Hollow Waveguide Research Articles

Low-Loss Flexible Polarization-Maintaining Hollow Waveguide for Linearly Polarized 100 GHz Radiation Transmission and Subwavelength Imaging

Terahertz imaging has been developed rapidly in recent years. However, conventional terahertz imaging systems are often bulky due to the free space propagation. Various subwavelength sized probes have been developed to realize high resolution. However the tiny probes often suffer from high loss. A flexible metallic polarization-maintaining (PM) hollow waveguides with elliptical cross section is investigated and fabricated. The waveguide is used for transmitting linearly polarized terahertz (THz) radiation and flexible imaging. Bore size of the waveguide is on the order of wavelength. Transmission loss is as low as 0.92 dB/m. High birefringence of 0.08 and measured polarization ratio of 97.2% are achieved by the waveguide at 100 GHz. Moreover, a scanning 100 GHz imaging system is established by using the waveguide as a flexible probe. The PM capability of the fabricated waveguide brings benefits compared to conventional non-PM circular waveguide. The linearly polarized fundamental modes are easily excited and can be propagated through the fabricated waveguides to one meter away with polarization maintenance, which results in compressing of output beam and realizes a high resolution. A resolution better than λ/3 is achieved. It is further experimentally verified that the waveguide is robust to bending and twisting deformation, and maintained the imaging quality during flexible imaging. A subwavelength bore-sized single-polarization single-mode waveguide is also fabricated to simultaneously achieve high horizontal and vertical resolutions. The discussed waveguide is easy to fabricate and may find important applications in THz microscopy and flexible polarization imaging.

Magnetostatic microwaves in circular metallic waveguides filled with uniaxial negative permeability media

The quasi-magnetostatic (briefly, magnetostatic) microwave propagation in a long, circular cross section waveguide having a metallic wall that is filled with an anisotropic uniaxial magnetic medium (here, a metamaterial consisting of split-ring resonators) is studied. Such waves do not exist in the case of a hollow metallic waveguide or a metallic waveguide filled with an isotropic medium. Physically, the present magnetostatic waves owe their existence to the anisotropic property of the system, where in the absence of this property, these magnetostatic waves disappear. A theoretical analysis is carried out and characteristics of these magnetostatic microwaves in circular metallic waveguides filled with uniaxial negative permeability media are determined. The presented results include the dispersion relation of magnetostatic waves and their corresponding cutoff frequencies, group velocities, transverse modal field distributions, power flows, and stored energies.

RobustATR: Substrate-Integrated Hollow Waveguide Coupled Infrared Attenuated Total Reflectance Sensors

Small and compact mid-infrared devices are of increasing importance, as there are several applications demanding on-site and real-time measurements in harsh real-world scenarios. The RobustATR, an innovative infrared attenuated total reflectance (IR-ATR) accessory, has been developed and tested with exemplary analytes integrating a single-wavelength Fabry–Pérot quantum cascade laser as light source for testing the feasibility of a potentially miniaturized overall sensor design. Successful direct coupling of the laser radiation via substrate-integrated hollow waveguide (iHWG) coupling elements to the sensor interface could be shown, whereby exemplary analytes of environmental and medical relevance were tested, revealing the future potential for real-world applications.

Wideband Dual-Polarized Hollow-Waveguide Slot Array Antenna

A wideband dual-polarized slot array antenna based on hollow waveguide (HW) is presented in this article. Its wide bandwidth and high performance are enabled by two separate feeding networks, from the side and the bottom, using square coaxial line and ridge waveguide structures, respectively. The full-corporate-feed networks excite the radiation slots in a one-to-one fashion without using backed cavities. Tight element spacing is made possible, suppressing the grating lobe and maintaining high cross-polarization discrimination (XPD) across the wideband. This resolves a usual tradeoff between bandwidth and grating lobe. A double-ridge rectangular waveguide cavity is devised as an orthogonal mode splitter for isolation enhancement. Several design features are used to minimize the signal path length and facilitate routing. An <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$8\times8$ </tex-math></inline-formula> element prototype in K-band is designed, fabricated, and measured. Experimental results show that the antenna has a wide bandwidth of 23.4% (from 17 to 21.5 GHz) free from grating lobes. More than 25.5 dBi peak gain higher than 51.2 dB isolation and over 43.6 dB XPD are also obtained for the dual polarizations.

