Terahertz Sensors Research Articles

Detection of carbendazim in oranges with metal grating integrated microfluidic sensor in terahertz

A novel microfluidic metal grating integrated terahertz sensor has been designed, which is composed of a metal microstructure array-dielectric layer-metal layer, where the dielectric layer is the microfluidic channel carrying the analyte. By adjusting the structural parameters of the metal grating sensor, a highly confined electromagnetic field can be obtained in the microfluidic channel, thereby significantly enhancing the interaction between the analyte and the terahertz wave and improving the terahertz detection sensitivity. The metal grating described in this paper is composed of an array of square holes, which is manufactured using laser micromachining technology, so that the measurement method is simplified and improved. The results show that the addition of different concentrations of carbendazim solution resulted in a redshift of the overall spectrum, with the highest sensitivity reaching 8.773 GHz/mg L−1, which is about eight times more sensitive than the traditional terahertz transmission sensor. The relative error of using this method to determine carbendazim levels in orange juice samples was less than 5.3%. The terahertz time-domain spectroscopy technology combined with the metal grating integrated microfluidic sensor can improve the sensitivity of sample detection and realize the rapid detection and analysis of trace elements.

Carbon nanotube-based, serially connected terahertz sensor with enhanced thermal and optical efficiencies

ABSTRACT Owing to their high thermal and optical performances, carbon nanotube (CNT) films are used in various photo-thermo-electric (PTE) applications, such as terahertz (THz) sensing and energy harvesting. To improve the performance of PTE devices, a device structure should be designed based on a deep understanding of the thermal and optical responses of the CNT film. However, the optical properties of CNT films in the THz frequency region remain unclear because of the difficulties associated with device processing and measurements. Herein, we report our findings on the thermal and optical characteristics of CNT films. The shape of the CNT film that maximizes the product of the thermal and optical factors (optimal structure of the PTE sensor) depends on the frequency of the irradiating electromagnetic wave. The optimal film thickness and width values for THz irradiation range from 300–600 nm and 50–70 µm, respectively. Subsequently, we fabricated a serially connected, multi-element PTE sensor with an optimal device structure and enhanced the detection sensitivity by approximately 13 times compared with a single-element PTE sensor. In addition, we demonstrated the first THz spectroscopy application using a PTE sensor. The findings of this study, thermal/optical factor enhancement, and micro-sized CNT film processing technology can be used to improve the performance of all CNT-based photothermal devices, including PTE sensors and thermoelectric generators.

Topological sensor on a silicon chip

An ultrasensitive photonic sensor is vital for sensing matter with absolute specificity. High specificity terahertz photonic sensors are essential in many fields, including medical research, clinical diagnosis, security inspection, and probing molecular vibrations in all forms of matter. Widespread photonic sensing technology detects small frequency shifts due to the targeted specimen, thus requiring ultra-high quality (Q) factor resonance. However, the existing terahertz waveguide resonating structures are prone to defects, possess limited Q-factor, and lack the feature of chip-scale CMOS integration. Here, inspired by the topologically protected edge state of light, we demonstrate a silicon valley photonic crystal based ultrasensitive, robust on-chip terahertz topological insulator sensor that consists of a topological waveguide critically coupled to a topological cavity with an ultra-high quality (Q) factor of Q=0.14×106. Topologically protected cavity resonance exhibits strong resilience against disorder and multiple sharp bends. Leveraging on the extremely narrow linewidth (2.3 MHz) of topological cavity resonance, the terahertz sensor shows a record-high figure of merit of 4000 RIU mm−1. In addition to the spectral shift, the intensity modulation of cavity resonance offers an additional sensor metric through active tuning of critical coupling in the waveguide-cavity system. We envision that the ultra-high Q photonic terahertz topological sensor could have chip-scale biomedical applications such as differentiation between normal and cancerous tissues by monitoring the water content.

