Far-field Excitation Research Articles

An innovative methodology for hybrid vibration control (MR+TMD) of buildings under seismic excitations

In this study, the combination of magnetorheological dampers and tuned mass dampers (MR + TMD) as a hybrid control system is investigated on a 15-story shear building where MR damper is attached to the TMD to generate active control force of TMD. The seismic responses of the structure are reduced by employing MR + TMD on rooftop of the structure. The MR damper's control voltage is generated by combining IT2FLC and FOPID. The FOPID + IT2FLC, TMD, and control voltage parameters are optimized using the observer-teacher-learner-based optimization (OTLBO) algorithm to minimize the maximum displacement of the building rooftop under far-field and near-field earthquake excitations. To conduct additional research, the same method was used to mitigate structural responses for PID, FOPID, IT2FLC, and a combination of fuzzy logic type-1 (FLC) and FOPID (FOPID + FLC). All of these controllers' performances in mitigating seismic responses are compared to those of the uncontrolled system and to each other. The results indicate that FOPID + IT2FLC outperforms PID, FOPID, and FOPID + FLC controllers. Additionally, the building's rooftop displacement was reduced by an average of 35.06% using the FOPID + IT2FLC system for sixteen far-field and near-field earthquake records. Moreover, the hybrid MR + TMD system performs better than other conventional controllers. 1 The combined application of the meta-heuristic algorithm and IT2FLC controller along with FOPID controller (FOPID + IT2FLC) to mitigate structural responses. 2 Combined application of MR damper and TMD (MR + TMD) as a hybrid control device. 3 Optimizing FOPID and IT2FLC to generate the control voltage of MR + TMD. 4 Investigating PID, FOPID, FOPID + FLC controllers to compare with the proposed method. 5 Investigating the performance of the suggested method by the aim of energy consumption and 5 novel criteria such as base shear, base moment and etc.

Topological multipolar corner state in a supercell metasurface and its interplay with two-dimensional materials

Second-order topological insulators (SOTIs) have recently attracted much attention due to their capability to support lower-dimensional topological states, namely, the corner states. Here, we demonstrate that properly designed supercell metasurfaces can support photonic corner states, meanwhile further serving as an ideal platform for the implementations of topological polaritons and dynamically reconfigurable corner states by assembling two-dimensional materials. Such metasurfaces consist of an array of finite-sized SOTIs mimicking the two-dimensional Su–Schrieffer–Heeger model. We reveal that the topological transition happens in unit cells without the bandgap, and nondegenerate multipolar corner states emerge in the supercell metasurface due to the inter- and intrasupercell coupling effects. Especially since these corner states are above the light line of the metasurface, we realize the collective stimulation of the two dipolar corner states and their superposition state via far-field excitation. By stacking monolayer hexagonal boron nitride film onto the metasurface, we further achieve the topological phonon polaritons through the strong coupling between the corner state and the phonon, which is confirmed by the Rabi splitting as well as anticrossing behavior emerging in the transmission spectra. Furthermore, we reveal the robustness of the corner state and strong coupling by introducing defects into the metasurface. Finally, tunable corner state and strong coupling with on-demand control are realized by assembling monolayer graphene onto the metasurface. Our theoretical study proposes a unique hybrid-material platform for topological polaritonics and reconfigurable topological photonics, which can promote large-area topological applications in practice.

Broadband Enhancement of the Spontaneous Emission by an Optical Dipole Nanoantenna on Metallic Substrate: an Intuitive Model of Surface Plasmon Polariton

