Metal Parallel Plate Waveguide Research Articles

Terahertz Bragg Resonator Based on a Mechanical Assembly of Metal Grating and Metal Waveguide

Terahertz (THz) waveguide Bragg grating is experimentally demonstrated on the basis of a mechanical assembly that includes a metal-grating sheet and a metal parallel-plate waveguide (PPWG). Bragg frequency waves, which are also called phase-matching waves, are efficiently coupled from the free space on the basis of the designed parameters of the air gap width, the metal grating thickness, and the PPWG taper angle. These optimal parameters are critical to reflect the Bragg frequency waves with a transversely resonant field across the metal grating. For other THz waves that are not phase matching to the meal grating vector, transverse resonance simultaneously occurs. However, these waves propagate forward as waveguide modes. PPWG-assembled metal grating thus performs highly distinct Bragg frequencies in the waveguide transmittance when the noisy resonance field, not following Bragg law, is exactly constrained by the waveguide scheme. Given that all components of the waveguide Bragg grating are made of metal without dielectric perturbation, a series of strong Bragg resonant modes in a wide bandwidth and a hollow-core chamber is potentially facilitated for THz gas sensors.

太赫兹平行平板金属波导T型腔中消逝模共振频率的理论分析

太赫兹平行平板金属波导T型腔中消逝模共振频率的理论分析

Compact Beam-Scanning Flat Array Based on Substrate-Integrated Waveguide

A compact beam-scanning flat antenna array with high gain based on the substrate- integrated waveguide (SIW) is presented in this article. The radiation elements are the continuous transverse stub (CTS) constructed by SIW, which are arrayed on the substrate. The SIW-CTS array has a 4 dB higher gain than the traditional CTS array with the metal parallel plate waveguide and the theoretical explanation is analyzed. A new compact corporate-feed network consisting of an SIW-horn and a flat lens is also proposed. The flat lens could convert the cylindrical wavefront generated by the feed probe and the SIW-horn to a planar wavefront for feeding the CTS array. The Taylor distribution of the feed level made the array has a low sidelobe and high gain. The beam scanning of the array could be realized by simply moving the small SIW-horn along the line across the focal point. An eight-element array is designed, simulated, and fabricated to verify the design. The measured results show that the scanning range is from -35° to 35° with the highest gain of being 20.6 dBi and the sidelobe level of -25.2 dB at the broadside.

Concise relation among the effective index of the TE01 mode of the metal rectangular waveguide and those of the TE1 and TM0 modes of the metal parallel-plate waveguide for terahertz waves

For terahertz waves, we analytically present a concise relation of the effective indices among the TE01 mode of the metal rectangular waveguides, the TE1 mode, and the TM0 mode of the infinite metal parallel-plate waveguides. With the assumption that the modal field in the metal rectangular waveguide is linearly polarized and separable, the effective index of the TE01 mode can be obtained from the combination of the effective indices of the associated TE1 mode and TM0 mode. We further find that the real part of the effective index of the TE01 mode is mainly determined by that of the TE1 mode, while for the imaginary part, the TM0 mode dominates.

Electrically tunable electromagnetic switches based on zero-index metamaterials

In this work, we design a heterojunction in a two dimensional parallel-plate metallic waveguide by using zero-index metamaterials and anisotropic dielectrics which are electrically tunable. We show analytically and numerically that by only controlling the direct current voltage applied to the anisotropic dielectrics, the phenomena of perfect transmission and near perfect reflection can be tuned electrically. Such designed junctions can service as electrically tunable switches in waveguide systems, with good performances and high sensitivities. Our work provides another way to electrically control the propagation of electromagnetic waves in waveguide systems.

Particle-in-cell simulation of multipactor discharge on a dielectric in a parallel-plate waveguide

An original 2D3V (two-dimensional in coordinate space and three-dimensional in velocity space) particle-in-cell code has been developed for simulation of multipactor discharge on a dielectric in a parallelplate metal waveguide with allowance for secondary electron emission (SEE) from the dielectric surface and waveguide walls, finite temperature of secondary electrons, electron space charge, and elastic and inelastic scattering of electrons from the dielectric and metal surfaces. The code allows one to simulate all stages of the multipactor discharge, from the onset of the electron avalanche to saturation. It is shown that the threshold for the excitation of a single-surface multipactor on a dielectric placed in a low-profile waveguide with absorbing walls increases as compared to that in the case of an unbounded dielectric surface due to escape of electrons onto the waveguide walls. It is found that, depending on the microwave field amplitude and the SEE characteristics of the waveguide walls, the multipactor may operate in two modes. In the first mode, which takes place at relatively low microwave amplitudes, a single-surface multipactor develops only on the dielectric, the surface of which acquires a positively potential with respect to the waveguide walls. In the second mode, which occurs at sufficiently high microwave intensities, a single-surface multipactor on the dielectric and a two-surface multipactor between the waveguide walls operate simultaneously. In this case, both the dielectric surface and the interwall space acquire a negative potential. It is shown that electron scattering from the dielectric surface and waveguide walls results in the appearance of high-energy tails in the electron distribution function.

