Stopband Properties Research Articles

Attenuation properties of a meta-pipe: Analytical, experimental, and numerical prediction

Flexural vibration characteristics of a meta-pipe are investigated in this paper. Initially, the metamaterial piping system consisting of a homogeneous pipe and periodic flexible supports is designed. Subsequently, the dispersion relationship which is a function of the wavenumber and the frequency is derived employing the transfer matrix method and Bloch’s theorem. The designed system reveals a zero-frequency stopband and a wide passband in the examined frequency range, which are verified by both experimental and numerical results. To enhance the performance of this system, it is essential to effectively control the obtained passband. For this, a single-degree-of-freedom localized resonator is designed, which is subsequently installed at the center of each unit-cell of the pipe. The dispersion properties of this system are first evaluated analytically and then validated using experimental and numerical methods. Finally, the effect of pipe system parameters and resonator properties on the stopband characteristics is studied in depth. It was observed that the stopband properties can be effectively tuned by appropriately designing the piping system and resonators. The results presented in this study offer valuable insights into the propagation characteristics of flexural waves and the strategies devised for controlling vibrations in pipes.

A study of acoustic focusing based on the taboo genetic algorithm

Acoustic metasurfaces have shown great promise for applications in many acoustic wave modulation fields, while the use of intelligent optimization algorithms has facilitated the design of acoustic metasurfaces. In this paper, we utilize the stop band properties of phononic crystals to design a metasurface that can achieve both transmission focusing and reflection focusing, as well as a metasurface that utilizes acoustic focusing to filter the incident acoustic wave. In order to achieve the best effect, this paper uses the taboo genetic algorithm to optimize the arrangement order of the structural units controlling different focuses in the acoustic metasurface, and the optimization achieves a good focusing effect, which expands the design of acoustic metasurfaces as well as the application scenarios of acoustic focusing.

A Limited Memory BFGS Based Unimodular Sequence Design Algorithm for Spectrum-Aware Sensing Systems

Unimodular sequences with good correlation and spectral properties are desirable in numerous applications such as active remote sensing and communication systems. Therefore, designing sequences with stopband and correlation sidelobe constraints has gained a lot of attention in the last few decades. In this paper, we propose a fast and efficient iterative algorithm to design unimodular and sparse frequency waveforms with low aperiodic/periodic autocorrelation sidelobes and desired stopband properties. In our approach, the bi-objective optimization problem which minimizes both the integrated sidelobe level (ISL) of the autocorrelation function and the power density in the spectral stopbands is first turned into an unconstrained single objective optimization problem and then is treated as a nonlinear large-scale problem. For the solution of the problem, we develop an algorithm based on Limited Memory Broyden–Fletcher–Goldfarb–Shanno (L-BFGS) Quasi-Newton optimization method. Unlike most gradient based algorithms which employ line searches to deduce the step length, owing to L-BFGS method, unit step length is taken as a general rule to avoid the cost of computation at every iteration with very few exceptions. The calculation of gradient is based on Fast Fourier Transform and Hadamard product operations and thus the algorithm is fast and computationally efficient. Moreover, the algorithm is space efficient and its low-memory feature makes it possible to generate long sequences. Several numerical examples are presented to validate the efficacy of the proposed method and to show its superiority over other state-of-art algorithms.

A tunable phononic crystal system for elastic ultrasonic wave control

Tunable phononic crystals (PCs) are burgeoning within the engineering communities due to their adjustable capabilities for manipulating elastic waves. In this Letter, a shape memory alloy (SMA) stubbed PC is elaborated to achieve the tunable bandgaps for the control of ultrasonic waves. The tunable mechanism stems from the phase transformation of the SMA between the martensite at low temperature and the austenite at high temperature. Both numerical modeling and experimental verification are conducted to validate the achievement of the controllable stop band properties. The proposed SMA PC design could be broadened to numerous promising applications, e.g., ultrasonic wave waveguiding, flat-lens focusing, and mechanical frequency filtering, possessing enabling potential for highly adjustable wave manipulation.

