THz Applications Research Articles

Low-Profile Frequency Reconfigurable Graphene-Based Dipole Antennas Loaded with Wideband Metasurface for THz Applications

Low-Profile Frequency Reconfigurable Graphene-Based Dipole Antennas Loaded with Wideband Metasurface for THz Applications

Frequency reconfigurable dielectric lens antenna with graphene nano-antenna feed for THz applications

This paper proposes a frequency reconfigurable dielectric lens antenna (DLA) to compensate high attenuation due to atmospheric losses in the THz region. An aperture coupled graphene plasmonic nanoantenna has been designed and used to feed DLA. Frequency tuning has been introduced by varying the chemical potential of graphene. A lumped elements equivalent circuit for graphene-based feed antenna has also been proposed. The values of the lumped elements are obtained using the vector fitting algorithm. The proposed DLA has high gain, narrow beamwidth, broadside radiation pattern and stable input impedance. Directivity achieved through simulation is up to 7.3 dBi with radiation efficiency of 70 % and 21.2 dBi with radiation efficiency of 65 % for feed antenna and DLA respectively. The performance of DLA has also been studied as the function of feed element, relative permittivity of lens material, radius of lens and off-axis displacement tolerances.

A Theoretical Proposal for an Actively Controlled Ultra-Wideband Absorber Based on Vanadium Dioxide Hybrid Metamaterials

In this study, an ultra-wideband actively tunable terahertz absorber composed of four identical arc-shaped structures made of phase transition material vanadium dioxide (VO2) is presented. A metal ground plane is placed at the bottom and an insulating spacer (quartz) as the middle dielectric layer. Simulation results demonstrate 90% absorption with a broad bandwidth spanning 3 THz (2.7 THz–5.7 THz) under normal incidence. The proposed structure transforms from a reflector to an absorber by changing the conductivity from 200 S/m to 2 × 105 S/m; the absorbance at peak frequencies can be consistently tuned from 4% to 100%. Absorption spectra demonstrate that the polarization angle does not affect the response of the proposed structure. Power loss density (PLD) and impedance-matching theory are further analyzed to learn more about the physical origin of ultra-wide absorption. The ultra-wide operating bandwidth, high absorption efficiency, active tunability, and independence of polarization make the proposed structure an excellent candidate for integration into profound THz applications such as sensors, modulators, and optic-electro switches.

A tunable frequency selective surface integrated high isolation MIMO antenna for THz applications

A novel tunable triple stop band frequency selective surface (FSS) is designed and integrated with the multiple inputs multiple outputs (MIMO) antenna as an isolation structure to verify its validity. The proposed FSS renders excellent isolation characteristics for the reported MIMO system which is verified from the ECC and DG plot. In FSS, the graphene materials are attached at the end of three stubs to realize tuning over the respective bands. The center frequency of the attained triple bands shows an obvious shift from 2.2 to 2.4 THz, 3.15 to 3.5 THz, and 4.6 to 4.9 THz on increasing the chemical potential of graphene from 0 to 1.5 eV. The tuning range can be enhanced by further increasing the applied potential and hence its service might extend to multi or wideband MIMO systems. The proposed FSS unit cell achieves miniaturization to the level of 0.17 λ o. The validity of the proposed FSS is tested with a linear array MIMO antenna. The designed MIMO antenna consists of two linear array antennas separated by 40 μm with each linear array is made up of three individual antenna elements. The introduction of FSS between the linear arrays provides the minimum isolation of 30 dB. The presented highly isolated linear array MIMO antenna realizes maximum efficiency and gain of 83%, 12.2 dBi respectively. The FSS unit cell structure is modeled and their electromagnetic characteristics are simulated by the 3D electromagnetic simulator CST Studio Suite. The auto meshing technology and frequency domain solver are used to acquire S-parameter. This wearable antenna is a significant component in tracking and monitoring systems including the miniaturized multiband antennas exhibiting stable radiation characteristics during bending, stretching, and crumpling conditions to suit military applications. Nowadays, wearable antennas are being designed for military applications by perfectly matching the industry requirements such as thin, low profile, lightweight, low maintenance, robust, conformal, easy integration into clothing without affecting the movement of the soldier.

