Terahertz Frequency Applications Research Articles

Comparing the plasmon dispersion in graphene and MoS2 nanoribbons array under Electromagnetic excitation

Terahertz properties of different materials have been recently studied due to their wide applications in optoelectronics, industry, product inspection, and spectroscopy. Terahertz frequency applications are promising for the medical field as they are considered safe frequencies. Previous terahertz plasma response focused on 2D materials like graphene and transition metal dichalcogenides (TMDs) due to their favourable electronic properties, high electric conductivity, and their band gap characteristics, so they can be used in electronic devices. Some of these materials showed good biocompatibility so they can be used in biomedical applications. Since graphene has zero band gap, researchers are continuously exploring methods to increase its band gap to be used in electronics. Graphene heterostructures or metamaterials are ways to enhance graphene characteristics for specific applications. This work investigates the possibility of using MoS2 with graphene in THz applications. The plasmon dispersion for graphene and MoS2 nanoribbon array structure is compared. Both graphene and MoS2 behave differently in response to terahertz radiation due to their different band gaps. The results showed that MoS2 exhibits a plasmonic response in the THz region at high carrier concentrations. This opens up opportunities for MoS2 to be employed in THz sensors, both independently and in conjunction with graphene within heterostructures or metamaterials for power sources and detectors. These advancements hold significant potential for the future THz imaging and communication technologies.

Low thermal SRR metamaterial design with multi-layered structured for terahertz frequency applications

The goal of this study is to examine the effects of split-ring resonators (SRR) with compact square shapes at terahertz frequencies. Therefore, a unique and miniaturised SRR metamaterial was constructed in this work. Every simulation was performed by using the Computer Simulation Technology software to explore the radar cross-section (RCS), reflection and transmission coefficients. Furthermore, MATLAB software was adopted to calculate the characteristics of an effective medium for the recommended SRR metamaterial. The initial metamaterial structure consists of a single layer that exhibits lower peak points with unsatisfactory magnitude values. Therefore, by adopting the trial and error technique, a few parametric studies were conducted to select the optimised metamaterial. For instance, various square-shaped design analyses, investigation of different substrate materials with a dielectric constant between 3.5 and 11.9, multi-layer structures, and larger-scale analyses were explored in this work. According to the observation, the quintuple layer SRR design was adopted since it possesses multiple resonances with reasonable magnitude values at 2.58, 3.78, 7.54, 7.77, 8.21, 8.5, and 8.88 THz. Moreover, the proposed quintuple-layer metamaterial helps to reduce the RCS values from −128.5 to −90.93 dBm2 at 2.58 THz when adopting an array cell structure. Moreover, this work also examines and analyses the thermal distribution of the proposed quintuple-layer metamaterial at all resonance frequencies. The observation revealed only small temperature distribution discrepancies occurred on the design structure compared to all peak points. In a nutshell, the proposed quintuple-layer SRR metamaterial achieved the purpose of this research and is practically applicable for the terahertz frequency.

Symmetric left-handed split ring resonator metamaterial design for terahertz frequency applications

This work focused on the novel symmetrical left-handed split ring resonator metamaterial for terahertz frequency applications. A compact substrate material known as Silicon with a dimension of 5 µm was adopted in this research investigation. Moreover, several parameter studies were investigated, such as clockwise rotation, array and layer structure designs, larger-scale metamaterials, novel design structure comparisons and electric field distribution analysis. Meanwhile, two types of square-shaped metamaterial designs were proposed in this work. The proposed designs exhibit double and single resonance frequencies respectively, likely at 3.32 and 9.24 THz with magnitude values of − 16.43 and − 17.33 for the first design, while the second design exhibits a response at 3.03 THz with a magnitude value of − 19.90. Moreover, the verification of these results by adopting High-frequency Structure Simulator software indicates only slight discrepancies which are less than 5%. Furthermore, the initial response of the proposed designs was successfully altered by simply rotating the design clockwise or even increasing the dimension of the design. For instance, the first resonance frequency is shifted to the lower band when the first proposed design was rotated 90°. On the other hand, by increasing the size of the metamaterial, more than nine resonance frequencies were gained in each symmetric design. Furthermore, the symmetric metamaterial with a similar width and length of 10 µm dimension was adopted for both design structures to construct an equivalent circuit model by utilising Advanced Design System software. Finally, both unit cell designs were utilised to explore the absorption performances which exhibit four and five peak points. Overall, the altering behaviour by changing physical properties and compact design with acceptable responses become one of the novelties of this research investigation. In a nutshell, the proposed designs can be utilised in terahertz frequency which gives optimistic or advantageous feedback and is relatively suitable for the adopted frequency range.