Band dynamics accompanied by bound states in the continuum at the third-order Γ point in leaky-mode photonic lattices

Bound states in the continuum (BICs) and Fano resonances in planar photonic lattices, including metasurfaces and photonic-crystal slabs, have been studied extensively in recent years. Typically, the BICs and Fano resonances are associated with the second stop bands open at the second-order Γ point. This paper addresses the fundamental properties of the fourth stop band accompanied by BICs at the third-order Γ point in one-dimensional leaky-mode photonic lattices. At the fourth stop band, one band edge mode suffers radiation loss, thereby generating a Fano resonance, while the other band edge mode becomes a nonleaky BIC. The fourth stop band is controlled primarily by the Bragg processes associated with the first, second, and fourth Fourier harmonic components of the periodic dielectric constant modulation. The interplay between these three major processes closes the fourth band gap and induces a band flip whereby the leaky and BIC edges transit across the fourth band gap. At the fourth stop band, an accidental BIC is formed owing to the destructive interplay between the first and second Fourier harmonics. When the fourth band gap closes with strongly enhanced radiative Q factors, Dirac cone dispersions can appear at the third-order Γ point.

A Wideband High-Voltage Longitudinal Output Structure for Ka-Band Sheet Beam Traveling-Wave Tubes

This article proposes a longitudinal output structure in Ka-band sheet beam traveling-wave tube (SBTWT) with an ultrawideband transmission and high breakdown collector voltage. A novel electromagnetic bandgap (EBG) cutoff structure is designed to separate the slow-wave structure (SWS) and the depressed collector. The large stopband property of the EBG structure is applied to achieve not only the high transmission and the low gap leakage but also the long gap distance to provide a high breakdown voltage of the depressed collector. The 3-D simulations of a single longitudinal structure show a full Ka-band (26.5–40 GHz) reflection of less than −20 dB and gap leakage less than −0.02 dB. The entire longitudinal output collector, including two large gaps cutoff structures and the over mode electron collecting parts, is designed and manufactured. The simulations show a bandwidth of 10.4 GHz with a reflection coefficient <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S} _{\mathbf {11}}$ </tex-math></inline-formula> of lower than −20 dB. The cold test demonstrates the wild bandwidth of ~11.1 GHz with a reflection coefficient <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S} _{\mathbf {11}}$ </tex-math></inline-formula> of lower than −15 dB. The hot test reveals a stable collector dc voltage of −14.2 kV.

A compact UWB FSS single layer with stopband properties for shielding applications

Wydawnictwo SIGMA-NOT wydaje czasopisma fachowe informujące swoich czytelników o najnowszych osiągnięciach naukowych i nowoczesnych rozwiązaniach technicznych w Polsce i na świecie, popularyzuje problemy techniczne oraz poszerza wiedzę i kulturę techniczną.

Plasmonic Nanocrystal Arrays on Photonic Crystals with Tailored Optical Resonances.

Hierarchical plasmonic–photonic microspheres (PPMs) with high controllability in their structures and optical properties have been explored toward surface-enhanced Raman spectroscopy. The PPMs consist of gold nanocrystal (AuNC) arrays (3rd-tier) anchored on a hexagonal nanopattern (2nd-tier) assembled from silica nanoparticles (SiO2NPs) where the uniform microsphere backbone is termed the 1st-tier. The PPMs sustain both photonic stop band (PSB) properties, resulting from periodic SiO2NP arrangements of the 2nd-tier, and a surface plasmon resonance (SPR), resulting from AuNC arrays of the 3rd-tier. Thanks to the synergistic effects of the photonic crystal (PC) structure and the AuNC array, the electromagnetic (EM) field in such a multiscale composite structure can tremendously be enhanced at certain wavelengths. These effects are demonstrated by experimentally evaluating the Raman enhancement of benzenethiol (BT) as a probe molecule and are confirmed via numerical simulations. We achieve a maximum SERS enhancement factor of up to ∼108 when the resonances are tailored to coincide with the excitation wavelength by suitable structural modifications.

