Broadband Excitation Research Articles

Excellent thermostability and emission properties of Sr2CaTeO6:Sm3+ red emitting phosphor for high color-rendering white LEDs

A novel phosphor, red emitting Sr2CaTeO6:Sm3+ has been developed for white LEDs. The crystal structure and luminescence properties of this phosphor were investigated systematically. The space group of phase Sr2CaTeO6 compounds with a double-perovskite structure was found to be monoclinic P21/n (No.14). The Sr2CaTeO6:Sm3+ showed broad excitation band from 350 to 500 nm (monitored with emission wavelength at 648 nm), which is well aligned with 365 and 400 nm LED chips. Upon 407 nm near-UV excitation, the Sr2CaTeO6:Sm3+ exhibited red emission peak at about 658 nm. We found that Sr2CaTeO6:Sm3+ with 2 mol% of Sm3+ doping achieved the optimal performance. Meanwhile, the Sr2CaTeO6:Sm3+ phosphor showed good thermal stability. Finally, a near-UV pumped warm w-LED with the Sr2CaTeO6:Sm3+ phosphor, blue-emitting BAM:Eu2+ and green-emitting Lu-AG:Ce3+ phosphors was fabricated, which indicated that the Sr2CaTeO6:Sm3+ can be a promising red-emitting material for pc-LEDs.

Multi-stimuli-responsive and dynamic color tunable security ink for multilevel anticounterfeiting

Luminescent security features have been used for anticounterfeiting for a long time. However, constant effort is required to strengthen these security features to be ahead of counterfeiters. Here, we developed a multi-stimuli-responsive luminescent security ink containing Tb(ASA)3Phen, K2SiF6:Mn4+, and NaYF4:Yb3+/Er3+ luminescent materials in PVC gold medium. Tb(ASA)3Phen complex shows a broad excitation band in the UV region; upon UV light radiation it shows strong greenish emission of Tb3+ ions through the antenna effect. K2SiF6:Mn4+, on the other hand, has three excitation bands with maxima at 248, 354, and 454 nm which emit red light after excitation through these bands. NaYF4:Yb3+/Er3+ is used as an upconverting nanophosphor showing green emission under 976 nm laser excitation. Thus, the multi-stimuli-responsive luminescent security ink shows greenish, red, and green emissions under 367 nm, 450 nm, and 976 nm excitations, respectively. Furthermore, the distinct lifetimes of the activators in Tb(ASA)3Phen and K2SiF6:Mn4+, i.e. 0.1708 ms and 8.165 ms, respectively, under 380 nm excitation make this ink suitable for dynamic anticounterfeiting as well. The ink shows a change in the emission color with time delay, after the removal of the 380 nm excitation source, from greenish yellow (at 0 delays) to reddish color after a delay of 7.5 ms. These unique optical features along with excellent photo-, chemical- and environmental stability make this ink useful for advanced-level anticounterfeiting.

Observation of Enhanced Generation of a Fifth Harmonic from Halide Perovskite Nonlocal Metasurfaces

Lead halide perovskites are widely employed in photonic and light-emitting devices because of their rich optoelectronic properties and simplicity of fabrication based on low-cost flexible technologies. Perovskite bulk crystals and films demonstrate outstanding nonlinear characteristics with large optical nonlinearities exceeding the nonlinear susceptibilities of conventional semiconductor materials by several orders of magnitude. One of the promising approaches for further enhancement of the nonlinear response of perovskites as dielectric photonic materials is to employ optical resonances of structured surfaces, or metasurfaces. Nonlocal metasurfaces supporting lattice modes over many unit cells provide a new approach to both spatial and spectral control of light fields, and they may deliver improved characteristics of nonlinear effects for a wide range of applications associated with broadband excitation of multiple high-quality resonances. Here we report on the first observation of enhanced fifth harmonic generation in MAPbBr3 halide perovskite nonlocal metasurfaces driven by high-quality resonances at the generated harmonic wavelength in the visible frequency range. The demonstrated enhancement is about 2 orders of magnitude compared to an unpatterned MAPbBr3 film of the same thickness, and is broadband by virtue of the excitation of multiple resonant modes in the highly nonlocal regime. Our work suggests a novel approach for achieving parametric processes in resonant dielectric structures with high efficiency.

