Excitation Symmetry Research Articles

Advanced Excitation Symmetry Analysis Method (ESAM) for improved automated ultrasonic testing of polished materials

One of the strategic plans of the international NDT community is to define standards for developing advanced and digitalized non-destructive testing for automated set-up in mass production [1,2]. Excitation Symmetry Analysis Method (ESAM) are new coded signal processing tools exploring symmetry properties [3]. ESAM method aims to extract the nonlinearity from an ultrasound output by studying the property of symmetry of the transfer function. More precisely, the extraction of nonlinearity is done using basis function associated to a group which is associated to an a priori transfer function. As soon as the linear tomography uses the invariant properties of amplitude dependence, nonlinear tomography should exploit the invariance properties (or symmetry properties) of the complex system. Consequently, the invariance of the stationary properties of a complex medium would be supposed to be associated to a signature of the degradation, that will be extracted with advanced signal processing tools. The idea remains the same: studying the symmetry and more precisely the time reversal one and the inversion. A signal processing is used to measure discrepancies in the autocorrelation function (output signal called chirp coda) for two inputs: a signal and its inversion (pulse inversion). The experiments were done on different sample with and without cracks. It has been shown that when the nonlinearities are extracted from the chirp coda amplitude are non- negligible in crack area and nonlinearities are out of phase. Then, mixing symmetries like time reversal and pulse inversion enable the detection of defect that are difficult to detect with conventional ultrasound method. Indeed, for such crack a destructive mechanical analysis called Jomini is done where the material is polished and examined with microscope. The sensitivity improvement of chirp-coded signal processing has been validated in various domains. Coded excitation techniques, used in communication systems such as radar and sonar provide improved SNR without increasing the amplitude of excitation. The typical test equipment consists of a preamplifier Juvitek TRA-02 (0.02 - 5 MHz) connected to a computer, an amplifier ENI model A150 (55 dB at 0.3-35 MHz), a shear wave transducer Technisonic (2.25 MHz), and a longitudinal wave transducer Panametrics V155 (5 MHz). The experimental device was tested with the V3 calibration block, improved, and specially scaled in order to access to a wide range of multivalued parameters: mechanical properties, ultrasonic parameters (celerity and attenuation) and local geometric data.

Terahertz Spectral Signatures of Ultrafast Spin Transports in Ferromagnetic Heusler Alloy

AbstractAlthough the Co‐based Heusler compounds are predicted to be half‐metals, their sub‐picosecond demagnetization dynamics upon laser excitation show a transition‐metal‐like behavior. Any possible role of ultrafast nonlocal spin transport on the ultrafast demagnetization of half‐metallic Heusler compounds has only been inferred indirectly by time‐resolved magneto‐optical Kerr effect. Here, an ultrafast optically driven spin current traveling from Co2FeSi half metal into an adjacent Pt layer by using terahertz (THz) emission time‐domain spectroscopy (TDS) is demonstrated. By varying the magnetic field and excitation symmetry, the sizable THz generation from Co2FeSi/Pt stack arises from optically induced spin transport across the Co2FeSi/Pt interface in conjunction with spin‐to‐charge current conversion, via the inverse spin Hall effect (ISHE). The chemical ordering, interface roughness and magnetization of the samples become vital to engineer the THz emission properties. The amplitude of THz emission is comparable to that of CoFeB/Pt stack, thereby indicating Co2FeSi as an efficient ultrafast light‐driven spin current injector. Finally, by varying the pumping energy, the contributions are distinguished to the ultrafast spin current from thermalization and optical excitation of majority spins in Heusler alloy. This work illustrates THz emission TDS as a powerful tool to investigate the ultrafast spintronic properties of Heusler alloy/normal‐metal bilayers.

Excitation symmetry and origin of dark states in π-conjugated molecules

Utilizing the exact diagonalization method, the optical absorption spectra of two π-conjugated molecules, anthracene and pyrene, are calculated in various dielectric environments. In a confined quantum system with an anisotropic geometry, it is commonly believed that the first excited state is localized along the elongated direction. In the meantime, the dipole approximation says that the transitions to those states localized along the elongated direction shall generally have higher intensities. In this work we report that anthracene and pyrene would respectively fail these intuitive expectations. It is found that the first active transition in anthracene is always polarized along its short axis direction. For pyrene, it is revealed that the transition of the highest intensity is the one polarized along the short axis direction of the molecule. Furthermore, the first excited state in either anthracene or pyrene is often found to be optically inactive, which is successfully attributed to the short-range interactions by examining the energy spectra in varying interaction environments.