Design and fabrication of a low-loss microstrip lowpass-bandpass diplexer for WiMAX applications

This work presents a compact low-pass-bandpass microstrip diplexer with a novel configuration. It consists of a lowpass filter integrated with a bandpass filter via a simple compact junction. The proposed bandpass filter consists of four rectangular patch cells and some thin strips. The step impedance structures, with a radial cell, are applied to achieve a lowpass frequency response. The lowpass channel of the introduced diplexer has 2.64 GHz cut-off frequency, whereas, the bandpass channel center frequency is 3.73 GHz for WiMAX applications and covers the frequencies 3.31 GHz to 4 GHz. In addition to having novel structures, both filters have other advantages in terms of high return loss, low insertion loss and high selectivity. The presented microstrip diplexer has the compact size of 29 mm × 13.8 mm × 0.762 mm, calculated at 2.64 GHz. The obtained insertion losses are 0.20 dB (for the first channel) and 0.25 dB (for the second channel), which make the proposed diplexer suitable for energy harvesting. The stopband properties of both bandpass and lowpass filters are improved by creating several transmission zeros. The comparison results show that the lowest insertion losses, the minimum gap between channels, good return losses, and good isolation are obtained.

Mass Transport Determined Silica Nanowires Growth on Spherical Photonic Crystals with Nanostructure-Enabled Functionalities.

A robust and facile method has been developed to obtain directional growth of silica nanowires (SiO2 NWs) by regulating mass transport of silicon monoxide (SiO) vapor. SiO2 NWs are grown by vapor-liquid-solid (VLS) process on a surface of gold-covered spherical photonic crystals (SPCs) annealed at high temperature in an inert gas atmosphere in the vicinity of a SiO source. The SPCs are prepared from droplet confined colloidal self-assembly. SiO2 NW morphology is governed by diffusion-reaction process of SiO vapor, whereby directional growth of SiO2 NWs toward the low SiO concentration is obtained at locations with a high SiO concentration gradient, while random growth is observed at locations with a low SiO concentration gradient. Growth of NWs parallel to the supporting substrate surface is of great importance for various applications, and this is the first demonstration of surface-parallel growth by controlling mass transport. This controllable NW morphology enables production of SPCs covered with a large number of NWs, showing multilevel micro-nano feature and high specific surface area for potential applications in superwetting surfaces, oil/water separation, microreactors, and scaffolds. In addition, the controllable photonic stop band properties of this hybrid structure of SPCs enable the potential applications in photocatalysis, sensing, and light harvesting.

Multilevel Spherical Photonic Crystals with Controllable Structures and Structure‐Enhanced Functionalities

AbstractSpherical photonic crystals (SPCs) with tailorable multiscale structure, versatile surface morphology, and controllable optical properties of both photonic stop band (PSB) and surface plasmon resonance (SPR), have been fabricated using a robust and facile method. The fabricated SPCs consist of well‐spaced gold nanocrystals (AuNCs) (3rd‐tier) anchored on silica nanopatterns (2nd‐tier) confined in microspherical templates (1st‐tier). Droplet microfluidics is used to produce microdroplets containing silica nanoparticles (SiO2NPs) which assemble to form two‐tier SPCs. Subsequently, three‐tier SPCs are obtained by thermal dewetting and evaporation of metal films deposited on the two‐tier SPCs, with the 3rd‐tier morphology being controlled by the deposited film morphology and programmed thermal annealing. Optical PSB and SPR properties of the prepared SPCs can be on‐demand tailored by the 2nd and 3rd‐tier morphology and their corresponding constituent materials. It is found that the scattering from AuNC arrays on the SPCs can be amplified by tailoring the PSB properties. The hierarchical SPCs manufactured by this method take advantages of low‐cost, high controllability, and further processability. The manufacturing of flexible film encapsulated well‐assembled SPCs as anticounterfeiting stamps, which are easy to be identified using the mobile phone with a flash, is demonstrated.

