Transmission Stopband Research Articles

Spatial control of hybridization-induced spin-wave transmission stop band

Spin-wave (SW) propagation close to the hybridization-induced transmission stop band is investigated within a trapezoid-shaped 200 nm thick yttrium iron garnet film using time-resolved magneto-optic Kerr effect microscopy and broadband spin-wave spectroscopy, supported by micromagnetic simulations. The gradual reduction of the effective field within the structure leads to local variations of the SW dispersion relation and results in a SW hybridization at a fixed position in the trapezoid where the propagation vanishes, since the SW group velocity approaches zero. By tuning external field or frequency, spatial control of the spatial stop band position and spin-wave propagation is demonstrated and utilized to gain transmission control over several microstrip lines.

Optimizing graded metamaterials via genetic algorithm to control energy transmission

Optimization of functionally graded metamaterial arrays with a high dimensional and continuous geometric design space is cumbersome and could be accelerated via machine learning tools. Mechanical metamaterials can manipulate acoustic or ultrasonic waves by introducing significant dispersive and attenuative effects near their natural frequency. In this work, functionally graded structures are designed and optimized to combine the energy attenuation performance of multiple unit cells with varying frequency responses and to reduce the interlayer mismatch effects. Optimization through genetic algorithms avoids many local minima related to high dimensionality of the design space, but requires many iterations. A reduced order model (ROM) is applied, which can reproduce the transmission response traditionally calculated with FEM in a fraction of the time. Pairing GA and the ROM together, an array of 6 unit cells (with a total of 18 independent geometric design variables) is optimized to have stop bands with extended width and sharper boundaries. Symmetric functionally graded structures are determined to have optimal geometric configurations. Measured 3D printed features are projected onto the ROM solutions to quantify the effect of printing uncertainty on array performance. Repeatability error of ±20μm is determined to reduce the mean depth of the transmission stop band by a factor of 102 and introduce small shifts in center frequency and band width. Proposed methods to improve the resolution of accessible points in the ROM space, reduce sensitivity to geometric uncertainty, and add design freedom include introducing out-of-plane perforations and varying constituent materials using tunable filled resin systems.

Hybridization‐Induced Spin‐Wave Transmission Stop Band within a 1D Diffraction Grating

AbstractSpin wave propagation is studied through a diffraction grating in a 200 nm thick YIG film by using scanning time resolved magneto‐optic Kerr microscopy (TR‐MOKE) and supported by micromagnetic simulations. Caustic‐like spin wave emission and the hybridization of Damon Eshbach (DE) type spin wave modes within the grating region, depending on the magnetic field and the dimensions of the grating, are observed. Hybridization leads to an increased attenuation length for propagating spin waves and consequently to a transmission stop‐band for spin waves at the grating for a certain magnetic field range.

Mutual Coupling Reduction in CP MIMO Crossed-Dipole Antenna Array

In this letter, a circularly polarized (CP) 1×2 multiple-input–multiple-output (MIMO) crossed-dipole antenna array is investigated. The antenna element consists of a pair of orthogonal linear dipoles, which are connected by two quarter phase-delay lines, generating CP fields. It is found that within the operating CP band, both the excited and coupled crossed-dipole antennas are of circular operation and the mutual coupling is rather strong, but at higher frequency there exists a self-decoupled band (transmission stopband with low | <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> |) in which the excited antenna is of elliptical polarization while the coupled antenna is of linear polarization. In order to reduce the mutual coupling level in the operating band and make it applicable, a single metallic post is added in the middle of the MIMO array, shifting the self-decoupled band downwards to the desired CP band. As a result, the port isolation is enhanced by 13 dB and becomes higher than 20 dB within the entire CP bandwidth of 13.2%.

Modeling and Simulation of Wideband Radio Waves Traveling Through Plasma Coupling With Uniform Magnetic Fields

Propagation behaviors of radio waves through plasma are an essential issue in both science and engineering research. Many numerical methods are proposed to simulate the behaviors, in which the scattering matrix method (SMM) is one of the most popular techniques. However, the SMM available still has important shortcomings existing in the physical modeling aspect, which will result in numerical divergence in calculations. With the analysis of the scattering matrix theory, a new physical modeling method is presented to eliminate this problem. Then, based on the reconstructed model, a modified SMM is developed. Numerical examples indicate its correctness and ability to overcome the divergence phenomenon. Finally, the method is applied to investigate the propagation behaviors of radio waves in plasma boards coupling with uniform magnetic fields, and some particular behaviors are found, such as transmission stopband of the plasma plate. These results can present a new approach for plasma simulation related to the electromagnetic computation and provide further insights into plasma effects on propagations of electromagnetic waves (EMs).