A compact graphene-based dual-band band-stop filter using new hook-shaped resonator for THz applications

This paper presents a plasmonic Dual-band band-stop filter for terahertz frequencies using new graphene hook-shaped resonators. The structure of this filter is formed by placing two novel symmetrical resonators in the main waveguide path. The finite-difference time-domain (FDTD) and transmission line methods are used to analyze the proposed filter. This adjustable filter has a high-quality factor and two resonance modes in frequencies of 4.1 THz and 13.8 THz. Furthermore, the simulation results show a shift in the resonant frequencies upon changing the base length of the resonators. The designed filter has very small dimensions (350 nm × 500 nm) and can be a proper candidate for integrated optical circuits due to its compact structure and simple design.

Design of film supported single mesa Schottky diodes for above 1THz application with submicron T-junction

A high-performance terahertz Schottky barrier diode (SBD) with a film-supported single mesa structure featuring a low structural parasitic with extremely low dielectric loss is reported in this brief. The fabricated substrate-free SBD is supported with a 2–3 µm polymer film using a low parasitic coupling capacitor of about 0.12 fF. It significantly reduces the electromagnetic coupling effect between the anode and cathode pads by 90% compared with the traditional structure of the same size. To reduce the Schottky junction capacitance at the anode, a submicron T-junction process is used with a low junction capacitance of 0.15 fF. The cutoff frequency calculated by the total capacitance of the diode is up to 4 THz. The substrate-free terahertz monolithic integration circuit based on this film-supported single mesa SBD has better performance in reducing the circuit dielectric loss, exhibiting a remarkable potential for high-frequency applications.

Implementation of a reconfigurable miniaturized graphene-based SIW antenna for THz applications

In this paper, a graphene-based substrate integrated waveguide antenna is presented. The designed structure is considered very desirable for THz applications. The antenna is operated at a central frequency of 1 THz. While the structure is realized on a silicon dioxide substrate. The SIW antenna displays frequency reconfigurable capability due to the special properties of graphene in the THz regime. The designed structure also provides the ability to miniaturize and compress. While the S11 frequency response is less than −12 dB for the target range (0.1 through 2 THz). Finally, a complete report of magnetic field distribution, current density, electric field distribution, radiation efficiency, 2D & 3D radiation pattern, and graphene-based substrate integrated waveguide antenna input impedance is investigated and studied.

Miniaturization and performance enhancement of super wide band four element MIMO antenna using DNG metamaterial for THz applications

A compact semi-hexagonal-shaped MIMO antenna loaded with a planar SRR (split-ring resonator) structure is presented for Terahertz applications. The single radiating element consists of a semi-hexagonal shape metallic patch loaded with SRR-CC (Split ring Resonator-Connected C shape) metamaterial unit cell to enhance the impedance bandwidth as well as gain of the proposed antenna. The proposed antenna element can be easily extended into two-port and four-port MIMO antenna configurations. Four radiating elements of the proposed antenna are printed on 122.4*122.4 μm2 quartz substrate having a thickness of 10 μm. The radiating elements are placed orthogonally to achieve high isolation and miniaturization in the MIMO system. The proposed antenna exhibits super-wide impedance bandwidth (S11 ≤ − 10 dB) in the range of 0.9–10.1 THz (with fractional bandwidth of 168% at f0 = 5.5 THz) while isolation of antenna is more than 20 dB in the whole operating band. The peak gain of the antenna is 7.51 dBi with more than 95% radiation efficiency in the entire band. The stable radiation pattern, high gain, high radiation efficiency, and good diversity performance of the proposed antenna make it suitable for many THz applications such as breast cancer detection, sugar measurement, drug detection.

High-performance tri-band graphene plasmonic microstrip patch antenna using superstrate double-face metamaterial for THz communications

Abstract Recently, graphene-patch antennas have been widely used in communication technology, especially in THz applications due to the extraordinary properties of graphene material. Herein, a graphene-based rectangular microstrip patch antenna is designed on an FR4 substrate material (ɛr = 4.3). A single and double-faced superstrate MTM is placed upon the radiating patch for di erent purposes, such as enhancing the overall antenna performance, protecting the patch from environmental jeopardies, and generating a multiband resonance frequency. A single face superstrate triangle SRR unit was used to produce a dual-band frequency at 3.5 and 4.331 THz. The S 11 of the dual-band structure is achieved to be −26.78 dB and −46.25 dB with a bandwidth of 400 GHz and 460 GHz, respectively. The double face superstrate MTM unit cell of the triangle SRR printed on the opposite face gives another resonant frequency, so, triple frequency bands of 2.32, 3.35, and 4.38 THz with a wide impedance bandwidth of 230, 520, and 610 GHz, were generated, respectively. The double-face superstrate MTM not only enhances the antenna performance but also generates another resonant frequency that could be used in the next 6G communications. The proposed antenna is designed and optimized using two commercial 3D full-wave software, CST Microwave Studio and Ansoft HFSS, to validate the results.

