Terahertz Waveguide Research Articles

Design of a Split Ring Resonator Integrated with On‐Chip Terahertz Waveguides for Colon Cancer Detection

AbstractFinite element method (FEM) simulations (employing ANSYS High Frequency Structure Simulator, HFSS) are used to investigate the response of terahertz (THz) frequency range split‐ring resonators (SRRs) integrated with on‐chip THz waveguides to cancerous tissues. Two‐port S‐parameter simulations are performed to obtain the transmission spectra (S21) of a planar Goubau line (PGL) integrated with an SRR. Permittivity and loss tangent of the colonic tissues are both taken into account in the numerical simulation. The transmission spectra of the SRR integrated PGL are obtained for cancerous and healthy tissues in close proximity to the SRR, and it is found that they can be distinguished by the resonant frequency shift of the SRR induced by dielectric loading. The electric field distribution and magnitude near the SRR for various capacitive gap widths of SRR are investigated to understand how the gap width affects the maximum electric field magnitude and the vertical extent of the electric field in the gap area. The simulated imaging of colonic tissue consisting of healthy and cancerous tissues using the SRR integrated PGL device with a protective layer on it is performed, showing how the technique could in principle be used to distinguish tumor margins with realistic THz dielectric parameters.

Supercritical fluid deposition for conformal Cu film formation on sub-millimeter-scale structures used to fabricate terahertz waveguides

A small-volume hot-wall batch reactor with excess precursor loading was proposed for supercritical fluid deposition of Cu during the fabrication of sub-millimeter-scale, metal-coated terahertz (THz) wave devices. Conformal film formation was experimentally demonstrated, validating our method. Our method enables a much higher precursor concentration (at least 20 mol m−3) than the conventional method (below 2 mol m−3), facilitating conformal film formation on sub-millimeter structures. Kinetic analysis revealed that our proposed method was applicable for fabricating rectangular metal-coated THz waveguides; furthermore, it was promising for monolithically integrated THz wave devices.

Terahertz Resonances of Transverse Standing Waves in a Corrugated Plate Waveguide

Terahertz (THz) propagation in periodic structures has attracted great attention because of its captivating electrical and optical properties. However, the created THz band gaps are usually considered to be caused by Bragg resonances. Here, we theoretically and numerically investigate THz resonances and their related band gap properties in a periodically corrugated plate waveguide with arbitrary wall profiles. It has been found that the corrugated structure can cause not only the interaction between the same transverse modes, called the Bragg resonance, but also the strong interference between different transverse standing wave modes, which occurs in a higher frequency range with quite different features. Unlike the well-known Bragg resonance, the excited resonances between different transverse modes can produce the stronger energy attenuation and wider frequency band gap. The dispersion diagram is depicted to clearly describe the relationship between these resonances and waveguide geometries. Using this method, we can properly estimate the band gap structure of THz waveguides, which has been confirmed by the simulated results. In addition, these two resonances can be easily manipulated by changing the symmetry of waveguide structures. These findings on THz propagation in periodic waveguides could be applicable in high performance devices, such as filters, modulators and switches.

Terahertz beam splitter based on mode coupling of subwavelength waveguides

A broadband terahertz waveguide type beam splitting scheme for multi-port outputs is proposed, which is realized by uniform mode coupling of the multiple subwavelength waveguides. The full-vectorial finite-difference beam propagation method (BPM) is adopted to numerically analyze the mechanism and splitting performance of the beam splitter, and the influence of the supporting structure between the waveguides and the outer dielectric ring on its transmission performance is studied. In particular, a 1 × 8 beam splitter is designed with a coupling zone length of 1.02 mm, which can couple the input THz wave into eight output ports uniformly. The insertion loss of the device is 9.3 dB at the center frequency of 0.54 THz and the bandwidth can reach 0.21 THz with insertion loss lower than 10 dB.

