Wide Frequency Region Research Articles

Tunable terahertz filter based on graphene photonic crystals with defective layers

In this paper, we design a high-precision tunable terahertz filter by using transfer matrix method. The filter structure mainly consists of graphene embedded photonic crystals (GPCs). The front part of the GPCs contains artificial synthetic material and air layer, the back part of the GPCs is composed by and periodic stack of isotropic dielectric slabs (MgF2) embedded with graphene sheets, where air defect layer is located in the middle of the GPC as a central layer. Our simulation reveals that graphene layer and air defective layer strongly affect the filter performance. And we can get a relatively pure transmission peak in a wide frequency region. Additionally, the influence of incidence angle of terahertz wave, thickness of air layer, the unit number of front periodic structure and chemical potential of the graphene sheets can also modulate the function of the filter. And the filter has strong stability when the temperature changes from 150 K to 350 K.The results indicate that single channel, dual and multiple channels filter in a narrow frequency can be obtained by optimizing the structure parameter.

Rapid-scan broadband frequency-domain terahertz spectroscopy via dynamic optical phase lock

Frequency-domain terahertz (THz) spectroscopy using photomixing devices has unique advantages such as high dynamic range and high spectral resolution. Thus, many applications for solid-state and gas-phase spectroscopy have been proposed. In this study, we developed a feedback controlled technique to dynamically compensate for the optical phase accompanied by frequency sweep, enabling both fast and high-resolution data acquisition across a wide frequency region. From gas-phase THz spectroscopy measurements of dilute acetonitrile gas in a wide frequency range up to 1.1 THz, fine structures with linewidths less than 10 MHz were clearly resolved, while the data acquisition rate was improved by two orders compared to the previously reported value.

Wind-induced vibration energy harvesters under base excitation: Analytical and numerical analysis

In this paper, analytical analysis of a wind-induced vibration energy harvester used for the bacterial disinfection of water is mainly considered. The complexification-averaging (CA) method is utilized to analytically calculate the results of the harvester, the model equations of the harvester satisfied are reduced into a set of complex variable equations. The relationship between the displacement amplitude and voltage amplitude is derived under the steady-state case. The amplitudes and phases of both the displacement and voltage are obtained. The analytical solutions of the harvester are then verified by direct numerical simulation. The response mechanism under different parameters is discussed. The frequency island and multi-valued phenomena are found. Meanwhile, it is found that a higher wind speed corresponds to a larger displacement amplitude and voltage. In addition, the RMS voltage and average power are studied. Both have the same variation rules under different excitation amplitudes. Under the same coupling coefficient, load resistance and piezoelectric patch capacitance, the lower the frequency, the easier it is to obtain the larger voltage output. However, the load resistance has the opposite effect on both of them. Finally, to obtain maximum output in a wider frequency region, the optimization and choice of the parameters is studied, and the critical boundary can be identified.

Controllable single-photon transport mediated by a time-modulated Jaynes–Cummings model

Controllable single-photon scattering in a one-dimensional waveguide coupled to a Jaynes–Cummings structure containing a time-modulated two-level atom interacting with a single-mode cavity is investigated. The photon transmission and reflection amplitudes are calculated by using an effective Floquet Hamiltonian in real space. The results show that the coupling between the atom and the cavity mode can dynamically be tuned via periodically modulating the atomic transition frequency. As a consequence, the scattering behaviors of the waveguide photons can be actively manipulated, and a controllable single-photon switch with high on-off ratio could be realized. More interestingly, the switch works well within a wide frequency region, i.e., the transmission of both resonant and off-resonant waveguide photons can be effectively switched on or off with appropriate system parameters. Furthermore, the proposed dynamically tunable switching scheme is robust against atomic dissipation associated with the help of atom-cavity coupling mismatch. Such single-photon device can be used as an elementary unit for various quantum information processing.

White-light interferometry immune to vibration disturbance over wide frequency region

White-light interferometry immune to vibration disturbance over wide frequency region

Zn,N-codoped mesoporous carbon particles/carbon fibers for wide-band electromagnetic wave absorption

