Linear Excitation Research Articles

Standing Wave Binding of Hemispherical Resonator Containing First-Third Harmonics of Mass Imperfection under Linear Vibration Excitation.

Due to complicated processing technology, the mass distribution of a hemispherical resonator made of fused silica is not uniform, which can affect the azimuth of the standing wave of a resonator under the linear vibration excitation. Therefore, the analysis of standing wave evolution of a resonator with mass imperfection under linear vibration excitation is of significance for the improvement of the output accuracy of a gyroscope. In this paper, it is assumed that the resonator containing the first–third harmonics of mass imperfection is excited by horizontal and vertical linear vibration, respectively; then, the equations of motion of an imperfect resonator under the second-order vibration mode are established by the elastic thin shell theory and Lagrange mechanics principle. Through error mechanism analysis, it is found that, when the frequency of linear vibration is equal to the natural frequency of resonator, the standing wave is bound in the azimuth of different harmonics of mass imperfection with the change in vibration excitation direction. In other words, there are parasitic components in the azimuth of the standing wave of a resonator under linear vibration excitation, which can cause distortion of the output signal of a gyroscope. On the other hand, according to the standing wave binding phenomenon, the azimuths of the first–third harmonics of mass imperfection of a resonator can also be identified under linear vibration excitation, which can provide a theoretical method for the mass balance of an imperfect resonator.

A Single Chiral Nanoparticle Induced Valley Polarization Enhancement.

Valley polarization is among the most critical attributes of atomically thin materials. However, increasing contrast from monolayer transition metal dichalcogenides (TMDs) has so far been challenging. In this work, a large degree of circular polarization up to 45% from a monolayer WS2 is achieved at room temperature by using a single chiral plasmonic nanoparticle. The increased contrast is attributed to the selective enhancement of both the excitation and the emission rate having one particular handedness of the circular polarization, together with accelerated radiative recombination of valley excitons due to the Purcell effect. The experimental results are corroborated by the optical simulation using the finite-difference time-domain (FDTD) method. Additionally, the single chiral nanoparticle enables the observation of valley-polarized luminescence with a linear excitation. The results provide a promising pathway to enhance valley contrast from monolayer TMDs and utilize them for nanophotonic devices.

Photoluminescence of single gold nanorods following nonlinear excitation.

The photoluminescence (PL) mechanisms of gold nanorods following nonlinear excitation are described. Using single-particle nonlinear optical measurements, we compare PL signals resulting from both the plasmon-resonant and non-resonant excitations. In both cases, spectrally broad interband PL emission was observed. However, we also show that resonant excitation of the longitudinal plasmon mode leads to an increased photonic density of states at energies corresponding to the transverse plasmon resonance. This increased density of states is achieved by a multi-step mechanism, which is initiated by three-photon excitation and followed by an Auger relaxation process. Importantly, the results show that nonlinear excitation can lead to energy and polarization modulation of nanoparticle optical signals that are not observed using linear excitation. This work also demonstrates the effectiveness of single-nanoparticle PL studies for understanding how plasmon-resonant excitations can be used to modify hot carrier distributions.

A Battery‐Less Arbitrary Motion Sensing System Using Magnetic Repulsion‐Based Self‐Powered Motion Sensors and Hybrid Nanogenerator

AbstractSelf‐powered arbitrary motion sensors are in high demand in the field of autonomous controlled systems. In this work, a magnetic repulsion‐assisted self‐powered motion sensor is integrated with a hybrid nanogenerator (MRSMS–HNG) as a battery‐less arbitrary motion sensing system. The proposed device can efficiently detect the motion parameters of a moving object along any arbitrary direction and simultaneously convert low frequency (<5 Hz) vibrations into useful electricity. The MRSMS–HNG consists of a central magnet for the electromagnetic (EMG)–triboelectric (TENG) nanogenerator and four side magnets for motion sensors. Because all the magnets are aligned in the same magnetization direction, the repulsive force owing to the movement of the central magnet actuates the side magnets to achieve self‐powered arbitrary motion sensing. These self‐powered motion sensors exhibit a high sensitivity of 981.33 mV g−1 under linear motion excitation and have a tilting angle sensitivity of 9.83 mV deg−1. The proposed device can deliver peak powers of 27 mW and 56 µW from the EMG and TENG, respectively. By integrating the self‐powered motion sensors and hybrid nanogenerator on a single device, real‐time wireless transmission of motion sensor data to a smartphone is successfully demonstrated, thus realizing a battery‐less arbitrary motion‐sensing system for future autonomous control applications.

