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Stimulant abuse enhances dopamine release, thereby causing increased excitation. Any extent of stimulant abuse can considerably harm the user. Thus, methods of detecting stimulants must be precise, accurate, and reliable. A novel terahertz (THz) photonic crystal fiber with a Topas substrate is designed and rigorously investigated for detecting liquid amphetamine, cocaine, and ketamine. The fiber structure has a pentagonal shape and comprises circular air holes in the core and cladding spatial extents. As shown in finite element simulation, the proposed fiber yields a high relative sensitivity of approximately 80 % when any of the liquid stimulants is infiltrated in the core air holes. At 1 THz operating frequency, the proposed fiber produces a large effective mode area, negligible confinement loss, and extremely low bending and effective material losses. Other THz waveguiding properties, such as core power fraction and total loss, are also studied. Lastly, a positive and negative 2 % fabrication tolerance is set to ensure seamless potential practical realization of the fiber. [ABSTRACT FROM AUTHOR]

In this paper, we present a novel terahertz plasmonic sensing concept that utilizes a metasurface coupler to excite spoof surface plasmon polaritons (SSPs) modes on graphene strips. The dielectric metasurface unit cell consists of two silicon cylindrical rods with different radii on a quartz substrate. Vertically incident terahertz waves are deflected by the metasurface, resulting in a transverse wave-vector within the substrate that matches the SSPs wave-vector supported by the graphene strips. This enables efficient excitation of SSPs modes on the graphene strips surface, leading to absorption resonances in the range of 0.420–0.430 THz with a high quality (Q) factor of 152. We then applied this device for refractive index sensing, and the investigation results indicate that the high Q resonances in the absorption spectrum experience shifts with changes in the refractive index of the target substance, with a sensitivity of 110 GHz/RIU. The designed structure in this study exhibits significant potential for applications in trace substance absorption spectrum detection.

In this work, THz photonic crystal fiber (PCF) is used to detect the creatinine level in the blood with high sensitivity. The sensing technique depends on increasing the light interaction with the analyte infiltrated into the air holes in the fiber core region. In this regard, most of the light power should be confined through the analyte region. This will increase the relative sensitivity coefficient that is proportional to the analyte power fraction. The operation of the suggested sensor is based on studying the light–analyte interaction at different creatinine concentrations by detecting the change of the analyte power fraction against the change of the creatinine level in the blood at the THz range 0.5–1.5 THz. The effective mode index (neff), effective material loss (EML), effective modal area (Aeff), and relative sensor sensitivity are calculated using the full vectorial finite element method. Additionally, the different geometrical parameters are studied to maximize the sensor's sensitivity. The proposed THz-PCF has a 93% and 95% relative sensitivity for x- and y-polarized modes, respectively. Therefore, the suggested THz-PCF biosensor gives a promising usage in measuring the creatinine level in the blood. [ABSTRACT FROM AUTHOR]

Highly sensitive triple-band metamaterial-based biosensor for different cancer cells detection is suggested and numerically analyzed. The reported sensor has a polyimide dielectric layer which is sandwiched between gold bottom plane and top metallic patches. The analyte sample covers the metallic patch where multiple resonances occur with high absorption. The resonance frequencies depend on the optical properties of the analyte sample. Therefore, the proposed sensor can distinguish between different cancer cell types such as skin cancer, blood cancer, and Breast cancer. Full vectorial finite element method is used to study the effects of the geometrical parameters with the aim to maximize the sensor sensitivity. The suggested sensor has high sensitivity of 2050 GHz/RIU (which is much higher than those suggested in the literature) with high quality factor of 55.34 in the frequency range from 4.25 THz to 4.75 THz. Further, the proposed biosensor is a label-free, and easy for fabrication using the state-of-the-art fabrication technologies.

A terahertz plasmonic sensor based on parallel bulk Dirac semimetals (BDSs) is proposed and numerically investigated in this paper. The physical origin of the sensor is induced by the electromagnetically induced reflectance (EIR)-like effect between two identical resonators. The performance of the sensor is influenced by changing the coupling distance between the waveguide and resonator, the Fermi levels of the BDSs, and the analyte permittivity. Due to the strong field confinement of surface plasmon polaritons (SPPs) between the parallel BDSs, the saturated thickness of the analyte is low enough to ensure the reliable performance. In addition, the performance can be further improved by increasing the resonant order of the resonator, and the maximum sensitivity and FOM values can attain 120 GHz/RIU and 4.2, respectively. The proposed sensor owns potential prospects in terahertz sensing applications. [ABSTRACT FROM AUTHOR]

In this study, a Hollow-Core Anti-Resonant Fiber (HC-ARF) for gas sensing purposes with ultra-high sensitivity responses and the fiber bending effects on sensing parameters have been investigated. The findings are based on the computer simulations run in COMSOL Multiphysics (version-5.3a) software which implements Finite Element Methods (FEM). The proposed optimized structure contains a large hollow core and six thin-walled capillaries. The proposed HC-ARF-based sensor also shows very low loss profiles. The values of propagation losses (mainly confinement loss) and material absorption loss of the sensor are in the order of 10–3. The maximum relative sensitivity response is 99.95%. As the fiber core is relatively large, fundamental modes (horizontally polarized mode and vertically polarized mode) and also a few Higher Order Modes (TM01 mode, TE01 mode, HE21 (even) mode, HE21 (odd) mode, etc.) are observed. But the fiber sensor also has a good Higher Order Mode suppression ratio (HOMER) response. Along with short-distance gas sensing, it can be used for long-distance sensing as well as transmission purposes with single-mode propagation. At the end of the article, the bending effects on relative sensitivity and propagation losses have also been demonstrated in detail when the fiber is bent with various bend-radii. It is observed that bending introduces sensitivity drops. [ABSTRACT FROM AUTHOR]

