Fundamental Resonance Research Articles

Frequency multiplication in a high-current relativistic gyrotron for obtaining high-power THz-band radiation

Using averaged equations and 3D particle-in-cell simulations, we investigate the frequency multiplication regime in a high-current relativistic gyrotron operating in 0.1 THz frequency band. We demonstrate that the ratio of the 3rd harmonic power and the fundamental cyclotron resonance power can reach 0.4–0.8%, which aloows for obtaining the 0.3 THz radiation with sub-MW output power level.

An electrical/thermal dual-controlled quad-functional terahertz metasurface absorber.

Although the design of graphene-based tunable broadband terahertz (THz) absorbers has attracted much attention, improving the functionality of the absorbers to adapt to different scenarios is still worth studying. This paper presents an innovative design of a quad-functional metasurface absorber (QMA) in the THz region, which can switch the absorption frequency/band by means of dual voltage/thermal manipulation. By electrically manipulating the chemical potential of graphene, the QMA can switch freely between the narrowband absorption mode ("NAM") and the broadband absorption mode ("BAM"), while thermally manipulating the phase transition of VO2 allows switching between the low-frequency absorption mode ("LAM") and the high-frequency absorption mode ("HAM"). Detailed mechanistic analysis shows that the "NAM" and "BAM" are due to the switching of the fundamental and second order graphene surface plasmon polariton (SPP) resonances, respectively, and the switching between "LAM" and "HAM" is due to the phase transformation of VO2. Furthermore, the QMA is polarization insensitive in all absorption modes and maintains excellent absorption performance at large angular incidence of TE- and TM-polarized waves. All the results indicate that the proposed QMA has great potential for stealth, sensing, switching, and filtering applications.

Dynamical Stability of a 3-DOF Auto-Parametric Vibrating System

PurposeThis article concentrates on the oscillating movement of an auto-parametric dynamical system comprising of a damped Duffing oscillator and an associated simple pendulum in addition to a rigid body as main and secondary systems, respectively.MethodsAccording to the system generalized coordinates, the controlling equations of motion are derived utilizing Lagrange's approach. These equations are solved applying the perturbation methodology of multiple scales up to higher orders of approximation to achieve further precise unique outcomes. The fourth-order Runge–Kutta algorithm is employed to obtain numerical outcomes of the governing system.ResultsThe comparison between both solutions demonstrates their high level of consistency and highlights the great accuracy of the adopted analytical strategy. Despite the conventional nature of the applied methodology, the obtained results for the studied dynamical system are considered new.ConclusionsIn light of the solvability criteria, all resonance scenarios are classified, in which two of the fundamental exterior resonances are examined simultaneously with one of the interior resonances. Therefore, the modulation equations are achieved. The conditions of Routh–Hurwitz are employed to inspect the stability/instability regions and to analyze them in accordance with the solutions in the steady-state case. For various factors of the examined structure, the temporary history solutions, the curves of resonance in terms of the adjusted amplitudes and phases, and the stability zones are graphically presented and discussed.ApplicationsThe results of the current study will be of interest to wide range experts in the fields of mechanical and aerospace technology, as well as those working to reduce rotors dynamical vibrations and attenuate vibration caused by swinging structures.

Spatial observation of highly directional radiation from a compact 3D photonic-crystal antenna in Ku band

A three-dimensional compact prototype of a photonic crystal directive antenna is constructed for beam steering and directional radiation in the microwave. Experiments demonstrate that the array of dielectric cylinders placed in a U-shape open metal cavity can be profitably exploited to act as a high-efficiency splitting directive antenna in particular modes. Highly directional radiation has been observed at the fundamental electromagnetic resonance of band edge crystal with a maximum gain of 20.73 dBi and an aperture efficiency of 49.77%. The side-lobes on the E-plane (yoz) are almost constrained in −20 dB benefitting from the band edge lattice-induced resonance on the H-plane (xoy), suppressing the effect of aperture diffraction, and retaining high directivity performance in space. The detected half-power beam widths on the E-plane and H-plane are 7.7° and 8.9°, respectively, agreeing well with the simulations. Different from traditional orientation approaches, the photonic crystal antenna delivers performance of high directivity gain with compact structure, facile preparation, low cost, and high efficiency, which make it suitable for spatial information transmission and modulation and offer potential applications in remote sensing and wireless communication.