Plasmon-induced photoluminescence and Raman enhancement in Pr:CaF2 crystal by embedded silver nanoparticles

Photoluminescence (PL) enhancement of rare earth ions is in high demand due to their crucial applications in optoelectronics devices and biomedical frequency up-convertors for endoscopy, which could be realized by a plasmonic field enhancement effect of metallic nanoparticles (NPs). In this work, both the PL emission and Raman scattering enhancement of Pr3+ ions in CaF2 crystal host have been observed for the first time by embedding silver NPs. By Ag+ ion implantation, Ag NPs (around 5 nm) have been encapsulated into the Pr:CaF2 matrix. Analysis of the difference between linear optical absorption and PL excitation spectra within the plasmon band, the mechanism of observed PL enhancement has been attributed to the local field enhancement induced by the plasmonic NPs. The classical energy transfer process nanoparticle-emitter was not present. The detailed analysis of the PL enhancement induced by unannealed and annealed Ag NPs is compared under the measurement of PL decay, showing the dominant mechanism of the increased absorption and radiative decay rate, respectively. This work shows an efficient method to enhance PL emission of rare earth ions in the crystal based on the embedded metallic NPs, which could be used to optimize the photonic sensors and light emitting devices.

Plug-and-play adaptive optics for commercial laser scanning fluorescence microscopes based on an adaptive lens.

In this Letter, we present a solution for simple implementation of adaptive optics in any existing laser scanning fluorescence microscope. Adaptive optics are implemented by the introduction of a multiactuator adaptive lens between the microscope body and the objective lens. Correction is performed with a sensorless method by optimizing the quality of the images presented on screen by the microscope software. We present the results acquired on both a commercial linear excitation confocal microscope and a custom-made multiphoton excitation microscope.

Characterization of hydrogel structural damping

Gels are composed of crosslinked polymer networks and solvent molecules imbibed into the networks. Gels are both ubiquitous in nature and important engineering materials widely used in many applications. Due to their biocompatibility, stimuli-responsiveness, and compliance, gels gain an edge over traditional materials, such as metals and composites in many modern engineering applications. In the past, the static and kinetic properties of gels have been widely studied. However, the dynamic properties of gels, particularly their structural damping, remain largely unknown even though gels are often under dynamic conditions in various applications. The literature of soft materials is lacking both damping data for hydrogels and a standard testing method to that end. This work reports experimentally identified structural damping data for a set of hydrogel samples via resonant vibration tests for the first bending mode. Beam-shaped samples of rectangular cross-section are clamped vertically at both ends and tested under linear base excitation. An analysis of the frequency response functions based on the Euler–Bernoulli beam theory is conducted to extract Young’s modulus and structural damping values. In the experiments, polyacrylamide gels of three different compositions and polydimethylsiloxane (PDMS) elastomers of two different compositions are prepared and tested. The remarkable result is that the hydrogels have 80% less damping than PDMS, even though hydrogels are an order of magnitude softer than PDMS. The molecular origins of the damping in hydrogels and PDMS are discussed. The low damping of gels may open new avenues of research and applications of soft materials in structural dynamics and wave propagation, such as metamaterials and topological insulators, among others.

Current-assisted Raman activation of the Higgs mode in superconductors

The Higgs mode in superconductors is a scalar mode without electric or magnetic dipole moment. Thus, it is commonly believed that its excitation is restricted to a nonlinear two-photon Raman process. However, recent efforts have shown that a linear excitation in the presence of a supercurrent is possible, resulting in a new resonant enhancement at $\Omega=2\Delta$ with the driving light frequency $\Omega$ and the energy of the Higgs mode $2\Delta$. This is in contrast to the usual $2\Omega = 2\Delta$ resonance condition found in nonlinear third-harmonic generation experiments. In this communication, we show that such a linear excitation can still be described as an effective Raman two-photon process, with one photon at $\omega=2\Delta$ and one virtual photon at $\omega=0$ which represents the dc supercurrent. At the same time we demonstrate that a straightforward infrared activation with a single photon excitation is negligible. Moreover, we give a general context to our theory, providing an explanation for how the excitation of the Higgs mode in both THz quench and drive experiments can be understood within a conventional difference-frequency generation or sum-frequency generation process, respectively. In such a picture, the observed new resonance condition $\Omega = 2\Delta$ is just a special case. With the same approach, we further discuss another recent experiment, where we find a suppression of odd order higher harmonics in the presence of a dc supercurrent.