Evaluation of fatigue cracks on rail material based on laser nonlinear wave modulation and adaptive particle swarm optimization - support vector machines

ABSTRACT The evaluation of fatigue cracks is essential for the detection of early damage to railway tracks. Laser nonlinear wave modulation spectroscopy using broadband excitation is an effective method for fatigue crack detection that can solve the problem of frequency combination optimisation. However, due to the complex components of laser nonlinear ultrasonic signals, the evaluation of fatigue cracks using laser nonlinear ultrasonic signals remains a challenge. In this study, wavelet packet decomposition and principal component analysis were used to extract various features of nonlinear ultrasonic signals in the frequency domain, and four new combinations of features were obtained. These new features were used as input for the evaluation model. Finally, a fatigue crack evaluation model based on adaptive particle swarm optimisation-support vector machines was proposed to accurately evaluate fatigue cracks of different lengths. The experiments conducted showed that the fourth new combination of features and evaluation model proposed in this paper can improve the accuracy of evaluating rail fatigue cracks of different lengths, reaching a classification accuracy of 97%.

Surface wave suppression in granular media using a partially embedded meta-barrier

Existing studies on meta-barrier designs for suppressing surface modes in inhomogeneous granular media are limited to meta-barriers comprising vertical or horizontal oscillators. However, a clear understanding of the surface mode hybridizations with the local resonances of the resonators is lacking and could help realize more realistic meta-barrier designs for surface wave control in granular media. In this work, we propose a meta-barrier comprising partially embedded rod-like resonators to study the hybridization of the fundamental surface modes: PSV1 and PSV2, with the longitudinal and flexural resonances of the resonators that enable surface mode suppression. The hybridized dispersion curves and frequency-domain finite element simulations with the meta-barrier demonstrate preferential hybridization of the PSV1 mode with the longitudinal resonance of the resonators, enabling PSV1 mode suppression above the hybridized longitudinal-resonance mode cut-off frequency. Unlike the PSV1 mode, PSV2 hybridizes with both longitudinal and flexural resonances, making it challenging to suppress the mode. However, we observe that enhancing the filling fraction of the meta-barrier promotes PSV2 mode suppression as the flexural-resonance hybridized modes are localized under the meta-barrier. The time-domain finite element simulation using a broadband excitation source further numerically validates the meta-barrier design for applications in vibration mitigation and seismic isolation of structures.

A dual-functional platform for plant cultivation and wide-range optical thermometry based on vibration sidebands

Due to the Sb5+ substituted by Mn4+ at the 2c position of the [SbO6] octahedron, novel Sr2ScSbO6: Mn4+ (SSSO: Mn4+) phosphors showcased a broad excitation band in the range of 200–600 nm. In this respect, an intense far-red emission peak originating from the 2Eg → 4A2g spin-forbidden transition for SSSO: Mn4+ phosphors was observed under the 345 nm excitation. Furthermore, the temperature-dependent emission spectra exhibited excellent thermal stability (70.106% emission intensity remained at 423 K relative to 298 K). The feasibility of the fabricated LED device for potential plant cultivation was also examined. Furthermore, the different temperature reactions for anti-Stokes and Stokes sidebands of the SSSO: Mn4+ phosphors endowed the constructed luminescence intensity ratio (LIR) optical thermometry with an excellent relative sensitivity of 1.21% K−1@83 K in a wide-range (23–648 K). Finally, the sensing sensitivity based on the temperature-dependent decay curves was also analyzed.

