Fano resonance-integrated metal nanoparticles' enhanced sensing for pesticide detection.

The combined application of metasurface and terahertz (THz) time-domain spectroscopy techniques has received considerable attention in the fields of sensing and detection. However, to detect trace samples, the THz wave must still be enhanced locally using certain methods to improve the detection sensitivity. In this study, we proposed and experimentally demonstrated a fano resonance metasurface-based silver nanoparticles (FaMs-AgNPs) sensor. AgNPs can enhance the sensitivity of the sensor by generating charge accumulation and inducing localized electric field enhancement through the tip effect, thereby enhancing the interaction between the THz waves and analytes. We investigated the effects of four different contents of AgNPs, 10 µl, 20 µl, 30 µl and 40 µl, on the detection of acetamiprid. At 30 µl of AgNPs, the amplitude change of the FaMs-AgNPs sensor was more pronounced and the sensitivity was higher, which could detect acetamiprid solutions as low as 100 pg/ml. The FaMs-AgNPs sensor has the advantages of a simple structure, easy processing, and excellent sensing performance, and has a great potential application value in the field of THz trace detection and other fields.

Ultrasensitive optical modulation in hybrid metal-perovskite and metal-graphene metasurface THz devices.

Implementation of efficient terahertz (THz) wave control is essential for THz technology development for applications including sixth-generation communications and THz sensing. Therefore, realization of tunable THz devices with large-scale intensity modulation capabilities is highly desirable. By integrating perovskite and graphene with a metallic asymmetric metasurface, two ultrasensitive devices for dynamic THz wave manipulation through low-power optical excitation are demonstrated experimentally here. The perovskite-based hybrid metadevice offers ultrasensitive modulation with a maximum modulation depth for the transmission amplitude reaching 190.2% at the low optical pump power of 5.90 mW/cm2. Additionally, a maximum modulation depth of 227.11% is achieved in the graphene-based hybrid metadevice at a power density of 18.87 mW/cm2. This work paves the way toward design and development of ultrasensitive devices for optical modulation of THz waves.

Metamaterial graphene sensors for the detection of two food additives.

Food safety is an important consideration for the food industry and for daily life, and food additives are essential in the modern food industry. Graphene-based metamaterial sensors are of great value and have potential applications in the detection of food additives, due to their ultra-sensitivity. This paper proposes a metasurface sensor consisting of graphene and dual elliptical ring resonators (Gr-DERRs) sensor for the detection of two common food additives. The limit of detection (LOD) for Sudan I solution is 581.43 fg/ml and, for taurine, 52.86 fg/ml. This ultra-sensitive detection is achieved by exploiting the unique electromagnetic properties of electromagnetically induced transparency (EIT) resonance, together with the Fermi energy level of graphene moving to the Dirac point, resulting in a dramatic change in the dielectric environment. The Gr-DERRs sensor has brings significant improvement in the detection of food additives with detection limits reduced to the femtogram level.

Ultrasensitive refractive index sensor based on stainless steel metamaterial.

Terahertz metamaterial technology, as an efficient nondestructive testing method, has shown great development potential in biological detection. This paper presents a stainless steel terahertz metamaterial absorber that achieves a near-perfect absorption of incident metamaterial waves with a 99.99% absorption at 2.937 THz. We demonstrate the theoretical discussion about the absorber and the application in sensing. The effect of the metamaterial absorber's structural parameters on the sensing performance is also analyzed. Simulation results show that the sensor can detect analytes with a refractive index between 1.0 and 1.8. Additionally, the performance of the sensor in detecting analytes in three states (solid, liquid, and gas) is analyzed in detail, and the sensitivity and the FoM of the sensor to detect methane are 22.727 THz/RIU and 568.175R I U -1, respectively. In addition, the terahertz sensor has the advantage of wide incident angle insensitivity, maintaining a good sensing performance within a wide manufacturing tolerance range of -10% to 10%. Compared to metal-dielectric-metal or dielectric-metal structures, the proposed sensor adopts stainless steel as the only manufacturing material, which has the advantages of simple structure, low manufacturing costs, and high sensitivity, and has potential application prospects in label-free high-sensitivity biomedical sensing.

