Bipolar charge trapping for absorption enhancement in a graphene-based ultrathin dual-band terahertz biosensor

Tunable High-Sensitivity Four-Frequency Refractive Index Sensor Based on Graphene Metamaterial

As graphene-related technology advances, the benefits of graphene metamaterials become more apparent. In this study, a surface-isolated exciton-based absorber is built by running relevant simulations on graphene, which can achieve more than 98% perfect absorption at multiple frequencies in the MWIR (MediumWavelength Infra-Red (MWIR) band as compared to the typical absorber. The absorber consists of three layers: the bottom layer is gold, the middle layer is dielectric, and the top layer is patterned with graphene. Tunability was achieved by electrically altering graphene's Fermi energy, hence the position of the absorption peak. The influence of graphene's relaxation time on the sensor is discussed. Due to the symmetry of its structure, different angles of light source incidence have little effect on the absorption rate, leading to polarization insensitivity, especially for TE waves, and this absorber has polarization insensitivity at ultra-wide-angle degrees. The sensor is characterized by its tunability, polarisation insensitivity, and high sensitivity, with a sensitivity of up to 21.60 THz/refractive index unit (RIU). This paper demonstrates the feasibility of the multi-frequency sensor and provides a theoretical basis for the realization of the multi-frequency sensor. This makes it possible to apply it to high-sensitivity sensors.

Multiband terahertz metamaterial perfect absorber for microorganisms detection

We report a multi-resonant terahertz (THz) metamaterial perfect absorber (MPA)-based biosensor in the working frequency range of [Formula: see text] for sensing of microorganisms (such as fungi, yeast) and wheat pesticides. Nearly [Formula: see text] absorption is realized at [Formula: see text] and [Formula: see text]. We designed our THz MPA sensor making resonators' gap area compatible with the microorganisms' size. To obtain optimum performance of the MPA, a mapping of amplitudes and shifts in the absorption resonance peaks with different structural parameters of the resonators is carried out. A very high-frequency shift is obtained for microorganisms such as Penicillium chrysogenum (fungi), yeast, and pesticides (Imidacloprid, N, N-Diethyldithiocarbamate sodium salt trihydrate, Daminozide, N, N-Diethyldithiocarbamate sodium salt hydrate, and Dicofol). An equivalent circuit model using Advance Design System (ADS) software is developed. The calculated results through the model show similar trends as obtained in the simulations using CST. Investigations of the effect of incidence angle of THz wave on the absorption spectra of the MPA are also carried out. It is found that incidence angle does not impact the stability of the lower resonance absorption peak (1.79THz). Due to the wide working frequency range, the proposed sensor is extremely suitable for the detection of all range of pesticides because their specific absorption fingerprint lies in the frequency range of 0-3.8THz. We believe that our sensor could be a potential detection tool for detecting pesticide residues in agriculture and food products. The THz MPA-based biosensor is capable of detecting a very small change in the effective dielectric constant of the MPA environment. Therefore, it can also offer huge opportunities in label-free biosensing for future biomedical applications.

Terahertz absorber based on vanadium dioxide with high sensitivity and switching capability between ultra-wideband and ultra-narrowband

The terahertz perfect absorber can be applied in the control, sensing and modulation of optical fields in micro- and nanostructures. However, they are only single function, complex device structure and low sensing sensitivity. Based on this, by introducing the bound state in the continuum (BIC) with infinite quality factor and field enhancement effect, and taking advantage of the phase transition characteristics of vanadium dioxide (VO2), we designed a terahertz perfect absorber device which can actively switch between ultra-wideband and ultra-narrowband. The absorption mechanism is explained by multipole analysis theory, impedance matching theory and electromagnetic field distribution. The broadband absorption is mainly due to the electric dipole resonance on metallic VO2 materials, and the absorption is more than 99% across 3.64-6.96 THz, and it has excellent characteristics such as robustness. Ultra-narrowband perfect absorption has a quality factor greater than 2200 due mainly to the implementation of symmetrically protected BIC with a sensing sensitivity of 2.575 THz per RIU. Therefore, this research could be widely used in the fields of integrated optical circuits, optoelectronic sensing and perceptual modulation of energy, as well as providing additional design ideas for the design of terahertz multifunctional devices.

Developing a Novel Terahertz Fabry-Perot Microcavity Biosensor by Incorporating Porous Film for Yeast Sensing

We present a novel terahertz (THz) Fabry-Perot (FP) microcavity biosensor that uses a porous polytetrafluoroethylene (PTFE) supporting film to improve microorganism detection. The THz FP microcavity confines and enhances fields in the middle of the cavity, where the target microbial film is placed with the aid of a PTFE film having a dielectric constant close to unity in the THz range. The resonant frequency shift increased linearly with increasing amount of yeasts, without showing saturation behavior under our experimental conditions. These results agree well with finite-difference time-domain (FDTD) simulations. The sensor's sensitivity was 11.7 GHz/μm, close to the optimal condition of 12.5 GHz/μm, when yeast was placed at the cavity's center, but no frequency shift was observed when the yeast was coated on the mirror side. We derived an explicit relation for the frequency shift as a function of the index, amount, and location of the substances that is consistent with the electric field distribution across the cavity. We also produced THz transmission images of yeast-coated PTFE, mapping the frequency shift of the FP resonance and revealing the spatial distribution of yeast.