Nonlinear frequency response curves estimation and stability analysis of randomly excited systems in the subspace framework

In this paper, the periodic solutions of nonlinear mechanical systems are studied starting from the nonlinear state-space model estimated using the nonlinear subspace identification (NSI) technique. In its standard form, NSI needs the input–output data from a nonlinear structure undergoing broadband excitation and requires the prior knowledge of the locations and kind of nonlinearities to be estimated. The method allows the estimation of the nonlinear features of the system and the indirect study of its periodic solutions using a single broadband excitation, without the need of feedback control loops. To this end, the nonlinear frequency response curves of the system are estimated merging the harmonic balance method with the NSI technique and using a continuation approach. Then, a monodromy-based stability analysis is developed in the nonlinear state-space framework to study the stability of the periodic solutions of the system and to track its bifurcations. The method is validated considering conservative nonlinearities on two numerical examples and one experimental application, the latter comprising a double-well oscillator with period-doubling phenomena. The effects of noise and nonlinear modeling errors are also evaluated.

Vibro-Acoustic Modulation with broadband pump excitation for efficient impact damage detection in composite materials

In the past few decades, the need for efficient and reliable Structural Health Monitoring strategies has led to the development of several approaches for damage detection and characterization purposes. Among them, the Nonlinear Vibro-Acoustic Modulation (VAM) exploits the modulation arising from the interaction of two concurrently applied driving waves, namely the probe and the pump excitations, in the presence of nonlinear scatters such as cracks and defects. Therefore, the VAM provides information on the emergence of internal damage by extracting the nonlinear modulated components of the response of a damaged system. Originally proposed for granular media, the method has shown to be effective in detecting the presence of defects also in metals and composite materials. Nonetheless, its efficacy is highly affected by the excitation frequencies, which are usually chosen among the system resonances. The need for a preliminary modal analysis and, at once, the risk of selecting pump-probe frequency combinations with low sensitivity to damage may make the procedure time-consuming and not fully reliable, preventing the VAM technique from being widely accepted as a robust monitoring tool. To overcome these limitations, a broadband excitation may be used. This study assesses the effectiveness of the VAM technique when a combination of a frequency-swept pump excitation and a mono-harmonic probe wave is applied to drive the sample. Experimental tests were conducted on a composite laminated beam mounted on an electrodynamic shaker and tested in both pristine and damaged conditions. Low-profile surface-bonded piezoceramic transducers were used for both probe excitation and sensing. Barely visible impact damage (BVID) was introduced in the composite beam to examine the potential of the approach for the detection of very small, localized damage. The results show that the use of VAM with a broadband low-frequency excitation may be an effective option for identifying nonlinearities associated with typical damage occurring in composite structures.

Modal Decomposition Analysis of Bluff-Body Stabilized Lean Premixed CH4/H2/air Flames Based on LES Data

Unsteady flow and flame dynamics of bluff body stabilized CH4/H2/air premixed flames are investigated numerically with LES and modal decomposition techniques. Energy ranked and frequency dependent modes are identified with proper orthogonal (POD), spectral proper orthogonal (SPOD) and dynamic mode decompositions (DMD). Flow and flame coherent structures are extracted based on the axial velocity and heat release rate contours. Two cases, namely 100% CH4/air (V-flame) and 43.4% CH4 + 56.6% H2/air (M-flame), are selected for the analysis. Each case is acoustically excited in large eddy simulation (LES) with harmonic excitation signals at distinct frequencies and with a broadband excitation signal, to produce snapshots for modal decomposition analysis. In POD, the frequencies of the relevant modes are extracted with the discrete Fourier transform (DFT) of time coefficients, while in SPOD and DMD the extracted modes are frequency dependent by nature. Results are compared and commented.