Ultra‐Wideband Integrated Graphene‐Based Absorbers for Terahertz Waveguide Systems

AbstractThis article presents novel graphene‐based absorber materials which can be directly integrated in terahertz waveguide systems. A simple, low‐cost integration method is developed, allowing graphene augmented inorganic nanofibers to be embedded inside a metallic waveguide. In contrast to existing absorbers, the ability to embed such materials in a metallic waveguide allows them to be integrated into complete terahertz systems for large‐scale applications. The electromagnetic properties of such materials are then examined using standard network analysis techniques. A wideband measurement setup is developed to enable measurement of a single sample from 67 to 500 GHz, eliminating the need to fabricate multiple samples. The porosity of the integrated material leads to excellent electromagnetic performance across a wide range of frequencies. The samples are found to have a reflection coefficient less than −10 dB for frequencies above 200 GHz, while their attenuation per unit length exceeds 35 dB mm−1. The low reflectivity of the material allows it to be used in systems applications where undesired reflections must be avoided. The electromagnetic shielding effectiveness of the material is assessed, with a total effectiveness of 20–45 dB observed for 0.84 mm thick samples.

Valley topological line-defects for Terahertz waveguides and power divider

Topological edge states exhibit low backscattering, unidirectional energy propagation, and robust immunity to sharp bends and defects in photonic topological waveguides. These properties of topological waveguides are expected to be applied in terahertz (THz) technology and expand to functional devices. Here, we introduce a line-defect into valley Hall photonic crystals (VPCs) and discuss the influence of line-defect to edge states of valley Hall photonic topological waveguide. The introduction of line-defect can greatly improve the design freedom of topological waveguides, and is expected to be extended to functional devices. We constructed a straight VPC waveguide and a Z-shaped VPC waveguide to verify the topological valley line-defect edge state and the single-mode transmission of topological line-defect states is excited by a rectangular waveguide with a tapered coupling structure. The VPC waveguide has low bending loss and a working bandwidth (return loss < −10 dB) from 0.4 to 0.422 THz. We propose a topological power divider (TPD) operating at THz frequencies and achieved −3.3 per port equal power divide. Furthermore, this TPD is robust against defects and is promising for on-chip THz integration.

Suitability of metallic materials for constructing metal-coated dielectric terahertz waveguides

We aimed to identify metallic materials that could be used to construct metal-coated dielectric terahertz (THz) waveguides. We examined seven different metals: gold (Au), copper (Cu), silver (Ag), aluminum (Al), nickel (Ni), chromium (Cr), and titanium (Ti). The propagation losses of our in-house metal-coated dielectric parallel-plate waveguide (PPWG) were experimentally determined. We developed a physical model to estimate the two key parameters determining the performance of metal-coated waveguides: the critical film thickness required for bulk material-like behavior and the propagation loss in a film with a thickness greater than critical film thickness. Film quality, as revealed by the thickness-dependent electrical conductivity of the metal film, was measured prior to experiments and used for model calculations because propagation loss is influenced by film conductivity, which differs from bulk conductivity and depends on film thickness. After experimentally validating the applicability of the model to different metals, suitable metals were identified based on the two key parameters calculated by the model, assuming the same high film quality. Cu was identified as the optimal metal. The effect of film quality on the two key parameters is discussed in this paper. The impact of the surface oxide (CuOx) layer on THz wave propagation was experimentally evaluated using CuOx/Cu-coated PPWG; no detectable transmittance decrease was observed regardless of the CuOx thickness (1.5–176 nm), when the underlying Cu film was of sufficient thickness. Our model also indicated that a CuOx layer &amp;lt;1 μm-thick had a negligible impact on THz wave propagation. Thus, native oxidation is not an issue when using Cu.