Generation of Intense Low-Divergence Isolated Soft-X-Ray Attosecond Pulses in a Gas-Filled Waveguide Using Three-Color Synthesized Laser Pulses

Isolated attosecond pulses (IAPs) in the soft-x-ray (SXR) region have been successfully produced via high-order harmonic generation in a gas medium at several laboratories, but pulse energies are much lower compared to the IAPs in the extreme ultraviolet. Here, we show that in a gas-filled hollow waveguide, an efficient gating method using a few-cycle three-color synthesizer is able to generate stronger SXR IAPs. We show that an 104-as IAP in the SXR region (central energy of 210 eV) with a higher pulse energy (0.9 nJ) and a lower divergence (within 1.5 mrad) can be generated with a three-color laser beam for input pulse energy at 0.62 mJ, under the condition of optimal gas pressure and waveguide length. The enhancement of the IAP is attributed to favorable phase matching of SXR high harmonics caused by the evolution of the three-color waveform during its propagation in the waveguide, which can be understood by analyzing the time shift of the waveform due to the interplay of the waveguide mode, atomic dispersion, and plasma dispersion. We check that the gating method is quite robust with respect to the phase jitter in the waveform. The advantage of this method is further demonstrated by comparing the generation of SXR high harmonics and attosecond pulses with two other schemes: the three-color waveform in a gas cell and a previously optimized two-color waveform in a gas-filled waveguide.

Substrateless Packaging for a D-Band MMIC Based on a Waveguide with a Glide-Symmetric EBG Hole Configuration.

This paper presents a novel substrateless packaging solution for the D-band active e mixer MMIC module, using a waveguide line with a glide-symmetric periodic electromagnetic bandgap (EBG) hole configuration. The proposed packaging concept has the benefit of being able to control signal propagation behavior by using a cost-effective EBG hole configuration for millimeter-wave- and terahertz (THz)-frequency-band applications. Moreover, the mixer MMIC is connected to the proposed hollow rectangular waveguide line via a novel wire-bond wideband transition without using any intermediate substrate. A simple periodical nail structure is utilized to suppress the unwanted modes in the transition. Additionally, the presented solution does not impose any limitations on the chip’s dimensions or shape. The packaged mixer module shows a return loss lower than 10 dB for LO (70–85 GHz) and RF (150–170 GHz) ports, achieving a better performance than that of traditional waveguide transitions. The module could be used as a transmitter or receiver, and the conversion loss shows good agreement in multiple samples. The proposed packaging solution has the advantages of satisfactory frequency performance, broadband adaptability, low production costs, and excellent repeatability for millimeter-wave- and THz-band systems, which would facilitate the commercialization of millimeter-wave and THz products.

TEM-wave propagation in a coaxial waveguide with impedance-matched RHM to LHM transition.

In this paper, we study TEM-wave propagation inside a hollow coaxial waveguide filled with an inhomogeneous metamaterial composite, with a graded transition between a right-handed material (RHM) and an impedance-matched left-handed material (LHM). The graded transition and the TEM-wave propagation occur in the direction perpendicular to the boundary between the two media, which has been chosen to be the z-direction. The relative permittivity ɛ(ω, z) and permeability µ(ω, z) of the RHM-LHM composite vary according to hyperbolic tangent functions along the z-direction. The exact analytical solutions to Maxwell's equations are derived, and the solutions for the field components and wave behavior confirm the expected properties of impedance-matched RHM-LHM structures. Furthermore, a numerical study of the wave propagation over an impedance-matched graded RHM-LHM interface, using the COMSOL Multiphysics software, is performed. An excellent agreement between the analytical results and numerical simulations is obtained, with a relative error of less than 0.1%. The present method has the ability to model smooth realistic material transitions, and includes the abrupt transition as a limiting case. Finally, the RHM-LHM interface width is included as a parameter in the analytical and numerical solutions, allowing for an additional degree of freedom in the design of practical devices using RHM-LHM composites.