Linear and Nonlinear Excitation of P3HT Induced by Spectral-Shaped Ultrafast Mid-IR Laser Radiation

This article discusses the linear vibrational and nonlinear electronic excitation of the organic material P3HT during processing by ultrafast laser radiation in the mid-IR spectral range. Therefore...

An Experimental and Modeling Study on Vibro-Acoustic Response of Double-Walled Steel Cylindrical Shells

Based on precise transfer matrix method (PTMM), the analytical model of the double-walled steel cylindrical shell was setup by taking into account of the annular plate and interlayer water. Under the linear sweep frequency excitation or the fixed frequency excitation, an experimental model of the double-walled steel cylindrical shell has been designed, which is performed to gain the natural frequencies, forced vibration in air and water, underwater acoustic radiation. The analytical model was established to calculate natural frequencies, vibration acceleration level and sound pressure level and compare with the relevant experimental results. The compared results show that analytical results coincide with the experimental value and prove that the analytical model established by PTMM is reliable and credible. Meanwhile, the forced vibration behaviour of measuring positions at inner and outer shells was investigated both theoretically and experimentally. Effects of different types of external excitations on the vibration and sound radiation of the double-walled cylindrical shell are discussed. As to acoustic radiation, the acoustic excitation plays a leading role in the low-frequency range. The force excitation is a major contributor in middle-high frequency range conversely.

The Second-Order-Polarization-Propagator-Approximation (SOPPA) in a four-component spinor basis.

A theoretical framework for understanding molecular structures is crucial for the development of new technologies such as catalysts or solar cells. Apart from electronic excitation energies, however, only spectroscopic properties of molecules consisting of lighter elements can be computationally described at a high level of theory today since heavy elements require a relativistic framework, and thus far, most methods have only been derived in a non-relativistic framework. Important new technologies such as those mentioned above require molecules that contain heavier elements, and hence, there is a great need for the development of relativistic computational methods at a higher level of accuracy. Here, the Second-Order-Polarization-Propagator-Approximation (SOPPA), which has proven to be very successful in the non-relativistic case, is adapted to a relativistic framework. The equations for SOPPA are presented in their most general form, i.e., in a non-canonical spin-orbital basis, which can be reduced to the canonical case, and the expressions needed for a relativistic four-component SOPPA are obtained. The equations are one-index transformed, giving more compact expressions that correspond to those already available for the four-component RPA. The equations are ready for implementation in a four-component quantum chemistry program, which will allow both linear response properties and excitation energies to be calculated relativistically at the SOPPA level.

Surface-enhanced Raman scattering from buffer layer under graphene on SiC in a wide energy range from visible to near-infrared

Raman signals from the buffer layer between graphene and a SiC substrate are enhanced through the formation of nano-structured Au deposited directly on graphene grown on SiC. This simple method makes it possible to deconvolute multiple graphene and buffer layer peaks in a wide energy range from 1.58 to 2.33 eV and find new buffer layer peaks that are not resolved in conventional Raman scattering spectroscopy. Furthermore, we clearly show a small linear excitation energy dependence for one of the buffer layer peak positions and an absence of energy dependence for the peaks at most of the other positions.

Single Molecule Nonlinearity in a Plasmonic Waveguide.

Plasmonic waveguides offer the unique possibility to confine light far below the diffraction limit. Past room temperature experiments focused on efficient generation of single waveguide plasmons by a quantum emitter. However, only the simultaneous interaction of the emitter with multiple plasmonic fields would lead to functionality in a plasmonic circuit. Here, we demonstrate the nonlinear optical interaction of a single molecule and propagating plasmons. An individual terrylene diimide (TDI) molecule is placed in the nanogap between two single-crystalline silver nanowires. A visible wavelength pump pulse and a red-shifted depletion pulse travel along the waveguide, leading to stimulated emission depletion (STED) in the observed fluorescence. The efficiency increases by up to a factor of 50 compared to far-field excitation. Our study thus demonstrates remote nonlinear four-wave mixing at a single molecule with propagating plasmons. It paves the way toward functional quantum plasmonic circuits and improved nonlinear single-molecule spectroscopy.

