Waveguide Cutoff Research Articles

Miniaturised multi-band substrate integrated waveguide filters using complementary split-ring resonators

A novel method for designing miniaturised multi-band filters using the complementary split-ring resonators (CSRRs) is proposed and demonstrated on the basis of substrate integrated waveguide (SIW) technology. By loading different types of CSRRs on the waveguide surface, multiple passbands propagating below the waveguide cutoff frequency can be generated separately. The proposed structures allow relatively independent control over the centre frequency and the coupling coefficient. The working principle is illustrated by the equivalent circuit and the filter coupling methodology is introduced for design purpose. Two types of dual-passband filters, a triple-band and a quadruple-band filter are designed and fabricated to validate the proposed concept. These filters are showing advantages in terms of compact size, good selectivity and stopband rejection, as well as the easy integration capability with other circuits.

Optical pulse dynamics for quantum-dot logic operations in a photonic-crystal waveguide

We numerically demonstrate all-optical logic operations with quantum dots (QDs) embedded in a bimodal photonic-crystal waveguide using Maxwell-Bloch equations in a slowly varying envelope approximation (SVEA). The two-level QD excitation level is controlled by one or more femtojoule optical driving pulses passing through the waveguide. Specific logic operations depend on the relative pulse strengths and their detunings from an inhomogeneouslly broadened (about 1$%$ for QD transitions centered at $1.5\phantom{\rule{0.222222em}{0ex}}\ensuremath{\mu}$m) QD transition. This excitation controlled two-level medium then determines passage of subsequent probe optical pulses. Envelope equations for electromagnetic waves in the linear dispersion and cutoff waveguide modes are derived to simplify solution of the coupled Maxwell-Bloch equations in the waveguide. These determine the quantum mechanical evolution of the QD excitation and its polarization, driven by classical electromagnetic (EM) pulses near a sharp discontinuity in the EM density of states of the bimodal waveguide. Different configurations of the driving pulses lead to distinctive relations between driving pulse strength and probe pulse passage, representing all-optical logic and, or, and not operations. Simulation results demonstrate that such operations can be done on picosecond time scales and within a waveguide length of about $10\phantom{\rule{0.222222em}{0ex}}\ensuremath{\mu}$m in a photonic-band-gap (PBG) optical microchip.

Average and instantaneous velocities of energy of evanescent modes

Many theoretical and experimental investigations have concluded that evanescent electromagnetic modes can propagate superluminally. However, in this paper we show that the average energy velocity of evanescent modes inside a cutoff waveguide is always less than or equal to the velocity of light in vacuum, while the instantaneous energy velocity can be superluminal, which does not violate causality according to quantum field theory: the fact that a particle can propagate over a spacelike interval does preserve causality provided that a measurement performed at one point cannot affect another measurement at a point separated from the first with a spacelike interval.

Self-consistent Maxwell-Bloch theory of quantum-dot-population switching in photonic crystals

We theoretically demonstrate the population switching of quantum dots (QD's), modeled as two-level atoms in idealized one-dimensional (1D) and two-dimensional (2D) photonic crystals (PC's) by self-consistent solution of the Maxwell-Bloch equations. In our semiclassical theory, energy states of the electron are quantized, and electron dynamics is described by the atomic Bloch equation, while electromagnetic waves satisfy the classical Maxwell equations. Near a waveguide cutoff in a photonic band gap, the local electromagnetic density of states (LDOS) and spontaneous emission rates exhibit abrupt changes with frequency, enabling large QD population inversion driven by both continuous and pulsed optical fields. We recapture and generalize this ultrafast population switching using the Maxwell-Bloch equations. Radiative emission from the QD is obtained directly from the surrounding PC geometry using finite-difference time-domain simulation of the electromagnetic field. The atomic Bloch equations provide a source term for the electromagnetic field. The total electromagnetic field, consisting of the external input and radiated field, drives the polarization components of the atomic Bloch vector. We also include a microscopic model for phonon dephasing of the atomic polarization and nonradiative decay caused by damped phonons. Our self-consistent theory captures stimulated emission and coherent feedback effects of the atomic Mollow sidebands, neglectedmore » in earlier treatments. This leads to remarkable high-contrast QD-population switching with relatively modest (factor of 10) jump discontinuities in the electromagnetic LDOS. Switching is demonstrated in three separate models of QD's placed (i) in the vicinity of a band edge of a 1D PC, (ii) near a cutoff frequency in a bimodal waveguide channel of a 2D PC, and (iii) in the vicinity of a localized defect mode side coupled to a single-mode waveguide channel in a 2D PC.« less

