Broadband Peak Research Articles

Steady and unsteady loading on a hydrofoil immersed in a turbulent boundary layer

The steady and unsteady loads acting on a hydrofoil immersed in a turbulent boundary layer have been investigated. Measurements were performed in a cavitation tunnel in which the hydrofoil was mounted from the test section ceiling, via a 6-component force balance. Two NACA0012 hydrofoil models with trapezoidal planforms and aspect ratios of 2.4 and 1.2 were examined. The ceiling turbulent boundary layer was artificially thickened via an array of transverse jets located upstream of the test section. Thickening the ceiling boundary layer allowed for varying levels of hydrofoil immersion (nominally up to 100%) to be studied. In addition to the level of immersion, the effect of varying incidence and Reynolds number on the hydrodynamic loading normal to the chord was also investigated. Steady forces were found to be significantly affected by the relative scale of the boundary layer, particularly in the stall region. A broadband peak in the unsteady normal force spectra was observed at a constant reduced frequency of 0.2. The relative peak amplitude was found to be dependent on the boundary layer thickness to hydrofoil span ratio and Reynolds number. As the incidence is increased past stall, a low-frequency power increase was observed which was superimposed over the existing broadband excitation induced by the ceiling boundary layer.

Novel Dy and Sm activated BaSi2O5 phosphors: Insights into the structure, intrinsic and extrinsic luminescence, and influence of doping concentration

Pure and a series of BaSi2O5:RE3+ (RE = Dy, Sm) phosphors varying with the substitution of Ba2+ were prepared using a gel-combustion method. X-ray diffraction results confirmed that all samples exhibited an orthorhombic structure. FT-IR spectra showed that the samples consist of BaCO3, SiO2 network and the presence of OH groups. The photoluminescence spectrum of non-doped BaSi2O5 showed a broad-band emission peak at 520 nm. The analysis of photoluminescence emission and comparison of the chromaticity diagrams of BaSi2O5:Dy3+ and BaSi2O5:Sm3+ phosphors were presented, and their implications are also discussed. The optimum quenching concentrations and critical distances of Dy3+ and Sm3+ ions in the BaSi2O5 were 2mol%, 22.57 Å and 1 mol%, 28.44 Å, respectively. The investigation of concentration quenching behaviour, fluorescence decay curves, and decay time indicates that the dominant mechanism type leading to concentration quenching was governed by electric dipole-dipole interactions. The concentration dependence of intensity ratio of yellow-blue emission indicated that the emission of BaSi2O5:Dy3+ phosphors fell in the white light region tuning the blue and yellow components. BaSi2O5:Sm3+ phosphors may allow one to consider as potential orange-red-emitting phosphors.

Structural, Vibrational, Optical and Improved Photoluminescence Properties of Dy3+ Doped Ca2KZn2V3O12 Phosphors

A series of dysprosium (Dy3+) ion doped Ca2KZn2V3O12 phosphors were explored which exhibits a broad band and sharp peaks in the visible region under the ultraviolet light excitation. Fourier transform infrared (FTIR) spectra and the optical diffuse reflectance spectra were ascribing the formation of the distorted VO4 tetrahedral group due to the influence of Dy3+ ions. Moreover, a tunable luminescence color was achieved by doping of Dy3+ ion in the Ca2KZn2V3O12 phosphor. Through doping concentration optimization, the Ca2KZn1.9Dy0.1V3O12 phosphor was observed for high color rendering index (CRI) and excellent correlated color temperature (CCT) with cool white emission. Hence, in the designing of the VO4 → Dy3+ energy transfer that is capable of converting ultraviolet light into efficient white light, this phosphor is suitable for solid-state lighting applications.

Growth, thermal, and polarized spectroscopic properties of Nd:CeF3 crystal for dual-wavelength lasers

A Nd:CeF3 crystal was grown successfully by the vertical Bridgman method. The polarized optical properties were investigated for the first time. Based on the Judd–Ofelt theory, the spectral parameters of Nd3+ were studied in detail by using the spectrum of the as-grown crystal. The peak absorption cross-section of σ-polarization at 791 nm is 1.59 × 10−20 cm2, the full width at half maximum (FWHM) is 10 nm. The peak emission cross-section of σ-polarization at 1070 nm is approximately 2.53 × 10−20 cm2, and the three peaks that make up the broadband peak have comparable value. The measured 4F3/2 level fluorescence lifetime is 0.406 ms. Moreover, segregation coefficient of Nd3+, thermal diffusivity coefficient, and thermal conductivity were also measured. We propose that the Nd:CeF3 crystal may be a promising material for 1 μm dual-wavelength laser applications.