An Adaptive Partial Feedback Linearizing Control Scheme: An Application to a Single Machine Infinite Bus System

A new form of nonlinear control scheme is presented in this brief where the concepts of feedback linearization and adaptive control schemes are combined together to design an adaptive partial feedback linearizing controller. The newly proposed scheme is exercised on a single machine infinite bus (SMIB) power system in order to design its excitation controller in order to eliminate the parameter sensitivity problems of the feedback linearization scheme. The SMIB system is first linearized using the partial feedback linearization technique and the parameters appearing in the feedback linearized model are estimated using the adaptive control scheme. The proposed adaptive partial feedback linearizing excitation controller of the SMIB system uses the estimated values of parameters. The stability of the SMIB system is also analyzed using the Lyapunov stability theory while using the proposed scheme. Simulation studies are carried out to demonstrate the effectiveness of the proposed control scheme over an existing partial feedback linearizing scheme.

TIRF Microscopy Studies of Axial Protein Organization using Photoswitchable Fluorescent Proteins

The spatial organization of proteins at the plasma membrane remains an intense focus of investigation and has prompted the development of numerous super-resolution Total Internal Reflection Fluorescence (TIRF) microscopy techniques. For example, some of the new technologies allow very high resolution in both axial and lateral directions, albeit often at the expense of performing live cell imaging. Live cell TIRF has been improved by multi-angle TIRF imaging providing more information in the axial direction and TIRF-Structured Illumination Microscopy methods improving lateral resolutions but these approaches are often incompatible. Here, we propose to utilize the photoswitching kinetics of photoswitchable fluorescent proteins located within the TIRF evanescent wave to determine their relative axial positions. Since the intensity of the evanescent wave formed at the interface between the glass coverslip and the sample decays exponentially in depth (the evanescent field is typically <200 nm in depth), we rely on the linear excitation intensity dependence of photoswitching to provide a readout of axial depth differences for proteins of interest. In testing its accuracy and utility, we performed photoswitching TIRF (psTIRF) imaging of several proteins located in focal adhesions, which we can compare with previously published interferometric Photoactivated Localization Microscopy (iPALM) studies of the same proteins.

Comparison study of DLOEC and H_Infinity control strategies for transient stability enhancement of power system

This paper studies the control strategies for enhancing the transient stability of a single machine infinite bus (SMIB) power system. First, the dynamical SMIB model is established, and the excitation control is discussed. Next, the damping linear optimal excitation controller (DLOEC) and the output feedback H_infinity control strategy are studied. Then, the DLOEC and the suboptimum H_infinity controller are designed for a specific SMIB power system. The effectiveness of the two presented methods is verified by simulations. The simulation results show that the presented methods improve the transient stability of the SMIB power system and that the H_infinity controller provides better performance than the DLOEC controller.

Magnetic excitations in assemblies of dipolar coupled nanoparticles

The equilibrium magnetization configurations and the associated microwave susceptibility spectra of dipolar coupled nanoplatelets are explored using three-dimensional (3D) micromagnetic simulations. First, the case of periodic arrangements of nanoplatelets on square arrays is considered. As a result, a macro-vortex state defined as a flux closure pattern spreading over the whole array with or without a vortex core can be stabilized starting from an initial orthoradial magnetization configuration. For macro-vortex states with a vortex core, the linear excitation spectrum exhibits a sub-GHz resonance line ascribed to the vortex core dynamics at the array center. The features of this line (spectral position and amplitude) depend on the array size and the strength of the dipolar coupling through the interplatelet spacing. This resonance is also observed for macro-vortex states without a vortex core but only in the regime of a strong dipolar coupling. The effect of disorder is then investigated by numerically generating assemblies of nanoplatelets with a position disorder and, shape and size distributions. The micromagnetic simulations reveal flux closure magnetization configurations as well but without a vortex core. A low-frequency resonance appears in the susceptibility spectra for quite high surface contents of nanoplatelets but its amplitude is weaker compared to the case of periodic arrays. This line arises from a collective mode extended over a few nanoplatelets. A large variety of static and dynamical behaviors is thus evidenced resulting in a great complexity even in such model systems.