Three-stimulus control ultrasensitive Dirac point modulator using an electromagnetically induced transparency-like terahertz metasurface with graphene.

This letter presents a fabricated Dirac point modulator of a graphene-based terahertz electromagnetically induced transparency (EIT)-like metasurface (GrE & MS). Dynamic modulation is realized by applying three stimulus modes of optical pump, bias voltage, and optical pump-bias voltage combination. With increasing luminous flux or bias voltage, the transmission amplitude undergoes two stages: increasing and decreasing, because the graphene Fermi level shifts between the valence band, Dirac point, and conduction band. Thus, an approximate position of the Dirac point can be evaluated by the transmission spectrum fluctuation. The maximum modulation depth is measured to be 182% under 1 V. These findings provide a method for designing ultrasensitive terahertz modulation devices.

Design of an all-optical multi-logic operation-integrated metamaterial-based terahertz logic gate.

Terahertz logic gates play a vital role in optical signal processing and terahertz digitization. Herein, a strategy to design an all-optical terahertz logic gate device composed of metamaterials with a semiconductor-metal hybrid is proposed; accordingly, a concrete logic gate composed of Ge embedded-in Au stripe supported by a Si board is presented theoretically. Simulation results reveal the dependence of the terahertz transmission spectra on the different illuminations in the device. Based on the illumination-transmission response, the designed device can realize the NOR or OR Boolean operation. The effects of the width of the Ge-Au stripe as well as the Si board on the transmission spectra and logic performance were also investigated.

Multispectral higher-order Fano resonant metasurface based on periodic twisted DNA-like split ring arrays with three modulation methods.

The active modulation of the Fano resonance is rare but desirable. However, recent studies mostly focused on a single modulation method and few reported the use of three photoelectric control methods. A tunable graphene DNA-like metamaterial modulator with multispectral Fano resonance is demonstrated. In experimentally fabricated metamaterials with six photoelectric joint modulation patterns, each joint shows different optoelectrical response characteristics. Ultrahigh modulation depth (MD) up to 982% was achieved at 1.5734 THz with a 1.040 A external laser pump by involving combined optoelectrical methods. These results show that the metasurface modulator is a promising platform for higher-order Fano resonance modulation and communication fields.

Graphene-integrated toroidal resonance metasurfaces used for picogram-level detection of chlorothalonil in the terahertz region.

Toroidal dipole resonance can significantly reduce radiation loss of materials, potentially improving sensor sensitivity. Generally, toroidal dipole response is suppressed by electric and magnetic dipoles in natural materials, making it difficult to observe experimentally. However, as 2D metamaterials, metasurfaces can weaken the electric and magnetic dipole, enhancing toroidal dipole response. Here, we propose a new graphene-integrated toroidal resonance metasurface as an ultra-sensitive chemical sensor, capable of qualitative detection of chlorothalonil in the terahertz region, down to a detection limit of 100 pg/mL. Our results demonstrate graphene-integrated toroidal resonance metasurfaces as a promising basis for ultra-sensitive, qualitative detection in chemical and biological sensing.

Time-frequency joint mappings of a terahertz metasurface for multi-dimensional analysis of biological cells.

Traditional fast Fourier transform is used to extract the frequency component at the cost of losing the time domain, which is critical for metasurface biosensing. In this Letter, a more comprehensive algorithm, continuous wavelet transform (CWT), to process signals from THz time-domain spectroscopy is introduced. By comparing the metasurface-enhanced 2D time-frequency mappings (TFMs) of HaCaT and HSC3 cells, the two types of biological cells can be clearly differentiated, showing the great potential of CWT in the label-free recognition of biological cells. Also, the 2D TFMs serve as effective visualization indicators, successfully detecting the concentration of cancer cells characterized by being label free and low cost. In addition, the 2D TFMs of different metasurfaces under the same cell concentration reveal the correlation of TFMs and localized fields. Such a feature provides evidence of an interaction between biological cells and electromagnetic waves, implying the absorption of THz radiation by biological cells can be effectively controlled by properly designing split ring resonators (SRRs) of metasurfaces.

Christiansen filters realized with cylindrical lenses of even symmetry.