In this paper, we design a novel photonic crystal fiber (PCF) chemical sensor wherein the cladding structure is made of a steering-wheel (SW)-shaped large non-circular air-hole. In this PCF, we introduce the rectangular air holes in the core and they are filled with any one of the analytes, namely, water, ethanol and benzene. We use the full-vectorial finite element method (FEM) to optimize the structural parameters. Based on the numerical results, we find that the proposed sensor exhibits a high relative sensitivity of 92.8%, 93.2% and 93.4% for water, ethanol and benzene, respectively, at THz. Furthermore, we study the various optical characteristics of the proposed PCF, namely, birefringence, effective area and numerical aperture (NA). [ABSTRACT FROM AUTHOR]

An unstable bilirubin level in the human blood causes many dangerous health problems, such as jaundice, coronary artery disease, ulcerative colitis, and brain lesions. Therefore, the accurate and early detection of bilirubin concentrations in the blood is mandatory. In this work, a highly sensitive biosensor based on photonic crystal fiber (PCF) for monitoring bilirubin levels is proposed and analyzed. The sensor parameters, including relative sensitivity, effective mode area, confinement loss, and effective material loss, are calculated. The geometrical parameters are studied, and a modal analysis of the suggested sensor is carried out using the full-vectorial finite element method (FEM). The fabrication tolerance of the geometrical parameters is also studied to ensure the fabrication feasibility of the reported design. High sensitivities of 95% and 98% are obtained for the x-polarized and y-polarized modes, respectively. Furthermore, a small material loss of 0.00193 cm−1, a small confinement loss of 2.03 × 10−14 dB/cm, and a large effective mode area of 0.046 mm2 are achieved for the y-polarized mode. It is believed that the presented sensor will be helpful in health care and in the early detection of bilirubin levels in the blood. [ABSTRACT FROM AUTHOR]

Localized Surface Plasmon Resonances (LSPR) based on highly doped semiconductors microstructures, such as antennas, can be engineered to exhibit resonant features at THz frequencies. In this work, we demonstrate plasmonic antennas with increased quality factor LSPRs from Fano coupling to dark modes. We also discuss the advances in the biofunctionalization of n-doped Ge antennas for specific protein immobilization and cell interfacing. Finally, albumin biolayers with a thickness of a few hundred nanometers are used to demonstrate the performance of the fano-coupled n-Ge antennas as sensors. A resonant change of over 10% in transmission, due to the presence of the biolayer, can be detected within a bandwidth of only 20 GHz. [ABSTRACT FROM AUTHOR]

Since the fingerprints of numerous crucial biologic materials can be identified by terahertz (THz) spectroscopy, THz sensing have become an important approach of biological and medical detections. Particularly, benefit from the excellent capability of metasurface, strong interactions between the metasurface and THz waves can be realized, thus the THz sensing with high sensitivity becomes reality. However, the common configuration of metasurface-based THz sensor is composed of metallic subwavelength structure. Due to the inherent resistive loss of metal, it is still a great challenge to further enhance the quality factor (Q-factor) of resonance and sensitivity of THz sensor. In this work, we designed an all-dielectric metasurface with high Q-factor for highly sensitive THz sensors. The metasurface is a windmill-like structure consisting of four cuboids, and every adjacent two cuboids are arranged alternately vertically and horizontally. The transmission spectrum of metasurface exhibits four polarization-independent and strong resonance peaks with high Q-factor in 0.1–2.5 THz, and all of them show high sensitivity related to ambient refractive index. The transmitted structure and polarization-independent resonances can relief the difficulty of measurement. We believe these studies will lay the theoretical and technical foundation for the design of high-sensitivity terahertz sensing.

We present a terahertz graphene metamaterial sensor based on dual polarized plasmon-induced transparency caused by the destructive interference between the bright mode and the two quasi-dark modes in the terahertz (THz) band. In addition, the sensor can achieve frequency modulation by adjusting the polarization direction of the THz wave and the Fermi level of graphene. Importantly, the sensor also has high sensitivity at each polarization direction, and the sensitivity of dual transmission peaks with x-polarization direction can reach about 1.1 THz/RIU (Refractive Index Unit). Furthermore, the theoretical results of the proposed three-particle model are in good agreement with the simulated transmission spectra. The sensor is also insensitive to the change of incident angles, and the transmission spectra of the sensor can remain roughly unchanged with the incident angle less than 60°, which is beneficial to the high-speed and high-sensitivity detection in a complex environment. Therefore, the proposed graphene metamaterial sensor exhibits numerous potential applications in THz biochemical sensing. [ABSTRACT FROM AUTHOR]