Вимушені коливання тришарової оболонки подвійної кривизни з пружним стільниковим заповнювачем

This paper presents a mathematical model of vibrations of a three-layered double-curved shell under geometrically nonlinear deformation. The middle layer is a honeycomb manufactured using FDM additive technologies. The mechanical properties of the honeycomb were assessed by a homogenization procedure. The outer layers of the shell are thin, and they are made of carbon-filled plastic. The model is based on a higher-order shear theory and accounts for the orthotropy of the mechanical properties of all the shell layers. Each layer of the shell is described by five variables (three displacement projections and two rotation angles of the normal to the middle surface). The properties of linear vibrations were studied using discretization by the Rayleigh?Ritz method. Because the middle layer of the shell is far lighter and more compliant in comparison with the outer layers, the computational process has some features. The eigenferquencies and eigenmodes of the shell were found for a further analysis of nonlinear vibrations. The mathematical model of forced vibrations of the shell under geometrically nonlinear deformation is a system of nonlinear ordinary differential equations derived by the assumed-mode method. Nonlinear periodic vibrations and their bifurcations were studied using a numerical procedure, which is a combination of the continuation method and the shooting technique. The properties of the nonlinear periodic vibrations and their bifurcations in the regions of fundamental and subharmonic resonances were studied numerically. A spherical panel and a hyperbolic paraboloid panel were considered. It was shown that when a disturbing force is applied at a point out of the panel’s center of gravity, the panel’s eigenmodes interact, and the frequency response and the bifurcation diagram change qualitatively in comparison with the case where that force is applied at the panel’s center of gravity. An agreement between the results was studied as a function of the number of terms in the Rayleigh-Ritz and assumed-mode expansions.

Near-perfect ultrabroadband metal-free ultrathin THz absorber

An ultrathin metal-free terahertz (THz) absorber is implemented and numerically analyzed. The absorber structure with a rectangular graphite resonator is designed to operate with the fundamental magnetic resonance in the lower- and higher-order magnetic resonance in the upper band. Carving the slots in the graphite resonator converts the magnetic resonance into the electric resonance at the upper frequency and merges the resonance spectra. The proposed absorber provides near-perfect flat broadband absorption of more than 99% in the 7.28–11.21 THz frequency range. Further, the absorber provides absorption of more than 90% and 80% in the 6.26–13.05 and 5.69–14.25 THz frequency range, respectively. The absorber structure utilizes a graphite-based resonator and backreflecting plane and provides the polarization insensitive response with the allowed incidence angle of more than 20° with the absorption more than 99%. The absorber response is validated through a transmission line method-based equivalent electrical circuit. Moreover, the reliability of the implemented absorber for its use in electromagnetic shields and stealth applications is calculated in terms of shielding effectiveness, which remains high in range of 50 to 250 dB.

An actuator with tuneable resonant frequency for active vibration damping

This paper explores implementing active resonance frequency tuning of adaptive tuned vibration absorbers (ATVA) by coupling a vibration absorber comprising a voice coil actuator (VCA) and a sprung inertial mass to a solenoid actuator (SA). In this device, the DC current flowing through the SA generates negative magnetic stiffness and varies the equilibrium position of the nonlinear spring of the VCA, thus changing the effective stiffness of the system and its resonant frequency. The paper details the analytical design steps and the parametric sizing process of designing the SA to maximize its force density within defined magnetic, dimensions and thermal constraints. The design of the SA was verified using finite element analysis (FEA), manufactured and coupled to a VCA to make the complete ATVA system. The experimental results demonstrate the tunability of the resonance frequency of the ATVA by controlling the SA’s DC current. It is shown that the system’s resonance frequency can be shifted by up to 23% of the fundamental resonance frequency, which improves the effectiveness of the ATVA at damping a broader range of excitation frequencies.