Effect of Spontaneous Emission in Hollow Waveguide on Gas Absorbance Measurement by Absorption Spectroscopy

Hollow waveguide (HWG) as an absorption cell for gas spectral sensing has the advantages of low transmission loss, fast response speed, and high path length to volume ratio. However, due to the tubular structure of the HWG, the spontaneous emission generated in the HWG will be transmitted together with the incident laser, which reduces the measured absorbance. We measured and verified the spontaneous emission at the outlet of the HWG by basing on the laser gas absorption spectroscopy and combining with the distinguished polarization characteristics of laser and spontaneous emission. Then, we quantitatively determined the intensity distribution of the spontaneous emission output from the HWG based on the infrared (IR) imaging using an IR camera. We also investigated the effect of the spontaneous emission on the measured absorbance as a function of the HWG length by comparative experiments. The effect exponentially decreases as the HWG length increases since the increased loss. The method could be used for designing or optimizing of HWG-based sensors.

Ultra-Low-Loss and Broadband All-Silicon Dielectric Waveguides for WR-1 Band (0.75–1.1 THz) Modules

This study presents ultra-low-loss and broadband all-silicon dielectric waveguides for the WR-1 band (0.75–1.1 THz). The waveguides are built in high-resistivity silicon (10 kΩ-cm) and integrated with supportive frames fabricated from the same silicon wafer in a single etch process to achieve a compact design. We pursued low-loss, broadband, substrateless, unclad and effective medium waveguides. Smaller propagation losses of 0.3 dB/cm and 0.1 dB/cm were achieved for the unclad and effective medium waveguides, respectively. The 3 dB bandwidth was not encountered in the frequency range of interest and was as broad as 350 GHz. An unclad waveguide was employed to devise a Y-junction to demonstrate its practical applications in terahertz imaging. An integrated circuit card was successfully scanned. In addition, we developed unclad waveguide, effective medium waveguide, and Y-junction modules. The modules incorporated an input/output interface compatible with a standard WR-1 flange (254 μm × 127 μm). Unlike the conventional hollow waveguide modules, the unclad waveguide and effective medium waveguide modules reported total loss improvements of 6 dB and 8 dB, respectively, across the operation band. Our results provided a systematic way of achieving low-loss, compact, and versatile modules in the WR-1 band based on all-dielectric-waveguide platforms.

Analytical and Numerical Models for TE-Wave Absorption in a Graded-Index GNP-Treated Cell Substrate Inserted in a Waveguide

In this paper, absorption phenomena in a hollow waveguide with an inserted graded dielectric layer are studied, for the case of transverse electric (TE) wave propagation. The waveguide model aims to be applicable to a study of a potential cancer treatment by heating of gold nanoparticles (GNPs) inside the cancer cells. In our previous work, general exact analytical fomulas for transmission, reflection, and absorption coefficients were derived. These fomulas are further developed here to be readily applicable to the calculation of the absorption coefficient within the inserted lossy layer only, quantifying the absorption in the GNP-fed cancer tissue. To this end, we define new exact analytic scale factors that eliminate unessential absorption in the surrounding lossy medium. In addition, a numerical model was developed using finite element method software. We compare the numerical results for power transmission, reflection and absorption coefficients to the corresponding results obtained from the new modified exact analytic fomulas. The study includes both a simple example of constant complex permittivities, and a more realistic example where a dispersive model of permittivity is used to describe human tissue and the electrophoretic motion of charged GNPs. The results of the numerical study with both non-dispersive and dispersive permittivities indicate an excellent agreement with the corresponding analytical results. Thus, the model provides a valuable analytical and numerical tool for future research on absorption phenomena in GNP-fed cancer tissue.

Influence of ultrasonic processing on the mechanical properties of metals subjected to intense plastic deformation

An original device for production of nanostructured materials by the method of severe plastic deformation with the imposition of ultrasonic vibrations is described, which provides the grinding of the structure of the material of the workpiece and eliminates the occurrence of defects on its surface and in the end areas. This effect is achieved through the use of an annular spacer made in the form of a hollow waveguide of resonant length, fixed on a fixed support in the displacement unit, connected to a concentrator and an ultrasonic transducer. The physical-mechanical properties of nanostructured samples of nickel and copper after ultrasonic treatment (UST) have been studied. It is shown that UST of nanostructured samples leads to an increase in their plastic properties and a slight decrease in the tensile strength. At small amplitudes of mechanical stresses within 17.5 MPa, a noticeable microhardness increase is observed in copper and nickel samples. A further increase in the mechanical stress amplitude up to 100 MPa during ultrasonic treatment leads to a microhardness decrease.