Scheme Schematic diagram of (a) the measurement system, (b) the sensing principle, and (c) the transmission spectra of TE and TM waves. [Display omitted] • A terahertz polarization-sensitive metamaterial sensor for the identification of chiral Ibuprofen is designed and fabricated. • The qualitative and quantitative identification of the ibuprofen enantiomers is achieved by their different sensitivities. • The chiral parameters of Ibuprofen molecules are extracted for simulation. • The recognition principle is verified by both the simulation and measurement. Chirality plays an important role in medicine, biology, and chemistry. Molecules of different chirality could display dramatically different medical effects, pharmacological activities, and physiological impacts. Ibuprofen is an important anti-inflammatory drug in clinics. The anti-inflammatory effect is almost solely attributed to the (S)-(+)-Ibuprofen, while its enantiomer (R)-(-)-Ibuprofen plays a negative effect on increasing the metabolic burden. In this work, a terahertz (THz) polarization-sensitive metasurface sensor is proposed for qualitative and quantitative identification of the chiral Ibuprofen. The chirality parameters of Ibuprofen are extracted from the circular-polarized transmission coefficients. The parameters are further used to simulate the coupling mechanism between the Ibuprofen and the sensor to explain the principle of recognition. The sensitivities of (R)-(-)-Ibuprofen and (S)-(+)-Ibuprofen are found to be 1.5 THz/(mg/L) and 1.8 THz/(mg/L) for the TM polarization, respectively, and 1.7 THz/(mg/L) and 2.1 THz/(mg/L) for the TE polarization, respectively. The difference enables the chirality identification according to the different frequency shift at the same concentration. The exceptional specificity and sensitivity provide a new avenue for chiral molecular recognition. [ABSTRACT FROM AUTHOR]

The surface conductivity of graphene can be precisely adjusted to approach the sigma-near-zero (SNZ) point by manipulating the quasi-Fermi level via optical pumping. This capability holds great promise for advanced graphene metamaterials-based terahertz (THz) sensors. In this study, we present a rational approach for designing two distinct types of patterned graphene THz sensors, each tailored to specific requirements regarding the quasi-Fermi level, resonance frequency, and Q-factor. The first sensor type operates in transmission mode and is well-suited for analytes with refractive indices ranging from 1 to 2. With a fixed analyte thickness of 40 nm, it achieves a frequency sensitivity of 1.036 THz/RIU, a Q-factor between 230 and 360, and a figure of merit (FOM) ranging from 51/RIU to 74/RIU. The second sensor type operates in reflection mode and is optimized for detecting biomolecules with refractive indices between 1.5 and 1.7. It exhibits a significant frequency shift of approximately 0.16 THz and a reflectance variation as high as 0.941 when the bio-analyte thickness is 100 nm. Numerical simulations of the sensing performances confirm the potential of these designed sensors for applications in detecting thin dielectric films and biosensing. [Display omitted] • Graphene metamaterials terahertz sensors based on sigma-near-zero (SNZ) point under optical pumping were proposed. • Relationship between the quasi-Fermi level and surface conductivity of graphene was discussed. • With a 40 nm analyte, TYPE I sensor reaches 1.036 THz/RIU sensitivity, with FOM from 51/RIU to 74/RIU. • TYPE II sensor shows significant 0.891 reflectance change with a 100 nm thick DNA molecule. [ABSTRACT FROM AUTHOR]

Terahertz sensing is one of the most promising methods for label-free and noninvasive detection of refractive index changes. However, the figure of merit (FOM) of terahertz sensors in practical applications has been low. In this paper, a metamaterial sensor based on simple stacking of gold and silicon dioxide is proposed, through whose structure not only narrow-band absorption with five absorption peaks is realized, but FOM is also improved to 1792. The excellent sensing performance and the mature manufacturing technology of this kind of structure provide a platform for the design of multi-band photodetectors and high-sensitivity sensors. [ABSTRACT FROM AUTHOR]

In recent years, novel terahertz (THz) metamaterial sensors are being actively explored by the research community. These structures absorb the incident THz waves, and their absorption spectra shift with the change in the physical parameter. In this paper, a terahertz metamaterial absorber is proposed for sensing both the temperature and the refractive index of the surrounding medium. The unit cell of the proposed structure consists of a concentric hexagonal ring resonator (CHRR) on an indium antimonide (InSb) substrate. The proposed sensor detects the changes in the refractive index with a sensitivity of 1.045 THz/RIU at room temperature, and it can detect the temperature of the surrounding medium having unity refractive index, with a sensitivity of 3.71 GHz/K. It also offers a near-unity absorption of 99.93% at 1.93 THz when the temperature is 300 K, and the refractive index of the surrounding medium is unity. Due to the simple design and dual-sensing capabilities, the proposed sensor can have profound bio-medical applications. [ABSTRACT FROM AUTHOR]

A novel design of a nearly perfect metamaterial absorber based on square split-ring resonators for terahertz sensing applications is proposed and analyzed. The design in this report is simulated and analyzed by using standard numerical simulation software. Magnetic and electric resonant field enhancement in the impedance matched absorber cavity enables stronger interaction with the dielectric analyte. The proposed structure is based on the simultaneous increase in the electromagnetic field and the surface current distribution at the resonance frequency. An absorptivity of 99% is achieved at 0.53 THz with a narrow resonance peak and a Q-factor of 44.17. At a fixed analyte thickness, the resonance frequency is sensitive to the refractive index of the surrounding medium. The influence of the thickness of the covering sample on the sensitivity and absorption coefficient of the absorber is comprehensively analyzed, and the reported design can be used as a refractive index sensor with a high sensitivity of 126.0 GHz/RIU and a figure of merit of 10.5 in the refractive index range from 1.0 to 2.0 at an analyte thickness of $15.0~\mu \text{m}$ . The results show that the sensor has high sensitivity to the analyte covering it. The sensor not only exhibits good sensitivity to thin analytes but also shows high sensitivity to analytes more than $10~\mu \text{m}$ thick in the terahertz low frequency band. Specifically, the sensitivity changes rapidly when the thickness of the sample changes in the range of 0- $6~\mu \text{m}$ , but slowly in the range of 6- $16~\mu \text{m}$ . In general, the response of the resonance frequency to changes in the refractive index of the sample becomes more sensitive as the thickness of the sample is increased from 0 to $16~\mu \text{m}$ . The reported terahertz sensor of a metamaterial absorber has potential applications in biomedical sensing and trace detection of substances.