Reconfigurable high-Q terahertz filtering of VO2-based metamaterials using optical tunneling

Up to date, reconfigurable terahertz (THz) band-pass filters based on the design of metamaterials with vanadium dioxide (VO2) have been investigated extensively. However, VO2-based THz metamaterials with high-quality tunable narrowband transmission and good stop-band rejection have yet to be reported, which are essential for THz wave filtering components or systems. Here, we propose a reconfigurable filter in combination with a dielectric metastructure and a VO2 thin film. Utilizing the optical tunneling effects of a cavity layer, the filter has a high band-pass transmission up to 98% at 0.246 THz with a narrow bandwidth of 0.0009 THz, an ultra-high Q-factor of 273, and two broad nearly-suppressed sidebands. The transmission intensity can be modulated by the phase transition of VO2, and the modulation depth is as high as 96.4%. By efficiently tuning the cavity layer thickness, the filter can be designed for a series of transmission wavelengths and further accomplish the switch between the narrow band-pass filtering and wideband reflection. Moreover, the manipulation of the incident angle and polarization can be adopted as an additional degree to achieve a similar switching function. Our study implies a potential for developing various THz applications, where reconfigurable high-Q THz filtering is required.

Mid-infrared spectral reconstruction with dielectric metasurfaces and dictionary learning.

Mid-infrared (MIR) spectroscopy has numerous industrial applications and is usually performed with Fourier-transform infrared (FTIR) spectrometers. While these work well for many purposes, there is currently much interest in alternative approaches that are smaller and lighter, i.e., MIR microspectrometers. Here we investigate all-dielectric metasurfaces as spectral filters for MIR microspectrometers. Two metasurface types are studied. For the first, we design, fabricate, and test a metasurface with a narrow and angularly tunable transmission stop band. We use it to reconstruct the transmission spectra of various materials. The second metasurface, investigated theoretically, possesses narrow passband features via symmetry-protected bound states in the continuum.

Solution‐Processed Barium Titanate Nonlinear Woodpile Photonic Structures with Large Surface Areas

Three dimensional (3D) nonlinear photonic crystals are one of the potential candidates for enabling photonic circuits. Most of the current top‐down fabrication approaches for such crystals are time consuming and suffer from small surface areas and limitations to specific substrates. Herein, a bottom‐up fabrication technique (UV‐soft nanoimprint lithography) together with solution processing of barium titanate (BaTiO3) nanoparticles is used to create large‐scale 3D nonlinear woodpile photonic crystals. Opposed to other 3D photonic crystals, the woodpile structure can be fabricated layer by layer. This property makes it suitable for roll‐to‐roll fabrication, which can be easily adapted in industry. The prepared photonic crystals with up to eight layers have surface areas of 1.0 cm2. The linear and nonlinear optical behavior of these 3D woodpile structures is characterized. Their transmission stop band lies around 757 nm and a relative linear minimum transmission of 48.4% for a four‐layer sample is obtained. Images of diffraction patterns together with polarization‐dependent second harmonic generation measurements prove a cubic photonic crystal structure in the linear and nonlinear regime. In addition to second‐order nonlinearity, BaTiO3 exhibits electro‐optic, elasto‐optic, and thermo‐optic effects. Therefore, it is especially suited for future active photonic crystal applications.