(INVITED) Design of symmetric nanoresonators to scale the ultrafast optical modulation in plasmonic metasurfaces

The light-driven control over the properties of optical nanostructured materials has been shown to be a promising route to achieve various functionalities, possibly on sub-picosecond time scales when ultrashort pulses are employed. Here, we discuss how to induce and reveal an ultrafast dichroism in a set of plasmonic metasurfaces with increasing symmetry by using femtosecond pump-probe spectroscopy. We show that such effect is general, promoted by the photogeneration of out-of-equilibrium electrons, and it can be induced in any sufficiently large nanoresonator, provided that plasmonic modes with well-defined symmetry are excited. The intensity of the photoinduced ultrafast dichroisms depends on the geometry of the nanoresonator, and scales up when the excitation symmetry of the plasmonic mode decreases. Our comparative analysis demonstrates the possibility to exploit light-matter interaction as an effective tool to manipulate transient optical symmetry of metasurfaces.

Active Feed Tuning for Excitation Symmetry in Simultaneous Transmit and Receive Antennas

We present an actively tuned balun to enable simultaneous transmit and receive (STAR) antennas to achieve greater isolation. Specifically, for the first time, we propose placing a chip into the antenna feed to control the amplitude of the current density. Nearly perfect balance of the feed currents enables STAR antennas to maintain strong isolation despite unpredictable fabrication imperfections. Two identical prototypes were fabricated for comparison, one passive and the other with active tuning inserted. Measurements demonstrate a minimum isolation of 42 dB across a 250 MHz bandwidth, representing an 11 dB improvement over the passive case. This is the best reported transmit/receive isolation enhancement to date. Further, this novel tuning method is significant because it enables STAR antennas with low-cost substrates, such as FR-4, to achieve greater isolation than passive designs. Notably, the balun was tuned across 1.68–1.93 GHz, part of the Federal Communication Commission's AWS-3 spectrum auction. Measurements show good comparison with simulation.

Vertical oscillation investigation of spatially elastically supported rigid plate – vehicle model

The article deals with introduction into the issue of the general asymmetry under vertical oscillation of systems of spatially elastically supported bodies with context of vehicle application. The various models are mentioned, quarter, half and full model (with different DOF) and advantages and disadvantages are discussed. The basic calculation procedures were presented for symmetry and asymmetry of geometry and excitation. The excitation is produced by jump change of plate support (analytically and numerically solved by Heaviside’s function, in the experimental part by jump down from the wedges). The procedure to solve the basic 3D model is introduced for selected types of asymmetry of geometry and excitation. The practical approach is presented on the two-axle railroad vehicle. The vertical displacement, velocities, accelerations and angles of rotation of symmetry axes are considered. The time trend of the vertical displacement of the general point the aim of the calculation (3 DOF considered).

Effects of stress on neighboring laser written waveguides in gallium lanthanum sulfide

We investigate an observed lack of excitation symmetry of discrete diffraction patterns in periodic arrays of waveguides written by ultrafast laser inscription (ULI) in gallium lanthanum sulfide glasses. We found experimentally that successive waveguides written with identical parameters are detuned from the previous one by Δβ ∼ 0.2–0.5 mm−1. We show that by varying the writing speed of successive waveguides, we increase the symmetry of the array and reduce the detuning by a factor of 2. After careful analysis of possible physical causes, observations suggest that the density of the laser irradiated material is affected by long range stresses induced by ULI.

Observation of linear plasmonic breathers and adiabatic elimination in a plasmonic multi-level coupled system

We provide experimental and numerical demonstrations of plasmonic propagation dynamics in a multi-level coupled system, and present the first observation of plasmonic breathers propagating in such systems. The effect is observed both for the simplest symmetric case of a thin metal layer surrounded by two identical dielectrics, and also for a more complex system that includes five and more layers. By a careful choice of the permittivities and thicknesses of the intermediate layers, we can adiabatically eliminate the plasmonic waves in all the intermediate interfaces, thus enabling efficient vertical delivery and extraction of plasmonic signals between the top layer and deeply buried layers. The observation relies on controlling the excited mode by breaking the symmetry of excitation, which is crucial for obtaining the results experimentally. We also observe this breathing effect for transversely shaped plasmonic beams, with Hermite-Gauss, Airy and Weber wavefronts, that despite the oscillatory nature of propagation in such systems, still preserve all their unique wavefront properties. Finally, we show that such approaches can be extended to plasmonic propagation in a general multi-layered system, opening a path for efficient three-dimensional integrated plasmonic circuitry.