An unstable bilirubin level in the human blood causes many dangerous health problems, such as jaundice, coronary artery disease, ulcerative colitis, and brain lesions. Therefore, the accurate and early detection of bilirubin concentrations in the blood is mandatory. In this work, a highly sensitive biosensor based on photonic crystal fiber (PCF) for monitoring bilirubin levels is proposed and analyzed. The sensor parameters, including relative sensitivity, effective mode area, confinement loss, and effective material loss, are calculated. The geometrical parameters are studied, and a modal analysis of the suggested sensor is carried out using the full-vectorial finite element method (FEM). The fabrication tolerance of the geometrical parameters is also studied to ensure the fabrication feasibility of the reported design. High sensitivities of 95% and 98% are obtained for the x-polarized and y-polarized modes, respectively. Furthermore, a small material loss of 0.00193 cm−1, a small confinement loss of 2.03 × 10−14 dB/cm, and a large effective mode area of 0.046 mm2 are achieved for the y-polarized mode. It is believed that the presented sensor will be helpful in health care and in the early detection of bilirubin levels in the blood.

In this article, a highly sensitive hollow-core photonic crystal fiber (PCF) is proposed and numerically investigated for the application of different chemical identification in the terahertz regime. Commercially available COMSOL Multiphysics software is used to evaluate the sensing and propagation characteristics of the proposed sensor. The numerical analysis informs that extremely high relative sensitivity around 98% can be achieved from the sensor at optimum structural condition. Moreover, at optimum structural dimension and operating frequency significantly low effective material loss (

Featured Application: An elementary method for MMW detection by GDD-LTF combination as single pixel for focal plane arrays. Performance enhancement of a very inexpensive millimeter-wave (MMW)/terahertz (THz) sensor for MMW/THz imaging systems is experimentally demonstrated in this study. The MMW sensor is composed of a glow discharge detector (GDD) and a light-to-frequency (LTF) converter combination. The experimental results given in this study show an improvement in the performance parameters of the detector element, such as the minimum detectable signal, as well as the signal to noise ratio (SNR) and the noise equivalent power (NEP), when a NIR long-pass filter was inserted between the GDD and the LTF combination. A detailed derivation of the NEP of this unique sensor is presented in this work. Based on this derivation and the experimental measurements, the NEP value was calculated. [ABSTRACT FROM AUTHOR]

A Mach–Zehnder interferometric sensor with a dual-core photonic crystal fiber structure with defect cores is designed, which is used to detect the content of crude refined or refined cottonseed oil in olive oil in terahertz band. The radius of the central air holes and the defect cores in the dual-core fiber structure are optimized, so that the interval between adjacent transmission peaks of mixed oil doped with different contents of crude refined or refined cottonseed oil is as large as possible, the transmission spectrum width is as small as possible, and the transmission depth is as deep as possible. The linear relationship between the resonance frequency and the content of doped cottonseed oil is obtained, and the average sensitivities are 5.976 × 10−4 THz/% and 2.212 × 10−4 THz/%. [ABSTRACT FROM AUTHOR]

[Display omitted] • A terahertz EIT metamaterial sensor is designed to distinguish chiral lactic acids. • The qualitative and quantitative identification of the chiral lactic acids is fixed by the frequency offset. • The chiral parameters of chiral lactic acids are extracted by theory analysis and used to analyze the coupling between lactic acids and the sensor. • The principle of the recognition of chiral lactic acids by metamaterial sensor is proved both in simulation and measurement. Lactic acid (LA) plays an essential role as a biochemical indicator in life, the detection and identification of chiral LAs have a wide range of applications. A method to detect chiral LA enantiomers (D-LA and L-LA) by using an achiral terahertz (THz) electromagnetically induced transparency (EIT) metasurface sensor is proposed. The chiral enantiomers of LA samples are measured, and the chiral parameters are extracted from the circular polarization transmission coefficients, theoretical analysis and experiment are combined to verify the principle of the achiral sensor to recognize the chiral LAs. The quantitative and qualitative recognition of LA enantiomers can be realized by the significant difference in the transmission spectrum at the characteristic frequency. The limit of detection reaches 0.01 μg/mL, and the sensing sensitivity of L-LA, D-LA, and DL-LA (achiral, racemate) is 0.28 THz/(mg/mL), 0.07 THz/(mg/mL) and 0.13 THz/(mg/mL), respectively. The proposed method can be further extended to a wide range of chiral bio-samples, showing a great potential in biomedical applications, food safety inspection, and environmental monitoring. [ABSTRACT FROM AUTHOR]