Frequency-division multiplexing in the terahertz range using a leaky-wave antenna

Using a leaky-wave antenna, free-space-to-waveguide frequency-division multiplexing and demultiplexing are demonstrated in the terahertz range. Both the frequency and the spectral bandwidth of multiplexed channels can be independently controlled. The idea of using radiation in the 0.1–1.0 THz range as carrier waves for free-space wireless communications has attracted growing interest in recent years, due to the promise of the large available bandwidth1,2. Recent research has focused on system demonstrations3,4, as well as the exploration of new components for modulation5, beam steering6 and polarization control7. However, the multiplexing and demultiplexing of terahertz signals remains an unaddressed challenge, despite the importance of such capabilities for broadband networks. Using a leaky-wave antenna based on a metal parallel-plate waveguide, we demonstrate frequency-division multiplexing and demultiplexing over more than one octave of bandwidth. We show that this device architecture offers a unique method for controlling the spectrum allocation, by variation of the waveguide plate separation. This strategy, which is distinct from those previously employed in either the microwave8 or optical9 regimes, enables independent control of both the centre frequency and bandwidth of multiplexed terahertz channels.

褶皱金属结构平板波导传输特性研究

褶皱金属结构平板波导传输特性研究

Wave Propagation in Parallel Plate Metallic Waveguide With Finite Conductivity and Three Dimensional Roughness

We study the effects of random roughness on wave propagation in a parallel plate metallic waveguide with finite conductivity. The rough surface is three dimensional (3D) with roughness heights varying in both horizontal directions. Integral equations are obtained from the extinction theorem formulated with layered medium Green's function. The second order small perturbation method is then applied to solve the integral equations. A closed form expression for the coherent wave is derived, which is expressed in terms of a three-fold Sommerfeld type integral due to the waveguide structure. Approximate methods are applied to calculate the Sommerfeld integral. The results based on the approximations are shown to agree with that of direct integration. The coherent wave enhancement factors of absorption are computed. Results of the present 3D roughness case are compared with the previous results of two dimensional (2D) roughness. The results for waveguides are also compared with the results obtained for a plane wave incident on a metal surface with 3D roughness. Results are illustrated for 3D roughness with a variety of power spectra. It is shown that enhancement factors of 3D roughness are larger than for 2D roughness. We also show that enhancement factors for a waveguide are larger than that of the plane wave case. Comparisons are also made between the theoretical results and measurements.

Coupling into tapered metal parallel plate waveguides using a focused terahertz beam

Coupling of focused terahertz radiation into tapered metal parallel plate waveguides with sub-wavelength gap widths is presented. A line-focus terahertz time-domain spectroscopy setup with flexible foci is used to investigate coupling of broadband terahertz radiation from free-space into waveguides for different gap widths. Various waveguide lengths are compared in experiment and calculation in terms of occurring loss and divergence phenomena. Amplitude coupling ratios of 80% from free-space into the waveguides are obtained.

Guided Wave THz Spectroscopy of Explosive Materials

One of the important applications of ㎔ time-domain spectroscopy (TDS) is the detection of explosive materials through identification of vibrational fingerprint spectra. Most recent ㎔ spectroscopic measurements have been made using pellet samples, where disorder effects contribute to line broadening, which results in the merging of individual resonances into relatively broad absorption features. To address this issue, we used the technique of parallel plate waveguide (PPWG) ㎔-TDS to achieve sensitive characterization of three explosive materials: TNT, RDX, and HMX. The measurement method for PPWG ㎔-TDS used well-established ultrafast optoelectronic techniques to generate and detect sub-picosecond THz pulses. All materials were characterized as powder layers in 112 ㎛ gaps in metal PPWG. To illustrate the PPWG ㎔-TDS method, we described our measurement by comparing the vibrational spectra of the materials, TNT, RDX, and HMX, applied as thin powder layers to a PPWG, or in conventional sample cell form, where all materials were placed in Teflon sample cells. The thin layer mass was estimated to be about 700 ㎍, whereas the mass in the sample cell was ～100 ㎎. In a laboratory environment, the absorption coefficient of an explosive material is essentially based on the mass of the material, which is given as: a(w)=[ln(I R (w)I S (w))]/m. In this paper, we show spectra of 3 different explosives from 0.2 to 2.4 ㎔ measured using the PPWG THz-TDS.