Silicon Dielectric Diplexer Module for 600-GHz-Band Frequency-Division Multiplexing Wireless Communication

We developed a silicon (Si) dielectric diplexer module for 600-GHz-band frequency-division multiplexing wireless communication. This diplexer was developed based on Si dielectric waveguide technology, which provides low-loss transmission characteristics compared with the conventional metallic terahertz-transmission platform, such as metallic hollow waveguides. It includes straight waveguides based on an effective medium composed of Si and air, and a coupler comprising of two adjacent unclad Si waveguides. Furthermore, for the communication system integration, we developed a practical and low-insertion loss module with WR1.5 metallic hollow waveguide input–output interfaces and modularized the diplexer. We performed transmission measurement experiments with the fabricated diplexer module and achieved 3-dB bandwidths of 101 and 37 GHz, and crosstalk of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula> 34 dB at 680 GHz and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$-$</tex-math></inline-formula> 41 dB at 618 GHz in the cross and bar directions, respectively. Finally, we performed a two-channel wireless communication experiment using the developed diplexer module in the 600-GHz band. We achieved error-free communication at 10 and 6 Gbit/s on the cross and bar channels, respectively. In addition, we achieved a data rate of up to 12.5 Gbit/s at the forward error correction limits for both channels.

Bovine serum albumin label-free concentration sensor based on silica corrosion quantitative monitoring system.

Bovine serum albumin (BSA) label-free concentration sensor based on silica corrosion quantitative monitoring system (SCQMS) has been proposed. Anti-resonance of hollow cylindrical waveguide (HCW) in SCQMS is simulated and investigated for monitoring corrosion rate quantitatively. Hydrofluoric acid (HF) samples with different concentrations are studied respectively, and the corrosion rate is obtained by demodulating the corresponding anti-resonance dips shift and free spectral range (FSR). Therefore, a high-precision SQCMS was prepared successfully. On this basis, a highly sensitive concentration sensor based on hole-assisted dual-core fiber (HADF) is prepared. The BSA samples with concentration from 0.2 mg/mL to 0.7 mg/mL are detected. The sensor has a high sensitivity of 30.04 nm/(mg/mL) and ultra-low limit of detection (LOD) of 0.05 mg/mL for the assisted core exposed to the target solution directly. We have demonstrated the SCQMS that can be a feasible tool for precise and quantitative corrosion of silicon structure safely. In addition, the concentration sensor structure has a wide application for ultra-low LOD, simple preparation process and high integration.

Nonlinear waves propagating in a hollow metamaterial waveguide

This work studies a hollow nonlinear Metamaterial–Air–Metamaterial (MAM) waveguide and investigates the propagation characteristics of transverse electric waves in the MAM waveguide. It is shown that the waves propagate in the waveguide in the form of guided waves or surface waves. Furthermore, the electromagnetic fields are distributed in a symmetric or an antisymmetric mode depending on the frequency response of nonlinear metamaterial to the incident waves. Moreover, the power distribution in the MAM waveguide can be controlled by adjusting the width of the guiding layer of the hollow waveguide. We hope the theoretical analysis can be verified by future experiments and provide ideas on potential applications.

A Low-Profile Hollow Waveguide Slot Array Antenna with Full-Corporate Feeding Network at K-Band

A hollow waveguide slot array antenna with a total of 338x106.5x28.9 mm3 is proposed as a sensor for radar applications at K-band. The proposed antenna consists of a radiating slot plate with 8x32 slots, a middle plate with 64 cavities and apertures, and a full-corporate-feeding network. The antenna is fabricated with these three parts in which the sub-array feeding technique is employed. The proposed antenna is designed for millimeter-wave short-range radar applications. For this purpose, the antenna radiates and achieves reasonable gain values between 23.72–25.30 GHz frequencies with 6.45% bandwidth. The measurement directivity is 32.1 dBi, and the realized gain of the antenna is measured as 30.83 dBi at 24.5 GHz center frequency with 74.8% total efficiency. The antenna achieves high gain and efficiency with straightforward fabrication. Although there are some fabrication errors and connector losses, antenna measurements are obtained sufficiently consistent with simulations.