Metamaterial-based terahertz dual-band sensor integrated with microfluidic channels

We present the design of an innovative dual-band terahertz sensor based on a metamaterial array and a microfluidic channel. The sensing performance of two ultra-high absorption peaks—with absorption rates of 99.96% and 99.85%—generated in the frequency band of 0.2 to 1.2 THz is analyzed. Refractive indexes of analytes are predicted to be in a specific refractive index range, demonstrating the very low prediction error rate of the proposed sensor. Further analyses show that the designed sensor has application potential in label-free and rapid detection.

Ultrahigh sensitivity nitrogen-doping carbon nanotubes-based metamaterial-free flexible terahertz sensing platform for insecticides detection

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.

Quantitative exploration of the absorber behavior of metal–insulator–metal metamaterials within terahertz via an asymmetric peak model

Terahertz (THz) metamaterials have been developed for THz sensing, detection, imaging, and many other functions due to their unusual absorbers. However, the unusual absorption spectra change with different incident angles. Thus, we designed and fabricated a focal plane array with metal-insulator-metal (MIM) structure metamaterial absorbers for further research. The absorption spectrum with incident angles from 20 to 60 was measured using THz time-domain spectroscopy (THz-TDS), and the experimental results reveal that the absorption spectrum changes with incident angle variations. A basic analytical asymmetric peak model for extracting absorption-frequency characteristics was developed in this study to quantitatively explore this variation in the absorber behavior with incident angles. The best result was that the frequency corresponding to the highest absorption can be easily found using this peak model. The experimental data was coherent with the validation of the asymmetric peak model. Moreover, a second model to quantitatively relate parameters to the incident angle was discovered, allowing for the prediction of absorption spectrum shifts and changes. The absorption spectrum was predicted to have a valley-like absorption curve at particular incident angles based on the secondary models deduction. The proposed extraction method's essential feature is that it can be applied to any physics-based MIM metamaterial system. Such a model will guide the design and optimization of THz metamaterial absorbers, sensors, imagers, and many others.

Thermally and magnetically controlled dual-band terahertz metamaterial absorber based on InSb

A thermally and magnetically controlled dual-band terahertz metamaterial absorber with a simple structure consisting of a patterned InSb film, a Teflon dielectric layer, and a metal board is designed and investigated. Numerical simulation results demonstrate that the absorption peak can reach 94% at 0.434 THz and 0.692 THz at 300 K, in the absence of an applied magnetic field. The absorber has the features of polarization insensitivity due to the symmetry of the structure and wide incidence angle absorption. The physical absorption mechanism can be analyzed by the impedance matching theory, the electric field distribution, the current distribution, and the excitation of LC resonance theory. Furthermore, the metamaterial absorber enables temperature-tunable characteristics due to the temperature-dependent variation of the permittivity of InSb in the terahertz region. By varying temperature from 295 to 320 K, the first resonance frequency gradually shifts from 0.413 to 0.529 THz with the second peak shifting from 0.654 to 0.863 THz. In addition, frequency dependent absorption of the InSb-based metamaterial absorber under a series of applied magnetic fields is discussed. When the magnetic field B is changed from 0.2 T to 1T, the dual resonant frequencies gradually shift from 0.4816 THz to 0.423 THz and 0.7804 THz to 0.716 THz. The InSb pattern film behaves as a resonant unit, thus providing an alternative method for designing independent bi-tunable absorbers in the terahertz band. The proposed absorber has the advantages of good tunability, simple structure, and high sensitivity, which has great potential in terahertz sensors. This work provides an effective method for implementing thermally and magnetically dual-controlled terahertz metamaterial absorbers.

Terahertz hybrid optical-plasmonic modes: tunable resonant frequency, narrow linewidth, and strong local field enhancement.