A robust PEEK/silver-coated hollow waveguide for terahertz bendable transmission in hot and cold environments

The rapid development of terahertz (THz) technology makes the study of THz waveguide indispensable. In the current research on terahertz waveguides, there is still a lack of flexible waveguides for stable transmission in hot/cold environments. In this work, a mechanically strong and high-and low-temperature resistant PEEK (polyether ether ketone) tube was selected as the structural tube. On the basis of plasma treatment on the surface of PEEK tube, a high-performance silver (Ag) coated hollow THz waveguide was prepared. The results indicate that the plasma treatment can greatly improve the surface bonding strength between the PEEK tube and the silver film, which enhances the bending performance of the PEEK/Ag-coated hollow waveguide. For the 3-mm-bore waveguide, the minimum bending radius can reach 3 cm without damaging waveguide structure. Meanwhile, the superior thermal properties of the PEEK tube and its good bonding with the silver layer allow the waveguide to transmit terahertz waves stably at high (150 °C) and low (−78.5 °C) temperatures with the no more than 5% fluctuation in output power. In addition, the PEEK/Ag-coated hollow waveguide has extremely low transmission loss compared to other previously reported terahertz waveguides. In the 4.25-mm-bore waveguide sample, the straight transmission losses for 0.3 and 0.1 THz wave is 0.67 and 1.26 dB/m at room temperature (25 °C). When the bending angle of the 4.25-mm-bore waveguide increases from 0 to 180°, the transmission loss increases to 1.02 dB/m at 0.3 THz.

All-optical terahertz logic gates based on coupled surface plasmon polariton sub-wavelength waveguiding in bulk Dirac semimetal

Bulk Dirac semimetals (BDS) are quantum fermionic system also identified as three-dimensional counterpart of graphene exhibit rich plasmonic response at terahertz (THz) frequencies. In this work, we report THz waveguiding in the BDS curved geometries, which confine electromagnetic field in the form of symmetrically coupled surface plasmon polariton (SPP) modes. We demonstrate how the sub-wavelength BDS-Air-BDS multilayer system with various bending and corners efficiently confines THz beam with minimal bending losses. The realized sub-wavelength THz waveguiding in BDS system is advantageous than other approaches like zero-refractive index metamaterials. Based on the symmetric coupled SPP waveguiding in BDS multilayers, all-optical logic gates including OR, AND, XOR, NOR and NAND are realized at THz waveband. The realized THz logic gates exhibit optimal threshold transmission (at least 50%) for defining ON-OFF states. We anticipate that the illustrated waveguiding and logic gate applications in BDS system could open an avenue for optoelectronic applications at THz frequencies.

Versatile metal-wire waveguides for broadband terahertz signal processing and multiplexing

Waveguides play a pivotal role in the full deployment of terahertz communication systems. Besides signal transporting, innovative terahertz waveguides are required to provide versatile signal-processing functionalities. Despite fundamental components, such as Bragg gratings, have been recently realized, they typically rely on complex hybridization, in turn making it extremely challenging to go beyond the most elementary functions. Here, we propose a universal approach, in which multiscale-structured Bragg gratings can be directly etched on metal-wires. Such an approach, in combination with diverse waveguide designs, allows for the realization of a unique platform with remarkable structural simplicity, yet featuring unprecedented signal-processing capabilities. As an example, we introduce a four-wire waveguide geometry, amenable to support the low-loss and low-dispersion propagation of polarization-division multiplexed terahertz signals. Furthermore, by engraving on the wires judiciously designed Bragg gratings based on multiscale structures, it is possible to independently manipulate two polarization-division multiplexed terahertz signals. This platform opens up new exciting perspectives for exploiting the polarization degree of freedom and ultimately boosting the capacity and spectral efficiency of future terahertz networks.

Highly birefringent hollow-core anti-resonant terahertz fiber with a thin strut microstructure.