Beyond fundamental resonance mode: high-order multi-band ALN PMUT for in vivo photoacoustic imaging

This paper reports on an aluminum nitride (AlN) piezoelectric micromachined ultrasound transducer (PMUT) array for photoacoustic (PA) imaging, where the high-order resonance modes of the PMUT are utilized to improve imaging resolution. A flexural vibration mode (FVM) PMUT is fabricated and applied in a photoacoustic imaging (PAI) system. Specifically, the microelectromechanical system (MEMS)-based PMUT is suitable for PA endoscopic imaging of blood vessels and bronchi due to its miniature size and high sensitivity. More importantly, AlN is a nontoxic material, which makes it harmless for biomedical applications. In the PAI system, the AlN PMUT array is used to detect PA signals, and the acousto–mechanical response is designed and optimized at the PMUT’s fundamental resonance. In this work, we focus on the high-order resonance performance of the PMUT PAI beyond the fundamental resonance. The acoustic and electrical responses of the PMUT’s high-order resonance modes are characterized and analyzed. The fundamental and three high-order resonance bandwidths are 2.2, 8.8, 18.5, and 48.2 kHz. Compared with the resolution at the fundamental resonance mode, the resolutions at third- and fourth-order resonance modes increase by 38.7% and 76.9% in a phantom experiment. The high-order resonance modes of the AlN PMUT sensor array provide higher central frequency and wider bandwidth for PA signal detection, which increase the resolution of PAI compared to the PMUT working at the fundamental resonance mode.

Resonance effects in the radiation transfer of thin-film intracavity devices

A great deal of interest has been recently directed at exploring how the performance of photovoltaic and thermophotovoltaic systems can benefit from the use of ultra-thin layers and near-field effects. Related questions on how radiation transfer is modified if both the source and sink of the radiation are located within an optical cavity have, however, received far less attention. This question is, nevertheless, particularly relevant in the field of electroluminescence-driven thermophotonics, which could substantially benefit from the possibility to boost the energy transfer by making use of optical cavities. To gain insight into this possibility, we deploy fluctuational electrodynamics and study the fundamental resonance effects in structures where the emitter and absorber layers are separated by a vacuum nanogap and bordered by high-efficiency mirrors. We obtain the expected result that resonance effects can strongly enhance the interactions at specific wavelengths and propagation angles. Moreover, we find that even after integrating over wavelength and propagation angle, (1) the total power emitted can be tuned by adjusting the cavity thickness and the optical cavity mode structure, and (2) thinning the active layer enhances its emission in the cavity, causing a sublinear dependence between the active layer thickness and its overall emission. In plain numbers, adjusting the cavity thickness produces non-monotonous changes of over 50% in the total emission of thin layers. These observations apply also to absorption, which can become remarkably efficient even for an extremely thin absorber layer, thanks to cavity effects.

Improved circuit model for plasmonic resonance of single split-ring resonators

In this work, an improved circuit model is presented for the single split ring resonator (SRR), taking into account non-uniform distribution of conduction current and surface charges. To determine the circuit elements of the SRR, the electron kinetic, electric, and magnetic energy have been derived theoretically. Moreover, in addition to gap and surface capacitances, coupling capacitance is introduced to characterize the electric coupling effect between them. A formula for the fundamental resonance wavelength has been obtained, which shows good agreement with numerical simulations in both optical and microwave frequency ranges. The model will be instructive for engineering resonant properties of the SRRs and provide valuable insight into other complex plasmonic structures.