Optical nanoantennas can achieve electromagnetic-field enhancement under far-field excitation or spontaneous-emission enhancement under excitation by radiating emitters. Among them, nanoantennas on a metallic substrate (i.e., the so-called nanoparticle-on-mirror antennas) have drawn great research interests due to their ease in forming metallic gaps of sizes down to a few nanometers or even subnanometer. Here we propose an optical dipole nanoantenna on a metallic substrate with a broadband enhancement of spontaneous emission. Its total and radiative emission-rate enhancement factors can be up to 5454 and 1041, respectively. In the near-infrared band, the wavelength range of spontaneous-emission enhancement (Purcell factor over 1000) can reach 260nm. By changing the width of the slit between the two antenna arms and changing the length of the antenna arms, the spontaneous-emission enhancement bandwidth and enhancement factors can be adjusted, respectively, which brings great freedom and simplicity to the design process. The antenna can achieve a strong far-field radiation within a central anglular zone (polar angle <i>θ</i>≤60°) corresponding to a certain numerical aperture of objective lens, and therefore can increase the intensity of the fluorescence collected by the objective lens. Based on the above performances, the antenna can provide a broadband enhancement of spontaneous emission for fluorescent molecules or quantum dots (whose fluorescence spectrum usually covers a certain wavelength range), which is of great significance for applications such as high-speed and super-bright nanoscale light sources and high-sensitivity fluorescent-molecule sensing.<br>To clarify the underlying physical mechanisms, we build up a semi-analytical model by considering an intuitive excitation and multiple-scattering process of surface plasmon polaritons (SPPs) that propagate along the antenna arms. All the parameters used in the model (such as the SPP scattering coefficients) are obtained via rigorous calculations based on the first principle of Maxwell’s equations without any fitting process, which ensures that the model has a solid electromagnetic foundation and can provide quantitative predictions. The SPP model can comprehensively reproduce all the radiation properties of the antenna, such as the total and radiative emission rates and the far-field radiation pattern. Two phase-matching conditions are derived from the model for predicting the antenna resonance, and show that under these conditions, the SPPs on the antenna arms form a pair of Fabry-Perot resonance and therefore are enhanced, and the enhanced SPPs propagate to the emitter in the nanogap (or scattered into the free space), so as to enhance the total spontaneous emission rate (or the far-field radiative emission rate). Besides, this pair of Fabry-Perot resonance result in a pair of resonance peaks close to each other, which then forms the broadband enhancement of spontaneous emission.

Broadband enhancement of spontaneous emission by optical dipole nanoantenna on metallic substrate: An intuitive model of surface plasmon polariton

<sec>Optical nanoantennas can achieve electromagnetic-field enhancement under far-field excitation or spontaneous-emission enhancement under excitation by radiating emitters. Among them, nanoantennas on a metallic substrate (i.e. the so-called nanoparticle-on-mirror antennas) have aroused great research interest due to their ease in forming metallic gaps of sizes down to a few nanometers or even subnanometer. Here we propose an optical dipole nanoantenna on a metallic substrate with a broadband enhancement of spontaneous emission. Its total and radiative emission-rate enhancement factors can reach up to 5454 and 1041, respectively. In the near-infrared band, the wavelength range of spontaneous-emission enhancement (Purcell factor over 1000) can reach 260nm. By changing the width of the slit between the two antenna arms and also the length of the antenna arms, the spontaneous-emission enhancement bandwidth and enhancement factors can be adjusted, respectively, which brings great freedom and simplicity to the design process. The antenna can achieve a strong far-field radiation within a central anglular zone (polar angle <i>θ</i>≤60°) corresponding to a certain numerical aperture of objective lens, and therefore can increase the intensity of the fluorescence collected by the objective lens. Based on the above performances, the antenna can provide a broadband enhancement of spontaneous emission for fluorescent molecules or quantum dots (whose fluorescence spectrum usually covers a certain wavelength range), which is of great significance for the applications such as in high-speed and super-bright nanoscale light sources and high-sensitivity fluorescent-molecule sensing.</sec><sec>To clarify the underlying physical mechanisms, we build up a semi-analytical model by considering an intuitive excitation and multiple-scattering process of surface plasmon polaritons (SPPs) that propagate along the antenna arms. All the parameters used in the model (such as the SPP scattering coefficients) are obtained via rigorous calculations based on the first principle of Maxwell's equations without any fitting process, which ensures that the model has a solid electromagnetic foundation and can provide quantitative predictions. The SPP model can comprehensively reproduce all the radiation properties of the antenna, such as the total radiative emission rate and the far-field radiation pattern. Two phase-matching conditions are derived from the model for predicting the antenna resonance, and show that under these conditions, the SPPs on the antenna arms form a pair of Fabry-Perot resonance and therefore are enhanced, and the enhanced SPPs propagate to the emitter in the nanogap (or scattered into the free space), so as to enhance the total spontaneous emission rate (or the far-field radiative emission rate). Besides, this pair of Fabry-Perot resonance results in a pair of resonance peaks close to each other, then enhancing the spontaneous emission with a broadband.</sec>