A novel highly birefringent and low transmission loss hollow-core anti-resonant (HC-AR) fiber with a central strut is proposed for terahertz waveguiding. To the best of our knowledge, it is the first time that a design of a highly birefringent terahertz fiber based on the hybrid guidance mechanism of the anti-resonant mechanism and the total internal reflection mechanism is provided. Several HC-AR fibers with both ultra-low transmission loss and ultra-low birefringence have been achieved in the near-infrared optical communication band. We propose a HC-AR fiber design in terahertz band by introducing a microstructure in the fiber core which leads to tremendous improvement in birefringence. Calculated results indicate that the proposed HC-AR fiber achieves a birefringence higher than 10-2 in a wide wavelength range. In addition, low relative absorption loss of 0.8% (8.6%) and negligible confinement loss of 1.69×10-4 dB/cm (9.14×10-3 dB/cm) for x-polarization (y-polarization) mode at 1THz are obtained. Furthermore, the main parameters of the fiber structure are evaluated and discussed, proving that the HC-AR fiber possesses great design and fabrication tolerance. Further investigation of the proposed HC-AR fiber also suggests a good balance between birefringence and transmission loss which can be achieved by changing strut thickness to cater numerous applications ideally.

Performance simulation of terahertz waveguide resonance biochemical sensor based on nanoporous gold films

A highly sensitive terahertz (THz) waveguide resonance biochemical sensor is designed and simulated. The sensor consists of a silicon prism, a dielectric layer and a nanoporous gold film. The nanoporous gold film acts as both a THz waveguiding layer and a biochemical molecular enrichment layer, which can enhance the interaction between the THz waveguide mode and the adsorbed biochemical molecules, consequently improving the sensor’s sensitivity. When the THz transverse electric (TE) or transverse magnetic (TM) modes are excited by the prism-coupling method, the THz absorption of the nanoporous gold film results in the sharp resonance dips in the THz reflection spectrum. The resonance frequencies of the THz waveguide modes and the sensitivity to either liquid refractive index (RI) or adsorbed molecules can be determined with the measured reflection spectra, and the sensor’s sensitivity and its figure of merit (FOM) can be improved by adjusting the thickness and RI of the dielectric layer. The simulation results at 45º incidence angle indicate that the resonance frequencies of the TE and TM modes of the sensor linearly change with increasing either liquid RI or the amount of adsorbed molecules, and the RI sensitivity and the FOM with the TM mode are 13.42 THz/RIU and 167.70/RIU, respectively. Compared with the TM mode, the TE mode has a lower sensitivity to lqiuid RI but a high sensitivity to adsorbed molecules. The reason for these differences is that with the TE mode the evanescent field penentrating out of the nanopous gold film is weaker than that with the TM mode.

Ultrafast imaging of terahertz electric waveforms using quantum dots

Microscopic electric fields govern the majority of elementary excitations in condensed matter and drive electronics at frequencies approaching the Terahertz (THz) regime. However, only few imaging schemes are able to resolve sub-wavelength fields in the THz range, such as scanning-probe techniques, electro-optic sampling, and ultrafast electron microscopy. Still, intrinsic constraints on sample geometry, acquisition speed and field strength limit their applicability. Here, we harness the quantum-confined Stark-effect to encode ultrafast electric near-fields into colloidal quantum dot luminescence. Our approach, termed Quantum-probe Field Microscopy (QFIM), combines far-field imaging of visible photons with phase-resolved sampling of electric waveforms. By capturing ultrafast movies, we spatio-temporally resolve a Terahertz resonance inside a bowtie antenna and unveil the propagation of a Terahertz waveguide excitation deeply in the sub-wavelength regime. The demonstrated QFIM approach is compatible with strong-field excitation and sub-micrometer resolution—introducing a direct route towards ultrafast field imaging of complex nanodevices in-operando.

Sensitivity analysis for detecting chemicals by the optical chemical sensor based Photonic Crystal Fiber (PCF) in the Terahertz (THz) regime

This research article demonstrates a good simulation result for identifying and detecting various industrial chemicals in a Terahertz (THz) waveguide with a new heptagonal, five layers of heptagonal photonic fiber elliptic form, heptagonal cladding shape (H-PCF). COMSOL 4.2 software based on finite element (FEM) methods and perfectly matched layers check our composition (PML). The different chemicals are also differentiated and identified by each other in different parameters H-PCF fibers show a high relative sensitivity of ethanol of approximately 86.50 percent after numerical analysis, Benzene around 89.35%, and water around 85.15% at a frequency of around 0.7 THz. In our experiment, we obtained very low confinement losses at 1 terahertz (THz) such as 5.95 × 10−08 dB/m for Ethanol 6.67 × 10−08 dB/m for Benzene, and 5.80 × 10−08 dB/m for water. Regarding these results, we can strongly recommend that our proposed heptagonal photonic crystal fiber (H-PCF) will be more congenial in biomedical, bio-medicine, and industrial areas for the identification and detection of various types of chemicals with the help of a THz waveguide.