Temperature dependent characterization of terahertz vibrations of explosives and related threat materials

Waveguide terahertz time-domain spectroscopy (THz-TDS) is used to characterize the temperature dependent vibrational properties of three threat-related materials: 4-amino-dinitrotoluene (4A-DNT), pentaerythritol tetranitrate (PETN), and octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). These materials are characterized as thin polycrystalline layers deposited in the 50 micron gap of a metal parallel plate waveguide. For each material waveguide THz-TDS at least partially resolves the underlying vibrational spectrum and reveals new features that have not been observed in previous free space measurements of these materials. Strong experimental evidence for a phase transformation is observed for 4A-DNT as the polycrystalline layer on the waveguide surface is cooled to near 200 K. For PETN a highly resolved spectrum containing eleven vibrational lines is observed at 11 K with full-width at half maximum linewidths ranging from 7 GHz to 40 GHz. Based on comparison to measurements in the literature, our PETN measurement suggests that it is possible to produce narrow linewidths from a polycrystalline layer that approach those from a single crystal. Finally, for HMX, a highly resolved vibrational spectrum is measured that is assigned to the metastable gamma polymorph.

Terahertz (Far-Infrared) Characterization of Tris(hydroxymethyl)aminomethane Using High-Resolution Waveguide THz-TDS

Waveguide THz-TDS using metal parallel plate waveguides is employed to obtain a high-resolution terahertz vibrational spectrum of tris(hydroxymethyl)aminomethane, commonly known as Tris or THAM, an important biological buffer component, a neuroinhibitor and an organic thermal energy-storing molecule. Dropcast and sublimated thin films are used to characterize the sample. The development (with cooling) of 3 broad resonances seen at room temperature into 12 highly resolved spectral features for the dropcast sample and 11 features for a sublimated sample at 13.6 K is tracked by measuring the temperature dependence of the transmission spectrum. Resonances with fwhm linewidths as narrow as 12 GHz are obtained. These results provide strong constraints on theoretical modeling.

Guided-wave terahertz spectroscopy of molecular solids [Invited

One of the outstanding problems of terahertz spectroscopy is the measurement of the underlying vibrational spectrum of a molecular solid, where individual vibrational transitions are often merged into broad absorption features by line-broadening processes. We address this problem using the technique of waveguide terahertz time-domain spectroscopy (THz-TDS), whereby a thin polycrystalline molecular film is contained within a single-mode metal parallel plate waveguide. Thin films of the molecular solids cyclotrimethylene trinitramine (RDX explosive) and 4-iodo-4′-nitrobiphenyl serve to demonstrate the ability of waveguide THz-TDS to resolve previously unseen complex underlying THz vibrational spectra at cryogenic temperatures, with linewidths as narrow as 7 GHz. With such narrow linewidths we are able to demonstrate the measurement of vibrational line-center frequencies to a precision of 1 GHz.

High-Q terahertz Bragg resonances within a metal parallel plate waveguide

One-dimensional (1D) Bragg waveguides are demonstrated at terahertz (THz) frequencies. Lithographically made 1D symmetric and asymmetric dielectric gratings on metallized high conductivity Si chips are incorporated within metal parallel plate waveguides to form Bragg waveguides. These waveguides have high throughput and have Bragg resonances with linewidths approaching 6 GHz and Q as high as 430. These high-Q resonant Bragg waveguides are excellent structures for applications in THz sensing.

Radiation admittance of an open‐ended parallel plate metallic waveguide with exterior planar dielectric coating

A methodology is presented for the calculation of the radiation admittance of an open‐ended, homogeneously filled, parallel plate metallic waveguide with exterior planar dielectric coating, operated at the fundamental (transverse electromagnetic wave) mode. The methodology relies on the definition of an auxiliary electromagnetic boundary value problem that is simple to solve using an integral equation approach. In order to provide for broadband calculation of the radiation admittance of the open‐ended waveguide from DC to several tens of gigahertz, a loop‐tree decomposition scheme is used in the method of moments solution of the resulting integral equation. A rational function‐fitting process with passivity enforcement is used to generate a broadband, passive macromodel for the calculated radiation impedance. Such a macromodel is suitable for use as a local truncation boundary condition in the finite difference/finite element modeling of electromagnetic interactions in multilayered signal and power distribution networks encountered in printed circuit boards and packages.