A type of Christiansen filter that takes the form of a smooth cylindrical lens of even symmetry is proposed. By varying the shape of the lens, the filter can be made to realize many common filtering responses, including the polynomial function response, the Gaussian function response, and the sinc function response. A systematic design technique based on inverse scattering is established, and a desired, prescribed response can be tailored by properly shaping the lens of the filter. Three prototypical Christiansen filters, namely, a second-order all-real-roots filter, a second-order sinc filter, and a Gaussian filter, are synthesized using the proposed method. A prescribed response at 545 nm with a FWHM of 2 nm is achieved systematically by all of the three Christiansen filters.

Analysis of graphene-based tunable THz four-band absorption sensors.

A novel, to the best of our knowledge, four-band tunable absorber sensor, based on a graphene layer, is presented. The proposed sensor configuration is composed of a single monolayer of graphene placed on top of a SiO2 dielectric substrate, whereas a gold grounding plane is placed beneath the SiO2. In addition, the resonant frequencies of the sensor can be directly controlled by adjusting the Fermi level of graphene, while the absorption rate reaches a value greater than 99% at all resonant peaks. The acquired calculation results of the refractive index sensitivity of our proposed sensor show that the four resonant peaks possess superior sensing characteristics. Additionally, by covering the measured objects with different refractive indices, the acquired results indicate that the sensing performance of the sensor exhibits good linearity. From our analysis, it is concluded that the absorbing sensor exhibits a broad range of potential applications in the biomedical field.

Graphene-bridged topological network metamaterials with perfect modulation applied to dynamic cloaking and meta-sensing.

Perfect state transfer of the bus topological system enables the sharing of information or excitation between nodes. Herein we report groundbreaking research on the transfer of the graphene-bridged bus topological network structure to an electromagnetic metamaterial setting, named "bus topological network metamaterials (TNMMs)." Correspondingly, the electromagnetic response imprints onto the topological excitation. We find that the bus-TNMMs display a perfect modulation of the terahertz response. The blue-shift of resonance frequency could increase to as large as 1075 GHz. The modulation sensitivity of the bus-TNMMs reaches 1027 GHz/Fermi level unit (FLU). Meanwhile, with the enhancement of modulation, the line shape of the reflection keeps underformed. Parabola, ExpDec1, and Asymptotic models are used to estimate the modulation of the resonance frequency. Besides, the bus-TNMMs system provides a fascinating platform for dynamic cloaking. By governing the Fermi level of graphene, the bus-TNMMs can decide whether it is cloaking or not in a bandwidth of 500 GHz. Also, the bus-TNMMs exhibit the immense potential for dynamically detecting the vibrational fingerprinting of an analyte. These results give a far-reaching outlook for steering dynamically the terahertz response with the bus-TNMMs. Therefore, we believe that the discovery of bus-TNMMs will revolutionize our understanding of the modulation of the electromagnetic response.

Electromagnetically induced transparency-like metamaterials for detection of lung cancer cells.

A biosensor based on electromagnetically induced transparent (EIT) metamaterials (MMs) is proposed owing to the low loss and high Q-factor. The theoretical sensitivity of the biosensor based on EIT-like MMs were evaluated up to 248.8 GHz/RIU (RIU, Refractive Index Unit). In experiments, the cancer cells A549, as an analyte, are cultured on EIT-like MMs surface. The results show that when the cell concentration increases from 0.5 × 105 to 5 × 105 cells/ml, the frequency shift Δf could change from 24 to 50 GHz. Moreover, the coupled oscillators model is applied to explain the effect of the refractive index of analyte in simulations and the cell concentration in experiments on the EIT-like MMs. The fitting results exhibit that the refractive index of analyte and cell concentration significantly affect the radiative damping of the bright mode resonator γ1. The proposed EIT-like MMs biosensors show great potentials for cell measurement because any change that results in the lineshape variation in EIT-like MMs can only be attributed to the change of external dielectric environment due to the suppression of radiative losses.

Dual-wavelength terahertz sensing based on anisotropic Fano resonance metamaterials.

