Terahertz Metamaterial Research Articles

Editorial: Advanced Terahertz spectrum and metamaterials for biochemical sensing and detection

Editorial: Advanced Terahertz spectrum and metamaterials for biochemical sensing and detection

A novel terahertz integrated sensor for liquid samples detections based on metamaterial and embedded microfluidic channels

THz metamaterial (MM) microfluidic sensors, which can be utilized for the detection of small volume liquid specimen, have attracted much attention in biosensing applications. In this paper, we propose a novel microfluidic channel embedded terahertz MM sensor, which employs split ring resonators (SRRs) and asymmetric two-gap SRRs for the detection of liquid samples. Unlike currently reported THz MM microfluidic sensors utilizing microfluidic superstrate-metal-substrate structure, our approach combines the microfluidic superstrate with substrate, by designing micro-channels underneath the gap of the SRRs within the polydimethylsiloxane substrate. It could be easily fabricated by standard photolithography techniques. The simulated results show that the performance of the sensor is dependent on the spacing angle between two gaps, the orientation of the gaps, as well as the polarization of electric fields. Furthermore, maximum Q factors of 9.5 and 44 are achieved for dipole resonance, with corresponding figure of merits of 0.63 and 2.89 RIU−1 for transmission and reflection scenarios. Thus, this concept and method not only provides sensitive biosensing for liquid-based samples, but also can be applied to other MM structures to further improve the sensitivity.

Silicon bowtie structure based adjustable nonrigid all-nonmetal metamaterial terahertz filter.

An all-nonmetal metamaterial (ANM) terahertz device with a silicon bowtie structure has been developed, which has comparable efficiency to that of its metallic counterparts, and better compatibility with modern semiconductor fabrication processes. Moreover, a highly tunable ANM with the same structure was successfully fabricated through integration with a flexible substrate, which demonstrated large tunability over a wide frequency range. Such a device can be used in terahertz systems for numerous applications, and is a promising substitute for conventional metal-based structures.

A Four-Band Terahertz Metamaterial Sensor Based on Symmetric E-Shaped Structure

To realize the multi-frequency selectivity of the analyte, a novel four-band terahertz metamaterial sensor is proposed in this work. In particular, the sensor performance is analyzed theoretically and numerically within a terahertz frequency range (0.8–1.5 THz) via the finite element method. According to the results, higher-order Fano resonance is the main cause of the four narrow and sharp transmission valleys in the operating band region of the sensor, yielding high resolution with Q values up to 177. Moreover, this sensor is polarization-insensitive over a wide polarization angle range of 0° to 50°. In addition, the sensor achieves refractive index sensitivity of 200 GHz/RIU and offers FOM values of up to 26.7. The sensor proposed in this study exhibits a simple structure, frequency selection characteristics, low cost, and enhances the interaction between terahertz waves and substances, which is of great theoretical and practical significance for the development of terahertz functional devices such as sensors and filters.

Thermo-optic modulator based on vanadium dioxide and nonlinear Kerr medium in terahertz region

Three-layer design of multi-functional terahertz (THz) metamaterials is proposed and numerically investigated, where the nonlinear Kerr-media is sandwiched between the phase change material of vanadium dioxide (VO2). Owing to the unique picosecond response of VO2 at different temperature, a high transmission is achieved before the phase transformation. When the vanadium dioxide is in the metallic phase, three band perfect absorption exist in the MIM structure. To fully dig out such high absorption, the impedance matching and effective medium theory are taken to obtain the effective dielectric and permeability. The underlying physical mechanism is further investigated with the normalized electric and magnetic field distribution at each resonant peak. In addition, a switcher is realized when the external pump light is utilized to irradiate nonlinear Kerr-media and its high sensing capability is explored with different Kerr medium. Therefore, our dynamic tunable modulator paves a new way in the design of tunable THz modulators, sensors and switchers.

Sensing Performance of Triple-Band Terahertz Metamaterial Absorber Based on Snowflake-Shaped Resonators

Terahertz metamaterial absorbers are important functional devices for liquid analyte detection. In contrast to general metamaterial absorbers with single-layer metasurfaces that possess only one resonant mode, a triple-band terahertz metamaterial absorber formed by a single layer of symmetrically arrayed snowflake-shaped resonators was proposed in this study. The simulation results showed that the absorption of the metamaterial absorber reached 97.43% at 0.550 THz, 79.22% at 1.249 THz, and 99.02% at 1.867 THz with narrow resonant peaks. The resonant frequencies were sensitive to the refractive index of the surrounding medium at a fixed analyte thickness, which would play an important role in the performance of the sensor for detecting changes in the surrounding refractive index. The maximum value of the refractive index sensitivity was 137.70 GHz/RIU, 306.25 GHz/RIU, and 473.86 GHz/RIU, with a figure of merit (FoM) of 3.14, 2.33, and 6.46, respectively, for refractive index values ranging from 1.0 to 2.2 under three resonant modes. It is worth noting that most of the liquid samples showed a refractive index ranging from 1.0 to 2.0. Furthermore, the identification of peanut oil, carbon disulfide, and turpentine was considered to verify that the proposed terahertz sensor could be used for high-sensitivity liquid detection and has broad development prospects in the field of detecting and sensing.