Fluorescence anisotropy of indole molecules under two-photon excitation in the spectral range of 485–510 nm

Decay of polarized fluorescence in indole dissolved in propylene glycol under two-photon excitation by femtosecond laser pulses in the wavelength range of 485–510 nm has been studied. It is shown that under the experimental conditions used the fluorescence decay signal can be well described by a single excited state lifetime τf and a single rotation diffusion time τrot. By processing the data obtained, the times τf and τrot as well as anisotropy parameter r 0 characterizing the symmetry of two-photon excitation of indole molecules have been determined. Decreasing of the anisotropy parameter r0 down to zero under two-photon excitation energy higher than 5.1 eV has been observed. Interpretation of the obtained results have been done on the basis of ab initio quantum-mechanical computations. A model of energy relaxation under the condition of twophoton excitation of indole in a polar solvent has been discussed.

Interferometric control of the photon-number distribution

We demonstrate deterministic control over the photon-number distribution by interfering two coherent beams within a disordered photonic lattice. By sweeping a relative phase between two equal-amplitude coherent fields with Poissonian statistics that excite adjacent sites in a lattice endowed with disorder-immune chiral symmetry, we measure an output photon-number distribution that changes periodically between super-thermal and sub-thermal photon statistics upon ensemble averaging. Thus, the photon-bunching level is controlled interferometrically at a fixed mean photon-number by gradually activating the excitation symmetry of the chiral-mode pairs with structured coherent illumination and without modifying the disorder level of the random system itself.

Composite particle theory of three-dimensional gapped fermionic phases: Fractional topological insulators and charge-loop excitation symmetry

Topological phases of matter are usually realized in deconfined phases of gauge theories. In this context, confined phases with strongly fluctuating gauge fields seem to be irrelevant to the physics of topological phases. For example, the low-energy theory of the two-dimensional (2D) toric code model (i.e. the deconfined phase of $\mathbb{Z}_2$ gauge theory) is a $U(1)\times U(1)$ Chern-Simons theory in which gauge charges (i.e., $e$ and $m$ particles) are deconfined and the gauge fields are gapped, while the confined phase is topologically trivial. In this paper, we point out a new route to constructing exotic 3D gapped fermionic phases in a confining phase of a gauge theory. Starting from a parton construction with strongly fluctuating compact $U(1)\times U(1)$ gauge fields, we construct gapped phases of interacting fermions by condensing two linearly independent bosonic composite particles consisting of partons and $U(1)\times U(1)$ magnetic monopoles. This can be regarded as a 3D generalization of the 2D Bais-Slingerland condensation mechanism. Charge fractionalization results from a Debye-H\"uckel-like screening cloud formed by the condensed composite particles. Within our general framework, we explore two aspects of symmetry-enriched 3D Abelian topological phases. First, we construct a new fermionic state of matter with time-reversal symmetry and $\Theta\neq \pi$, the fractional topological insulator. Second, we generalize the notion of anyonic symmetry of 2D Abelian topological phases to the charge-loop excitation symmetry ($\mathsf{Charles}$) of 3D Abelian topological phases. We show that line twist defects, which realize $\mathsf{Charles}$ transformations, exhibit non-Abelian fusion properties.

Electronic Raman scattering and the renormalization of the electron spectrum in LuB12

The electronic Raman scattering in LuB12 single crystals of various isotope compositions is studied in the temperature range 10–650 K. The shape and the energy position of spectral maxima depend on the direction and magnitude of a probe wavevector, the temperature, and the excitation symmetry and remain unchanged when the isotope composition changes. Experimental spectra are compared with the spectra simulated on the basis of a calculated electronic structure. The experimental results are successfully described when the electron spectrum renormalization effects caused by electron–phonon coupling are taken into account. This confirms that the origin of the observed spectra in LuB12 is due to Raman scattering by electrons. A comparison of the calculated and experimental data makes it possible to determine the coupling constant (λep = 0.32) that gives the correct superconducting transition temperature.