Electric and photoluminescence properties of Eu3+-doped PZN-9PT single crystal

Eu3+-doped 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3 (PZN-9PT: Eu) single crystals were grown by the flux technique. The phase structure, ferroelectric, dielectric, and photoluminescence properties of the crystal were investigated. The PZN-9PT: Eu crystal has a pure perovskite structure. The residual polarization Pr, coercive field Ec, total bipolar strain, and unipolar stain are 36.7664 μC/cm2, 11.0597 kV/cm, 0.35%, and 0.12%, respectively. The coercive field Ec of PZN-9PT: Eu crystal is three times higher than that of PZN-9PT crystal. The Curie temperature Tc of the PZN-9PT: Eu crystal is about 172 °C which is slightly higher than that of PZN-PT single crystal. Under the excitation of 464 nm laser, four broadband peaks appear around 593 nm, 613 nm, 657 nm, and 716 nm, corresponding to the 5D0 → 7FJ (J = 0, 1, 2, 3, 4) transition emission, respectively. The luminescence intensity of all the emission peaks decreases with increasing temperatures range from 298 to 393 K. Our results can provide a new way for the design of multifunctional materials used in optical-electrical devices.

Optical properties of various graphitic structures deposited by PECVD

In this article, we report on the comparative study of the optical properties of different graphitic structures such as stacked graphite (GR), graphene oxide (GO) and few-layers of graphene (FLG). Plasma enhanced chemical vapour deposition technique employing acetylene (C2H2) as precursor gas used to deposit the different graphitic nanosheets on copper substrates. The influence of mixture gas ratio (C2H2/H2) on the structure of nanosheet was examined. Investigation of detailed structure of nanosheets was performed by different characterization techniques such as X-ray diffraction, attenuated total reflectance-Fourier transform infrared and Raman spectroscopy. Optical properties of stacked graphite, graphene oxide and few layers of graphene sheets were studied. The present results reveal that how the optical properties change due to the variations in structure parameters of sheets. The refractive index results indicate the anomalous dispersion due to strong absorption featured for graphitic structures. The real dielectric constant of GR and FLG sheets reveals independent frequency behavior and approach to zero but GO sheet shows a broadband peak from 1.75 to 4.75 eV which attributed to oxygen functional group behavior.

Nonlinear Charge Transport in InGaAs Nanowires at Terahertz Frequencies.

We probe the electron transport properties in the shell of GaAs/In0.2Ga0.8As core/shell nanowires at high electric fields using optical pump/THz probe spectroscopy with broadband THz pulses and peak electric fields up to 0.6 MV/cm. The plasmon resonance of the photoexcited charge carriers exhibits a systematic redshift and a suppression of its spectral weight for THz driving fields exceeding 0.4 MV/cm. This behavior is attributed to the intervalley electron scattering that results in the doubling of the average electron effective mass. Correspondingly, the electron mobility at the highest fields drops to about half of the original value. We demonstrate that the increase of the effective mass is nonuniform along the nanowires and takes place mainly in their middle part, leading to a spatially inhomogeneous carrier response. Our results quantify the nonlinear transport regime in GaAs-based nanowires and show their high potential for development of nanodevices operating at THz frequencies.

Dynamic mode decomposition analysis of the flow characteristics in a centrifugal compressor with vaned diffuser

To understand the flow dynamic characteristics of a centrifugal compressor, the dynamic mode decomposition (DMD) method is introduced to decompose the complex three-dimensional flow field. Three operating conditions, peak efficiency (OP1), peak pressure ratio (OP2), and small mass flow rate (near stall, OP3) conditions, are analyzed. First, the physical interpretations of main dynamic modes at OP1 are identified. As a result, the dynamic structures captured by DMD method are closely associated with the flow characteristics. In detail, the BPF/2BPF (blade passing frequency) corresponds to the impeller–diffuser interaction, the rotor frequency (RF) represents the tip leakage flow (TLF) from leading edge, and the 4RF is related to the interaction among the downstream TLF, the secondary flow, and the wake vortex. Then, the evolution of the dynamic structures is discussed when the compressor mass flow rate consistently declines. In the impeller, the tip leakage vortex near leading edge gradually breaks down due to the high backpressure, resulting in multi-frequency vortices. The broken vortices further propagate downstream along streamwise direction and then interact with the flow structures of 4RF. As a result, the 8RF mode can be observed in the whole impeller, this mode is transformed from upstream RF and 4RF modes, respectively. On the other hand, the broken vortices show broadband peak spectrum, which is correlated to the stall inception. Therefore, the sudden boost of energy ratio of 14RF mode could be regarded as a type of earlier signal for compressor instability. In the diffuser, the flow structures are affected by the perturbation from the impeller. However, the flow in diffuser is more stable than that in impeller at OP1–OP3, since the leading modes are stable patterns of BPF/2BPF.