Hybrid optical-plasmonic modes have the characteristics of low loss and small mode volume, which will result in the strong localization and enhancement of electromagnetic field. Such advantages of hybrid optical-plasmonic mode are important for the enhancement of light-matter interactions. Here, terahertz (THz) hybrid modes of Fabry-Perot resonances (FPRs) and spoof surface plasmon polaritons (SSPPs) in the modified Otto scheme are investigated both in theoretical and experimental aspects. The device structure is composed of a metal grating silicon waveguide (MGSW) and a metal slit grating (MSG). The two components are vertically stacked with a variable air gap between them. The THz hybrid modes are originated from the far-field coupling of the FPRs and the SSPP supported by the air gap and the MSG, respectively. By changing the thickness of the air gap, the resonant frequency of the FPR-SSPP modes can be tuned in a frequency range of about 0.1 THz. An anti-crossing behavior between two reflection dips corresponding to the guided-mode resonance in the MGSW and the FPR-SSPP mode is observed, which leads to the narrowing of the reflection dips in the anti-crossing region. Numerical simulations show that at the resonant frequencies of FPR-SSPP mode, there is a huge volume-averaged electromagnetic energy enhancement of about 1600 times in the grooves of the MSG, which is around 8.7 times larger than that induced by the SSPP directly launched by free-space electromagnetic field. The hybrid FPR-SSPP modes can be used to construct THz sensors and detectors with high sensitivity.

High-Sensitivity Multiband Detection Based on the Local Enhancement Effect of an Electric Field at Terahertz Frequency

The terahertz detection sensitivity interaction with other substances has received extensive attention and remains challenging due to the loss of terahertz waves. Herein, a terahertz, multiband sensor with high sensitivity based on the local enhancement effect of an electric field at terahertz frequency is proposed. The designed terahertz sensor is based on a common metamaterial sandwich absorber of electromagnetic energy that only employs an asymmetric metallic strip, which produces three resonance peaks with an absorption coefficient of greater than 95% in the 0.2-1.5 THz range and a maximum Q -factor of 46 at 1.397. Based on the principle of impedance matching, the high absorption of electromagnetic wave energy is reasonably analyzed. Furthermore, the influence of the top metal structure on the absorptivity and resonance frequency is analyzed by simulation optimization, and the optimum structural parameters are obtained. In addition, the influence of the refractive index (RI) change of the sample and dielectric layer on the resonance characteristics of the sensor is analyzed. When the thickness of the surface sample of the terahertz sensor metamaterial-based absorber is changed so that the RI changes in 1-2, the RI sensitivity increases up to 208 GHz/RIU, which can be used as an RI sensor. Furthermore, the influence of the sensor sensitivity and reflection coefficient on the thickness parameters of the sample covered by the sensor surface is analyzed, and the optimal sample thickness is 6 μm. The influence of the incident angle of the THz wave on the detection performance in the TM polarization mode is analyzed, and it is determined that the change in the incident angle in the range of 0-40 degrees has minimal influence on the resonance peak and absorption rate of multiband detection. Therefore, we suggest that our proposed sensor has considerable potential in biomedicine and trace detection applications.

Pesticide detection with covalent-organic-framework nanofilms at terahertz band

Development of rapid molecular detection technologies is critical for the safer and smarter urban agriculture, medicine, and pro-environment. The emergent terahertz (THz) spectroscopy has its distinct advantages of being non-destructive, label-free and able to trace intermolecular and intramolecular vibrations, yet it suffers from the low performance of sensing materials available and their high fabrication cost. Here, we introduce a reticular material -- two dimensional (2D) covalent organic frameworks (COFs) and prepare their nanofilms as the lossy layer for THz absorbers. The COF film can be directly deposited on the dielectric layer of THz absorbers via an in-situ wet-chemistry growth. It possesses designable hierarchical structures, high specific areas of 736–971 m2/g, and precise nanopores of 1.6–2.1 nm, depending on its 2D COF constituents. The resulting THz absorber has been tested for pesticide detection. It presents a limit of detection at 2.2 ng and a selective response of 2.7–7.8 times that of interferents such as saccharides, antibiotics, and dyes, satisfying the need for practical application. Such flexible filmy sensor can measure the pesticide residue on the surface of apple for practical application. The THz sensor also demonstrates high stability over 1000 cycles of bending. Use of reticular nanofilm as the responsive layer may permit the future development of high-performance THz absorbers and other sensors for rapid molecular recognition.