Carbon fibers with low density, high stability and unique structural characteristics are a huge potential electromagnetic wave absorber. However, it is difficult to achieve wide-band absorption of the carbon fibers due to their impedance mismatching and limited loss factors. Herein, we develop a facile method to fabricate Zn,N-codoped carbon particles/carbon fiber (N,Zn-PNP/NCF). The Zn,N-codoped strategy can cause the distortion of electron clouds of C atoms and the generation of defect sites, which improve the electromagnetic wave absorption performance of carbon fibers. As a result, the optimal N,Zn-PNP/NCF show excellent absorption property with an effective absorption bandwidth of 5.89 GHz at the thickness of 2.0 mm, which is significantly superior to the ordinary N-doped carbon material and most of reported carbon-based absorbers. Theoretical calculations and the experimental results suggest that the Zn,N-codoping along with the interfaces between carbon particles and carbon fibers can effectively enhance the polarization loss capability of carbon fibers due to the charge redistribution caused by the Zn and N heteroatoms. Besides, based on the dielectric characteristic of the optimal N,Zn-PNP/NCF, a pressure response absorber (PSA) is designed, which can realize the selective absorption in the wide frequency region.

Вторичный магнитоупругий резонанс в структуре: тонкая магнитная пленка – толстая упругая подложка

The task about excitation of connected magnetic and elastic vibrations in flat-parallel structure consisted of normal magnetized thin magnetic film applied on thick nonmagnetic substrate is considered. As a result of estimated task solution the amplitude-frequency characteristics of magnetic and elastic vibrations are found. It is shown that in conditions when the ferromagnetic resonance frequency is higher then own elastic vibration of the structure on the amplitude-frequency characteristics of magnetic vibrations applied the equidistance net of elastic resonances of structure. For the interpretation of observed resonance net it is proposed the model of phase synchronization between initial excitation and elastic wave which experience two passages along the thickness of structure. It is shown that the frequency of maximum elastic vibrations rounding exceeds frequency of maximum magnetization characteristic. The existence of own rounding elastic vibrations characteristic is named the second elastic system resonance. It is investigated the dependence of maximum second resonance characteristic from value of magnetoelastic interaction constant. It is shown that by increasing of this constant the frequency distance between both characteristic maxima increases. As a hypothesis of this increasing it is proposed the analogy with splitting of amplitude-frequency characteristic of system consisted of two connected resonators. It is investigated the amplitude-frequency characteristics magnetic and elastic vibrations in wide frequency region. On the equidistance elastic resonances net amplitude-frequency characteristic it is found the narrow equidistance disposed resonance depressions which are correspond to decreasing of amplitude more then two orders of value. It is shown that the frequency interval between neighbouring depressions is back proportional to distance between point of elastic vibrations registration and surface of substrate opposite to magnetic film. For the interpretation of depressions formation it is proposed the model which takes into account the phase opposition in registration point between wave which goes out of the excitation point and wave which is reflected from the opposite end of structure. Briefly noted some physical analogies of investigated phenomena.

Hierarchical polyimide-polyborosiloxane host-guest structure with impact resistance, acoustic and thermal insulation performance for electro-stability applications

The ubiquitous existence of mechanical impact, sound pollution and thermal damage conferred severe risks to both individuals and devices, which catalyzed increasing demand for advanced protective materials. Though biomimetic structures have motivated the development of high-performance protective composites against a simple threat, the realization of multi-mode interference protection remained a challenging issue. This work proposed a host-guest structural composite, consisting of viscoelastic polyborosiloxane (PBS) dispersively distributed in porous polyimide (PI) matrix. The resulting PI-PBS composite presented effective impact force attenuation (85.8%-92.3%), arising from the multi scale energy dissipation mechanism of strain-rate enhancement, structural densification and synergistic stress transfer, which was intuitively demonstrated by an innovative mechano-discoloration approach. The viscous dissipation effect endowed PI-PBS with extraordinary acoustic insulation effectiveness in a wide frequency region (1–6.3 kHz). Due to the low thermal conductivity (0.051 W/(m·K)), PI-PBS presented good thermal insulation performance over a wide temperature range (∼ −80 – 300 °C). The structure dependence of thermal conduction behavior was revealed via FE simulation, while demonstrating various applications in dynamic infrared thermal camouflage. PI-PBS, as a functional wrapping, provided reliable guarantee for electrical stability of conductive elements against impact, ultrasonic and thermal interferences, which indicated significant value in electromagnetic shielding, sensing and electrothermal fields. Therefore, PI-PBS could assist in efforts to lighten, integrate and intelligentize protective materials in the future.