The underlying terahertz vibrational spectrum of explosives solids

Using waveguide terahertz time-domain spectroscopy, we demonstrate the measurement of the underlying terahertz vibrational spectrum of the explosive solids hexahydro-1,3,5-trinitro-1,3,5-triazine, and 2,4,6-trinitrotoluene. Each explosive is cast as a polycrystalline thin film with planar ordering on the inner surface of metal parallel plate waveguide. For measurements near 10K, each explosive reveals a complex spectrum of approximately 20 vibrational modes between 0.5 and 3.5THz. The explosive films are of sufficient quality to produce vibrational linewidths as much as an order of magnitude sharper compared to conventional terahertz measurements of corresponding pellet samples of explosives.

High-Resolution Waveguide THz Spectroscopy of Biological Molecules

Low-frequency vibrational modes of biological molecules consist of intramolecular modes, which are dependent on the molecule as a whole, as well as intermolecular modes, which arise from hydrogen-bonding interactions and van der Waals forces. Vibrational modes thus contain important information about conformation dynamics of biological molecules, and can also be used for identification purposes. However, conventional Fourier transform infrared spectroscopy and terahertz time-domain spectroscopy (THz-TDS) often result in broad, overlapping features that are difficult to distinguish. The technique of waveguide THz-TDS has been recently developed, resulting in sharper features. For this technique, an ordered polycrystalline film of the molecule is formed on a metal sample plate. This plate is incorporated into a metal parallel-plate waveguide and probed via waveguide THz-TDS. The planar order of the film reduces the inhomogeneous broadening, and cooling of the samples to 77K reduces the homogenous broadening. This combination results in the line-narrowing of THz vibrational modes, in some cases to an unprecedented degree. Here, this technique has been demonstrated with seven small biological molecules, thymine, deoxycytidine, adenosine, D-glucose, tryptophan, glycine, and L-alanine. The successful demonstration of this technique shows the possibilities and promise for future studies of internal vibrational modes of large biological molecules.

High-Resolution Waveguide Terahertz Spectroscopy of Partially Oriented Organic Polycrystalline Films

We have characterized the terahertz (THz) vibrational spectroscopy of organic polycrystalline thin films using the new experimental technique of waveguide terahertz time domain spectroscopy (waveguide THz-TDS). The organic materials used in this study are tetracyanoquinodimethane (TCNQ) and 1,3-dicyanobenzene (13DCB). For each material, a thin film is cast onto one of the inner surfaces of a metal parallel plate waveguide (PPWG), followed by measurement of the low-frequency vibrational spectrum using waveguide THz-TDS. The vibrational spectra of the waveguide films are compared to corresponding vibrational spectra of standard pellet samples made by dispersing the organic solid in transparent polyethylene. We show how the waveguide films produce significantly narrower THz vibrational line shapes and reveal additional spectral lines that are obscured by inhomogeneous broadening effects in the pellet samples. When TCNQ waveguide films are cooled to 77 K, vibrational line widths as sharp as 25-30 gigahertz (0.83-1.0 cm(-1)) at the full width at half-maximum are observed, which are among the narrowest far-infrared line widths measured for this material. The origin of the line-narrowing effect for the waveguide films is the suppression of inhomogeneous broadening due to the planar ordering of the film on the waveguide surface. The TCNQ waveguide films are further characterized using optical microscopic evaluation to understand how film morphology affects the THz vibrational spectrum. X-ray diffraction is used to determine the orientation of the polycrystalline TCNQ films on the PPWG surface and to qualitatively explain the different vibrational line strengths observed for the ordered waveguide film relative to the random pellet.

2-D Terahertz Metallic Photonic Crystals in Parallel-Plate Waveguides

2-D metallic photonic crystals without defects, with point defects, and with a Fabry-Perot (F-P) defect are characterized by terahertz time-domain spectroscopy. The metal parallel-plate waveguide (PPWG) with single TEM-mode propagation is used as a tool to simulate 2-D photonic crystals in free space. The 2-D metallic photonic-crystal structures were fabricated by coating Au on an SU-8 polymer cylinder array. Wide terahertz bandgaps were observed in the photonic crystals within the PPWG. The experimental measurements have excellent agreement to 2-D photonic-crystal theory without defects. Defect modes are observed in the samples with defects and show the F-P defect has a strong localization effect