Sub-thermal to super-thermal light statistics from a disordered lattice via deterministic control of excitation symmetry

Monochromatic coherent light traversing a disordered photonic medium evolves into a random field whose statistics are dictated by the disorder level. Here we demonstrate experimentally that light statistics can be deterministically tuned in certain disordered lattices, even when the disorder level is held fixed, by controllably breaking the excitation symmetry of the lattice modes. We exploit a lattice endowed with disorder-immune chiral symmetry in which the eigenmodes come in skew-symmetric pairs. If a single lattice site is excited, a "photonic thermalization gap" emerges: the realm of sub-thermal light statistics is inaccessible regardless of the disorder level. However, by exciting two sites with a variable relative phase, as in a traditional two-path interferometer, the chiral symmetry is judiciously broken and interferometric control over the light statistics is exercised, spanning sub-thermal and super-thermal regimes. These results may help develop novel incoherent lighting sources from coherent lasers.

Laser-induced jetting and controlled droplet formation

AbstractThe article reports, in the general context of developing techniques to generate microjets, nanojets and even individual nanodroplets, a new method to obtain such formations by interaction of a single laser pulse at 532 nm with an individual/single mother droplet in pendant position in open air. The beam energy per pulse is varied between 0.25 and 1 mJ, the focus diameter is 90 μm, and the droplet’s volumes are either 3 μl or 3.5 μl. Droplet’s shape evolution and jet emission at impact with laser pulse was visualised with a high speed camera working at 10 kfps. Reproducible jets and/or separated microdroplets and nanodroplets are obtained which shows potential for applications in particular in jet printing. It is demonstrated that it becomes possible to play with the geometrical symmetry of both laser excitation and liquid in order to manage the number and the orientation of an induced microjet and consequently to actuate the orientation and the production of nanodroplets by light.

Ultra-wide band magneto-electric dipole antenna for radio frequency integrated circuits

A novel differentially fed magneto-electric dipole antenna is proposed and analysed, which is simpler and better in performance than previously designed antennas by various counterparts. The antenna has achieved an impedance bandwidth of 133.3% 0.5-2.5 GHz for a differential reflection coefficient less than −10 dB. The maximum broadside gain 7.5 dBi has been achieved. A unidirectional radiation pattern with stable gain has also been observed for the entire range of frequency operation. Symmetric radiation pattern for both E-plane and H-plane has also been confirmed owing to the symmetry of antenna structure and feed excitation. The cross-polarisation level lower than −30 dB has also been observed for better signal purity.

A photonic thermalization gap in disordered lattices

A theoretical study looks at the interplay between disorder and chiral symmetry in the photon statistics in a one-dimensional photonic lattice, predicting that for increased disorder coherent light becomes thermal. The formation of gaps—forbidden ranges in the values of a physical parameter—is common to a variety of physical systems: from energy bandgaps of electrons in periodic lattices1 and their analogues in photonic2, phononic3 and plasmonic4 systems to pseudo-energy gaps in aperiodic quasicrystals5. Here, we predict a thermalization gap for light propagating in finite disordered structures characterized by disorder-immune chiral symmetry6—the appearance of the eigenvalues and eigenvectors in skew-symmetric pairs. In these systems, the span of sub-thermal photon statistics is inaccessible to input coherent light, which—once the steady state is reached—always emerges with super-thermal statistics no matter how small the disorder level. We formulate an independent constraint of the input field for the chiral symmetry to be activated and the gap to be observed. This unique feature enables a new form of photon-statistics interferometry: the deterministic tuning of photon statistics via controlled excitation symmetry breaking realized by sculpting the amplitude or phase of the input coherent field.