Effect of material variability on the seismic response of reinforced concrete box-girder bridges for different pier heights

RC bridges situated in regions prone to seismic activity are susceptible to undesirable pier column movements and deck displacements. The component-level nonlinear response evaluation can give a better picture of the seismic safety of the existing bridge structures. Many times, the material strength variability also affects the inelastic response of the structures significantly. The present study looks into the effects of column heights and material strength variability on the seismic performance of reinforced concrete bridge models adopted from past investigations, subjected to bi-directional loading. Chosen suite of 40 far-field ground excitations are employed to simulate the seismic response of chosen models in OpenSees and their non-linear response is evaluated. The seismic response of the structure is evaluated as curvature ductility of piers (primary component), and deck displacements (secondary component). Fragility curves so obtained across the damage states considered depict the difference in seismic performance of the model considering only record variability and considering record and material strength uncertainties. The results showed that for the primary component, damage probability for different states is found to be increased under material variability irrespective of the column pier height, while secondary components showed an increase in the damage state due to material strength variability for the shorter pier.

Verification of a Stiffness-Variable Control System with Feed-Forward Predictive Earthquake Energy Analysis.

Semi-active isolation systems with controllable stiffness have been widely developed in the field of seismic mitigation. Most systems with controllable stiffness perform more robustly and effectively for far-field earthquakes than for near-fault earthquakes. Consequently, a comprehensive system that provides comparable reductions in seismic responses to both near-fault and far-field excitations is required. In this regard, a new algorithm called Feed-Forward Predictive Earthquake Energy Analysis (FPEEA) is proposed to identify the ground motion characteristics of and reduce the structural responses to earthquakes. The energy distribution of the seismic velocity spectrum is considered, and the balance between the kinetic energy and potential energy is optimized to reduce the seismic energy. To demonstrate the performance of the FPEEA algorithm, a two-degree-of-freedom structure was used as the benchmark in the numerical simulation. The peak structural responses under two near-fault and far-field earthquakes of different earthquake intensities were simulated. The isolation layer displacement was suppressed most by the FPEEA, which outperformed the other three control methods. Moreover, superior control on superstructure acceleration was also supported by the FPEEA. Experimental verification was then conducted with shaking table test, and the satisfactory performance of the FPEEA on both isolation layer displacement and superstructure acceleration was demonstrated again. In summary, the proposed FPEEA has potential for practical application to unexpected near-fault and far-field earthquakes.

Metallic nanosphere-assisted coupling ultrafast surface plasmon polaritons background-free tip nanofocusing.

Plasmonic tip nanofocusing has gained much attention owing to its wide application in the field of nanospectroscopy. Here, we present the Au nanosphere (AuNS)-assisted coupling ultrafast surface plasmon polaritons (SPP) background-free tip nanofocusing. The plasmonic tip was prepared by attaching an AuNS on the shaft of an Au conical tip fabricated by electrochemical etching. The AuNS was adopted as an antenna to couple the far-field excitation light to the propagating SPP along the shaft to the tip apex for achieving power compression. Importantly, we experimentally and theoretically demonstrate that such a plasmonic tip can realize background-free ultrafast SPP tip nanofocusing with radially polarized features in a wide spectral range based on the localized SPP resonance effect supported by AuNS. Furthermore, the intensity of the tip nanofocusing light field has strong polarization dependence under linearly polarized light excitation, providing a powerful platform for spatiotemporal light control on the nanoscale. Our technique realizes remote excitation of background-free tip nanofocusing with a structured light feature, and it holds promising potential for tip-enhanced nanospectroscopies, nonlinear nanophotonics, etc.