Displacement current guided control of electromagnetically induced transparency in nano-discretized terahertz metamaterials

It is difficult for nano-scale optical devices to resonate with terahertz waves. By using a nano-discretized metamaterial (NDMM), we converted a gap-localized electromagnetic response into terahertz spectroscopy. A switch of an electromagnetically induced transparency (EIT) analog is acquired by a displacement current in NDMMs and is strongly dependent on the discretization of the nanogap. By controlling the distance of the nanogap, the switch of the EIT can be determined, which, in turn, is linked to the polarization of the electric field. If the electric field is perpendicular to the nanogap, the switch of the EIT can be tuned. While the electric field is parallel to the nanogap, the EIT would exist on all occasions, no matter how the nanogap changes. The proposed NDMMs may ultra-sensitively detect the vibrations of the nano-world using the spectroscopic information of terahertz (THz) response.

Terahertz Metamaterials Broadband Perfect Absorber Based on Molybdenum Disulfide

In this letter, a terahertz metamaterials broadband perfect absorber based on molybdenum disulfide (MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) is proposed. The absorber is composed of a single layer of MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> and a metal plate separated by thin polyimide. The single-layer MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> structure consists of four rectangular strips transformed by fillets. Numerical simulations using the full-vector finite element method show that the new hybrid mode can be excited and the electric fields of the modes can be redistributed due to the introduction of fillet MoS <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> metamaterial structure, which effectively improves the absorption performance of devices. In the range of 1.8 to 2.2 THz, the absorptivity is close to 100%, and the absorption curve is much flatter, which provide a useful reference for terahertz modulation equipment.

Analysis of reaction between vitamin B6 and bovine serum albumin based on a terahertz metamaterial sensor.

A four-peak terahertz metamaterial sensor was used to detect the reaction between different concentrations of vitamin B6 (VB6) and bovine serum albumin (BSA), which achieves a concentration range (0.015-0.125 mg/µl) of VB6 and a maximum binding concentration (0.05 mg/µl) of VB6 and 0.0875 mg/µl BSA. To understand the combination of VB6 and BSA, the reactants between VB (VB1, VB3, and VB5) with the same concentration (0.05 mg/µl) and a BSA solution with a concentration of 0.0875 mg/µl were carried on the surface of the sensor. Experimental results show that the reactants cause the four resonance peaks of the sensor to produce the coincident redshift, which is the same as the order of their binding coefficients determined by the fluorescence method. The experimental process indicates that it is feasible to use terahertz metamaterials to detect the reaction process of organic matter.

Numerical Investigation of a Gear-Shaped Triple-Band Perfect Terahertz Metamaterial Absorber as Biochemical Sensor

In this work, a triple-band perfect metamaterial absorber (MA) in the terahertz (THz) regime is demonstrated for different biochemical sensing applications. The metasurface’s unit cell is made up of a distinct metallic gear-shaped core with eight identical teeth and two concentric circular rings (CCRs) on the top layer. Finite-difference time-domain (FDTD) method-based software is utilized for modeling and numerical analysis, and a genetic algorithm (GA) is employed to optimize some geometric parameters of the absorber. The proposed MA shows almost 100% absorption in the frequency range from 0.5 to 4.5 THz. The MA becomes almost insensitive to polarization angle when the number of gear tooth has been increased. Resonance frequencies can be tweaked by adjusting the top layer’s geometric parameters while maintaining the absorption constant. The influence of different metals and forms in the top layer, such as solid and hollow circular metallic cores, on absorption spectra is also illustrated in this article. The modeled MA can detect glucose, ethanol, oils, and cancer cells (BCCs) by changing the resonance frequency and peak absorption of the spectra due to the variation in refractive index (RI) of the sample. When detecting ethanol, the absorber achieves a maximum sensitivity and figure of merit (FOM) of 56.67 THz/RIU and 358.7 RIU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> , respectively. In addition, the effects of incident angle of light and bending of MA on absorption are analyzed in this study. With this unique feasible design, structural tunability, and high sensitivity, the proposed absorber exhibits good promise in detecting biochemicals in the THz region.