Multifunctional optical materials based on transparent inorganic glasses embedded with PbS QDs

Quantum dot (QD)-embedded glass, which is a high-performance multifunctional optical material, has the combined advantages of excellent optical properties of QDs and easy processing of glass. In this study, an inorganic PbS QD embedded multicomponent glass, which produces variable emission at different ultraviolet (UV) excitation wavelengths, is synthesized. The samples are fabricated via a facile melt-quenching method and their structural, optical, photoluminescent, and morphological properties are discussed. The QD glass exhibits broad-band UV excitation in the range of 250–400 nm, and the full width at half maximum of the emission peaks of the QD glasses reaches 177.8 and 158.5 nm at room temperature under an excitation of 365 and 310 nm, respectively. Notably, white light emission with a high color rendering index is obtained under UV excitation at 350 and 365 nm by doping blue light emitting rare earth (RE) Tm3+ ions into the system. The doped samples exhibit excellent color stability and their CIE coordinates shift along the Planck blackbody locus with increasing temperature. Further, based on the differences in the luminescence mechanisms of the PbS QDs and Tm3+ ions, the application potential of the RE-doped PbS QD-embedded glasses in optical thermometry is discussed. The results demonstrate the application potential of these optically multifunctional inorganic QD glasses in the fields of optical conversion, anti-counterfeiting, white light illumination, and non-contact optical temperature measurement.

Synthesis, structure, and luminescence properties of Europium/Cerium/Terbium doped strontium-anorthite-type green phosphor for solid state lighting

Novel phosphor exploration and luminescence property regulation are two important strategies in pursing high performance phosphors for white light emitting diodes, and have attracted great attention from the researchers. Herein, novel green phosphors Sr2Ga2SiO7:Eu2+ and Sr2Ga2SiO7:Ce3+,Tb3+ had been obtained by high-temperature solid-state reactions and their luminescence properties had been investigated in detail. Powder X-ray diffraction and Rietveld structure refinement results verified the phase purity and gave the crystal structure of the prepared samples. Due to the electric dipole transition between inter configurations of 4fN and 4fN-15d1, Sr2Ga2SiO7:Eu2+ and Sr2Ga2SiO7:Ce3+ exhibited intense broad excitation and emission bands, giving out green and blue emitting light under UV excitation, respectively. By codoping Tb3+ with Ce3+ in the host and utilizing the energy transfer, tunable blue to green emission had been obtained. The energy transfer mechanism had been determined to be electric dipole-quadrupole interaction through dynamic luminescence analysis using I-H model. The prepared phosphors exhibited good thermal stability with integral emission intensity at 150 °C remaining more than 80 % of the emission intensity at 25 °C. Moreover, by coating Sr2Ga2SiO7:Eu2+ and Sr2Ga2SiO7:Ce3+,Tb3+ on UV chips, green LED devices had been obtained. The investigation results indicated that the Eu2+ singly doped and Ce3+-Tb3+ codoped Sr2Ga2SiO7 might be potential UV excited green phosphors for solid state lighting.

Utilization of BaAl1.4Si0.6O3.4N0.6:Eu2+ Green-emitting Phosphor to Improve Luminous Intensity and Color Adequacy of White Light-emitting Diodes

BaAl1.4Si0.6O3.4N0.6:Eu2+ exhibiting broad excitation and emission bands with intense green emission centred at 510 nm is applied to produce high-performance white-light-emitting diodes (WLEDs). The preparation of the green phosphor utilizes NaNO3

Vocal Tract Resonance Detection at Low Frequencies: Improving Physical and Transducer Configurations

Broadband excitation introduced at the speaker's lips and the evaluation of its corresponding relative acoustic impedance spectrum allow for fast, accurate and non-invasive estimations of vocal tract resonances during speech and singing. However, due to radiation impedance interactions at the lips at low frequencies, it is challenging to make reliable measurements of resonances lower than 500 Hz due to poor signal to noise ratios, limiting investigations of the first vocal tract resonance using such a method. In this paper, various physical configurations which may optimize the acoustic coupling between transducers and the vocal tract are investigated and the practical arrangement which yields the optimal vocal tract resonance detection sensitivity at low frequencies is identified. To support the investigation, two quantitative analysis methods are proposed to facilitate comparison of the sensitivity and quality of resonances identified. Accordingly, the optimal configuration identified has better acoustic coupling and low-frequency response compared with existing arrangements and is shown to reliably detect resonances down to 350 Hz (and possibly lower), thereby allowing the first resonance of a wide range of vowel articulations to be estimated with confidence.