Reduction of Radar Cross Section by Adopting Symmetrical Coding Metamaterial Design for Terahertz Frequency Applications.

This work focused on the novel and compact 1-bit symmetrical coding-based metamaterial for radar cross section reduction in terahertz frequencies. A couple of coding particles were constructed to impersonate the elements '0' and '1', which have phase differences of 180°. All the analytical simulations were performed by adopting Computer Simulation Technology Microwave Studio 2019 software. Moreover, the transmission coefficient of the element '1' was examined as well by adopting similar software and validated by a high-frequency structure simulator. Meanwhile, the frequency range from 0 to 3 THz was set in this work. The phase response properties of each element were examined before constructing various coding metamaterial designs in smaller and bigger lattices. The proposed unit cells exhibit phase responses at 0.84 THz and 1.54 THz, respectively. Meanwhile, the analysis of various coding sequences was carried out and they manifest interesting monostatic and bistatic radar cross section (RCS) reduction performances. The Coding Sequence 2 manifests the best bistatic RCS reduction values in smaller lattices, which reduced from -69.8 dBm2 to -65.5 dBm2 at 1.54 THz. On the other hand, the monostatic RCS values for all lattices have an inclined line until they reach a frequency of 1.0 THz from more than -60 dBm2. However, from the 1.0 THz to 3.0 THz frequency range the RCS values have moderate discrepancies among the horizontal line for each lattice. Furthermore, two parametric studies were performed to examine the RCS reduction behaviour, for instance, multi-layer structures and as well tilt positioning of the proposed coding metamaterial. Overall it indicates that the integration of coding-based metamaterial successfully reduced the RCS values.

Development and Analysis of Coding and Tailored Metamaterial for Terahertz Frequency Applications.

This study represents the development and analysis of the types of metamaterial structures for terahertz frequency. Recently, investigations about unique coding metamaterial have become well-known among the scientific community since it can manipulate electromagnetic (EM) waves by utilizing various coding sequences. Therefore, several coding and tailored metamaterial designs were compared and numerically analyzed the performances in this research work. The 1-bit coding metamaterial made up of only “0” and “1” elements by adopting two types of unit cells with 0 and π phase responses were analyzed for the coding metamaterial. Moreover, for the numerical simulation analyses, the well-known Computer Simulation Technology (CST) Microwave Studio software was adopted. This investigation focused on the frequency ranges from 0 to 5 THz. On the other hand, the proposed designs were simulated to find their scattering parameter behavior. The comparison of coding and tailored metamaterial revealed slight differences in the RCS values. The coding metamaterial designs manifested RCS values less than −50 dBm2, while tailored metamaterial designs exhibited less than −60 dBm2. Furthermore, the proposed designs displayed various transmission coefficient result curves for both types of metamaterial. Moreover, the bistatic far-field scattering patterns of both metamaterial designs were presented in this work. In a nutshell, the 1-bit coding metamaterial with a unique sequence can influence the EM waves and realize different functionalities.