Circularly polarized tunable graphene-patch over SiO2 substrate for THz applications

In this communication, a graphene-based radiator is designed and analyzed at THz frequency. A tilted dumbbell-shaped aperture is loaded on a graphene patch for creating degenerated orthogonal mode with 90° phase shift, which in turn produces circularly polarized (CP) waves. Graphene patch is placed over SiO2 substrate. Slot loading in graphene patch shifts the frequency band due to a reduction in effective permittivity. Another important feature of the proposed antenna is to provide a tunable feature in the frequency response and axial ratio by changing the chemical potential. The proposed antenna operates from 5.85 to 6.05 THz. 3-dB axial ratio is obtained from 5.85 to 5.95 THz. Good value of gain, as well as left-handed circular polarization characteristics, makes the proposed THz antenna in sensing applications.

Progress in THz Rectifier Technology: Research and Perspectives.

Schottky diode (SD) has seen great improvements in the past few decades and, for many THz applications, it is the most useful device. However, the use and recycling of forms of energy such as solar energy and the infrared thermal radiation that the Earth continuously emits represent one of the most relevant and critical issues for this diode, which is unable to rectify signals above 5 THz. The goal is to develop highly efficient diodes capable of converting radiation from IR spectra to visible ones in direct current (DC). A set of performance criteria is investigated to select some of the most prominent materials required for developing innovative types of electrodes, but also a wide variety of insulator layers is required for the rectification process, which can affect the performance of the device. The current rectifying devices are here reviewed according to the defined performance criteria. The main aim of this review is to provide a wide overview of recent research progress, specific issues, performance, and future directions in THz rectifier technology based on quantum mechanical tunneling and asymmetric structure.

Graphene-based double-loaded complementary split ring resonator (CSRR) slotted MIMO patch antenna for spectroscopy and imaging THz applications

Graphene-based double-loaded complementary split ring resonator (CSRR) slotted MIMO patch antenna for spectroscopy and imaging THz applications

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 the ac conductivity of the graphene flakes and homogenized graphitic films as being around 1200 S cm−1 and 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.

2022 IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications

2022 IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications

Comparison of individual and common wavelength-operation for 785 nm Y-branch DBR ridge waveguide diode lasers with adjustable spectral distance.

An experimental comparison between individual and common wavelength-operation of a Y-branch distributed Bragg reflector (DBR) ridge waveguide (RW) laser at 785 nm with an electrically adjustable spectral distance is presented. The dual-wavelength Y-branch laser combines two laser cavities via a Y-section to a common output section. DBR gratings with different grating periods are associated with the two cavities, which set the emission wavelengths of the two branches. Implemented resistive heater elements allow separate wavelength tuning of the two branches, which can be operated individually for alternating emission wavelengths in applications such as differential absorption spectroscopy or shifted excitation Raman difference spectroscopy. Common wavelength operation simultaneously generates two emission lines suitable for the generation of THz radiation using difference frequency mixing. Hereby, the devices could potentially be used as single-chip light sources for a combination of Raman and THz applications. For the wavelength-operation comparison presented, the devices were operated up to optical output powers of about 105 and 185 mW in individual and common wavelength-operation mode, respectively. In individual operation mode, the devices show spectral single-mode emission over the whole operation range. In common operation mode, the spectral emission is predominantly single mode up to an optical output power of 65 mW. In both operation modes, mode hops typical for DBR lasers occur. At an optical output power of 50 mW, tuning of the spectral distance between the two wavelengths using the implemented resistor heaters is demonstrated. In both modes of wavelength operation, a flexible frequency difference between 0 and 0.8 THz (0 and 1.6 nm) with predominantly single-mode spectral emission is obtained.