In this paper, we present a novel mechanism based on metasurface coupling to efficiently excite SSP on graphene strips, and further investigate its sensing applications. In comparison with previous studies, this work has several advantages and innovative points: (1) The metasurface unit cell structure consists of two cylindrical dielectric gratings with different sizes on the substrate, which can defect vertically incident terahertz waves at specific angles. This ensures that the wave-vector of the deflected wave couples with the graphene strips, thereby exciting SSPs. (2) Replacing the commonly used metal groove arrays with graphene strips to excite SSPs offers the advantage of reducing device volume and enhancing the system integration. (3) This structure enables highly sensitive sensing of different target analytes, with a high sensitivity of 110 GHz/RIU. [Display omitted] • Efficient excitation of spoof surface plasmon polaritons (SSPs) modes can be realized on the graphene strip surface. • SSP can be excited using vertically incident terahertz waves without the need for specific incident angles. • A high sensitivity of 110 GHz/RIU can be achieved based on the designed structure. In this paper, we present a novel terahertz plasmonic sensing concept that utilizes a metasurface coupler to excite spoof surface plasmon polaritons (SSPs) modes on graphene strips. The dielectric metasurface unit cell consists of two silicon cylindrical rods with different radii on a quartz substrate. Vertically incident terahertz waves are deflected by the metasurface, resulting in a transverse wave-vector within the substrate that matches the SSPs wave-vector supported by the graphene strips. This enables efficient excitation of SSPs modes on the graphene strips surface, leading to absorption resonances in the range of 0.420–0.430 THz with a high quality (Q) factor of 152. We then applied this device for refractive index sensing, and the investigation results indicate that the high Q resonances in the absorption spectrum experience shifts with changes in the refractive index of the target substance, with a sensitivity of 110 GHz/RIU. The designed structure in this study exhibits significant potential for applications in trace substance absorption spectrum detection. [ABSTRACT FROM AUTHOR]

Ultra-high sensitivity sensor has significant application for micro-nano optical devices in terahertz. Here, we propose a simple graphene metasurface, which can achieve obvious graphene plasmon-induced transparency (PIT) phenomenon. We can find that PIT, reflectivity, and absorbance can be effectively tuned by the Fermi level. Moreover, the finite-different time-domain (FDTD) numerical results are well agreement with the coupled mode theory (CMT) results. Interestingly, an ultra-high sensitivity sensor performance based on tunable PIT in terahertz bands can be realized in our proposed metasurface, the sensitivity and Figure of merit (FOM) can reach up to 1.7745 THz/RIU and 23.61, respectively. Hence, these results can provide theoretical guidance for terahertz dynamic integrated photonic devices.

This paper presents an analysis on a hexagonal cladding with a rotated-hexa core in photonic crystal fiber (H-PCF) based optical sensor formation with simultaneously minimal confinement loss and higher sensitivity for chemical sensing functions. The numerical assessment of the designed structure is achieved with the procedure of finite element method (FEM) and perfectly matched layers (PML) boundary condition in the comsol multiphysics software tool. As per FEM numerical analysis, the proposed PCF sensor presents the extreme relative sensitivity at 81.46%, 82.26% and 79.22% for three chemicals at 1 terahertz (THz) such as Ethanol (n = 1.354), Benzene (n = 1.366) and Water (n = 1.330), respectively. Moreover, at 1 terahertz (THz) the low confinement losses are 5.85 × 10−08, 6.07 × 10−08 and 5.84 × 10−08 dB/m for similar three chemicals. The effective area, the effective mode index and the total power fraction are also elaborately explained. Furthermore, we hope that our proposed structure can be operated intensely in the field of biomedical, bio-sensing experiments and industrial applications in terahertz (THz) waveguide technology.

The article presents a rectangular core photonic crystal fiber (RCPCF) structure to detect the major components of blood. We used Topas as bulk fiber material for its extraordinary properties in the terahertz (THz) frequency region and the investigation is accomplished from 0.8 THz to 1.8 THz. The RCPCF sensor provides enriched relative sensitivity (almost 94.38%, 93.72%, 92.94%, 92.14%, and 90.8% for red blood cells (RBCs), hemoglobin (HB), white blood cells (WBCs), plasma, and water respectively at 1.8 THz frequency) with negligible confinement loss (only 2.75 × 10−13 cm−1, 2.87 × 10−13 cm−1, 3.06 × 10−13 cm−1, 3.27 × 10−13 cm−1, and 3.48 × 10−13 cm−1 for RBCs, HB, WBCs, plasma, and water respectively at 1.8 THz frequency). Besides, the proposed sensor offers a large effective area (approximately 3.382 × 105 μm2, 3.436 × 105 μm2, 3.493 × 105 μm2, 3.551 × 105 μm2, and 3.613 × 105 μm2for RBCs, HB, WBCs, plasma, and water respectivelyat 1.8 THz frequency). Also, this RCPCF sensor structure can be fabricated by utilizing the current fabrication techniques.

Low density lipoprotein (LDL) cholesterol is the leading cause of heart diseases, peripheral artery diseases, and stroke. An accurate, flexible, and efficient detection process is very urgent for identifying the cholesterol. In this context, an octagonal shaped hollow core with eight head star cladding structured photonic crystal fiber (PCF) has been proposed in this paper for cholesterol sensing in liquid analytes (human blood, cocking oil, liquid foods, etc.). The sensing performances of proposed PCF are evaluated through the COMSOL multiphysics software where the finite element method (FEM) has been used as a solver. The numerical investigation of the presented PCF structure exhibits highly sensitive characteristics for cholesterol detection in liquid samples and the proportion is 98.75% at 2.2 THz frequency. It also reveals negligible confinement loss of 3.14 × 10−20 cm−1 and low effective material loss of 0.0008 cm−1 at the same operating point. Furthermore, the commercial development feasibility of designed PCF in the existing manufacturing environment and other crucial optical properties such as effective area, dispersion, numerical aperture are also discussed in detail.