Performance Study of 90° Bend Substrate Integrated Waveguide Band-Pass Filter with Multi-transmission Zeroes for MIC Applications

Design of microwave components over planar integrated circuits requires flexibility in terms of compactness and adept performance. To aid this requirement design and analyses of multi-band microwave bandpass filter operating in frequencies 13.8–15.0 GHz, 15.1–17.95 GHz and 18.0–19.7 GHz using substrate integrated waveguide (SIW) has been proposed in this paper. Transmission zeroes arising due to periodic rectangular slots are intended to create separation between the successive passbands to eliminate possibility of co-channel interference. Periodic reactive slots are analysed in details to provide greater insight over the controlling parameters of the BPF. The experiments are carried out over 90° bend SIW structure which provide more compact layout as required in microwave integrated circuits. Nearly 35% compactness is achieved by using 90° bend compared to planar structure without waveguide bend. Transmission bandwidth and stopband performances of the proposed BPF are enhanced on the basis of intense study of the waveguide bend as well as periodic slots. Insertion loss within all passbands is obtained in between 0.3 and 0.9 dB. The designs are fabricated using Neltec 3.2 NH9320 substrate with tanδ = 0.0024 and thickness of 30 mils. Measured results testify the simulated outcomes.

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.

Fabry–Pérot Cavities for the Terahertz Spectral Range Based on High-Reflectivity Multilayer Mirrors

We constructed Fabry–Perot cavities using well-characterized broadband dielectric multilayer mirrors based on z-cut quartz with a clear aperture of 20 mm. Each quartz wafer thickness was measured with an error significantly less than 1%, which enabled the accurate simulation of the cavity mode frequency positions. Characterization methods in transmission geometry involved white-light interferometry, terahertz Fourier transform, and time-domain spectroscopy. The latter enabled the determination of the cavities phase response. The first transmission stopband with strong attenuation of the multilayer mirrors appears between 0.5 and 0.9 THz. With four dielectric layers, a reflectivity of at least 98.6% and a full-width at half-maximum of more than 53% with respect to the center design frequency at 0.75 THz are obtained. A loaded 2-mm cavity made from two mirrors, each with two layers, reached a Q-factor of at least 54. In all cases, we find a good agreement between the experimental transmission and phase response of our structures and the simulations based on the transfer matrix formalism.

Low-loss, high reflectivity, first-order, pitch-by-pitch fiber Bragg grating fabrication in truly free-standing single-mode fiber

Low-loss, high reflectivity, first-order fiber Bragg gratings are achieved by pitch-bypitch fabrication methodology in a truly free-standing fiber. Slit beam-profiling adopted attain transversely-extended index modification for each grating period defined by single laser pulse. Fiber Bragg grating of 0.52µm period with >27dB transmission stopband and <0.2dB/cm insertion loss are demonstrated.

Investigation of Microstrip Meander-Line Traveling-Wave Tube Using EBG Ground Plane

A novel electromagnetic bandgap meander-line (EBG-ML) slow-wave structure (SWS) is proposed for improvement of the interaction impedance and reduction of circuit attenuation by in-phase reflection of EBG for traveling-wave tube applications. With high on-axis interaction impedance, the EBG-ML SWS can interact with a low-current-density electron beam. The backward-wave mode of EBG-ML SWS can be suppressed by the transmission stopband of EBG. Particle-in-cell simulations are carried out for the proposed structure with a sheet beam source. The simulation results show that a 50% improved interaction impedance and a 20% reduced circuit attenuation of EBG-ML SWS can be achieved, in comparison with those of traditional ML SWS. With a small input signal, the RF efficiency of EBG-ML is improved by 137.5% over that of the ML SWS.

Tunable Omnidirectional Broadband Band-Stop Filter in Symmetric Hybrid Plasmonic Structures

We numerically investigate a symmetric hybrid waveguide-plasmon system composed of a periodic metallic nanowires pairs array embedded in symmetric dielectric sandwich layers for band-stop filtering in the optical frequency range. The proposed symmetric system shows an omnidirectional broadband absorption enhancement with flat band-stop transmission induced by the coupling and hybridization of photonic and plasmonic modes. The transmission stop band bandwidth ranging from 80 to 585 nm is observed. The bandwidth of the proposed system can be further manipulated by tailoring their geometrics for potential applications in plasmonic-assisted broadband optical filtering.

Analogue of Electromagnetically Induced Transparency in a Magnetic Metamaterial

In this paper, the analogue of electromagnetically induced transparency is achieved in a magnetic metamaterial consisting of coupled “radiative” square closed loop (SCL) and “dark” square spiral resonator (SSR). Full-wave numerical simulations are carried out to validate the EIT-like effect of the magnetic metamaterial. Transmission spectrums and surface current distributions for the metamaterial are presented. It is shown that placing the SSR close to the SCL causes the electromagnetic field energy to be coupled back and forth between them. This leads to destructive interference and the transparency window in the transmission stop-band of the metamaterial. In addition, it is numerically demonstrated that the magnetic metamaterial can be employed as a refractive-index based sensor with a sensitivity of 41.3 mm/RIU, which means that the resonance wavelength of the sensor shifts 41.3 mm per unit change of refractive-index of the surrounding medium.