2022 IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications

2022 IEEE MTT-S International Microwave Workshop Series on Advanced Materials and Processes for RF and THz Applications

Compact dual-core liquid crystal photonic crystal fiber polarization splitter for terahertz applications

Compact terahertz dual core photonic crystal fiber (THz-DC-PCF) polarization splitter is presented and analyzed. The dual core is infiltrated with a nematic liquid crystal (NLC) of type E7 that produces high birefringence between the two polarized modes. The suggested design has a TOPAS as a background material with low material loss. The effect of the structure geometrical parameters is investigated to achieve compact polarization splitter. In addition, the impact of the temperature dependence of the NLC in the THz range on the performance of the suggested splitter is also studied. The numerical results show that the THz-DC-PCF has a device length of 3.848 cm with a low crosstalk of −62 dB for the x- and y-polarized modes. Further, large bandwidth of 58 GHz is achieved around the central frequency of 0.6 THz where the crosstalk is better than −10 dB with a good fabrication tolerance. Therefore, the design is less sensitive to the perturbation introduced during the fabrication process with a strong potential for THz applications.

Graphene-based ultra-wideband absorber for terahertz applications using hexagonal split ring resonators

In this paper, a hexagonal split ring ultra-wideband absorber is proposed at THz frequency. The proposed structure consists of four graphene based hexagonal split rings, a dielectric substrate and graphene layer at the bottom. The proposed absorber achieves an ultra-wideband absorption characteristics from 0.95 THz to 2.96 THz with percentage bandwidth of 102.8% and bandwidth of 2.1THz with absorptivity beyond 90%. Also, 100% absorption is achieved from 2.07 to 2.33 THz making this unique feature for THz applications. Ultra-wideband response is mainly resulted because of overlapping of strong Electromagnetic (EM) resonance of the four split rings. The modes generated within the graphene split rings are merged for achieving ultra wide band response. Furthermore, the proposed absorber is polarization insensitive because of symmetry geometry and also exhibits absorption greater than 90% for incidence angle up to 75° for both TE and TM waves. In addition, tunability is achieved by varying the graphene chemical potential. These features make the proposed metal free absorber useful for terahertz applications and future nanoscale systems. The proposed absorber shows narrow absorption characteristics for higher graphene chemical potential values which can also be utilized for sensor applications.

Wideband Slotline-to-Microstrip Transition for 210–375 GHz Based on Marchand Baluns

This article describes the design and cryogenic measurement of a novel slotline-to-microstrip transition based on Marchand baluns. The proposed transition is an attractive solution for numerous THz applications due to its remarkable broadband performance and compactness. For instance, such transition could be considered for wideband devices covering the frequency band 210–375 GHz. The suggested transition is designed on a thin silicon substrate and employs superconducting Nb as the electrode for the slotline and microstrip lines. In order to verify the performance of the designed transition, we fabricated a dedicated test structure consisting of two transitions connected back-to-back and integrated with E-probes at the waveguide interfaces. Due to the inherent bandwidth limitation of the E-probes, two different test structures for 210–295 GHz and 295–375 GHz were employed to characterize the proposed transition over the whole frequency band. The experimental verification performed at cryogenic temperatures showed results consistent with the simulation. Moreover, the cryogenic measurements indicated a remarkable 56% fractional bandwidth with an insertion loss as low as 0.3 dB for the fabricated slotline-to-microstrip transition.

A Multi-Layered Borophene-Silica-Silver Based Refractive Index Sensor for Biosensing Applications Operated at the Infrared Frequency Spectrum

We have presented the borophene based refractive index sensor for the infrared frequency spectrum of 188 to 250 THz (1.2–1.6 µm) range. The proposed structure was formed by using the Silver-borophene-silica-Ag layered structure. The behaviour of the different analyte (with a different refractive index) material is numerically calculated by placing it on the top of the structure. The behaviour of the structure is identified in terms of absorption, reflectance, physical parameter variation, and oblique angle incident conditions. The presented results provide the basic idea of selecting optimized structure dimensions to get the specific resonating response. This sensor offers the Figure of Merit (FOM) of 444 RIU−1 with high sensitivity of 660 THz/RIU (4471 nm/RIU). The refractive index sensor also provides wide-angle stability for (0° to 80°) for the wide frequency range (239 to 245 THz and 207 to 209 THz). This sensor is developed on the silver metal layer (not required to separate borophene from its origin metal deposition process) and easily fabricated using standard boron fabrication and layered deposition techniques. The results of the proposed structure make it possible to design a basic biosensor structure. This device is also applicable for various THz and biomedical applications.