Sb3+ Doping-Induced Triplet Self-Trapped Excitons Emission in Lead-Free Cs2SnCl6 Nanocrystals.

Doped halide perovskite nanocrystals (NCs) have opened new opportunities for the emerging optical and optoelectronic applications. Here, we describe a hot-injection synthesis of all-inorganic lead-free Cs2SnCl6 and Sb3+ doped Cs2SnCl6 NCs. Cs2SnCl6 NCs present a blue emission peak at 438 nm, whereas a new broad-band emission peak appears at 615 nm for the Sb3+ doped NCs. Comparative structural and spectral characterizations of Sb3+ doped Cs2SnCl6 NCs with micrometer-sized undoped and Sb3+ doped crystals show that the formation of broad-band orange emission is originted from triplet self-trapped excitons, attributed to the 3Pn-1S0 transitions (n = 0, 1, 2). Our results in Sb3+ doped Cs2SnCl6 materials provide insights into the machanisms of doping-induced emission centers, and it extends the existing knowledge of optical properties of doped halide NCs for further studies.

Broadening sound absorption coefficient with hybrid resonances

In the last years, a great research effort has been focused on the noise mitigation at low frequencies. Membrane-type acoustic metamaterials (AMM) are one of the most promising solutions to meet the growing demand for low frequency sound absorbers. Typically, acoustic membrane absorbers require large back cavities to achieve low frequency sound absorption, which is usually categorised by a single narrow absorption peak. This paper presents an acoustic resonator unit cell, comprising of a thin elastic silicone plate with an air gap cavity with broadband absorption in a frequency range between 250 and 400 Hz. The broadband and multiple peak sound absorption showed by the proposed resonator is due to hybrid resonances which occur in the frequency range due to coupling of the structural dynamic response of the plate with the acoustic response of the air cavity. A numerical model based on acoustic-structural interaction, validated for experimental data, has been used to explain how the broadening gain in the sound absorption level is strictly related to the hybrid resonances of the unit cell resonator. We demonstrated that hybrid resonances are a function of the geometrical parameters and the ratio between the Young’s modulus and the density of the material plate, thus the proposed resonators absorption frequency range is tuneable at low frequencies allowing a wider broadband not achievable with acoustic membrane absorbers.

Detection of epileptic seizures in EEG signals during long-term monitoring of patients after traumatic brain injury

Long-term (several days) monitoring of epileptiform activity in scalp EEG of posttraumatic brain injury patients is an important task. EEG signals contain epileptiform seizures and similar signals of myographic activity associated with chewing. Both epileptiform activity and chewing artifacts appear in the same frequency range, which complicates their differentiation. To distinguish epileptiform activity from chewing artifacts, a method based on the wavelet spectrogram analysis of EEG is proposed. EEG wavelet spectrogram contains broadband peaks at times corresponding to peak-wave epileptiform activity on the one hand, and peaks of myographic activity at chewing on the other hand. The periodicity of these peaks is investigated. The difference in the period dispersion of epileptiform peaks and chewing peaks are found.

Spectroscopic investigation of red Eu3+ doped ceria nanophosphors and promising color rendition for warm white LEDs

Europium-ion doped CeO2 phosphors with different Eu3+ concentrations were synthesized via sol-gel combustion synthesis. X-ray diffraction patterns revealed the formation of pure cubic structure of CeO2 even after the incorporation of 35 mol% Eu3+. The excitation spectra of CeO2:Eu3+ displayed a broad band (330–370 nm) and several sharp peaks (393–532 nm) owing to the charge transfer (CT) from O2− to Ce4+ ions and f-f transitions of Eu3+ ions respectively. The evaluation of CT band was explained on the basis of environmental factors. The phosphors were illuminated with blue light and showed the characteristic red luminescence corresponding to the 5D0→ 7FJ transitions of Eu3+ as confirmed from XPS. The emission was observed to increase with the increase in the Eu3+ concentration up to 35 mol% without any quenching. Judd-Ofelt parameters were evaluated using the emission spectroscopy to explain the variation in ligand environment and enhancement of red color purity to 80%. The thermal stability of present phosphors was measured by estimating activation energy and TG-DSC analysis. Robust blue-excitation and sharp red-emission with high color-purity and promising thermal stability in fabricated WLEDs indicated the suitability of CeO2:Eu3+ phosphors for white light emitting diodes with quantum yield efficiency up to 84.6%.