High frequency noise calculation in Schottky metal-semiconductor-metal structure and parameter retrieval of nanometric CdTe structure

An analytical model to calculate the noise spectral density in Metal-Semiconductor-Metal (M-S-M) structure of Schottky contacts has been developed. The model based on the linearization of Langevin approach of carrier motion inside a bulk semiconductor, and taking into account the fluctuations in the leakage current through the structure due to the barrier lowering by the image force effect. Particularly, the calculations describe two quantities of noise: electric field and total current spectral densities. The results obtained from the Au-CdTe-Au Schottky structure exhibit sharp resonances due to the effect of plasma frequency oscillations and the relative thickness of the depletion region below the anode. Moreover, the noise current spectrum exhibits Lorentzian behavior when the M-S-M has the form of a homogenous structure (thickness of depletion region≈0). These results are in agreement with that reported by Monte Carlo technique of metal Schottky structure. The discussion of the noise spectra as a function of structure parameters revealed that the nanometric M-S-M structures with undoped CdTe can be used as Schottky detectors/emitters in Terahertz frequency applications.

Efficient coupling of double-metal terahertz quantum cascade lasers to flexible dielectric-lined hollow metallic waveguides.

The growth in terahertz frequency applications utilising the quantum cascade laser is hampered by a lack of targeted power delivery solutions over large distances (>100 mm). Here we demonstrate the efficient coupling of double-metal quantum cascade lasers into flexible polystyrene lined hollow metallic waveguides via the use of a hollow copper waveguide integrated into the laser mounting block. Our approach exhibits low divergence, Gaussian-like emission, which is robust to misalignment error, at distances > 550 mm, with a coupling efficiency from the hollow copper waveguide into the flexible waveguide > 90%. We also demonstrate the ability to nitrogen purge the flexible waveguide, increasing the power transmission by up to 20% at 2.85 THz, which paves the way for future fibre based terahertz sensing and spectroscopy applications.

Advances in Polarizer Technology for Terahertz Frequency Applications

As investigations into potential applications of terahertz technology grow, there is an increasing need for improved terahertz optical components such as polarizers. To determine the optical properties of a sample accurately, the polarization properties of the light must also be known accurately. Many terahertz emitters will have both horizontal and vertical polarization components and often assumptions are made about device characteristics without measuring them-even the position of excitation beam on the photoconductive emitter can affect the resulting terahertz electric field and so the exact optical properties of a given device will vary depending on how they are configured. Polarizers operating at terahertz frequencies can be used to characterize the electric field accurately or remove unwanted components as long as the polarizer is of sufficiently high performance. In this paper we review the key properties of polarizers and look at recent advances in their design and development at terahertz frequencies.

Off-Axis Measurements on a mm-Wave Optically Controlled Lens Antenna

We present initial experimental data demonstrating the off-axis scanning performance and beam fidelity of a range of optically controlled transmission-type photo-injected Fresnel zone plate antenna (tx-piFZPA) designs at 94 GHz, based on a novel architecture. Several tx-piFZPA designs were characterized at a range of off-axis beam locations, up to 45 ° from boresight. Initial results indicate scan losses up to 5 dB at 10 ° deflection and increasing up to 12 dB at 45 ° deflection, across several tx-piFZPA designs. In addition, measured tx-piFZPAs have demonstrated highly repeatable and precise beam control, achieving a minimum pointing resolution of Δθ = 0.005°. The piFZPA method proves to be a promising cost-effective nonmechanical beam-steering solution for high millimeter- and terahertz-frequency applications.

Study of Goubau lines for submillimetre wave and terahertz frequency applications

The performance of Goubau lines at submillimetre and terahertz frequency are studied. Both ultra-wide and narrow bandwidth applications are examined and selections of parameters are provided. Characteristics of lines with different dimensions are calculated and displayed. Comparisons with other types of transmission lines are also performed.

Low-loss monolithic transmission lines for submillimeter and terahertz frequency applications

The design and construction of low-loss monolithic transmission lines are critical to systems which require that terahertz-power be guided to the antenna front ends. Two types of novel monolithic guiding structures, designed for the 0.3-2.0 THz and 0.1-0.3 THz ranges, respectively, are proposed. The novel waveguides are constructed from dielectric materials and structures which are available in monolithic technology so that the integration of active devices is possible. Propagation in each of the waveguides is characterized over relevant frequency ranges by applying a mode-matching technique, which takes into account all forms of electromagnetic coupling as well as losses in the dielectrics. The structures are predicted to exhibit excellent power confinement and low losses. >