Higher order Fano resonance metamaterials provide a desirable platform for biosensing applications. In this work we exhibit higher order Fano modes by designing an elliptical metallic ring. The simulation results show that for Fano resonant metamaterials, the higher order modes lead to improved sensitivity to refractive index change and larger frequency shifts. Numerically, the sensitivity of dip A (quadrupole mode) is 112 GHz/RIU, whereas the sensitivity of dip B (octupole mode) is 234 GHz/RIU, over 2 times that of dip A. According to our experimental results, the Fano resonant frequencies of dip A and dip B exhibit redshift as the concentration of the anti-cancer drug methotrexate decreases from 120 to 90 µM, with the cell analyte concentration of A549 cells at 5×105 cell/ml. For dip A (quadrupole mode), there is a frequency shift of 0.84 GHz for drug concentration of 120 µM and a frequency shift of 22 GHz for a 90 µM drug-treated sample. For dip B (octupole mode), there is a frequency shift of 6.767 GHz for the drug concentration of 120 µM treated sample and a frequency shift of 51.815 GHz for the drug concentration of 90 µM treated sample. Furthermore, the frequency shift of dip A is always smaller than that of dip B for both 90 µM and 120 µM drug concentrations. Such phenomena indicate that dip B is much more sensitive than dip A. The enhanced sensitivity of higher order Fano metamaterials makes it possible to realize high-performance terahertz sensing for biomedical applications.

Sensitive detection of the concentrations for normal epithelial cells based on Fano resonance metamaterial biosensors in terahertz range.

In this paper, we have cultured normal epithelial cells (HaCaT) as analytes to detect the sensitivity of a biosensor based on Fano resonance metamaterials (FRMMs). The frequency shift Δf of the transmission spectrum was experimentally measured at three different concentrations (0.2×105, 0.5×105, and 5×105 cell/ml) of HaCaT cells. By employing the FRMMs-based biosensor, the detection concentration of HaCaT cells can approximately arrive at 0.2×105 cell/ml; further, the corresponding Δf is 25 GHz, which reaches the measurement limit of the THz-TDS system. Additionally, the increase of HaCaT cell concentration causes a different redshift of Δf from 24-50 GHz, and the maximum of Δf can reach 50 GHz when the HaCaT cell concentration is at 5×105 cell/ml. Similarly, the simulated results show that the Δf depends on the numbers of analytes with a semiball shape and the refractive index of analytes. The theoretical sensitivity was calculated to be 481 GHz/RIU. The proposed FRMMs-based biosensor paves a fascinating platform for biological and biomedical applications and may become a valuable complementary reference for traditional biological research.

[Research Progress of Electromagnetic Metasurface Used for Radar Cross Section Reduction in Microwave and Terahertz Wave].

Electromagnetic metasurface with many special electromagnetic properties can be utilized to manipulate electromagnetic wave propagation and reflection. If the metasurfaces were designed for coding, random, phase discontinuities, perfect absorber and so on，they can manipulate the scattering or the reflection of electromagnetic wave and achieve the reduction of the radar cross section(RCS). This paper mainly presents the recent progress concerning the reduction of RCS using non - directional scattering or the absorption characteristic in microwave and terahertz wave. The analysis results show that coding electromagnetic metasurface can disperse the reflection into a variety of direction by designing the specific coding sequences for different elements. The coding metasurface which are composed of different digital elements, and the reflection phase difference of these digital elements is constant in a wide frequency range, and higher bit coding metasurface can flexible manipulate electromagnetic wave. The random electromagnetic metasurface can achieve broadband phase shifter by adjusting the size parameters of array element, and can diffuse characteristic by scattering into random wave of the reflection peaks for metal target. Phase discontinuities metasurface can achieve anomalous or diffuse of wave because the phase distribution is not uniform at the surface. The absorber metasurfaces which are designed reasonably with the physical dimensions of the devices can reduce reflection by absorbing electromagnetic wave energy. So, the electromagnetic metasurfaces have a large potential application for radar stealth, broadband communications, imaging and so on. Finally, we discussed the future development of RCS reduction by using the electromagnetic metasurface. In order to satisfy the needs of practical application, the research of metasurface will continue development in broadband, flexible, large angle and other aspects.

Broadband, wide-angle, low-scattering terahertz wave by a flexible 2-bit coding metasurface.