Passband modes beyond waveguide cutoff in metallic tilted-woodpile photonic crystals

The infrared propagation character of metallic woodpile photonic crystals made of gold in the <110> direction was studied. The bandgap in metallic woodpile was found to be interrupted by a new propagating mode far beyond the waveguide cutoff. This new passband cannot be explained by the defect mode of the photonic crystal or the waveguide theorem. However, studies of surface plasmons within the structure showed that 3-D metallic photonic crystals can expand the propagating mode far beyond waveguide cutoff. This passband mode can be adjusted by design allowing for new optical filtering devices using 3-D photonic crystals. Finite-difference-time-domain (FDTD) simulations were applied to explain this phenomenon and matched well with experimental results.

Tight-binding analysis of coupling effects in metamaterials

We established a generalized tight-binding method (TBM) to study the coupling effects in metamaterials. All parameters involved in our theory can be calculated from first principles, and the theory is applicable to general photonic systems with both dielectric and magnetic materials. As an illustration, we applied the theory to study the cutoff waveguides loaded with resonant electric/magnetic metamaterials. We not only accurately computed the coupling strengths between two resonant metamaterials, but also revealed a number of interesting coupling-induced phenomena. Microwave experiments and full-wave numerical simulations were performed to successfully verify all predictions drawn from the TBM.

Experimental study on water-wave trapped modes

We present an experimental study on the trapped modes occurring around a vertical surface-piercing circular cylinder of radius a placed symmetrically between the parallel walls of a long but finite water waveguide of width 2d. A wavemaker placed near the entrance of the waveguide is used to force an asymmetric perturbation into the guide, and the free-surface deformation field is measured using a global single-shot optical profilometric technique. In this configuration, several values of the aspect ratio a/d were explored for a range of driving frequencies below the waveguide's cutoff. Decomposition of the obtained fields in harmonics of the driving frequency allowed for the isolation of the linear contribution, which was subsequently separated according to the symmetries of the problem. For each of the aspect ratios considered, the spatial structure of the trapped mode was obtained and compared to the theoretical predictions given by a multipole expansion method. The waveguide–obstacle system was further characterized in terms of reflection and transmission coefficients, which led to the construction of resonance curves showing the presence of one or two trapped modes (depending on the value of a/d), a result that is consistent with the theoretical predictions available in the literature. The frequency dependency of the trapped modes with the geometrical parameter a/d was determined from these curves and successfully compared to the theoretical predictions available within the frame of linear wave theory.

Progress on CRLH metamaterials based on dielectric resonators

Combination of magnetically-resonant dielectric resonators and TE cut-off waveguide structures provides unbalanced or balanced composite right/left handed transmission lines. This idea has been expanded into 1-D, 2-D, and 3-D composite structures, and can be applied to potential microwave and millimeter wave circuits and antennas in the same manner as conventional composite right/left handed transmission lines.

Development of a 100 GHz Grooved Circular Empty Cavity for Complex Permittivity Measurements in W Band

A 100GHz grooved circular empty cavity is proposed for the low loss dielectric substrate measurements by the cut-off circular waveguide method in W band. The influence of the excitation holes for the coaxial cable with a small loop are revealed by an FEM based 3D electromagnetic simulator. And also, the diameter of the excitation hole is determined based on the calculated results and the manufacturing accuracy. Then, two kinds of four 100GHz grooved circular empty cavities are fabricated. Comparative experiments of the cavities with the different excitation holes validate the simulated results. Moreover, the complex permittivity of a PTFE plate is measured using the fabricated four cavities by the cut-off circular waveguide method around 84GHz. The measured results agree within measurement error about 0.5% for er and 5% for tanδ. Also, these results accord with results measured by the Whispering-Gallery mode resonator method in 85-110GHz band. It verifies that the proposed 100GHz cavity for the cut-off waveguide method is useful for the complex permittivity measurement of low loss dielectric substrates in W band.