Can a terahertz metamaterial sensor be improved by ultra-strong coupling with a high-Q photonic resonator?

When a metamaterial (MM) is embedded in a one-dimensional photonic crystal (PC) cavity, the ultra-strong coupling between the MM plasmons and the photons in the PC cavity gives rise to two new polariton modes with high quality factor. Here, we investigate by simulations whether such a strongly coupled system working in the terahertz (THz) frequency range has the potential to be a better sensor than a MM (or a PC cavity) alone. Somewhat surprisingly, one finds that the shift of the resonance frequency induced by an analyte applied to the MM is smaller in the case of the dual resonator (MM and cavity) than that obtained with the MM alone. However, the phase sensitivity of the dual resonator can be larger than that of the MM alone. With the dielectric perturbation theory - well established in the microwave community - one can show that the size of the mode volume plays a decisive role for the obtainable frequency shift. The larger frequency shift of the MM alone is explained by its smaller mode volume as compared with the MM-loaded cavity. Two main conclusions can be drawn from our investigations. First, that the dielectric perturbation theory can be used to guide and optimize the designs of MM-based sensors. And second, that the enhanced phase sensitivity of the dual resonator may open a new route for the realization of improved THz sensors.

High-Q Toroidal Dipole Metasurfaces Driven By Bound States in the Continuum for Ultrasensitive Terahertz Sensing

A novel metallic toroidal dipole (TD) metasurface driven by Friedrich-Wintgen bound states in the continuum (FW-BIC) is theoretically proposed for terahertz (THz) sensing. By tuning the middle gap distance without breaking the symmetry of unit cell, the FW-BIC and quasi-BIC mode can be excited via resonance coupling between dipole modes. Based on the cyclic distribution of anti-aligned magnetic dipoles and the calculated scattering powers, TD resonance is demonstrated qualitatively and quantitatively; also FW-BIC is verified by far-field transmission spectrum and near-field enhancement spectrum. More importantly, it is the first time to exploit this quasi-BIC TD resonance for THz sensing to the best of our knowledge. For micron film sensing with frequency shift ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FS</i> ) method, numerical results show the sensitivity, the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</i> -factor and the corresponding figure of merit (FoM) can simultaneously reach 775.7 GHz/RIU, 1016, and 284, respectively. Moreover, for nano film sensing where <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FS</i> method is inapplicable, the amplitude difference method is utilized and the simulated results show it has superior sensing capability. Our proposed structure opens up an avenue to develop multifunctional and ultrasensitive THz sensors.

Ultra-high sensitivity terahertz sensor based on a five-band absorber

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.

Terahertz immunosensing assisted by functionalized Au NPs based on all-dielectric metasurface

Terahertz (THz) sensors have attracted significant attention in the biological field due to their non-destructive and harmless interaction with biochemical samples, but how to achieve efficiently specific sensing for the small molecular weight samples is still challenging. In this work, we incorporated an all-dielectric metasurface with the functionalized gold nanoparticles (Au NPs) to realize an immunosensing of the target biomolecule at nanomole-level concentration. These Au NPs are modified by the Anti-HA tag antibody, which realizes the specific immunobinding of the human influenza hemagglutinin tag protein (HA antigen). By comparing with and without the functionalized Au NPs, the experimental results show that this sensor can quantitatively detect the concentration of HA antigen, and its sensitivity can be greatly improved by the functionalized Au NPs. The maximum sensitivity can be achieved to 2.96 GHz∙ml/nmol, which is 2.66 times higher than that without the Au NPs, and the highest detection accuracy reaches 1.05 nmol/ml. Furthermore, by comparing three other nonspecific proteins, only the HA antigen can be sensitively distinguished, which proves the specificity of this sensing method. Our results illustrate a reliable platform for THz immunosensing applications, and it is expected that the proposed sensing strategy opens new avenues for developing high-efficient, low-cost, and real-time THz all-dielectric sensors for biomolecular detection.