On the unsteady behaviour of cavity flow over a two-dimensional wall-mounted fence

The topology and unsteady behaviour of ventilated and natural cavity flows over a two-dimensional (2-D) wall-mounted fence are investigated for fixed length cavities with varying free-stream velocity using high-speed and still imaging, X-ray densitometry and dynamic surface pressure measurement in two experimental facilities. Cavities in both ventilated and natural flows were found to have a re-entrant jet closure, but not to exhibit large-scale oscillations, yet the irregular small-scale shedding at the cavity closure. Small-scale cavity break-up was associated with a high-frequency broadband peak in the wall pressure spectra, found to be governed by the overlying turbulent boundary layer characteristics, similar to observations from single-phase flow over a forward-facing step. A low-frequency peak reflecting the oscillations in size of the re-entrant jet region, analogous to ‘flapping’ motion in single-phase flow, was found to be modulated by gravity effects (i.e. a Froude number dependence). Likewise, a significant change in cavity behaviour was observed as the flow underwent transition analogous to the transition from sub- to super-critical regime in open-channel flow. Differences in wake topology were examined using shadowgraphy and proper orthogonal decomposition, from which it was found that the size and number of shed structures increased with an increase in free-stream velocity for the ventilated case, while remaining nominally constant in naturally cavitating flow due to condensation of vaporous structures.

Dual-band switchable terahertz metamaterial absorber based on metal nanostructure

In this paper, a tunable terahertz metamaterial absorber with two bands is proposed. It is composed of Au reflector, dielectric layer and periodic array of Au. By adjusting different materials and optimizing parameters, it can achieve the characteristics of both broadband and single peak absorption. In the range of 3.5–4.1 terahertz, the broadband absorption rate can reach over 90%, the relative bandwidth ratio is 17%, and the absorption peak at 7.15 THz can reach 98.9%. The coupling mechanism of broadband and unimodal absorption is studied by coupled mode theory. In addition, the designed absorber has broad application prospects in terahertz imaging and sensing due to its broadband and tunable characteristics.

Influence of apple phytochemicals in ZnO nanoparticles formation, photoluminescence and biocompatibility for biomedical applications

The new green-synthesised ZnO nanoparticles (NPs) using apple (var. Starking) phytochemicals present a great potential for bioimaging applications. These NPs, when compared with ZnO microparticles synthesised with pure phytochemicals (quercetin or sucrose), and water, revealed that sizes and shapes were widely dependent on the organic precursors used. Based on these findings, new insights into the synthesis of ZnO NPs using apple phytochemicals were presented.The photoluminescent properties, characterized by steady-state and time resolve photoluminescence measurements, revealed that besides the intense sharp near-UV band edge emission observed for all particles, with sub-nanosecond lifetime, a strong broad emission band peak at 2.20 eV was detected for microparticles, with longer decay times being associate to crystal defects. Additionally, the photoluminescent properties of ZnO particles, further explored by fluorescence lifetime imaging microscopy (FLIM), suggested adequacy for imaging applications. The cytotoxicity, evaluated in human dermal fibroblasts, proving the biosafety of ZnO NPs by an unaffected cellular viability, total mitochondrial activity and F-actin cytoskeleton organization, contrasted with some degree of cytotoxicity depicted for microparticles. The influence of the phytochemicals in ZnO cytotoxicity was discussed.To the authors' best knowledge this is the first report of ZnO NPs synthesised with apple extracts. The novelty of choosing a fruit widely used in the food industry will render affordable NPs through the concept of circular economy. The proved biosafety of these ZnO NPs together with their intrinsic photoluminescent properties, open perspectives for the development of cost-effective bioimaging materials with potential to be further directed into biomedical applications.

Trap distribution and mechanism for near infrared long-afterglow material AlMgGaO4:Cr3.