A comparative study on seismic fragility analysis of RC frame structures with consideration of modeling uncertainty under far-field and near-field ground motion excitation

A comparative study on seismic fragility analysis of RC frame structures with consideration of modeling uncertainty under far-field and near-field ground motion excitation

Seismic design and performance evaluation of low-rise steel buildings with self-centering energy-absorbing dual rocking core systems under far-field and near-fault ground motions

This research focuses on seismic design and performance evaluation of low-rise steel buildings with self-centering energy-absorbing dual rocking core systems (denoted as SEDRC systems) under far-field and near-fault ground motions. The SEDRC system consists of two rocking cores (RCs) and several shear friction spring dampers (SFSDs). These SFSDs are located between the two RCs and arranged over the height of the SEDRC system to absorb energy and provide a self-centering feature. The two RCs are considered as two buckling restrained braced frames in this paper, which are utilized to obtain uniform inter-story drift distribution in buildings. The two RCs can also act as a backup lateral force-resisting and energy-dissipation system if necessary. A direct displacement-based design (DDBD) procedure is introduced for the SEDRC system. A physical test was conducted to investigate the stability of the SEDRC system under repeated loading with near-fault loading protocol. The test results show that the SEDRC specimen can get stable self-centering behavior under repeated rounds of tests, indicating that the SEDRC system can be fully recoverable against multiple earthquakes before the development of the bearing action in SFSDs. The computational model of the SEDRC system was created and verified with the test results. A three-story representative building was designed through the proposed DDBD method, and the numerical models of the building were generated using the verified modeling method. Forty far-field ground motions and twenty near-fault ground motions were selected to perform Nonlinear Dynamic Analyses (NDA) on the designed building. The NDA results show that the designed three-story SEDRC system can satisfy the drift limit under DBE far-field excitations. And the designed SEDRC system can also survive and achieve small residual drifts subjected to the fault-normal components of the selected near-fault excitations and MCE far-field excitations.

Assessment and mitigation on near-fault earthquake wave effects on seismic responses and pile-soil interactions of soil-pile-bridge model

A 1/70-scale soil-pile-bridge model was designed and constructed according to an extremely long-span cable-stayed bridge with a 1400 m-main span. Shaking table tests were performed to estimate the effects of the near-fault earthquake waves on the seismic responses and pile-soil interactions of the bridge model under near-fault and far-field earthquake waves in the longitudinal direction. A three-dimensional analytical model was established to represent the test results and track the response variations resulting from the near-fault earthquake waves. Finally, both control strategies were numerically studied to decrease the effects of the near-fault earthquake waves on the seismic responses of the bridge model. The results showed that the longitudinal near-fault earthquake excitation significantly increased the acceleration and displacement of the towers and girder, and augmented the curvature and strain at the bottom of the towers as compared to the far-field earthquake excitation, resulting in the bridge model suffering from more severe damage. The near-fault earthquake excitation considerably amplified the pile-soil interaction effects on the seismic responses of the towers and piers. Additionally, the analytical model could approximately represent the test results and reliably take the response variations induced by the near-fault earthquake waves. The elastic cables and supporting piers control strategies could effectively mitigate the effects of the near-fault earthquake waves on the seismic responses the bridge model except for the longitudinal acceleration and vertical displacement of the girder, preventing the towers from experiencing more significant damage.

Far-field controllable excitation of phonon polariton via nanostructure engineering.

Hexagonal boron nitride (h-BN) as a natural mid-infrared (mid-IR) hyperbolic material which supports a strong excitation of phonon polariton (PhP) has enabled a new class of photonic devices with unprecedented functionalities. The hyperbolic property of h-BN has not only brought in new physical insights but also spurred potential applications. However, most of the current h-BN devices are designed repying on near-field excitation and manipulation of PhP. For fully realizing the potentials of h-BN, research on far-field controllable excitation and control of PhP is important for future integrated photonic devices. In this work, we exploit the designs of controllable far-field excitation of PhP in nanostructure-patterned h-BN thin film for deep subwavelength focusing (FWHM∼λ0/14.9) and interference patterns of 1D (FWHM∼λ0/52) and 2D standing waves (FWHM∼λ0/36.8) which find great potential for super-resolution imaging beyond diffraction limit. These polaritonic patterns could be easily tuned remotely by manipulating the polarization and phase of incident laser. This approach provides a novel platform for practical IR nanophotonic devices and potential applications in mid-IR bio-imaging and sensing.