Preparation of hierarchical carbon/CoFe2O4/Co3Fe7 composite with super electromagnetic absorption by innovative ball-milling assisted chemical precipitation instead of traditional solvothermal method

To overcome the serious situation of electromagnetic pollution at present stage, herein the hierarchical carbon/CoFe2O4/Co3Fe7 composites were prepared via an innovative ball milling assistant chemical precipitation and subsequent annealing technique. By changing the solvent ratio of DW/DMF and the additive amount of milling balls, the micro-nano structures, electromagnetic parameters and response behavior of hybrids can be adjusted, in which the N-6 composite obtains a maximal RL of −42.70 dB at 16.42 GHz with the broadest EAB of 8.68 GHz for only 2.38 mm. Based on the well matched impedance within wide frequency region, the excellent attenuation properties of this composite generated from the strong dielectric polarizations, magnetic resonances, eddy current and structure-scattering effect could play roles to absorb incident electromagnetic wave. In short, this investigation offers an innovative route to synthesize the hierarchical carbon decorated with CoFe2O4 and Co3Fe7 nanoparticles for super electromagnetic absorption.

Development of a new base isolation system using the concept of metamaterials

Metamaterials are making a breakthrough in seismic wave manipulation with their ability to yield wave filtration properties that cannot be realized inherently through conventional construction materials. The popularly used seismic vibrational control techniques, such as base isolation, fall short at low-frequency regions of the spectrum and fail to encompass both flexible and rigid structures. Using the concept of metamaterials, the phenomena of local resonance and Bragg scattering can create a stop band region of frequencies that undergo vibrational wave filtration. This study proposes a two-dimensional periodic foundation system with steel-silicone embedded assemblies to serve as local resonators. The proposed foundation can cover a wide and ultra-low frequency region in its band gap, enveloping the principal frequencies of seismic waves (i.e., 0.2 Hz to 10 Hz). This two-dimensional periodic foundation can also exhibit a Bragg region without local resonators but is made more efficient with the help of steel-silicone resonators (SSR). Upon experimental testing, the scaled-down prototype of the proposed foundation with three locally resonant panels embedded with SSR showed an average of 50 % attenuation of displacement within the frequency band gap (i.e., 2.54 Hz to 8.08 Hz). Moreover, the experimentation showed a 73 % attenuation at the resonant frequency of SSR. Experimental testing affirmed the frequency band gap obtained through mathematical formulation. Furthermore, the theoretical results demonstrate that the full-scale proposed periodic foundation would be able to achieve a band gap that covers frequencies from as low as 0.3 Hz to 7.35 Hz. This study is the first attempt to set the baseline for periodic foundations capable of fully mitigating the entire spectrum of seismic vibrational effects on structures.

Design of High Slot Fill Aluminum Winding in a Permanent Magnet Synchronous Machine With Reduced Winding Loss

This paper presents a design of high slot fill aluminum winding in permanent magnet machines for reducing winding losses. Generally, there is a trade-off between the winding weight and the winding losses. The winding DC loss is reduced with an increase of cross-sectional areas of conductors. In general, the aluminum winding is effective for reducing the winding weight, on the other hand, the winding loss is high because the electrical conductivity is low. In this paper, a high slot fill aluminum winding of 83.8% is proposed. It overcomes the trade-off, as a result, the winding losses are significantly reduced. In particular, reducing winding DC loss contributes the decrease of the whole winding loss including winding AC losses. Two prototype machines with the proposed aluminum winding and comparable round copper winding are fabricated and tested. In experiments, both winding DC loss and whole winding losses are measured in the proposed aluminum winding and the round copper winding machines. Consequently, it is demonstrated that the proposed aluminum winding machine can reduce the whole winding losses in wide frequency region. Furthermore, efficiency improvements can be also demonstrated in the proposed aluminum winding.

Shaking table experimental investigations on dynamic characteristics of CFRP cable dome

Carbon fiber reinforced polymer (CFRP) bars have the feature of lower density, higher strength and better anti-fatigue performance compared with steel bars. Therefore, CFRP is a kind of ideal material for a long-span spatial structure. To explore the dynamic performance of spatial structures and take full advantages of CFRP bars in long span cable dome structures, a CFRP cable dome scale model with a diameter of 5.4 m was designed for a shaking table test. In this model, struts and cables were made of CFRP material except that the internal ring beams were fabricated with steel. In the test, new types of joint forms and anchoring configurations for CFRP cable dome structures were proposed because the existing forms for steel domes cannot be applied to CFRP dome structures. In addition, three kinds of seismic waves including Elcentro wave, Kobe wave and Pasadena wave were selected to generate 12 load cases and the maximum acceleration was 900 Gal. The acceleration and strain response of the model under different types of seismic waves was researched. It is demonstrated that middle-ring upper joints have the largest acceleration response because the control vibration modes of the CFRP cable dome structure are anti-symmetric mode. In addition, a CFRP dome numerical model was established to carry out modal analysis by program ANSYS. By comparing the results of finite element modal analysis with the measured frequencies, the natural vibration modes of the structure are analyzed. Based on the results of seismic wave spectrum analysis and structural modal analysis, the Pasadena wave, which has a wide low frequency region for high Fourier amplitude, can excite the greatest acceleration response.