The Feature Recognition of CFRP Subsurface Defects Using Low-Energy Chirp-Pulsed Radar Thermography

In this article, a low-energy chirp-pulsed radar thermography (CP-RT) is used to detect the subsurface delamination of carbon fiber reinforced polymer (CFRP) composite as nondestructive testing and evaluation (NDT&E) techniques. The CFRP specimen with artificial flat-bottom holes (FBHs) is prepared for NDTandE by chirp-pulsed radar thermography. Two lasers are employed to be external excitation heat sources. The laser intensities are modulated according to a chirp-pulsed radar signal that combines linear frequency modulation and pulse excitation, and the temperature rise is controlled within 2 °C in the experiment. The thermal-wave response signal is processed by a series of different postprocessing characteristic extraction algorithms. These algorithms include time–frequency algorithms [crosscorrelation algorithm (CC), fast Fourier transform (FFT), and dual-orthogonal demodulation algorithm (DOD)] and statistical analysis approaches [principal component analysis (PCA) and PCA-based reconstructed independent component analysis (PCA-RICA)]. The signal-to-noise ratio (SNR) of defects is employed to evaluate the defect detectability for different size defects by different postprocessing algorithms. A three-dimensional (3-D) tomography method based on the FFT phase characteristic is proposed. A truncated-correlation photothermal tomography based on DOD is also introduced to enable the 3-D tomography of CFRP specimen. The FFT phase presents a relatively high SNR and has good correlation with the depth of the defect. The FFT-based CP-RT has the potential to provide a rapid NDT&E and 3-D tomography approach for CFRP with subsurface defects under the low-energy excitation condition.

Group dynamics and wave resonances in a narrow gap: modes and reduced group velocity

The spatial and temporal structure of the resonant fluid response in a narrow gap (the so-called gap resonance) between two identical fixed boxes is investigated experimentally. Transient wave groups are used to excite the gap resonance from different wave approach directions. This shows a strong beating pattern and a very long duration, reflecting that gap resonance is a multi-mode resonant and weakly damped phenomenon. For head sea excitation the linear transfer function of the gap mode is as significant as that of the mode. Gap resonance can be driven through different mechanisms, e.g. linear excitation and a nonlinear frequency-doubling process. Significant wave group structure is shown in the gap, and the group structure is more distinct in the case with frequency doubling, i.e. long wave, excitation. Then it is clearer visually that the groups originate at the end of the gap, propagate along the gap and are then partially reflected from the other end. The groups within the gap are very clear because the group velocity is close to constant for the first few gap resonance modes, and much smaller than that for free waves on the open sea. In contrast, the phase speed of waves in the gap is larger than that for free waves outside. Only in the limit of short waves do the group velocity and phase speed of the gap modes tend to those of deep-water free waves. The group and phase speeds from these experiments match well the theoretical forms given by Molin et al. (Appl. Ocean Res., vol. 24 (5), 2002, pp. 247–260), albeit for a slightly different box cross-sectional shape.

Noninvasive light focusing in scattering media using speckle variance optimization

Optical imaging deep inside scattering media remains a fundamental problem in bio-imaging. While wavefront shaping has been shown to allow focusing of coherent light at depth, achieving it non-invasively remains a challenge. Various feedback mechanisms, in particular acoustic or non-linear fluorescence-based, have been put forward for this purpose. Non-invasive focusing at depth on fluorescent objects with linear excitation is, however, still unresolved. Here we report a simple method for focusing inside a scattering medium in an epi-detection geometry with a linear signal: optimizing the spatial variance of low contrast speckle patterns emitted by a set of fluorescent sources. Experimentally, we demonstrate robust and efficient focusing of scattered light on a single source, and show that this variance optimization method is formally equivalent to previous optimization strategies based on two-photon fluorescence. Our technique should generalize to a large variety of incoherent contrast mechanisms and holds interesting prospects for deep bio-imaging.