an innovative concept to develop a home-made equipment to make a 2D circular-shaped polyvinylidene fluoride (PVDF) fiber piezoelectric sensor. The fibers spun by direct-write near-field electrospinning can be collected orderly to enhance the orientation of dipole moments and the piezoelectric effect. The electrospinning system mainly consists of a high voltage power supply system, a digitally controlled $X$–$Y$ table, a solution injection system, and a rotating flat disc collector system. PVDF piezoelectric fibers were spun for the sensing element of the sensor with a diameter of $\sim$16 $\mu$m under a high electric field ($\sim 1.6\times 10^7$ V/m). The optimal parameters of electrospinning process were investigated and determined by the uniform design (UD) method in terms of the working high voltage, feeding rate, and gap distance between the needle and collector. In addition, a prototype of this newly designed rotating flat disc collector system was utilized to make 2D circular-shaped PVDF piezoelectric fibers. Besides, the newly-designed radial electrodes were used to characterize multi-directional sensing ability. The 2D circular-shaped fibers were then packaged with this radial electrodes and polyethylene terephthalate (PET) film together to measure the dynamic behavior characteristics.