VO2-metallic hybrid metasurfaces for agile terahertz wave modulation by phase transition

The combination of VO2 and metasurfaces has opened an attractive route to dynamically control terahertz (THz) waves based on the giant conductivity change. However, the high-precision control of microfabrication and single performance of conductivity change limit the multifunctional application of VO2-based metasurfaces. Here, we proposed a VO2-metallic hybrid metasurface by in situ depositing high-quality VO2 thin films onto a metasurface composed of asymmetric Fano resonance units. It exhibits agile frequency and amplitude modulation for THz transmission across tuning the dielectric constant and conductivity of VO2. The metallic metasurface is designed as a matrix to achieve high transmission at 0.61 and 0.78 THz due to the split-ring resonance. During the thermally triggered phase transition of VO2, we found that the resonance frequency and amplitude can be tuned dominantly by the change of dielectric constant and conductivity, respectively. In particular, the increased dielectric constant enables red shift of the frequency by around 0.48 THz and the conductivity increases lead to a giant THz amplitude modulation of 88%. These results provide a route for developing VO2-based THz smart devices combined with functional metasurfaces and hold great promise for applications in THz sensor and modulation.

Terahertz bound states in the continuum with incident angle robustness induced by a dual period metagrating

Metasurface-empowered bound state in the continuum (BIC) provides a unique route for fascinating functional devices with infinitely high quality factors. This method is particularly attractive to the terahertz community because it may essentially solve the deficiencies in terahertz filters, sensors, lasers, and nonlinear sources. However, most BIC metasurfaces are limited to specified incident angles that seriously dim their application prospects. Here, we propose that a dual-period dielectric metagrating can support multiple families of BICs that originate from guided mode resonances in the dielectric grating and exhibit infinite quality factors at arbitrarily tilted incidence. This robustness was analyzed based on the Bloch theory and verified at tilted incident angles. We also demonstrate that inducing geometric asymmetry is an efficient way to manipulate the leakage and coupling of these BICs, which can mimic the electromagnetically induced transparency (EIT) effect in our dual-period metagrating. In this demonstration, a slow-light effect with a measured group delay of 117 ps was achieved. The incidence-insensitive BICs proposed here may greatly extend the application scenarios of the BIC effect. The high Q factor and outstanding slow-light effect in the metagrating show exciting prospects in realizing high-performance filters, sensors, and modulators for prompting terahertz applications.

An Ultra-Thin Multiband Refractive Index-Based Carcinoma Sensor Using THz Radiation

A novel multiband terahertz (THz) refractive index-based carcinoma sensor is designed and investigated in this paper. The proposed THz sensor is constructed using meandered sectoral stubs enclosed in a square loop. The proposed sensor has a footprint of <inline-formula> <tex-math notation="LaTeX">$1.019 \lambda _{eff} 1.019 \lambda _{eff}$ </tex-math></inline-formula> and operates at three distinct frequencies, viz. 1.095 THz, 2.865 THz, and 4.065 THz. The absorption characteristics of the sensor are analyzed to estimate the performance of the proposed sensor. The free space absorptivity is estimated as 99.6&#x0025;, 99.1&#x0025; and 99.2&#x0025; at the said three resonant frequencies. The peak Q factor of 22.58 is realized at 4.065 THz under free space unloaded condition. Further, a study on different design parameters on absorption peaks and polarization independency is conducted and the results are presented. Followed by these, the sensitivity performance of the proposed carcinoma THz sensor is analyzed for healthy and cancerous cells of blood, skin and breast based on their unique refractive index profile. The peak sensitivity of 514.28 GHz/RIU at 2.865 THz, 476.35 GHz/RIU at 4.065 THz, and 476.91 GHz/RIU at 4.065 THz is theoretically observed for blood, skin and breast cancer cells respectively. Similarly, the high Q factor of 21.85, 18.06, 19.21 at 4.065 THz with FoM of 2.8, 2.38 and 2.384 are also observed for blood, skin and breast cancer respectively. Furthermore, the influence of the sample thickness on the THz sensor performance is analyzed. Thus, the proposed THz sensor is a promising entity for the detection and early diagnosis of cancer cells using THz radiation.