Measurement of thermal properties of liquid analytes using microfluidic resonators via photothermal modulation

Understanding the thermal properties of fluids at the nanoscale with high spatial, temporal, and thermal resolution are on great demand in the fields of MEMS, drug development, biomedical devices, and analytical systems. Microfluidic channel-integrated cantilevers are considered as an established benchtop sensor platform for physical and thermal characterization of materials at the picogram amount of analytes. This paper reports analysis of thermal characteristics of liquid analytes using photothermal heating effect of the microfluidic cantilever. Using real-time tracking of frequency shift of microfluidic resonator, the thermal properties of liquid samples can be estimated whilst the liquid analytes in the cantilever are locally heated by laser-induced irradiation. The heating results in the expansion of the liquid inside the channel which induces thermal stress on the walls of the channel. This thermal stress contributes to the rise of the resonance frequency of the microfluidic resonator and, therefore, the frequency shift is linearly dependent on the volumetric coefficient expansion of the liquid. A threefold improved sensitivity is observed when the second order flexural vibration mode is analyzed compared to that of the fundamental resonance. This approach, which combines photothermal heating and the dynamic mode of operation, can serve as a platform for the development of a portable, lab on a chip device for the use of real time detection of thermomechanical properties of fluids at low cost.

Reticular chemistry for the rational design of mechanically robust mesoporous merged-net metal-organic frameworks

Reticular chemistry for the rational design of mechanically robust mesoporous merged-net metal-organic frameworks

Metamorphic bedrock geometry of Santorini using HVSR information and geophysical modeling of ambient noise and active-source surface-wave data

Santorini is the most active volcano of the Southern Aegean Volcanic Arc, with volcanic material that has accumulated during the last 600+ Kyrs on top of the pre-volcanic Santorini island. The geometry of the pre-volcanic basement not only provides constraints on the volcanic history, but is also fundamental for seismic hazard assessment, as it partly controls site-effects on strong ground motions. We investigate the geometry of the metamorphic (Cycladic) basement of Santorini using information from both passive (noise) and active source data. We performed a large number (∼280) of single-station ambient noise measurements and collected additional data from 42 previous studies to compute the Horizontal-to-Vertical Spectral Ratio (HVSR) curves. The majority of the recovered HVSR curves show prominent spectral ratio peaks (locally larger than 7–8) indicating a large impedance contrast between pyroclastic volcanic strata and bedrock metamorphic formations. We also performed active (MASW) and passive (Noise Array) surface wave measurements at 4 selected sites to determine local dispersion curves. To find the 1D Vs structure from the surface down to the metamorphic bedrock we performed a joint Monte Carlo inversion of the dispersion curves and the fundamental frequency (f0HVSR) values from the HVSR curves. These models, as well as deep borehole information and HVSR data along the caldera cliff, where the thickness and stratigraphy of the volcanic formations above the bedrock can be directly observed, allowed us to determine a linear relationship between the thickness of the pyroclastic formations and their fundamental resonance period for central-southern Santorini. The resulting geometry of pre-volcanic island shows deep basins around the pre-Alpine bedrock outcrop (Profitis Ilias) and generally agrees with recent results from larger-scale tomographic and gravity data but with some significant local differences. The basins are 200+ meters thick in the Kamari and Perissa area (southeastern Santorini) and >400 m thick in the central (Fira-Imerovigli) area, while there is a thick (∼70-130 m) pyroclastic layer on top of a metamorphic basement “plateau” in the Megalochori area.

Asymptotics of the meta-atom: plane wave scattering by a single Helmholtz resonator.

Using a combination of multipole methods and the method of matched asymptotic expansions, we present a solution procedure for acoustic plane wave scattering by a single Helmholtz resonator in two dimensions. Closed-form representations for the multipole scattering coefficients of the resonator are derived, valid at low frequencies, with three fundamental configurations examined in detail: the thin-walled, moderately thick-walled and extremely thick-walled limits. Additionally, we examine the impact of dissipation for extremely thick-walled resonators, and also numerically evaluate the scattering, absorption and extinction cross-sections (efficiencies) for representative resonators in all three wall thickness regimes. In general, we observe strong enhancement in both the scattered fields and cross-sections at the Helmholtz resonance frequencies. As expected, dissipation is shown to shift the resonance frequency, reduce the amplitude of the field, and reduce the extinction efficiency at the fundamental Helmholtz resonance. Finally, we confirm results in the literature on Willis-like coupling effects for this resonator design, and connect these findings to earlier works by several of the authors on two-dimensional arrays of resonators, deducing that depolarizability effects (off-diagonal terms) for a single resonator do not ensure the existence of Willis coupling effects (bianisotropy) in bulk.This article is part of the theme issue ‘Wave generation and transmission in multi-scale complex media and structured metamaterials (part 2)’.