Surface plasmon polaritons (SPPs) with noble metals lose the capacity to confine the optical field in the long wavelength region, while surface phonon polaritons (SPhPs) in polar dielectrics show greater ability to achieve high local field, sub-diffraction confinement, and low optical losses in the mid-IR to THz spectral ranges. We propose a hybrid structure based on SPhPs mode with CsI and SPPs mode with graphene in THz spectral ranges. The characteristics of the coupling of these two modes have been analyzed and strong coupling has been confirmed theoretically. The results are applied to an intensity-interrogated terahertz sensor; the highest imaging sensitivity as high as 954 RIU-1 and a large figure of merit of 74 740 RIU-1 have been demonstrated. The results could find potential applications in gas detection in the THz band.

We propose a plasmon induced tunable metasurface for multiband superabsorption and terahertz sensing. It consists of a graphene sheet that facilitates perfect absorption where the graphene pattern at the top layer creates an enhanced evanescent wave that facilitates the metasurface to work as a sensor. The modelling and numerical analysis are carried out using Finite Element Method (FEM) based software, CST microwave studio where a genetic algorithm (GA) is used to optimize the geometric parameters, and metasurface tunability is achieved via an external gate voltage on the graphene. By exploiting graphene's tunable properties we demonstrate a multiband superabsorption spectra having a maximum absorption of 99.7% in a frequency range of 0.1–2.0 THz that also maintain unique optical performance over a wide incidence angle. Further results show how the superabsorber can be used as a sensor, where the resonance frequency shifts with the refractive index of the surrounding environment. Image 1 [ABSTRACT FROM AUTHOR]

Performance enhancement of a very inexpensive millimeter-wave (MMW)/terahertz (THz) sensor for MMW/THz imaging systems is experimentally demonstrated in this study. The MMW sensor is composed of a glow discharge detector (GDD) and a light-to-frequency (LTF) converter combination. The experimental results given in this study show an improvement in the performance parameters of the detector element, such as the minimum detectable signal, as well as the signal to noise ratio (SNR) and the noise equivalent power (NEP), when a NIR long-pass filter was inserted between the GDD and the LTF combination. A detailed derivation of the NEP of this unique sensor is presented in this work. Based on this derivation and the experimental measurements, the NEP value was calculated.

In this paper, a novel partial type-b crystalline core with more compact cladding in hexagonal packing photonic crystal fiber (CC-PCF)-based optical sensor has been proposed for sensing different blood components. This fiber has investigated in terahertz (THz) region from 1.5 to 3.50 THz, intending to superior relative sensitivity with low confinement loss (CL). Circular air holes have been employed in the formation of the partial type-b crystalline core in a symmetric manner. A significant relative sensitivity response of 80.93%, 80.56%, 80.13%, 79.91%, and 79.39% are achieved for the targeted analytes such as RBCs, hemoglobin, WBCs, plasma, and water at frequency ${f} = 1.5$ THz. In X-polarization mode, a negligible CL of $1.23\times 10^{-11}$ dB/m, $8.63\times 10^{-12}$ dB/m, $4.93\times 10^{-12}$ dB/m, $2.93\times 10^{-12}$ dB/m, and $1.13\times 10^{-12}$ dB/m are also gained, respectively, for same analytes and at same THz frequency. Moreover, effective area (Aeff), V-Parameter ($V_{\mathrm{ eff}}$), dispersion ($\beta _{2}$), spot size ($W_{\mathrm{ eff}}$), and beam divergence ($\theta $) have been determined over the investigated region. The improved outcomes are anticipated that the proposed CC-PCF sensor will be opened a new epoch in biomedical sensing purposes. [ABSTRACT FROM AUTHOR]

Surface plasmon polaritons (SPPs) with noble metals lose the capacity to confine the optical field in the long wavelength region, while surface phonon polaritons (SPhPs) in polar dielectrics show greater ability to achieve high local field, sub-diffraction confinement, and low optical losses in the mid-IR to THz spectral ranges. We propose a hybrid structure based on SPhPs mode with CsI and SPPs mode with graphene in THz spectral ranges. The characteristics of the coupling of these two modes have been analyzed and strong coupling has been confirmed theoretically. The results are applied to an intensity-interrogated terahertz sensor; the highest imaging sensitivity as high as 954 RIU−1 and a large figure of merit of 74 740 RIU−1 have been demonstrated. The results could find potential applications in gas detection in the THz band. [ABSTRACT FROM AUTHOR]

We present a polarization-insensitive dual plasmon-induced transparency (PIT) sensor based on graphene metamaterial in the terahertz band. The double PIT window results from the coupling of the bright mode and quasi-dark modes, and the theoretical results fit well with the simulated results by using the three-particle coupling system. The sensor exhibits excellent sensing performance with sensitivity and the figure of merit (FOM) of the two transmission peaks can reach 1.3 THz/RIU and 6.259 RIU−1. The sensor is insensitive to the changes in the polarization angle, and the transmission spectra of the sensor remain essentially constant in two polarization directions for incident angles less than 60°, which reduces the impact of the experimental environment on the results. The resonant frequency of the PIT window can be turned by adjusting the Fermi level of graphene. The sensor proposed in this paper is an active advance in improving the reliability of analyte detection results. • A refractive index sensor based on graphene metamaterial is proposed. • The dual plasmon-induced transparency (PIT) is excited and applied to highly sensitive sensing in the terahertz band. • The sensitivity and the figure of merit (FOM) of the dual transmission peaks can reach 1.3 THz/RIU and 6.259 RIU−1. • The metamaterial for terahertz PIT is insensitive to polarization wave angle. [ABSTRACT FROM AUTHOR]

In this paper, we propose a high-sensitivity optical sensor at terahertz frequencies based on a composite structure containing a one-dimensional photonic crystal (1D PC) coated with a layer of monolayer graphene. Between the 1D PC and the graphene there is a sensing medium. This high-sensitivity phenomenon originates from the excitation of optical resonance between the graphene and the 1D PC. The proposed sensor is highly sensitive to the Fermi energy of graphene, the thickness and refractive index of the sensing medium, and the number of graphene layers. By selecting appropriate parameters, the maximum sensitivity ( 407.36 ∘ / RIU ) is obtained. We believe the proposed configuration is promising for fabricating graphene-based biosensor- or gas-sensor devices and other related applications in the terahertz band.