Folding metamaterials with extremely strong electromagnetic resonance

Aqueous solutions cannot be detected using transmissive terahertz metamaterials because water strongly absorbs terahertz waves. Transmissive terahertz metamaterials are easier to integrate terahertz emitters and receivers into single and compact devices than reflective terahertz metamaterials. The detection of aqueous solutions using transmissive terahertz metamaterials is a big challenge. This work fabricates a transmissive terahertz metamaterial using a folding metamaterial comprising split-ring resonators (SRRs) with nano-profiles with a high aspect ratio of 41.4. The folding metamaterial has a small transmittance of − 49 dB at its resonance frequency, large transmittance contrast of approximately 6 × 10 4 with respect to the transmittance of its substrate, large refractive index sensitivity of 647 GHz/RIU, and large quality factor of 37. This result arises from the nano-profiles of the SRRs. The nano-profiles increase the surface areas of the SRRs, increasing their surface currents and enhancing the electromagnetic resonance of the folding metamaterial. The folding metamaterial detects a 188-μm-thick rabbit-blood layer that is deposited on it, which cannot be detected by using a common metamaterial. This result reveals that folding metamaterials have potential in detecting the products of live microorganisms with geometrical sizes up to several hundreds of micrometers, such as hydrogen gas, hydrocarbons, and antibodies.

Design and Simulation of a Dual-Band Terahertz Metamaterial Absorber Based on a Slot-Patterned Monolayer Graphene Structure

Design and Simulation of a Dual-Band Terahertz Metamaterial Absorber Based on a Slot-Patterned Monolayer Graphene Structure

Terahertz Metamaterial Sensor With Ultra-High Sensitivity and Tunability Based on Photosensitive Semiconductor GaAs

We proposed and demonstrated an ultra-high sensitive refractive index metamaterial sensor within terahertz region. The sensor consists of a gallium arsenide (GaAs) structure on the top, aluminum in the middle, and a polyimide as the bottom substrate. The simulation results show that such sensor has an absorption peak at 2.0 THz with the Q-factor up to 444. The sensor is very sensitive to the change of the refractive index (RI) of the surrounding medium. The metamaterial sensor with optimized structure has a sensitivity of 1.762 THz/RIU and a figure of merit (FoM) value of 392 RIU <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> , respectively. Meanwhile, both the absorption peak position and intensity of such metamaterial sensor could be adjustable. The proposed terahertz metamaterial sensor with such high sensitivity and easily tunable properties has broad application prospects in research fields including biomedical sensing, disease diagnosis, and also trace detection of hazardous substances.

Dynamically tunable slow light effect based on graphene terahertz metamaterial

Recently, the terahertz metamaterials with tunable EIT effect have a wide application range, especially in slow light applications area. However, the traditional tuning means are very difficult to implement, such as superconductor and MEMS, which limit their application range. For this reason, we proposed a terahertz EIT metamaterial structure consisting of two metallic strips, a split ring resonator and a graphene ribbon. The numerical results demonstrate that a transparency window is induced by the destructive interference of the bright mode and dark mode and moreover, the amplitude of the transparency window can be actively modulated by changing Fermi energy of graphene via electrostatic doping. The tunable response can attribute to the increasing conductivity of graphene, weakening near field coupling between the bright mode and dark modes. Therefore, these results have important reference for future development of slow light devices and sensors.

Flexible Terahertz Metamaterial Biosensor for Ultra-Sensitive Detection of Hepatitis B Viral DNA Based on the Metal-Enhanced Sandwich Assay.

The high sensitivity and specificity of terahertz (THz) biosensing are both promising and challenging in DNA sample detection. This study produced and refined a flexible THz MM biosensor for ultrasensitive detection of HBV in clinical serum samples based on a gold magnetic nanoparticle-mediated rolling circle amplification (GMNPs@RCA) sandwich assay under isothermal conditions. Typically, solid-phase RCA reactions mediated by circular padlock probes (PLPs) are triggered under isothermal conditions in the presence of HBV DNA, resulting in long single-stranded DNA (ssDNA) with high fidelity and specificity. Then, the resultant ssDNA was conjugated with detection probes (DPs) immobilized on gold nanoparticles (DP@AuNPs) to form GMNPs-RCA-AuNPs sandwich complexes. The HBV DNA concentrations were quantified by introducing GMNPs-RCA-AuNPs complexes into the metasurface of a flexible THz metamaterial-based biosensor chip and resulting in a red shift of the resonance peak of the THz metamaterials. This biosensor can lead to highly specific and sensitive detection with one-base mismatch discrimination and a limit of detection (LOD) down to 1.27E + 02 IU/ml of HBV DNA from clinical serum samples. The HBV DNA concentration was linearly correlated with the frequency shift of the THz metamaterials within the range of 1.27E + 02∼1.27E + 07 IU/ml, illustrating the applicability and accuracy of our assay in real clinical samples. This strategy constitutes a promising THz sensing method to identify virus DNA. In the future, it is hoped it can assist with pathogen identification and clinical diagnosis.

