Design of a Penta-Band Graphene-Based Terahertz Metamaterial Absorber with Fine Sensing Performance

Asymmetric metamaterial sandwich structure with NIM characteristics for THz imaging application

This study presented a unique, miniaturised asymmetric interconnected vertical stripe (IVS) design for terahertz (THz) frequency applications. Therefore, this research aimed to achieve a frequency response of 0 to 10 THz using a 5 × 5 µm2 Silicon (Si) substrate material. Meanwhile, various parametric examinations were conducted to investigate variations in the performance. For example, the unit cell selection process was carefully examined by using various design structures and modifying the structure by adding split gaps and connecting bars between vertical stripes. Furthermore, the proposed sandwich structure design was used to compute the absorbance and reflectance properties. All the analytical examinations were executed utilising the Computer Simulation Technology (CST) 2019 software. The introduced IVS metamaterial exhibits negative index behaviour and has a single resonance frequency of 5.23 THz with an acceptable magnitude of - 24.38 dB. Additionally, the quadruple-layer IVS structure exhibits optimised transmission coefficient behaviour between 3 and 6 THz and 7 to 9 THz, respectively. However, the magnitude of the transmission coefficient increased with the number of material layers. Besides that, the absorbance study shows that using a quadruple-layer structure obtains unique and promising results. Overall, the proposed asymmetric IVS metamaterial design achieves the required performance by using a compact structure rather than extending the dimensions of the design.

High FOM PCF-SPR refractive index sensor based on MgF2-Au double-layer films

A simple twin-core D-shape photonic crystal fiber sensor based on surface plasmon resonance (SPR) is designed for the measurement of refractive indices (RI). The twin-core D-shape structure enhances the SPR effect, and the M g F 2-Au dual-layer film narrows the linewidth in the loss spectrum, consequently improving both the sensitivity and figure of merit (FOM). The properties of the sensor are analyzed by the finite element method. In the RI range of 1.32-1.42, the maximum wavelength sensitivity, FOM, and resolution are 62,000 nm/RIU, 1281R I U -1, and 1.61×10-6, respectively.

Modified 3D Graphene for Sensing and Electrochemical Capacitor Applications

Less defective, nitrogen-doped 3-dimensional graphene (N3DG) and defect-rich, nitrogen-doped 3-dimensional graphene (N3DG-D) were made by the thermal CVD (Chemical Vapor Deposition) process via varying the carbon precursors and synthesis temperature. These modified 3D graphene materials were compared with pristine 3-dimensional graphene (P3DG), which has fewer defects and no nitrogen in its structure. The different types of graphene obtained were characterized for morphological, structural, and compositional assessment through Scanning Electron Microscopy (SEM), Raman Spectroscopy, and X-ray Photoelectron Spectroscopy (XPS) techniques. Electrodes were fabricated, and electrochemical characterizations were conducted to evaluate the suitability of the three types of graphene for heavy metal sensing (lead) and Electric Double-Layer Capacitor (EDLC) applications. Initially, the various electrodes were treated with a mixture of 2.5 mM Ruhex (Ru (NH3)6Cl3 and 25 mM KCl to confirm that all the electrodes underwent a reversible and diffusion-controlled electrochemical process. Defect-rich graphene (N3DG-D) revealed the highest current density, followed by pristine (P3DG) and less-defect graphene (N3DG). Further, the three types of graphene were subjected to a sensing test by square wave anodic stripping voltammetry (SWASV) for lead detection. The obtained preliminary results showed that the N3DG material provided a great lead-sensing capability, detecting as little as 1 µM of lead in a water solution. The suitability of the electrodes to be employed in an Electric Double-Layer Capacitor (EDLC) was also comparatively assessed. Electrochemical characterization using 1 M sodium sulfate electrolyte was conducted through cyclic voltammetry and galvanostatic charge-discharge studies. The voltammogram and the galvanostatic charge-discharge (GCD) curves of the three types of graphene confirmed their suitability to be used as EDLC. The N3DG electrode proved superior with a gravimetric capacitance of 6.1 mF/g, followed by P3DG and N3DG, exhibiting 1.74 mF/g and 0.32 mF/g, respectively, at a current density of 2 A/g.

Design of a High Q-Factor Label-Free Optical Biosensor Based on a Photonic Crystal Coupled Cavity Waveguide

In recent years, there has been a significant increase in research into silicon-based on-chip sensing. In this paper, a coupled cavity waveguide (CCW) based on a slab photonic crystal structure was designed for use as a label-free biosensor. The photonic crystal consisted of holes arranged in a triangular lattice. The incorporation of defects can be used to design sensor devices, which are highly sensitive to even slight alterations in the refractive index with a small quantity of analyte. The plane wave expansion method (PWE) was used to study the dispersion and profile of the CCW modes, and the finite difference time domain (FDTD) technique was used to study the transmission spectrum, quality factor, and sensitivity. We present an analysis of adiabatically coupling light into a coupled cavity waveguide. The results of the simulation indicated that a sensitivity of 203 nm/RIU and a quality factor of 13,360 could be achieved when the refractive indices were in the range of 1.33 to 1.55.

Design of metamaterial perfect absorbers in the long-wave infrared region

We designed a narrow-band metamaterial absorber (NMA) and an ultra-broadband metamaterial perfect absorber (UMPA) based on the impedance matching theory. The narrow-band metamaterial absorber mainly consists of Si3N4 cylinders with Si3N4 and Ti substrates. Numerical analysis shows that the absorption peak of the NMA is about 99.9% and the absorption bandwidth with more than 90% absorption is about 4.8 μm (9.5-14.3 μm). To further extend the absorption bandwidth, an ultra-broadband absorber was designed by integrating a Ti hyperbolic rectangle into the Si3N4 cylinder of the NMA. Numerical analysis shows that the absorption bandwidth of the UMPA is up to 10 μm (7-17 μm) with an average absorption rate of 96.6%. The designed UMPA has polarization insensitive properties with wide-angle absorption characteristics, and the average absorption can reach 85% and 76% in transverse magnetic (TM) and transverse electric (TE) modes, respectively, at 60° oblique incidence. The high absorption and wide band are mainly dominated by localized surface plasmon resonance, Fabry-Perot resonance and inter-resonance interactions. The designed absorber achieves excellent absorption in the long infrared wavelength band, which has potential applications in energy absorption, infrared sensing and other fields.

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.