Interval Type-2 Fuzzy Hybrid Control of a High-Rise Building Including Soil–Structure Interaction Under Near-Field and Far-Field Ground Motions

The interval type-2 fuzzy logic controller (IT2FLC) has emerged as one of the most effective high-tech control strategies to improve the performance of structures under near-field and far-field ground motions. The present study discusses the application of this strategy in the hybrid control of the behaviour of a real high-rise shear building by considering soil–structure interaction. In this regard, a hybrid control technique, consisting of a tuned mass damper (TMD) and a magneto-rheological (MR) damper is proposed. Using an MR damper on a TMD (MR + TMD) helps to reduce the seismic responses of a high-rise building in soil–structure interaction. The utilization of this hybrid device is very useful because it does not require huge power sources. By applying the observer–teacher–learner–based optimization (OTLBO) method, the fuzzy rules of IT2FLC, the TMD parameters, and the voltage of the MR damper have been optimized for a fifteen-storey shear building under four seismic near-field and far-field excitations. Minimization of the peak displacement of the top floor by using the square root of sum of squares (SRSS) of each response to each earthquake was utilized as the optimization criterion. Then, the optimized type-2 fuzzy rules and optimized TMD parameters were examined under seven new earthquake records in the fifteen-storey building. The results obtained by comparing the proposed control and uncontrolled systems in terms of the structure interaction with soft and dense soils show that the IT2FLC- using OTLBO is an efficient control method. It is also shown that the responses of this building to the hybrid controller have been effectively improved.

Geometric Control of Collective Spontaneous Emission.

Dipole spin-wave states of atomic ensembles with wave vector k(ω) mismatched from the dispersion relation of light are difficult to access by far-field excitation but may support rich phenomena beyond the traditional phase-matched scenario in quantum optics. We propose and demonstrate an optical technique to efficiently access these states. In particular, subnanosecond laser pulses shaped by a home-developed wideband modulation method are applied to shift the spin wave in k space with state-dependent geometric phase patterning, in an error-resilient fashion and on timescales much faster than spontaneous emission. We verify this control through the redirection, switch off, and recall of collectively enhanced emission from a ^{87}Rb gas with ∼75% single-step efficiency. Our work represents a first step toward efficient control of electric dipole spin waves for studying many-body dissipative dynamics of excited gases, as well as for numerous quantum optical applications.

Near- and Far-Field Excitation of Topological Plasmonic Metasurfaces

The breathing honeycomb lattice hosts a topologically non-trivial bulk phase due to the crystalline-symmetry of the system. Pseudospin-dependent edge states, which emerge at the interface between trivial and non-trivial regions, can be used for the directional propagation of energy. Using the plasmonic metasurface as an example system, we probe these states in the near- and far-field using a semi-analytical model. We provide the conditions under which directionality was observed and show that it is source position dependent. By probing with circularly-polarised magnetic dipoles out of the plane, we first characterise modes along the interface in terms of the enhancement of source emissions due to the metasurface. We then excite from the far-field with non-zero orbital angular momentum beams. The position-dependent directionality holds true for all classical wave systems with a breathing honeycomb lattice. Our results show that a metasurface in combination with a chiral two-dimensional material, could be used to guide light effectively on the nanoscale.

Numerical Analysis of Seismic Site Effects in Loess Region of Western China under Strong Earthquake Excitations

The Loess Plateau is one of the most tectonically and seismically active areas in the world. Observations from past strong earthquakes, particularly the Minxian–Zhangxian and Wenchuan earthquakes, have shown distinctive evidence of seismic site effects in the mountainous area of southeastern Gansu province. In this study, seismic damage in the loess areas of southeastern Gansu province induced by these earthquakes was investigated and briefly described. Different types of ground motion were selected, and the one-dimensional equivalent linear method was used for numerical analysis of the ground motion effects in the loess regions. Moreover, seismic response analysis of a typical loess tableland was conducted. The results showed that the seismic responses of a typical loess tableland under different seismic excitations have totally different dynamic characteristics. Moreover, the seismic damage in loess regions was more serious under far-field seismic excitation compared with near-field seismic excitation with the same peak acceleration. Through this study, the quantitative assessment of ground motion effects can be approximately estimated and the mechanism of site amplification effects on ground motion is further explained.