Terahertz antenna with tunable filtering

A terahertz (THz) antenna with tunable filtering is designed and numerically studied. A slotted monopole radiator with defected ground structure is used for operating with wideband response with an impedance bandwidth in the range of 3.80-11.98 THz for S ₁₁≤-10d B. A slot is engraved in the radiator for obtaining the filtering characteristics in the antenna response. By varying its chemical potential, the frequency band 5.05-6.69 THz with graphene material can provide the tunability in the frequency and bandwidth of the filtered band in the antenna response. Furthermore, the antenna can provide radiation efficiency of more than 90% and gain with a peak value of 6.6 dBi in the passband.

Graphene-based terahertz bias-driven negative-conductivity metasurface

A graphene-based terahertz negative-conductivity metasurface based on two types of unit cell structures is investigated under the control of an external bias voltage. Electrical characterization is conducted and verification is performed using a finite-difference time-domain (FDTD) and an optical-pump terahertz (THz)-probe system in terms of simulation and transient response analysis. Owing to the metal-like properties of graphene, a strong interaction between the metasurface and monolayer graphene yields a short-circuit effect, which considerably weakens the intensity of the resonance mode under passive conditions. Under active conditions, graphene, as an active load, actively induces a negative-conductivity effect, which enhances the THz transmission and recovers the resonance intensity gradually because of the weakening of the short-circuit effect. The resulting resonance frequency shows a blue shift. This study provides a reference value for combining graphene exhibiting the terahertz bias-driven negative-conductivity effect with metasurfaces and its corresponding applications in the future.

Tunable Optimal Dual Band Metamaterial Absorber for High Sensitivity THz Refractive Index Sensing

We present a simple dual band absorber design and investigate it in the terahertz (THz) region. The proposed absorber works in dual operating bands at 5.1 THz and 11.7 THz. By adjusting the graphene chemical potential, the proposed absorber has the controllability of the resonance frequency to have perfect absorption at various frequencies. The graphene surface plasmon resonance results in sharp and narrow resonance absorption peaks. For incident angles up to 8°, the structure possesses near-unity absorption. The proposed sensor absorber's functionality is evaluated using sensing medium with various refractive indices. The proposed sensor is simulated for glucose detection and a maximum sensitivity of 4.72 THz/RIU is observed. It has a maximum figure of merit (FOM) and Quality factor (Q) value of 14 and 32.49, respectively. The proposed optimal absorber can be used to identify malaria virus and cancer cells in blood. Hence, the proposed plasmonic sensor is a serious contender for biomedical uses in the diagnosis of bacterial infections, cancer, malaria, and other diseases.

Fragmented graphene synthesized on a dielectric substrate for THz applications

Fragmented multi-layered graphene films were directly synthesized via chemical vapor deposition (CVD) on dielectric substrates with a pre-deposited copper catalyst. We demonstrate that the thickness of the sacrificial copper film, process temperature, and growth time essentially influence the integrity, quality, and disorder of the synthesized graphene. Atomic force microscopy and Kelvin probe force microscopy measurements revealed the presence of nano-agglomerates and charge puddles. The potential gradients measured over the sample surface confirmed that the deposited graphene film possessed a multilayered structure, which was modelled as an ensemble of randomly oriented conductive prolate ellipsoids. THz time domain spectroscopy measurements gave theacconductivity of the graphene flakes and homogenized graphitic films as being around 1200 S cm^-1and 1000 S cm^-1, respectively. Our approach offers a scalable fabrication of graphene structures composed of graphene flakes, which have effective conductivity sufficient for a wide variety of THz applications.

Graphene-based dual-functional chiral metamirror composed of complementary 90° rotated U-shaped resonator arrays and its equivalent circuit model

An equivalent circuit model (ECM) using a MATLAB code to analyze a tunable two-layered graphene-based chiral dual-function metamirror, is proposed in this work. The investigated metastructure is composed of complementary U-shaped graphene resonator arrays in the terahertz (THz) region. The ECM analysis could be used for any two-layered chiral metastructure for any frequencies, containing resonators with a thickness less than λ/50. The characteristics of the proposed tunable metamirror were analyzed numerically using the finite element method (FEM) in CST Software to verify the ECM analysis. The proposed metamirror can be used in polarization-sensitive devices in the THz region with simpler biasing without a need for ion gels or similar. It works as a broadband TE and multiband (four bands) TM mirror in the 0.3-4.5 THz bandwidth with a strong linear dichroism (LD) response (up to 96%). The designed mirror is a dynamically tunable, dual-functional structure, requiring only 90° rotation of the incident electromagnetic fields to switch between broadband and multiband spectral behavior making it a promising candidate for future THz intelligent systems. The proposed ECM is in agreement with the FEM results. The ECM analysis provides a simple, fast, and effective way to understand the metamirror's behavior and guides for the design and analysis of graphene-based chiral metastructures in the THz region.

Monolayer-Graphene-Based Tunable Absorber in the Near-Infrared

In this paper, a tunable absorber composed of asymmetric grating based on a graphene-dielectric-metal structure is proposed. The absorption of the absorber can be modified from 99.99% to 61.73% in the near-infrared by varying the Fermi energy of graphene, and the absorption wavelength can be tuned by varying the grating period. Furthermore, the influence of other geometrical parameters, the incident angle, and polarization are analyzed in detail by a finite-difference time-domain simulation. The graphene absorbers proposed in this paper have potential applications in the fields of stealth, sense, and photoelectric conversion. When the absorber that we propose is used as a gas sensor, the sensitivity of 200 nm/RIU with FOM can reach up to 159 RIU-1.