Thickness-scaling phonon resonance: A systematic study of hexagonal boron nitride from monolayers to bulk crystals

Phonons are important lattice vibrations that affect the thermal, electronic, and optical properties of materials. In this work, we studied infrared phonon resonance in a prototype van der Waals (vdW) material—hexagonal boron nitride (hBN)—with the thickness ranging from monolayers to bulk, especially on ultra-thin crystals with atomic layers smaller than 20. Our combined experimental and modeling results show a systematic increase in the intensity of in-plane phonon resonance at the increasing number of layers in hBN, with a sensitivity down to one atomic layer. While the thickness-dependence of the phonon resonance reveals the antenna nature of our nanoscope, the linear thickness-scaling of the phonon polariton wavelength indicates the preservation of electromagnetic hyperbolicity in ultra-thin hBN layers. Our conclusions should be generic for fundamental resonances in vdW materials and heterostructures where the number of constituent layers can be conveniently controlled. The thickness-dependent phonon resonance and phonon polaritons revealed in our work also suggest vdW engineering opportunities for desired thermal and nanophotonic functionalities.

Automatic Resonance Tuning Technique for an Ultra-Broadband Piezoelectric Energy Harvester

The main drawback of energy harvesting using the piezoelectric direct effect is that the maximum electric power is generated at the fundamental resonance frequency. This can clearly be observed in the size and dimensions of the components of any particular energy harvester. In this paper, we are investigating a new proposed energy harvesting device that employs the Automatic Resonance Tuning (ART) technique to enhance the energy harvesting mechanism. The proposed harvester is composed of a cantilever beam and sliding masse with varying locations. ART automatically adjusts the energy harvester’s natural frequency according to the ambient vibration natural frequency. The ART energy harvester modifies the natural frequency of the harvester using the motion of the mobile (sliding) mass. An analytical model of the proposed model is presented. The investigation is conducted using the Finite Element Method (FEM). THE FEM COMSOL model is successfully validated using previously published experimental results. The results of the FEM were compared with the experimental and analytical results. The validated model is then used to demonstrate the displacement profile, the output voltage response, and the natural frequency for the harvester at different mass positions. The bandwidth of the ART harvester (17 Hz) is found to be 1130% larger compared to the fixed resonance energy harvester. It is observed that the proposed broadband design provides a high-power density of 0.05 mW mm−3. The piezoelectric dimensions and load resistance are also optimized to maximize the output voltage output power.

Evaluation of acousto-optic effect in liquid using high-frequency and high-intensity ultrasound

Piezoelectric thin film transducers that can receive and transmit high-frequency and high-intensity ultrasound are powerful tools for future medical and industrial applications. Our group has developed acousto-optic devices where the refractive index can be controlled at high speed and high precision using ultrasound, but there are few reports on the interaction between high-frequency (tens to hundreds MHz) and high-intensity (MPa) ultrasound and visible light in liquids. In this study, we evaluated the acousto-optic interaction using polarized CW laser (532 nm) and a piezoelectric KNbO3 thin film transducer formed by hydrothermal method. The transducer was driven continuously at the fundamental or higher resonance frequencies (32, 96, 160, or 210 MHz) in a water cell, and the projection light of a laser beam penetrating perpendicularly to the sound axis was observed. The length of the projection light was elongated in the sound axis on the screen as the driving frequency increased. These results implies that the relationship between the wavelength of ultrasound and the laser beam width is the critical parameter for the acousto-optic phenomenon; the profile of the projection light is dependent on a ratio of the wavelength of ultrasound and the beam width of the laser beam.