On the theoretical limitations in estimating thickness of a plate-like structure from a full-field single-tone response Lamb wave measurement

A persistent question in wavenumber analysis in the estimation of thickness from a steady-state wave field is identifying the theoretical sensitivity of the system. A well-known trade-off between spatial frequency/wavenumber resolution and thickness resolution exists. The current work presents a calculation of the Cramer-Rao Lower bound (CRLB), specifically as applied to thickness estimates, for a 2-dimensional multi-mode waveform in Gaussian noise. Cases of near-field and far-field excitation are considered, and transducer position with respect to the scan area is also varied. Additionally, we consider the CRLB in a plate with multiple sources, simulated as sources placed on the boundary of the plate. We conclude by presenting the CRLB values in terms of frequency for various thicknesses, and by presenting optimal excitation frequencies for a nominal thickness, based on the CRLB.

Understanding the Electromagnetic Response of Graphene/Metallic Nanostructures Hybrids of Different Dimensionality

Plasmonic excitations such as surface-plasmon-polaritons (SPPs) and graphene-plasmons (GPs), carry large momenta and are thus able to confine electromagnetic fields to small dimensions. This property makes them ideal platforms for subwavelength optical control and manipulation at the nanoscale. The momenta of these plasmons are even further increased if a scheme of metal-insulator-metal and graphene-insulator-metal are used for SPPs and GPs, respectively. However, with such large momenta, their far-field excitation becomes challenging. In this work, we consider hybrids of graphene and metallic nanostructures and study the physical mechanisms behind the interaction of far-field light with the supported high momenta plasmon modes. While there are some similarities in the properties of GPs and SPPs, since both are of the plasmon-polariton type, their physical properties are also distinctly different. For GPs we find two different physical mechanism related to either GPs confined to isolated cavities, or large area collective grating couplers. Strikingly, we find that although the two systems are conceptually different, under specific conditions they can behave similarly. By applying the same study to SPPs, we find a different physical behavior, which fundamentally stems from the different dispersion relations of SPPs as compared to GPs. Furthermore, these hybrids produce large field enhancements that can also be electrically tuned and modulated making them the ideal candidates for a variety of plasmonic devices.

Image polaritons in boron nitride for extreme polariton confinement with low losses

Polaritons in two-dimensional materials provide extreme light confinement that is difficult to achieve with metal plasmonics. However, such tight confinement inevitably increases optical losses through various damping channels. Here we demonstrate that hyperbolic phonon polaritons in hexagonal boron nitride can overcome this fundamental trade-off. Among two observed polariton modes, featuring a symmetric and antisymmetric charge distribution, the latter exhibits lower optical losses and tighter polariton confinement. Far-field excitation and detection of this high-momenta mode become possible with our resonator design that can boost the coupling efficiency via virtual polariton modes with image charges that we dub ‘image polaritons’. Using these image polaritons, we experimentally observe a record-high effective index of up to 132 and quality factors as high as 501. Further, our phenomenological theory suggests an important role of hyperbolic surface scattering in the damping process of hyperbolic phonon polaritons.

Far-field excitation of single graphene plasmon cavities with ultracompressed mode volumes.

Acoustic graphene plasmons are highly confined electromagnetic modes carrying large momentum and low loss in the mid-infrared and terahertz spectra. However, until now they have been restricted to micrometer-scale areas, reducing their confinement potential by several orders of magnitude. Using a graphene-based magnetic resonator, we realized single, nanometer-scale acoustic graphene plasmon cavities, reaching mode volume confinement factors of ~5 × 1010 Such a cavity acts as a mid-infrared nanoantenna, which is efficiently excited from the far field and is electrically tunable over an extremely large broadband spectrum. Our approach provides a platform for studying ultrastrong-coupling phenomena, such as chemical manipulation via vibrational strong coupling, as well as a path to efficient detectors and sensors operating in this long-wavelength spectral range.

Effectiveness of Seismic Isolation for Long-Period Structures Subject to Far-Field and Near-Field Excitations

Effectiveness of Seismic Isolation for Long-Period Structures Subject to Far-Field and Near-Field Excitations