Ultra-broadband asymmetric transmission and linear polarization conversion based on terahertz metamaterials

Ultra-broadband asymmetric transmission and linear polarization conversion based on terahertz metamaterials

Fabrication of frequency-selective surface by picosecond laser direct writing

In this paper, a terahertz (THz) frequency-selective surface (FSS) is fabricated through picosecond (ps) laser direct writing on the silicon-based silver film. The time-domain spectroscopy (TDS) technique is used to test its electromagnetic modulation performance. Based on the finite-difference time-domain method, CST microwave studio 2016 was used to simulate its resonance characteristic. By optimizing the process parameters, the square ring structures on the FSS with the size error below 2% are fabricated. It is found that the center frequencies of the experimental and simulated resonance peaks are nearly both at 0.293 THz, and the transmission is about 0.01%. From the simulation results, when the size error is controlled below 2 μm, the resonant frequency shift is about 2.2 GHz. Combining the simulation and experimental results, if the size error of inner square is controlled below 2%, it has little effect on the THz metasurfaces’ modulation performance. By optimizing the laser scanning strategy and processing parameters, ps laser direct writing provides a new choice for the convenient fabrication of THz metamaterials.

Tune the resonance of VO2 joined metamaterial dimers by adjacent cut wires.

Two terahertz metamaterials were joined by a conductivity variable VO2 patch to obtain a metamaterial dimer. By applying voltage or heat to the VO2 patches, active modulation of terahertz wave could be achieved. A cut-wire metamaterial was placed adjacent to the VO2 joined dimer to affect its electromagnetic response. It was found that the cut wire could heavily impact the resonance mode of the VO2 joined dimer, which gives dual resonance dips in transmission spectrum for both insulating and conducting states of VO2 patches. As a result, by tuning the conductivity of VO2, active dual band phase modulation could be achieved with high transmission window by this dimer-cut wire coupling system.

VO2-enabled transmission-reflection switchable coding terahertz metamaterials.

Coding metamaterials have offered unprecedented degrees of freedom to manipulate electromagnetic waves in time and frequency domains in terms of various coding sequences, however, it is still challenging to realize dynamic coding metamaterials in the terahertz range. Here, we propose VO2-enabled transmission-reflection switchable coding terahertz metamaterials consisting of multilayered gold and VO2 patterns. The insulator-to-metal transition of VO2 leads to switch between the refractive and reflective scattering beams by changing the temperature. The four 2-bit elements are used to construct coding metasurface-based OAM generator with l = 1. Remarkably, the transmission-reflection switching functionality of the coding metasurface can be achieved at different frequencies. In addition, the novel designs in our work can achieve EM waves manipulation and provide a useful method to dynamically switch transmission-reflection response in the THz frequency regime.

Study on the Spectral Characteristics of Plant Growth Regulators Based on the Structure Difference of Terahertz Metamaterial Sensor

In this study, in order to solve the problems of low sensitivity and complex sample pretreatment in traditional detection, a method was proposed to detect plant growth regulators using terahertz combined with metamaterial sensors. Two metamaterial sensors with different structures are designed. The differences of two kinds of metamaterial sensors for detecting plant growth regulators were analyzed from three aspects of frequency domain, time domain and absorption characteristics. It is further proved that the difference of metamaterial structure will lead to the change of frequency shift, time domain waveform and absorption characteristics of measured samples, and the influence of sample concentration on the detection characteristics of the metamaterial sensor. The results show that both metamaterial sensors can effectively detect the four plant growth regulator solutions with concentrations as low as 0.05 mg/L. In terms of detection and sensing, the frequency shift, sensitivity, figure of merit value, vibration amplitude and change rate of absorption intensity of metamaterial sensor 2 are higher than each parameter of metamaterial sensor 1, which has better detection performance. However, in terms of identification between samples, metamaterial sensor 1 is more suitable for the identification of daminozide and naphthaleneacetic acid solutions, and metamaterial sensor 2 was more suitable for the detection of glyphosine and gibberellic acid solutions. It provides a new method for multi-target fusion to detect plant growth regulators with high sensitivity and high accuracy.