Photophysical studies for Cu(i)-based halides: broad excitation bands and highly efficient single-component warm white-light-emitting diodes.

Designing and synthesizing cuprous halide phosphors unifying efficient low-energy emission and a broad excitation band is still a great challenge. Herein, by rational component design, three novel Cu(i)-based metal halides, DPCu4X6 [DP = (C6H10N2)4(H2PO2)6; X = Cl, Br, I], were synthesized by reacting p-phenylenediamine with cuprous halide (CuX), and they show similar structures, consisting of isolated [Cu4X6]2- units separated by organic layers. Photophysical studies uncover that the highly localized excitons and rigid environment give rise to highly efficient yellow-orange photoluminescence in all compounds with the excitation band spanning from 240 to 450 nm. The bright PL in DPCu4X6 (X = Cl, Br) originates from self-trapped excitons due to the strong electron-phonon coupling. Intriguingly, DPCu4I6 features a dual-band emissive characteristic, attributed to the synergistic effect of halide/metal-to-ligand charge-transfer (X/MLCT) and triplet cluster-centered (3CC) excited states. Benefiting from the broadband excitation, a high-performance white-light emitting diode (WLED) with a high color rendering index of 85.1 was achieved using single-component DPCu4I6 phosphor. This work not only unveils the role of halogens in the photophysical processes of cuprous halides, but also provides new design principles for high-performance single-component WLEDs.

High performance of broad excitation and narrow emission green phosphor NaAl11O17:Eu2+, Mn2+for backlight displays

A high performance narrow-band green-emitting phosphor with a broad excitation band ranging from 250 to 470 nm was obtained by the introduction of Eu2+in NaAl11O17:Mn2+, which is a promising candidate in backlight displays.

A broadband pulse EPR spectrometer for high-throughput measurements in the X-band

We present an X-band pulse EPR spectrometer with high throughput and excellent sensitivity in the 8.5-11.5GHz range. It is designed for high stability and low noise Fourier Transform measurements for applications in pulse dipolar spectroscopy, pulse hyperfine spectroscopy, and spin relaxation from cryogenic temperatures to room temperature. An arbitrary waveform generator is used to generate pulses of any frequency and shape for multiple resonance experiments or for uniform broadband excitation with bandwidths exceeding 350 MHz. We illustrate the capabilities and performance of the spectrometer by measurements on free radicals and biradicals in solids and liquids. Relaxation times of radicals in liquid solution are measured for fewer than 30,000,000 spins (less than 3 nanomoles per liter). Non-uniform acquisition provides higher throughput for mixtures of radicals with quite different relaxation rates. Conventional DEER measurements on a rigid biradical have good modulation depth. Broadband SIFTER with chirped adiabatic WURST pulses demonstrates versatility for the latest broadband pulse schemes. A broadband ESEEM measurement correlates ESEEM and EPR frequencies which characterize the conformation of a nitroxide radical. The entire EPR spectrum with a width approaching 300 MHz was excited and detected throughout the measurement. The spectrometer supports the operator in tuning, setting up experiments and monitoring their progress so that even novice users consistently can obtain optimal results.

Six‐Photon Excited Self‐Trapped Excitons Photoluminescence in Lead‐Free Halide Perovskite

AbstractMultiphoton absorption (MPA) effects are suitable for biophotonic applications due to the long penetration depth and high spatial confinement. Compared with lead halide perovskites, relevant studies on MPA effects of lead‐free perovskites are still scarce, especially for the high‐order MPA effects. Herein, an all‐inorganic lead‐free Cs2TeCl6 perovskite crystal is reported that exhibits three‐ to six‐photon absorption (3PA to 6PA) effects over a wavelength range of 800–2200 nm. The results of steady and transient optical experiments indicate that the single‐ and multiphoton excited broadband orange emission in Cs2TeCl6 crystals is attributed to the self‐trapped excitons (STEs) recombination. What is more, the 6PA cross‐section (δ6) has been determined to be 1.87 × 10−174 cm12 s5 photon−5 (at 1980 nm). As far as it is known, the high‐order 6PA effect is unprecedented in lead‐free perovskites, which paves the way for the future development of lead‐free perovskite high‐order multiphoton application.