Cost-effective technique for the relocation of transmission characteristics of epitaxially grown Fabry–Pérot filter

A simulation model relying on Transfer Matrix Method is established to describe the working mechanism of an epitaxially grown multilayer Fabry–Pérot filter, which is formed by Metal-organic Chemical Vapor Deposition on GaAs substrate. By employing this model as theoretical guidance, a particular technique is proposed for the relocation of transmission characteristics (i.e. transmission peak wavelength and stop-band) of this filter. Two experiments are carried out to demonstrate this technique. In comparison with the typical one, this technique presents much higher efficiency, and consequentially is remarkably cost-effective. Moreover, this technique offers a referable methodology for the relocation of operation wavelengths of Fabry–Pérot micro-cavity-based devices.

A NOVEL COMPACT ULTRA-WIDEBAND BANDPASS FILTER WITH SIMULTANEOUS NARROW NOTCHED BAND AND OUT-OF-BAND PERFORMANCE IMPROVEMENT

A compact ultra-wideband (UWB) bandpass fllter (BPF) with simultaneous narrow notched band and out-of-band performance improvement is presented. The UWB BPF is built up using the hybrid microstrip and coplanar waveguide (CPW) structure. By employing the split-ring resonator (SRR) which is defected on the lower plane, the narrow notched band was introduced. The center and bandwidth of the notched band can be controlled by adjusting the length and width of the SRRs. A novel cross-shape patch is constructed to implement transmission stopband in the upper out-of-band so as to suppress the spurious passband. The novel UWB BPF is designed and fabricated. The measured insertion loss is less than 1.6dB, the return loss is more than 13dB and the variation of group delay is less than 0.12ns. The width of narrow notched band is about 0.15GHz and the attenuation is 18dB at the center frequency of 5.76GHz. The upper stopband is up to 15.2GHz with rejection greater than 20dB.

Angle and polarization tolerant midinfrared dielectric filter designed by genetic algorithm optimization

We report a dielectric frequency selective surface filter with an angularly tolerant, polarization independent stop band designed at a midinfrared wavelength of 3.0 μm. The structure, consisting of a doubly-periodic amorphous silicon grating supported by a polyimide membrane, was optimized using a genetic algorithm to have a transmission stop band with a 3.33% 10 dB bandwidth for incidence angles as large as 10° from normal. The measured properties of the fabricated filter were within 1% of theoretical predictions. This strategy provides a practical and efficient approach to creating advanced dielectric filters for highly customized infrared optical device and coating applications.

Multiresonant Metamaterial Based on Asymmetric Triangular Electromagnetic Resonators

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> A new multiresonant metamaterial composed of asymmetric triangular electromagnetic resonators (ATERs) is presented in this letter. The ATERs' structure has one electric resonance and two magnetic resonances. The parametric retrieval and eigenmode analysis based on simulations and experiments are utilized to gain an insight into the resonance mechanisms. A parallel-plate waveguide (PPW) measurement system is introduced to detect the characteristics of multiresonant ATERs. The transmission stopband obtained by the PPW system effectively reveals that one resonance is caused by an electric mode and the other two resonances are caused by magnetic modes. </para>

Optical properties of silica opal templates in the infrared and visible

Measurements of transmission and reflection from thick silica opal slabs were performed in the spectral range 0.3–3.5 μm. A wide transmission stop band due to photonic band gap with no signs of reentrance to high-transmission regime down to 0.3 μm was observed. In addition, a new stop band in the range 2.7–3.6 μm was observed, where the transmittance oscillates fringe like about a small level in spite of negligible dispersion and absorption. In the reflection measurements two strong rejection bands were observed around 9.0 μm and 20.9 μm, which are due to anomalous dispersion in SiO 2. The results obtained were explained using concepts of photonic band structure and photonic transport in disordered medium. An effective-medium theory was used to compute the infrared optical properties. A semi-quantitative agreement between the simulated and measured infrared reflectance was found.