Band Pass Response on Left-Handed Ferrite Rectangular Waveguide

This paper investigates characteristics of periodic structure of ferrite and dielectric slabs in cutoff waveguide which include left-handed operation. Transmission line model and finite element simulation are used to get dispersion characteristics and scattering parameters. Band pass response of left-handed ferrite mode at negative permeability region are discussed with backward wave phenomenon. Theoretical results show that by choosing appropriate ratio of (1) ferrite width and dielectric width, and (2) ferrite length and dielectric length, band pass response with steep edge characteristics can be obtained by the LH ferrite mode, which are confirmed with experiments using single crystal of yttrium iron garnet ferrite. Good band pass and phase shift responses are observed in S band.

Design and implementation of a band-pass filter using dielectric resonators with a new excitation structure

A Tchebychev band-pass filter is constructed by placing high-quality factor TE01δ mode dielectric ring resonators inside a cut-off rectangular waveguide. The focus is on the design process and practical realisation of the filter, which includes choosing optimum geometry of a dielectric resonator, enclosure dimensions and the excitation structure. A novel excitation structure is introduced, which provides high coupling. Also, a new orientation in mounting dielectric resonators in cavity is suggested, which helps in making the filter response more symmetric. The three-pole band-pass-fabricated filter exhibits a symmetrical response with small band-pass ripple.

Evanescent waveguide-based polarisation-agile antenna elements

Polarisation-agile, evanescent waveguide-based antenna elements are proposed, which allow below waveguide cutoff operation from 1.07 to 1.49 GHz or below and above waveguide cutoff operation from 1.54 to 1.89 GHz for return loss &lt;−10 dB. Here square evanescent waveguide with 1.7 GHz cutoff is shunt loaded, respectively, with orthogonal pairs of coaxially excited cylindrical metal posts or flat metal strips positioned across the aperture of the waveguide. It is shown that linear or circular polarisation may be achieved depending on the excitations supplied to the input ports of the antenna.

Study on the Chirped Waveform of the USPR Pulse using the Impulse Response of a Waveguide

In ultrashort-pulse reflectometry (USPR), a chirped waveform transformed from the USPR source impulse signal via waveguide makes it possible to employ millimeter-wave mixers for the frequency up-conversion process. Consequently, the frequency bandwidth of the USPR system is sufficiently wide to cover a large portion of the electron density profile of the plasma. Some physical aspects of the chirped waveform, such as maximum amplitude and length, are critical factors to determine the performance of the system. In this paper, the propagation of the USPR impulse signal through a rectangular waveguide is numerically studied to derive the chirped waveform using the impulse response of the waveguide. The results of numerical computation show that the chirped waveform significantly depends on the waveguide cutoff frequency as well as the waveguide length.

Bandwidth Tunable and Compact Band-Pass Filter (BPF) Using Complementary Split Ring Resonators (CSRRS) on Substrate Integrated Waveguide (SIW)

In this paper, we present a novel bandwidth tunable bandpass filter (BPF) loaded by complementary split ring resonators (CSRRs). On substrate integrated waveguide (SIW), the CSRRs are modeled and deployed to implement BPFs with tunable bandwidth capability. The microwave tunable circuits are designed in order to observe the various coupling coefficients and external quality factors by changing the orientations of the CSRRs. Four different configurations of filters are proposed and fabricated. These structures provide compact size as well as high frequency selectivity due to the use of the subwavelength resonators preserving sharp rejection and an evanescent wave propagation mode below the waveguide cutoff. Attractive performances of tunable BPFs are validated theoretically and experimentally.

Measurement of Complex Permittivity for Liquid Materials Using the Open-Ended Cut-Off Waveguide Reflection Method

Various studies of specific absorption rates (SARs) using liquid phantoms imitating human body tissues have been widely carried out in electromagnetic compatibility (EMC) research fields. In order to establish accurate SARs for measurement, a faithful mockup of human body tissue is needed. Therefore, knowledge of the accurate measurement of sample materials with high permittivity and high loss is very important. In this study, the complex permittivity of tap water, ethanol, methanol and isopropanol is measured by the open-ended cut-off circular waveguide reflection method. The effectiveness of the method presented here of measuring a liquid phantom with high-permittivity and high-loss is also confirmed by comparing the measured results with the results obtained by the TM010 circular cavity resonator method. At this time, the effects on the input impedance under variations of the insertion length and termination conditions were studied. Then the complex permittivity of tap water, ethanol, methanol and isopropanol was measured at frequencies ranging from 0.5 to 3.0GHz using the measurement procedure above. As a result, we confirmed the frequency characteristics of the complex permittivity for a wide variety of high-loss liquid materials.