Expanding bandwidths and arbitrary control of technology remain key issues in the field of electromagnetic waves, especially in terahertz (THz) wave. In this paper, we propose a novel method to achieve broadband low-scattering THz characteristics with wide-angle and polarization independence by a 2-bit flexible and nonabsorptive coding metasurface. The coding metasurface is composed of four digital elements based on double cross metallic line for "00", "01", "10", and "11." The reflection phase difference of neighboring elements is about 90° over a broad THz frequency band and wide incident angles. The low scattering coefficients below -10 dB were achieved over a wide frequency band from 0.8 THz to 1.5 THz when the incident angle is less than 50° by coding the four elements sequences. This superior property is maintained when the flexible coding metasurface is wrapped around a metallic cylinder with different dimensions. These results present a novel method to control THz waves freely and demonstrate significant scientific value in practical applications.

[Research progress in the application of biosensors by using metamaterial in terahertz wave].

In the present paper, the recent progress in terahertz metamaterials-based sensing is reviewed with the principle of metamaterial biosensor,metamaterial substrate, and structure design, respectively. The paper introduces the principle in detail, analyzes the sensitivity of the biosensor with the material and the thickness of the substrate and the structure of metamaterial. The analysis shows that we can enhance the sensitivity and resolution of biosensor by designing specific metamaterial structure, using low dielectric constant and low loss thin substrate, especially many materials have a specific response in the terahertz frequency. So, there is a large potential application for label-free sensing by using the terahertz metamaterials. This paper also presents the future development of THz metamaterial sensors.

Terahertz Liquid Biosensor Based on A Graphene Metasurface for Ultrasensitive Detection with A Quasi-Bound State in the Continuum.

The concept of a quasi-bound state in a continuum (QBIC) has garnered significant attention in various fields such as sensing, communication, and optical switching. Within metasurfaces, QBICs offer a reliable platform that enables sensing capabilities through potent interactions between local electric fields and matter. Herein, a novel terahertz (THz) biosensor based on the integration of QBIC with graphene is reported, which enables multidimensional detection. The proposed biosensor is distinctive because of its ability to discern concentrations of ethanol and N-methylpyrrolidone in a wide range from 100% to 0%, by monitoring the changes in the resonance intensity and maximum wavelet coefficient. This approach demonstrates an excellent linear fit, which ensures robust quantitative analysis. The remarkable sensitivity of our biosensor enables it to detect minute changes in low-concentration solutions, with the lowest detection concentration value (LDCV) of 0.21 pg mL-1 . 2D wavelet coefficient intensity cards are effectively constructed through continuous wavelet transforms, which presents a more accurate approach for determining the concentration of the solution. Ultimately, the novel sensing platform offers a host of advantages, including heightened sensitivity and reusability. This pioneering approach establishes a new avenue for liquid-based terahertz biosensing.

Terahertz Biosensor Engineering Based on Quasi-BIC Metasurface with Ultrasensitive Detection.

Terahertz (THz) sensors have attracted great attention in the biological field due to their nondestructive and contact-free biochemical samples. Recently, the concept of a quasi-bound state in the continuum (QBIC) has gained significant attention in designing biosensors with ultrahigh sensitivity. QBIC-based metasurfaces (MSs) achieve excellent performance in various applications, including sensing, optical switching, and laser, providing a reliable platform for biomaterial sensors with terahertz radiation. In this study, a structure-engineered THz MS consisting of a "double C" array has been designed, in which an asymmetry parameter α is introduced into the structure by changing the length of one subunit; the Q-factor of the QBIC device can be optimized by engineering the asymmetry parameter α. Theoretical calculation with coupling equations can well reproduce the THz transmission spectra of the designed THz QBIC MS obtained from the numerical simulation. Experimentally, we adopt an MS with α = 0.44 for testing arginine molecules. The experimental results show that different concentrations of arginine molecules lead to significant transmission changes near QBIC resonant frequencies, and the amplitude change is shown to be 16 times higher than that of the classical dipole resonance. The direct limit of detection for arginine molecules on the QBIC MS reaches 0.36 ng/mL. This work provides a new way to realize rapid, accurate, and nondestructive sensing of trace molecules and has potential application in biomaterial detection.