Resonant ultrasound spectroscopy measurement and modeling of additively manufactured octet truss lattice cubes

Advancement of additive manufacturing technological readiness requires high throughput evaluation capabilities that can keep pace with the development of complex parts. Resonant ultrasound spectroscopy (RUS) is an acoustic technique that provides rapid holistic probing of a part by tracking fundamental mechanical resonance modes. In this work, the RUS responses of additively manufactured Ti-5553 octet truss lattice cubes were characterized using experimental measurements and three-dimensional finite element models. Varying percentages of missing struts were designed into the lattices as controlled defects and were verified using X-ray computed tomography. Experimental measurements of density and Young’s modulus were treated as input parameters in a homogenous anisotropic continuum model. The continuum model was compared with experimental RUS measurements, thus evaluating the potential for a simplified approximation of the octet truss lattice. [This work was supported by US DOE LLNL-LDRD 20-SI-001 and was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.]

Laser-based resonant ultrasound spectroscopy in samples with compliant boundary conditions

Laser-based resonant ultrasound spectroscopy (LRUS) is a promising technique for measuring elastic properties in samples of exceedingly miniature sizes where traditional piezoelectric transducers present a limitation. However, the performance of an LRUS system is highly dependent on the geometry and boundary conditions of the specimen. More specifically, samples manufactured from a substrate, such as micro-pillars, have a compliant boundary at the base which needs to be accounted for. In this work, sub-millimeter, free-standing rectangular parallelepipeds were manufactured from tungsten material. These samples provided excellent measurements of resonances as well as mode shapes. A similar pillar was manufactured and left attached to the tungsten block to study the outcome due to alterations on boundary conditions. The results show a significant attenuation on all signal content in modes with higher frequency than the fundamental bending resonance. However, excellent images of mode shapes were obtained once the sample was detached from the substrate. Furthermore, silicon cantilevers were manufactured from wafers, and similar effects were observed. The theoretical justification for poor performance in higher-order modes resulted in sample redesigns that provided superior fidelity resonance measurements.

Multi-harmonic electron cyclotron heating and current drive scenarios for non-inductive start-up and ramp-up in high field ST-40 spherical tokamak

Non-inductive start-up and ramp-up is an important topic for spherical tokamak reactor design as the central solenoid implementation is highly restrictive particularly for the low-aspect-ratio tokamak configuration. In the high field spherical tokamak (ST), ST-40 with B T0 ⩽ 3 T, a preparation is underway for high power ECH and ECCD current start-up/ramp-up experiments utilizing two MW-class 140/105 GHz gyrotrons. Here, we explored various ECH/ECCD scenarios for a low-field-side (LFS) launch-angle steerable waveguide launcher placed near the mid-plane region. Due to the large toroidal field variation of ST configuration, multiple cyclotron harmonic resonance layers could exist within the plasma. In this start-up and ramp-up regime, both fundamental and second harmonic ECH resonances must be considered. We find that even with the presence of X-II resonance layer in the plasma, an efficient X-I ECH and ECCD regime can be accessed for the low electron temperature T e0 as low as 200 eV which is a typical starting temperature of ECH heated plasmas in an open-field-line configuration. The presence of X-II resonance could become significant at higher T e0 as X-II absorption increases with T e0 which could reduce the current ramp-up efficiency as the power reaching X-I is reduced. Finally for the pure X-I regime where the 2Ωe resonance is moved outside the plasma with B T0 ∼ 3.4 T, we find that it is possible to reach the full current of I p ∼ 1 MA fully non-inductively with the ECH power of ∼1 MW at n e0 ∼ 1.0 × 1019 m−3 using 105 GHz frequency gyrotron. By reducing the outer limiter position R L ∼ 78 cm to 70 cm, the pure X-I regime is recovered at the rated ST-40 magnetic field of B T0 ∼ 3.0 T. This X-I regime is accessible with a relatively broad range of launched n II or the launching angles. A survey of X-mode X-II ECH and ECCD at higher density regimes is also shown for completeness.