Analysis of Dielectric Characteristics of Lead Oxide/p-type Silicon Structure in a Wide Frequency and Voltage Region

Analysis of Dielectric Characteristics of Lead Oxide/p-type Silicon Structure in a Wide Frequency and Voltage Region

Transmission properties of grating-gate GaN-based HEMTs under different incident angles in the mid-infrared region.

The absorption tunability of grating-gate GaN-based HEMTs in the mid-infrared region has been confirmed in wide frequency regions. However, the application potential of grating-gate GaN-based HEMTs is limited due to a lack of study on transmission properties under different incident angles. Therefore, this paper studied the transmission characteristics of grating-gate GaN-based HEMTs under different incident angles in the mid-infrared region. By using the optical transfer matrix approach to model the dispersion characteristics in the structure, we found that the stronger plasmon polaritons and phonon polaritons occur in conductive channel and GaN layer. The variation of different incident wave vectors with incident angle affects the plasmon polaritons and phonon polaritons excitation intensities, resulting in the angular tunability transmission properties of grating-gate GaN-based HEMTs. After simulating the electric field distribution in COMSOL, the different transmission properties of grating-gate GaN-based HEMTs occur under different incident angles. Simulated results reveal the excellent angle-selectivity in grating-gate GaN-based HEMTs. The research into these characteristics shows that the structure has a lot of promise for designing mid-infrared angle selection filters, sensors, and other subwavelength devices in the future.

Design of miniaturize flexible wideband frequency selective surface for electromagnetic shielding application

In this paper, a flexible, ultra-thin, miniaturized broadband frequency selective surface (FSS) is demonstrated for wideband electromagnetic shielding application. The unit cell of the proposed FSS structure consists of cross dipoles loaded with the metallic circular strip. The proposed miniaturized FSS structure has an extremely low profile with a thickness of and a unit cell size of . This FSS structure provides wideband shielding of more than 20 dB from 1.1 to 2.6 GHz frequency region. Besides this wide stopband, the FSS structure acts as a transparent window in the frequency band which can be used for the 5G communication purpose. Moreover, by comprehending the interaction between the incident electromagnetic wave and unit cell, a conceptual equivalent circuit model is developed that helps to interpret the mechanism of the miniaturization as well as the broadband response of FSS structure. We also compared the proposed FSS with other recently reported structures and observed more than 55% improvement in size reduction with several other miniaturized flexible FSS structure. Furthermore, this FSS structure exhibits 20 dB attenuation over a wide frequency region with a fractional bandwidth of 111%. Moreover, excellent resonant stability is achieved in regards to different polarization and incident angle up to .

A self-vibration-control tensegrity structure for space large-scale construction

Tensegrity is a very attractive structure due to its unique advantages of self-equilibrium, lightweight, small compression volume, and low control cost. However, both the shape and the dynamic behavior of tensegrity are susceptible to environmental change, which has limited the development of the structure in high-accuracy applications. We develop a self-vibration-control tensegrity structure for the space large-scale construction that requires high accuracy and shape retention ability. In the control algorithm, a wave spectrum element method is established first for the dynamic, wave, and power flow analysis of the self-vibration-control tensegrity structure, and the relationship between the global displacement response and the local traveling waves is revealed quantificationally. Then, the power flow vibration control method, which has great robustness and adaptability to the environment, is established using both centralized control strategy and distributed control strategy. Eventually, some numerical simulations and experimental investigations are carried out. The result shows that the vibration of the self-vibration-control tensegrity structure can be effectively suppressed in a wide frequency region. The result of this study can also be applied to other fields.

Polarization properties in grating-gated AlN/GaN HEMTs at mid-infrared frequencies.

The plasmon resonances of grating-gated AlN/GaN HEMTs can occur in wide frequency regions at mid-infrared frequencies. However, the lack of polarization properties research in grating-gated AlN/GaN HEMTs prevents the application potential. In order to solve the problem, the polarization properties in grating-gated AlN/GaN HEMTs at mid-infrared frequencies were studied in the paper. After using the optical transfer matrix method to calculate the dispersion curves in grating-gated AlN/GaN HEMTs, the plasmon polaritons in conductive channel and phonon polaritons in GaN layer occur under TM incident waves rather than TE incident waves. The phenomenon illustrates the potential of polarization-selectivity has existed in grating-gated AlN/GaN HEMTs. To study the polarization properties of grating-gated AlN/GaN HEMTs in detail, the electric field distribution and transmission properties of the structure were simulated in COMSOL. The results show the excellent polarization-selectivity at mid-infrared frequencies in grating-gated AlN/GaN HEMTs. The studies of these characteristics indicate the vast potential for using grating-gated AlN/GaN HEMTs to design mid-infrared polarizers, mid-infrared polarization state modulators and other devices in the future.