Composite pulse combinations for chirp excitation

Composite pulses are the efficient method for broadband excitation to get control of the limitations of high field NMR, such as resonance offset effects with constraints on rf power that leads to signal intensity distortion. Phase-modulated chirp pulses are used as ordered composite pulse sequences in this paper as CHORUS sequence in a high-field NMR spectrometer (BRUKER 750 MHz) for broadband excitation. The composite pulse sequence applies chirp pulses with the forward and the reverse sweep mechanisms. A single excitation pulse combines adiabatic and non-adiabatic rotation, explained as a three-phase rotation, which leaves the magnetizing vectors to a non-uniform phase dispersion as a function of the offset frequency. One adiabatic refocusing pulse of the double sweep rate after the excitation pulse cannot satisfactorily compensate for the phase dispersion. Hence, composite self-refocussing CHORUS excitation pulse, with forward, reverse, and their combinations are used to remove the non-uniform phase dispersion generated due to offset resonance frequency. Four such combinations of composite pulses are produced with analytical explanation in this paper. MATLAB simulation results and experimental verification on the BRUKER 750 MHz NMR spectrometer of the composite pulses are also presented in this paper.

High-Fidelity Quantum Control by Polychromatic Pulse Trains

We introduce a quantum control technique using polychromatic pulse trains, consisting of pulses with different carrier frequencies, i.e., different detunings with respect to the qubit transition frequency. We derive numerous polychromatic pulse trains, which generate broadband, narrowband, and passband excitation profiles for different target transition probabilities. This makes it possible to create high-fidelity excitation profiles which are either (i)robust to deviations in the experimental parameters, which is attractive for quantum computing, or (ii)more sensitive to such variations, which is attractive for crosstalk elimination and quantum sensing. The method is demonstrated experimentally using one of IBM's superconducting quantum processors, in a very good agreement between theory and experiment. These results demonstrate both the excellent coherence properties of the IBM qubits and the accuracy, robustness, and flexibility of the proposed quantum control technique. They also show that the detuning is a control parameter which is as efficient as the pulse phase that is commonly used in composite pulses. Hence the method opens a variety of perspectives for quantum control in areas where phase manipulation is difficult or inaccurate.

Active control of human-induced vibrations on lightweight structures via electrodynamic actuator dynamics inversion

The active vibration absorber represents an effective means to mitigate excessive vibrations in low-damping structures. Nevertheless, the dynamics of the actuators employed in such systems may negatively affect their performance and stability restricting their operational frequency range. This article presents an application of dynamics inversion techniques to the force control of electrodynamic proof-mass actuators employed as active vibration absorbers in lightweight pedestrian structures. The dynamics inversion approach is applied to enhance the classical direct velocity feedback scheme. Additionally, a novel method relying on dynamics inversion is presented: the Broadband Force Cancellation Algorithm. This procedure consists in estimating, in real-time, the equivalent force acting on the system to later apply it back to the structure with the opposed sign. The effectiveness of the proposed methods is assessed via numerical simulations carried over a realistic model of an existing lightweight footbridge and an electrodynamic proof-mass actuator. Two load cases are analyzed: a fixed swept-sine force and a walking load. Both cases account for the actuator-structure interaction. The human-structure interaction is considered in the latter scenario due to its importance when dealing with lightweight pedestrian structures. Simulation results demonstrate that the dynamics inversion techniques effectively cancel out actuator dynamics leading to an excellent tracking of the reference force output by the suggested or other vibration control algorithm. The proposed schemes are proved promising since they substantially outperform the widespread direct velocity feedback approach. In particular, the Broadband Force Cancellation Algorithm minimizes the action of the external forces within their estimation frequency range, thus being especially suited to tackle broadband excitations, important in lively pedestrian structures, whereas the velocity feedback methodology performs best at the structural resonant frequencies.