Microwave propagation in a metamaterial-ferromagnetic waveguide

We theoretically investigate the propagation characteristics of transverse electric (TE) waves in a waveguide consisting of a nonlinear metamaterial and a linear ferromagnet. The investigation shows that the TE waves exist in a form of a surface wave localized at the interface between the metamaterial and ferromagnet and propagate backward in the metamaterial and forward in the ferromagnet. The effective wave power in the waveguide can be well balanced for some incident frequencies. It is manifested that the guiding structure exhibits the properties of passband and stop band that can be adjusted and switched into each other by controlling the intensity-dependent parameter at the interface. It is expected that such wave characteristics could provide ideas for novel device design and potential applications in information processing.

Reconfigurable frequency selective surface with tunable characteristics depending on intensity of atmospheric light

The authors presented a reconfigurable frequency selective surface (FSS), loaded with light-dependent resistors (LDRs). In this proposed FSS, no biasing circuit is used to control the transmission characteristics. Variation in intensities of the light that falls upon is used as the control parameter for variation of resistance values of LDRs. Thus tunable transmission characteristics of the FSS are achieved. The simulated results show that in the absence of atmospheric light, the FSS has band stop properties at the frequency range of lower S -band (2–3 GHz) and band pass properties at the frequency range of C -band (4–8 GHz). On the other hand, in the presence of full bright light, it has stop band properties at the frequency range of C -band (4–8 GHz). Experimental results are in good agreement with the simulated results. Another novelty about the proposed FSS is the polarisation-independent (TE and TM polarisation) transmission behaviour. The proposed FSS also has a stable transmission behaviour for variation in the incidence angle up to 30°.

Design of a novel microstrip four-channel diplexer for multi-channel telecommunication systems

This paper presents a novel microstrip quad-channel diplexer. The introduced diplexer operates at close resonance frequencies of 1.68 GHz, 1.9 GHz, 3.9 GHz and 4.43 GHz for multi-service wireless applications. The frequency responses of S21 and S31 are sharp at the edges of all passbands. To improve the stopband properties, several transmission zeros are created. Also, the low insertion losses of 0.68 dB, 0.57 dB, 0.71 dB, and 0.5 dB are obtained at the resonance frequencies. The proposed diplexer has a new structure with a compact size of 0.051 × λ g 2 . To verify the design method and simulation results, the proposed structure is fabricated and measured. There is a good agreement between the simulation and measurement results.

Band flips and bound-state transitions in leaky-mode photonic lattices

Leaky-mode photonic lattices exhibit intricate resonance effects originating in quasi-guided lateral Bloch modes. Key spectral properties are associated with phase-matched modes at the second (leaky) stop band. One band edge mode suffers radiation loss generating leaky-mode resonance whereas the other band edge mode becomes a bound state in the continuum (BIC). Here, we present analytical and numerical results on the formation and properties of the leaky stop band. We show that the frequency of the leaky-mode resonance band edge, and correspondingly the BIC edge, is determined by superposition of Bragg processes chiefly generated by the first two Fourier harmonics of the spatial modulation. We derive conditions for the band closure and band flip wherein the leaky edge and the bound-state edge transit across the band gap. Our work elucidates fundamental aspects of periodic photonic films and has high relevance to the burgeoning field of metamaterials.

Fast algorithm for designing periodic/aperiodic sequences with good correlation and stopband properties

Periodic/aperiodic sequences with low autocorrelation sidelobes are widely used in many fields, such as communication and radar systems. Besides the correlation property, the frequency stopband property is often considered in the sequence design when the systems work in a crowded electromagnetic environment. In this paper, we aim at designing periodic/aperiodic sequences with low autocorrelation sidelobes and arbitrary frequency stopbands, and propose an efficient algorithm named FFT (fast Fourier transform)-based conjugate gradient algorithm. To calculate the step size efficiently, a method based on Taylor series expansion is developed. By changing the number of FFT points, the proposed algorithm can be easily used to generate periodic/aperiodic sequences. Since the gradient and step size can be implemented by FFT operations and Hadamard product, the whole algorithm is computationally efficient and can be used to design very long sequences. Numerical experiments show that the proposed algorithm has better performance than the state-of-the-art algorithms in terms of the running time.