Terahertz graphene modulator based on hybrid plasmonic waveguide

As a key component of on-chip interconnection, optical modulator with large modulation depth and tiny footprint has always been studied. Profiting by high carrier mobility and flexible adjustability of graphene, numerous graphene modulators at optical communication band are proposed to overcome inherent flaws of traditional semiconductor waveguide modulators. Here, a terahertz waveguide modulator combing noble metal and graphene is presented. When Fermi level changes from 0 eV to 1 eV, intensity distribution of electric field becomes dispersed. Interaction area of graphene and wave increases, which results in larger propagation loss. On the premise of the existence of the allowed mode, the size of metal and the thickness of dielectric should be small. Besides, modulation capability of this device can also be improved by multilayer graphene with relaxation time of 0.1 ps. After optimizing structure parameters, the designed graphene waveguide modulator obtains modulation depth of 6.1 dB μm−1 at the frequency of 5 THz, and keeps effective mode area below 10−5. With the increase of frequency, modulation depth decreases. Modulation depth of 1.5 dB μm−1 is achieved at 10 THz, but the corresponding effective mode area remains in an ideal range. Because the allowed mode is confined in a tiny room, cross-sectional area of device is less than 4 μm2.

Design and analysis of heptagonal cladding with rotated-hexa elliptical core based PCF for the applications of communication sectors in the THz region

Photonic crystal fiber-based heptagonal cladding with the elliptical rotated-Hexa core is reported in the terahertz waveguide for the applications of communication area in this paper. By utilizing the photonic crystal fiber concept, all numerical results have been obtained by the finite element method with perfectly matched layers based on the COMSOL Multiphysics computer software tool. This H-PCF fiber shows a low effective material loss of 0.0076 cm−1, with a larger effective area of 5.95 × 10–8 m2, power flows within the core area of 90%, confinement loss of 3.35 × 10–16 cm−1, and scattering loss of 1.24 × 10–10 cm−1 at 1 THz. Besides, other optical properties like confinement loss, scattering loss, power fraction, V-parameter, and effective area have also been considered briefly. So, it is expected that the mentioned H-PCF structure-based waveguide will significantly improve many kinds of communication fields of the THz technology.

All-silicon, low-cross-talk terahertz waveguide crossing based on effective medium.

All-silicon effective-medium-clad waveguides are a promising candidate for an integrated terahertz platform with high efficiency and broad bandwidth. Waveguide crossings are essential circuit components, allowing for wave routing over shorter paths to increase circuit density. However, the simple intersection of two orthogonal effective-medium-clad waveguides results in terahertz wave scattering, leading to relatively high cross talk. In this work, a low-loss, 40% fractional bandwidth crossing utilizing Maxwell-Garnet effective-medium theory and wavefront planarization techniques is proposed. This monolithic structure is fabricated on a single high-resistivity float-zone silicon wafer using a deep reactive ion etching process with a modest 4.4 mm diameter (4.03λ0) structure footprint. Experimentally verified results show low insertion loss, less than 1 dB, and average cross talk level of -39dB for both E11x and E11y operating modes, over 220-330 GHz with a 40% fractional bandwidth. This waveguide crossing can be foreseen as a useful routing component for terahertz all-silicon integrated circuits. The proposed techniques are applicable to other dielectric waveguide platforms at infrared and optical frequencies.