Cyanide (CN) is a highly toxic chemical agent that is considered extremely harmful to humans. Considering the level of toxicity and harmfulness to human it is important to have an efficient and flexible detection method of CN. Based on this requirement, we propose a photonic crystal fiber (PCF) based terahertz sensor for the detection of CN. In a Zeonex substrate we use symmetrical and asymmetrical core structures inside a suspension type cladding. The possible sensor architecture and methodology of sensing is also addressed. The fabrication of the proposed sensors is feasible employing existing fabrication technology. [ABSTRACT FROM AUTHOR]

A Terahertz protein sensing concept based on subwavelength Ge resonators is presented. Ge bowtie resonators, compatible with CMOS fabrication technology, have been designed and characterized with a resonance frequency of 0.5 THz and calculated local intensity enhancement of 10.000. Selective biofunctionalization of Ge resonators on Si wafer was achieved in one step using lipoic acid-HaloTag ligand (LA-HTL) for biofunctionalization and passivation. The results lay the foundation for future investigation of protein tertiary structure and the dynamics of protein hydration shell in response to protein conformation changes. [ABSTRACT FROM AUTHOR]

We propose a graphene-dielectric hybrid metamaterial (GDHM) that supports the quasi-bound state in the continuum (BIC). By rotating one of the LiTaO 3 bars, the quasi-BIC resonance with high quality (Q) factor can be excited. The analysis of multipole decomposition confirms that electric quadrupole dominates this resonance. Moreover, the Q factor of quasi-BIC significantly decreases as the chemical potential of graphene increases, and the tuning transmission is achieved by changing the chemical potential of graphene. A low-energy THz-wave modulator is designed based on this scheme, and the designed modulator shows 97% modulation depth with a chemical potential shift of only 16 meV. In addition, the sensing performance of GDHM is investigated, and it is found that the sensitivity can be controlled by chemical potential. The minimum value of the sensing sensitivity can reach up to 308.91 GHz/RIU, and the maximum figure of merit (FOM) is 257.43 RIU−1. When the losses of LiTaO 3 are considered. The modulation depth, sensitivity, and maximum FOM achieve 90%, 309.54 GHz/RIU, and 121.39 RIU−1, respectively. This work provides a valuable reference for designing dynamic optical modulators and sensors in the THz region. • The graphene dielectric hybrid-metamaterial can achieve the quasi-bound state in the continuum resonant modes. • The graphene-dielectric hybrid metamaterial can be used for optical modulation and refractive index sensing. • The transmission spectra can be dynamically controlled by the chemical potential of graphene. [ABSTRACT FROM AUTHOR]

We present a design of electromagnetically induced transparency operating in terahertz regime based on near-field coupling between a metal strip and split ring resonators (SRR) made of superconducting film. When the SRR work in superconducting state, the transparency window can be thermally controlled. A pair of SRR has been introduced as the dark mode to enhance the coupling, which results in a broadening and high transmittance of near unity of the transparency window. These results may lead to potential applications in tunable terahertz devices, slowing light, and sensing technology. [ABSTRACT FROM AUTHOR]

Recently, resonance sensing based on the terahertz metamaterials has attracted considerable attentions due to the enhancement and confinement of local fields in microsize gap of resonant structures. Here, a high-sensitive dual-band sensor based on the complementary terahertz metamaterial composed of microsize circular ring gap array is proposed to detect the thickness and refractive index of thin film. The structure of this sensor is polarization insensitive dual-band operating mode and can be easily filled by the biomolecules. Simulation results demonstrate that the dual-band resonances are very sensitive to the dielectric variation, and the sensitivity levels of 99 GHz/refractive index unit (RIU) for modef1and 242 GHz/RIU for modef2are obtained and further enhanced by optimizing structures. Therefore, this structure can exhibit promising application for monitoring thin film thickness. [ABSTRACT FROM AUTHOR]

Organic/inorganic hybrid conductive organohydrogel prepared by rapid room-temperature polymerization strategy shows good stretchability, adhesion, self-healing, environmental stability, and achieves terahertz radiation shielding and terahertz displacement sensing. [Display omitted] • Conductive organohydrogel is prepared by rapid room-temperature polymerization. • Organohydrogel has excellent stretchability, self-healing and adhesion. • Organohydrogel exhibits absorption-dominated terahertz shielding performance. • Its terahertz responsive signal is linearly dependent on tensile strain. • Organohydrogel is assembled into strain sensor and terahertz displacement sensor. Conductive organohydrogel with better environmental stability than conductive hydrogel, has more promising applications in the fields of soft robots, wearable electronics and electromagnetic interference shielding. However, the current organohydrogels still face the trade-off dilemma between environmental stability and conductivity, and their preparation is usually time-consuming and complicated. Here, we report a room-temperature rapid polymerization strategy to prepare conductive organohydrogel using PEDOT:PSS and MXene nanosheets as conductive fillers as well as cross-linking sites, which is applicable to a variety of binary solvent systems. The synergistic effect between PEDOT:PSS and MXene nanosheets activates the formation of abundant hydrogen bonds, chelation interaction and electrostatic interaction between different components, thus significantly shortening the polymerization time from several hours to less than five minutes. Meanwhile, the organic–inorganic hybrid network constructs efficient conductive paths and strengthens the mechanical properties. Over a wide temperature range (−18 to 70 °C), this composite organohydrogel shows excellent stretchability, self-healing, adhesion, environmental stability. More interestingly, the organohydrogel with reasonably designed binary solvent system and conductive network achieves absorption-dominated shielding performance and wireless displacement sensing in the frequency of 2–10 terahertz. The revealed contributions of binary solvent system and conductive network to the absorption and reflection of terahertz waves are conducive to promote the development of organohydrogel-based terahertz responsive materials. [ABSTRACT FROM AUTHOR]