Switchable absorbing, reflecting, and transmitting metasurface by employing vanadium dioxide on the same frequency

In this work, we proposed a switchable metasurface based on phase transmission material of vanadium dioxide (VO 2 ) with metal and insulation mode. Simulation results demonstrated that the metasurface can switch perfectly at frequencies of 1.89 THz and 2.67 THz from two perfect absorbing peaks to a reflecting peak and a transmitting peak. When VO 2 serves as metal mode, there were two absorption peaks of 99.93% and 99.92% respectively. When VO 2 serves as insulation mode, the reflection and transmission magnitudes were 92.50% and 90.50% respectively. Simultaneously, the structure was insensitive to the incident angle from the simulation result. The proposed metasurface could therefore provide potential application prospects for the terahertz band switcher or sensor. • The proposed metasurface can switch at two frequency bands from the perfect absorber to transmitter and reflector. • The structure of a gold ring with a crack can generate a wide transmission frequency band. • The proposed metasurface can quickly respond and switch between two modes. • The proposed metasurface has insensitivity of incidence angle and cross-polarization.

Microresonator Frequency Reference for Terahertz Precision Sensing and Metrology

Highly sensitive terahertz (THz) sensors for a myriad of applications are rapidly evolving. A widespread sensor concept is based on the detection of minute resonance frequency shifts due to a targeted specimen in the sensors environment. Therefore, cutting-edge high resolution continuous wave (CW) THz spectrometers provide very powerful tools to investigate the sensors' performances. However, unpredictable yet non negligible frequency drifts common to state-of-the-art CW THz spectrometers limit the sensors' accuracy for ultra-high precision sensing and metrology. Here, we overcome this deficiency by introducing an ultra-high quality (Q) THz microresonator frequency reference. Measuring the sensor's frequency shift relative to a well-defined frequency reference eliminates the unwanted frequency drift, and fully exploits the capabilities of modern CW THz spectrometers as well as THz sensors. In a proof-of-concept experiment, we demonstrate the accurate and repeated detection of minute resonance frequency shifts of less than 5MHz at 0.6THz of a THz microresonator sensor.

Studies of leaf water content in smart agriculture using THz technologies: a review

In recent years, the concept of smart agriculture has entered our collective daily routine by radically modifying the methods by which crop monitoring was previously carried out. More precisely, the term smart farming focuses its attention on the use technologies already present on the market like, sensors (multispectral, hyperspectral, thermal and terahertz sensors), WNS (wireless sensor network) and drones capable to reduce the human work in the fields, optimizing at the same time the quantity and quality of the products limiting the use of resources, as water, fertilizers, pesticides and herbicides. Recently, one of the most important problem in the field of precision farming is the availability of water; unfortunately, this factor becomes more critical from year to year. To resolve this problem, the first step is to reduce consumption and rationalize the use of water by adjusting the water supply to the needs of the systems in order to increase its yield while saving money. In this respect, the development of non-destructive techniques operating in the THz spectral region has allowed to monitor in real time the water content present in leaves and plants. In fact, due to the strong water absorption and reflection in this spectral region, this feature can be exploited to detect the water content of leaves and plants helping us to intervene promptly in cases where the plant needs water, avoiding so does water stress, but above all trying to use our primary resource adequately by reducing waste. About this, Imaging and Time-Domain Spectroscopy (THz-TDS) techniques have been applied to monitor soil conditions, drought stress and presence of pathogens on the plants. In this review, we focus our attention on the latest research carried out on monitoring the water content present in leaves through THz technologies. Moreover, we pose particular attention in the description of our system set composed by a 97 GHz transmitter-receiver able to analyze the spatial distribution of the water inside a leaf.