Enhanced dielectric and electrical properties of PbS nanostructures facilely synthesized by low-cost chemical route: An effect of Ce doping concentrations

Herein the authors are presenting the facile preparation and characterization of Ce doped PbS nanostructures (Ce@PbS NSs). Phase analysis was studied by X-ray diffraction which approves major cubic phase of PbS along with CeClS phase peaks. Vibrational analysis also confirm major cubic phase of PbS of the final products. EDX/SEM e-mapping showed the Ce doping with homogeneity in PbS. SEM analysis gives a clear morphology of nanoparticles along with some nanoflakes in respective pure and doped PbS. Optical study reveals the variation in energy gap values of PbS with Ce content from 1.45 eV (pure PbS) to 1.084 eV (2.5 wt% Ce@PbS) and also there is another energy gap varies between 3 and 3.5 eV in pure and doped PbS NSs. The capacitance and impedance were studied over wide frequency region. The dielectric constant of PbS has been improved from 17.5 to 24.5 with Ce content addition. The ac electrical conductivity was improved with Ce content addition in PbS. The improvement in dc-electrical conductivity was also noticed with Ce addition in PbS. The improved dielectric and electrical characteristics makes the synthesized Ce@PbS NSs useful in optoelectronic.

Hierarchical HCF@NC/Co Derived from Hollow Loofah Fiber Anchored with Metal-Organic Frameworks for Highly Efficient Microwave Absorption.

Hierarchical electromagnetic wave (EMW) absorption materials with a dielectric-magnetic dual-loss mechanism are promising candidates for highly efficient EMW attenuation. Herein, hierarchical dielectric-magnetic composite hollow carbon fiber@nitrogen-doped carbon/Co (HCF@NC/Co) was successfully synthesized via in situ growth of two-dimensional (2D) Co metal-organic framework (MOF) (ZIF-67) nanosheets on the surface of hollow loofah fiber (HLF), followed by a calcination process, where the aggregation of carbonized MOFs was effectively avoided to construct a homogeneous hierarchical one-dimensional structure. Based on the advantages of the carbon/Co dielectric-magnetic dual-loss mechanism that results in good impedance matching and multiple polarization loss arising from the extensive heterointerfaces (e.g., HCF-NC/Co, air-carbon, nitrogen-carbon, and Co-carbon interfaces), dipole active sites (e.g., doped N, Co particle, and crystalline defects in graphitic carbon), and hierarchical porous structures, optimal EMW absorption performance of HCF@NC/Co is achieved through regulating the calcination temperature and filler content, where the HCF@NC/Co calcinated at 700 °C exhibits a minimum reflection loss (RLmin) value of -50.14 dB with only 14% filler loading and 2.25 mm thickness, and the maximum effective absorption bandwidth (EABmax) also reaches 7.36 GHz. Meanwhile, adjustable EAB can also be achieved by optimizing the sample thickness, making it applicable in a wider frequency region. It is expected that our prepared HCF@NC/Co might shed light on designing lightweight and highly efficient EMW MOF-derived EMW absorbing materials.

Wideband side-lobe level suppression metamaterial based on foldable spoof surface plasmon polaritons

In this work, we propose a method of suppressing the side-lobe level for transmitted waves based on foldable spoof surface plasmon polariton (SSPP). To this end, an SSPP structure is designed and folded with different angles to manipulate the amplitudes of co-polarization transmission coefficients, while the phases remain unchanged. In order to obtain the corresponding relationship between the transmission amplitudes and the folded angles, a nonlinear fitting was performed to get the mapping. After this, SSPP structures are arrayed according to the corresponding amplitudes of the discrete Taylor-distribution, so as to suppress the side-lobe level. To verify this idea, a transmissive metamaterial with wideband side-lobe level suppression (SLLS) was designed, fabricated and measured. The results demonstrated that the metamaterial has a good SLLS performance for transmitted waves, with approximately -20 dB SLL in a wide frequency region. This work provides an effective alternative to achieving SLLS and may find applications in antennas, radomes and stealth techniques, etc.