Designing PAR-constrained periodic/aperiodic sequence via the gradient-based method

Periodic/aperiodic sequence with good correlation and stopband properties has received widespread attention and been widely applied in radar and communication systems. To meet the hardware requirement and maximize the transmitter efficiency, the unimodular or peak-to-average ratio (PAR) constraint is always required in sequence design. In this paper, we consider the problem of designing PAR-constrained periodic/aperiodic sequence with good properties. After establishing the corresponding criterions for both correlation and stopband properties, the unified PAR-constrained problem is formulated and then transformed into an unconstrained minimization problem via sequence synthesis. To solve the problem, an efficient gradient-based algorithm is proposed to minimize the objective function directly. As the main steps can be implemented by fast Fourier transform (FFT) operations and Hadamard product, the whole algorithm is computationally efficient. In addition, the proposed algorithm can be applied to design both the periodic and aperiodic sequences by choosing proper parameters. Numerical experiments show that the proposed algorithm has better performance than the state-of-the-art algorithms in terms of the sequence quality and the running time.

Active control of elastic wave propagation in nonlinear phononic crystals consisting of diatomic lattice chain

This work studies the active control effects on nonlinear phononic crystals by the piezoelectric spring model. Both negative and positive proportional control actions are considered. Based on the Lindstedt–Poincaré method, the approximate solution is derived and the stop band properties are presented. Numerical calculations show that the structural stiffness and negative proportional control of the piezoelectric spring can create a new stop band which is under the acoustic branch. But the positive proportional case has a different influence because of a critical wave number appearing. Moreover, the optic branch can be uplifted by the elastic wave amplitude. Different from the soft nonlinear characteristic, the hard nonlinear property can increase the stop band width.

Cognitive Phase-Only Sequence Design With Desired Correlation and Stopband Properties

This paper considers the phase-only sequence design problem for a cognitive radar in order to achieve both the desired autocorrelation and the stopband properties, which is useful to obtain good range compression as well as the spectral compatibility. We develop a new objective function accounting for the weighted integrated sidelobe level and the spectral power within specific stopbands. An iteration algorithm based on pattern search (PS) is proposed to minimize the nonconvex multidimensional objective function with the continuous phase case, which is split into multiple one-dimensional problems that are simplified and solved with closed-form solutions. In particular, the PS algorithm can be applied to the discrete phase case. Finally, we evaluate the effectiveness of the PS algorithm compared with the weighted-stopband cyclic algorithm new algorithm via numerical simulations in terms of the autocorrelation function, the spectral power function, and the convergence speed.

Low-bias flat band-stop filter based on velocity modulated gaussian graphene superlattice

Transport properties of biased planar Gaussian graphene superlattice (PGGSL) with Fermi velocity barrier is investigated by transfer matrix method (TMM). It is observed that enlargement of bias voltage over miniband width breaks the miniband to WSLs leads to suppressing resonant tunneling. Transmission spectrum shows flat wide stop-band property controllable by external bias voltage with stop-band width of near 200meV. The simulations demonstrate that strong velocity barriers prevent tunneling of Dirac electrons leading to controllable enhancement of stop-band width. By increasing ratio of Fermi velocity in barriers to wells υc stop-band width increase. As wide transmission stop-band width (BWT) of filter is tunable from 40meV to 340meV is obtained by enhancing ratio of υc from 0.2 to 1.5, respectively. Proposed structure suggests easy tunable wide band-stop electronic filter with a modulated flat stop-band characteristic by height of electrostatic barrier and structural parameters. Robust sensitivity of band width to velocity barrier intensity in certain bias voltages and flat band feature of proposed filter may be opens novel venue in GSL based flat band low noise filters and velocity modulation devices.