Single‐Step Tabletop Fabrication for Low‐Attenuation Terahertz Special Optical Fibers

Amid the search for efficient terahertz transmission and gas sensing, all-polymer terahertz waveguides attract significant interest due to their compactness and capability for providing environmentally robust systems. The high loss within metals and dielectrics in the terahertz range makes it challenging to build a low loss, mechanically stable, and broadband terahertz waveguides. In this context, hollow waveguides made of Zeonex are promising for attaining low transmission loss in the terahertz range. Herein, a microstructured hollow hexagonal-core fiber (HCF) is investigated, which exhibits low loss, near-zero dispersion, wide operating bandwidth, and is suitable as a gas sensor. Notably, HCF fabrication is carried out by exploiting an efficient single-step extrusion method—by a standard filament extruder and a puller; hence the production cost is low compared with conventional extrusion methods. This introduces a novel way of fabricating complex and low-loss terahertz fibers. The experiments demonstrate that an HCF can achieve remarkably low attenuation and near-zero flattened dispersion as compared with any other terahertz fibers. The resulting HCFs are easy to handle and have high thermal and chemical stability. These results bring significant advancements for terahertz fiber fabrication, low-loss ultrafast short-distance terahertz transmission, and sensing in the terahertz spectral domain.

Optimization of Terahertz Spoof Surface Plasmon Polariton Waveguides for Maximum °/dB Performance

In this paper, we present an optimization study for the terahertz Spoof Surface Plasmon Polariton waveguides (THz-SSPP WGs) for the first time in the literature. The optimization study employs a selection method for the optimum dimensions for the THz-SSPP WGs, which targets maximizing the insertion phase and minimizing the insertion loss levels, and hence, finding the optimum set of dimensions that result in the best °/dB performance. The method first utilizes the analytical examination of the dispersion diagram to estimate a target range of dimensions for minimum insertion loss, which is then scrutinized by numerous simulations of the THz-SSPP WGs within the selected dimension range. In order to verify the optimization method, a set of THz-SSPP WGs are designed using the proposed method, fabricated, and measured in the 0.22-0.32 THz band. The measurements result in a record-low insertion loss per unit length of 0.75 dB/mm with the optimized THz-SSPP WGs. Additionally, the utilization of the optimized THz-SSPP WGs to compose a terahertz phase shifter enables a record-high figure of merit (FoM) of 102.8 °/dB, which is the most fundamental performance measure of the terahertz phase shifters. It should be indicated that both insertion loss per unit length and FoM levels make THz-SSPP WGs superior to the state-of-the-art planar terahertz waveguides. The presented method provides the most concrete evidence for the potential of the planar SSPP WGs for the terahertz band.

Development of a model for evaluating propagation loss of metal-coated dielectric terahertz waveguides

A simple physical model for evaluating propagation loss of a metal-coated dielectric terahertz (THz) waveguide with different metal film thicknesses was developed for those fabricated by three-dimensional printing and film coating techniques. Our model enables a comprehensive understanding of the propagation loss mechanism and two key values: the critical film thickness to behave like the bulk material and loss in a sufficiently thick film. To develop the model, in addition to reflection at the metal–dielectric interface, the thickness-dependent electrical conductivity of the metal film was considered. The model was validated by an in-house multi-channel Au-coated THz parallel-plate waveguide in the lowest transverse-electric mode. The estimated critical thickness of our Au film was 171–207 nm at 0.72–1.4 THz. Our model clarified the contribution of three loss components to the overall loss: penetration loss, ohmic loss by bulk conductivity, and ohmic loss by a decrease in conductivity due to thin-film effects. Evaluation of loss over a broader frequency range (0.03–3.0 THz), which corresponds to fifth- to sixth-generation mobile network, revealed that the critical thickness decreased by up to 1.0 THz but increased above this range due to the transition of the dominant loss component from penetration loss to ohmic loss by a decrease in conductivity. As all three loss components and the critical thickness depend on film quality, a deposition process to yield high-quality films is necessary for high-performance waveguides. Our model is applicable to various waveguides, including rectangular waveguides, at any frequency and with any metal film.