Plasmonic devices based on split ring resonator (SRR) of ultra-low-loss performance (high Q-factor) are significant for a broad range of applications and studies. However, due to the inevitable ohmic and radiation losses, it has always been a challenge to design various types of plasmonic devices with high Q-factor. Here, we theoretically present a unique E-shaped SRR that can destructively suppress dipole radiation by adjusting the grating-arm position to produce asymmetry. Meanwhile, the proposed terahertz metasurface structure formed by periodically arranged E-shaped SRRs can give rise to high Q-factor resonances. The results show that an ultra-high Q-factor beyond 104 can be obtained in the terahertz frequency region by optimizing the structural parameters. The sensing characteristics of such a metasurface-based structure served as a refractive-index sensor are investigated as well, demonstrating its advantage of high sensitivity. Unique radiation properties of the metasurface-based structure enable it to use in the design of novel low-loss, ultra-sensitive sensors, narrowband filters, and other plasmonic devices. • We theoretically present a metasurface structure that can obtain ultra-high Q-factor and modulation depth at a high level. • The deviation of the grating-arm can adjust the modulation depth and q factor. • We had access to the optimal geometric configuration with an ultra-high Q-factor beyond 104 for the proposed metasurface structure. • The result shows that the design of this metasurface sensor we proposed can guarantee the high sensitivity (734GHz/RIU). [ABSTRACT FROM AUTHOR]

In this paper, we theoretically propose a highly sensitive terahertz sensor based on a graphene metamaterial absorber in the THz region. The numerical analysis of the proposed sensor is numerically evaluated using the finite element method. The numerical results agree well with the analytical results of the coupled mode theory. Furthermore, the proposed sensor has no polarization sensitivity. For refractive index sensing, the simulation results show that sensing. The simulation results show that the sensor has the maximum sensitivity, quality factor, and figure of merit of 2.372 THz/RIU, 179.95, and 64.62 RIU − 1 , respectively. As a result, the proposed high performance sensor can be useful in biological diagnosis and environmental monitoring. • The sensitivity of the proposed sensor is completely independent of the polarization angle in the THz region. • The proposed sensor has high sensitivity, quality factor, and figure of merit. • The proposed graphene metamaterial absorber is polarization insensitive. [ABSTRACT FROM AUTHOR]

Spatial overlap between the electromagnetic fields and the analytes is a key factor for strong light-matter interaction leading to high sensitivity for label-free refractive index sensing. Usually, the overlap and therefore the sensitivity are limited by either the localized near field of plasmonic antennas or the decayed resonant mode outside the cavity applied to monitor the refractive index variation. In this paper, by constructing a metal microstructure array-dielectric-metal (MDM) structure, a novel metamaterial absorber integrated microfluidic (MAIM) sensor is proposed and demonstrated in terahertz (THz) range, where the dielectric layer of the MDM structure is hollow and acts as the microfluidic channel. Tuning the electromagnetic parameters of metamaterial absorber, greatly confined electromagnetic fields can be obtained in the channel resulting in significantly enhanced interaction between the analytes and the THz wave. A high sensitivity of 3.5 THz/RIU is predicted. The experimental results of devices working around 1 THz agree with the simulation ones well. The proposed idea to integrate metamaterial and microfluid with a large light-matter interaction can be extended to other frequency regions and has promising applications in matter detection and biosensing. [ABSTRACT FROM AUTHOR]

• A metamaterial-free and flexible THz sensor based on the N -CNTs was developed. • It realized high sensitivity detection and visible to the naked eye assay for NRIs. • It exhibited high sensitivity, simplicity, low cytotoxicity and good biocompatibility. • It was combined with smartphone-integrated sensing platform for real time analysis. • It was used potential applications for food safety and environment monitoring. With the rapid advances in terahertz (THz) spectroscopy, metamaterial-free THz sensors have been of importance due to efficient cost, high sensitivity and overcoming the limited tunability of the optical constants of metals. Here, a metamaterial-free and flexible THz sensor based on nitrogen-doping carbon nanotubes (N -CNTs) coupled with signal-enhancing Au NPs was proposed for detecting nereistoxin-related insecticides (NRIs). Sensitivity and selectivity for NRIs detection have been realized over the range of 3.3–100 μg/L with good linear fitting (R2 ≥ 0.9003) and LOD was 1.33 μg/L. Accuracy was validated by the recovery rates of 105.87–109.75% of NRI in spiked food-matrix sample. These results indicated the developed signal-enhancing THz method, validated by LC-MS/MS, exhibited high sensitivity and simplicity detection, which has noteworthy potential for applications in food safety and environment monitoring. [ABSTRACT FROM AUTHOR]