Near infrared (NIR) long-afterglow materials have attracted much attention due to their high penetration and low destruction in biological tissues. Here, a series of deep red and near infrared materials, AlMgGaO4:xCr3+, were successfully synthesized by a high temperature solid state method. AlMgGaO4 was selected as the host considering its rich antisite defects, which can effectively capture electrons. The emission spectra of AlMgGaO4:xCr3+ range from 680 nm to 1100 nm, which can be nicely decomposed into four Gaussian bands with peaks centered at 706 nm, 723 nm, 916 nm, and 938 nm, respectively. At low temperature (10 K), the emission spectra show there are four emission peaks: a sharp line (peak 1) and broad emission band (peak 2) come from Cr3+ substituting for the regular octahedron [AlO6], and two broad emission bands (peaks 3 and 4) which originate from the spin-allowed transition 4T2(4F) → 4A2(4F) of Cr3+ in the disordered [GaO6] and [MgO6] octahedra, respectively. Remarkably, after removing the excitation source, it exhibited more than 10 hours of afterglow emission which decreased sharply in the first 30 min and then decreased slowly. With an increase in the Cr3+ concentration, the trap depth became shallower due to the generation of the electronic trap centers . The distribution of trap centers and the mechanism of the persistent luminescence have been carefully analyzed and are also discussed.

Spectral and Luminescence Properties of Manganese Doped Sodium Lead Alumino Borosilicate Glass System

Na2O-PbO-Al2O3-B2O3-SiO2 glasses mixed with different concentrations of MnO (ranging from 0.3 to 0.9 mol %) was prepared by melt quenching technique. The samples were characterized by X-ray diffraction technique, Optical absorption and Photo luminescence technique. A prepared glass sample is confirmed by X-ray diffraction spectra. The optical absorption spectra of these glasses exhibited a predominant broad band peak at 21,052 cm-1 (475 nm) is identified due to 6A1g (S) → 4T1g (G) octahedral transition of Mn2+ ion. From the spectral analysis the optical band gap (Eopt) and Urbach energy (ΔE) are evaluated. The emission spectra of Mn2+ doped titled glasses have shown single and broad emission band at about 600 nm assigned to electronic transition 4T1g (G) → 6A1g (S) displaying red emission upon excitation at 413 nm. Various principle physical properties were also evaluated.

Coherent structures in a screen cylinder wake

While the solid cylinder wake shows apparent and stable shedding frequency, the screen cylinder wake shows a broad-band peak with a trend of decreasing frequency, indicating that amalgamation of vortices has taken place. This trend implies that the merging process does not occur at a fixed location.

Bluish-White Luminescence in Rare-Earth-Free Vanadate Garnet Phosphors: Structural Characterization of LiCa3MV3O12 (M = Zn and Mg).

Extensive attention has been focused toward studies on inexpensive and rare-earth-free garnet-structure vanadate phosphors, which do not have a low optical absorption due to the luminescence color being easily controlled by its high composition flexibility. However, bluish emission phosphors with a high quantum efficiency have not been found until now. In this study, we successfully discovered bluish-white emitting, garnet structure-based LiCa3MV3O12 (M = Zn and Mg) phosphors with a high quantum efficiency, and the detailed crystal structure was refined by the Rietveld analysis technique. These phosphors exhibit a broad-band emission spectra peak at 481 nm under near UV-light excitation at 341 nm, indicating no clear difference in the emission and excitation spectra. A very compact tetrahedral [VO4] unit is observed in the LiCa3MV3O12 (M = Zn and Mg) phosphors, which is not seen in other conventional garnet compounds, and generates a bluish-white emission. In addition, these phosphors exhibit high quantum efficiencies of 40.1% (M = Zn) and 44.0% (M = Mg), respectively. Therefore, these vanadate garnet phosphors can provide a new blue color source for LED devices.

Pulsed UV laser-induced modifications in optical and structural characteristics of alpha-irradiated PM-355 SSNTD

Pre- and postalpha-exposed PM-355 detectors were irradiated using UV laser with different number of pulses (100, 150, 200, 300, and 400). UV laser beam energy of 20mJ per pulse with a pulse width of 9ns was incident on an area of 19.6mm2 of the samples. XRD spectra indicated that for both reference and UV-irradiated samples, the structure is amorphous, but the crystallite size increases upon UV irradiation. The same results were obtained from SEM analysis. Optical properties of PM-355 polymeric solid-state nuclear track detectors were also investigated. Absorbance measurements for all PM-355 samples in the range of 200–400nm showed that the absorption edge had a blue shift up to a certain value, and then, it had an oscillating behavior. Photoluminescence spectra of PM-355 at 250nm revealed a decrease in the broadband peak intensity as a function of the number of UV pulses, while the wavelengths corresponding to the peaks had random shifts.