Frequency measurement of 0.14 THz high-power terahertz pulse

The subharmonic heterodyne combined with cut-off waveguide method is proposed to measure the frequency of 0.14 THz high-power nanosecond pulse in this paper. Using the cut-off waveguide method, the frequency range of the terahertz pulse is determined, in which two non-standard rectangular waveguides with cut-off frequencies of 0.125 and 0.15 THz are separately used as a receiver. According to the preliminary result, the subharmonic order is chosen to be 8, while the frequency of local oscillator is set to be in a range of 15—20 GHz. A series of tests on the 8th harmonic mixer is done using the pulses in Ka-band and the 0.14 THz continuous wave source, separately. Experimental results indicate that the mixer can be used in the heterodyne measurement of 0.14 THz nanosecond pulse. Finally, the heterodyne measurement is carried out on the high-power terahertz puls source, and an accurate frequency of 0.1465 THz is determined in this experiment. This novel method, which greatly reduces the frequency requirement of the local oscillator, is proved to have a good performance in the frequency measurement of 0.14 THz nanosecond pulse, and probably provides a new idea for frequency measurement of pulses in the long-wavelength terahertz-band.

A new approach for monitoring the 27‐day solar rotation using VLF radio signals on the Earth's surface

Typical solar parameters such as sunspot number, Lyman alpha radiation, and 10.7 cm radio flux exhibit 27‐day variations associated with the rotation of the Sun. We have discovered a robust indicator of this 27‐day rotation from measurements of VLF radio signals produced by lightning around the globe. The solar rotation signal is found only at VLF frequencies close to the Earth‐ionosphere waveguide cutoff frequency (∼2 kHz). Furthermore, the 27‐day solar rotation is detected only during daylight hours, implying a 27‐day periodicity in the daytime collision frequency between free electrons and neutral air molecules in the lower ionosphere (∼80 km). We propose that continuous monitoring of VLF radio noise at frequencies close to the waveguide cutoff could provide a new method of continuously monitoring changes in the solar rotation rate.

Narrow asymmetric waveguide semiconductor lasers for improved temperature wavelength stability

We show theoretically that Fabry–Pérot semiconductor lasers using narrow asymmetric waveguides can offer much higher temperature stability of the lasing wavelength compared to structures with standard symmetrical waveguides. The mechanism of the low thermal sensitivity is the uniquely strong wavelength dependence of the confinement factor near the waveguide cutoff, which increases the blueshift of the modal gain peak with carrier density and helps offset the bandgap shrinkage with temperature.

Numerical study on lateral mode behavior of 660-nm InGaP/AlGaInP multiple-quantum-well laser diodes

Theoretical analysis for InGaP/AlGaInP laser diodes with different ridge waveguide structures is performed to investigate the lateral mode behavior using advanced device simulation. The internal physical mechanisms including temperature-induced changes in the refractive index profile, spatial hole burning effect, lateral carrier distribution, and gain profile variation with increasing input current are discussed by theoretical calculation to analyze the effects of different ridge structures on the lateral mode behavior of 660-nm AlGaInP laser diodes. The simulation results show that the use of narrow and shallow ridge geometry is the approach to obtaining single mode operation. Furthermore, it is found that the different values of the ridge height cause the lateral carrier distribution within the active region to be varied, which is also an important factor in determining the emergence of the first order lateral mode in addition to the geometry-dependent waveguide cutoff condition.

Cutoff wavelength of ridge waveguide near field transducer for disk data storage

The electromagnetic eigenmodes of and light transmission through a C-aperture to the far field, and to a storage medium, have been studied based on the full vectorial finite difference method. It is found that the cutoff wavelength of C-aperture waveguides in a gold film is much longer than that in a perfect electric conductor, and the fundamental mode is confined in the gap and polarized with the electric field along the gap. The light transmission resonance through C-apertures to far field and to a storage medium occurs at wavelengths below the waveguide cutoff wavelength. Measurements on the fabricated C-apertures confirm the mode confinement and transmission resonance.