Compact acoustic-resolution photoacoustic imaging system with fiber-based illumination

Acoustic-resolution photoacoustic imaging enables unique high-resolution visualization of chromophores in deep tissue. However, most current systems are bulky and use complex optical components for illumination. In this study, we developed a compact acoustic-resolution photoacoustic imaging system with simple fiber-based illumination. Around the ultrasound sensor attached to an acoustic lens, four optical fibers were placed in four directions 90° apart, and the sensor plane was covered with an optically and acoustically transparent plastic spacer. The imaging head is compact (diameter, 32 mm; height, 27 mm) and lightweight (32 g) and is alignment-free, thus being suitable for in vivo applications. We assessed the uniformity and symmetry of photoacoustic excitation both by Monte Carlo simulation and experiments. A phantom study showed lateral and axial resolutions of the system to be 148 and 66 µm, respectively. With the system, high-contrast visualization of blood vessels in rat skin was demonstrated in vivo.

Study of Wheel-Rail Vibration in Railways

Together with the rapid development of high-speed railways, extensive research into railway technologies is compulsory, especially on the aspect of dynamic interactions between vehicle and railway line, in order to guarantee the operational security of vehicles at high speed and improve the passengers comfort. The increase in the severity of the dynamic interaction between wheel and rail arises from the increasing speed of trains. As a result, it is necessary to analyze for the characteristics of wheel-rail vibration. The curve is an important component of a railway line and is usually the very source of wheel-rail vibration, especially at the time when a vehicle rapidly passes through a railway line, and will seriously affect the safety and comfort of the vehicle. At present, the usual practice is to only take some limit values to design the vertical section of railway lines, such as the maximum slope and the minimum slope length, according to the grade of a certain right line. Moreover, the existing studies on the vertical curve focused more on the vertical section parameters without considering the influence of these parameters on the dynamic interaction between the vehicle and line. The aforementioned studies involve either static or quasi-static analysis. There is little literature available on using a systematic method based on dynamics to study wheel-rail vibration, and some related issues are mainly the assessment making for a certain line. Based on the multi-body dynamics and the existing achievements, this paper aims to systematically investigate the influence of vertical section parameters on the characteristics of wheel-rail vibration and discuss the relationship between the acting region of wheel-rail vibration and these parameters. Furthermore, the characteristics of wheel-rail vibration at different velocities are investigated. The evaluation of the vibrations behavior in a railway vehicle is one of the matters taken into consideration even from the design stage. The decrease of vibrations to an acceptable level in terms of running behavior, safety, passengers comfort and track fatigue is required by regulations at European level for vehicle homologation and their admission into traffic.The vibrations of railway vehicles are mainly produced by the interaction between the track and the rail. Regarding a track with irregularities or deviations from the ideal geometry it creates vibrations of vehicles, which are developed both vertically and horizontally. These two types of vibrations are decoupled, though, due to the construction symmetries (inertial, elastic, plastic, linear and geometric). As for the vertical vibrations, the bounce and pitch vibrations are the main reasons of the vehicles dynamic behavior. They can be studied on simple models, one or two degrees of freedom, based on the hypothesis of excitation symmetry, by considering only one mobile base as if the wheel-sets had identical motions . This is the reason why the results with these models can be overestimated.This paper presents the influence of vehicle wheel bases and of the steel tire in running conditions, depending on some geometric characteristics. The influence will be further reflected by the vehicles response to the crossing over the rolling track random irregularities and in the magnitude of the vertical accelerations. This is the reason why the complete model of a passenger vehicle has been accounted for, including the car body bending vibrations. The movement equations have been treated in an original manner and brought to a form that points out the symmetrical and anti-symmetrical decoupled movements of vehicle and their excitation modes.

High-purity transmission of a slow light odd mode in a photonic crystal waveguide

We demonstrate a novel scheme to control the excitation symmetry for an odd mode in a photonic crystal waveguide and investigate the spectral signature of this slow light mode. An odd-mode Mach-Zehnder coupler is introduced to transform mode symmetry and excite a high-purity odd mode with 20 dB signal contrast over the background. Assisted by a mixed-mode Mach-Zehnder coupler, slow light mode beating can be observed and is utilized to determine the group index of this odd mode. With slow light enhancement, this odd mode can help enable novel miniaturized devices such as one-way waveguides.

Electronic energy levels of RbCdBr3 crystal

AbstractThe RbCdBr3 crystal that consists of edge shared [CdBr6]4− octahedrons was grown by using the Bridgman growth technique and its electronic energy levels are investigated by measuring the excitation and the emission spectra. An energy level diagram for electronic energy levels of RbCdBr3‐crystal is constructed by using the observed excitation spectra and symmetry considerations.