Variational Quantum-Based Simulation of Waveguide Modes

Variational quantum algorithms are one of the most promising methods that can be implemented on noisy intermediate-scale quantum (NISQ) machines to achieve a quantum advantage over classical computers. This article describes the use of a variational quantum algorithm in conjunction with the finite difference method for the calculation of propagation modes of an electromagnetic wave in a hollow metallic waveguide. The two-dimensional (2D) waveguide problem, described by the Helmholtz equation, is approximated by a system of linear equations, whose solutions are expressed in terms of simple quantum expectation values that can be evaluated efficiently on quantum hardware. Numerical examples are presented to validate the proposed method for solving 2D waveguide problems.

Space-variant polarization conversion with artificial birefringent metallic elements

We present an artificial birefringent space-variant polarization converter for the near infrared, λ = 1550 nm. Each hollow waveguide has a rectangular shape with lateral dimensions of 1550 nm in the x-direction and 1034 nm as the largest length in the y-direction. The whole device consists of approximately 2000 × 2500 hollow waveguides realized in a 2-µm-thick gold structure. They are separated by sidewalls with a width of less than 500 nm. By proper choice of the lateral widths of the individual holes, a pixel-wise polarization conversion of an incoming wave field is possible. By suitable choice of the fabrication parameters, a birefringent phase shift up to 2π can be achieved. Hence, the structure is able to fully convert the state of polarization, e.g., from linear to circular. For fabrication of the device, femtosecond 3D direct laser writing was combined with electroplating. Here, we describe the operation of our device as a space-variant polarization converter by measuring the angle-dependent transmitted power and by calculating the ellipticity and the phase delay dependent on position as well as the azimuth angle from the experimentally determined powers.

Optical System Design of a Planar Waveguide Spectrometer

In this paper, an optical design for a hollow planar waveguide spectrometer with Czerny-Turner is proposed. To decrease the propagation loss of the spectrometer, the design strategy of designing the tangential plane and the sagittal plane separately is proposed, corresponding to resolution and energy, respectively. The Czerny–Turner optical path is designed on the tangential plane, and the sagittal design theory and method are analyzed in detail. The ray tracing results show that the resolution of the spectrometer is better than 4 nm on the tangential plane, while on the sagittal plane, the detector receives the highest energy when the detector pixel height matches the distance between the two mirrors.

Quantitative Determination of Loss Properties of Semiconductor Laser Beams Propagating in Hollow Waveguides

The loss propertiesof semiconductor laser beams propagating in hollow waveguides (HWGs) are crucial in several applications. The attenuation coefficient of the distributed feedback quantum cascade laser (DFB-QCL) in the HWG was quantitatively determined by measuring its beam profile and laser power along the length of the HWG. The experimental reliability was evaluated, with measurement uncertainties as low as 2.6&#x0025; and 0.5&#x0025; for the full width at half maximum (FWHM) of beam profile and the laser power, respectively. We derived an empirical formula for the attenuation coefficient, which is exponential rather than linear with the length of the HWG. The formula was verified to be highly accurate, with a 76.2&#x0025; reduction in deviation compared to the data in the datasheet. We propose a new concept for a free optical path (FOP) that depends on the inner diameter of the HWG and the incident angle of the beam. The FOP could be an excellent parameter for designing or optimizing HWG-based sensors.

Hollow Waveguide Arrays as Quarter- and Half-Wave Plates

We present hollow waveguide arrays acting like quarter- and half-wave plates. Approximately 2000 × 2500 individual rectangular hollow waveguides filled with air and surrounded by a gold cladding form the basis of the wave plates. The hollow waveguides fabricated by combining femtosecond 3D direct laser writing with electroplating have sub-micron dimensions of less than 500 nm in the cladding area. Both wave plates show the characteristic properties of quarter- and half-wave plates converting a linear polarized input field into a circular (quarter-wave plate) and a linear 180 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$^\circ$</tex-math></inline-formula> phase shifted polarized output field (half-wave plate). This is confirmed experimentally by measuring the angle-dependent phase shift, the Stokes vectors and polarization ellipses for both wave plates.