Excitation Intensity Research Articles

Enhanced temperature sensing properties in LiYGeO4: Eu3+ phosphors via Bi3+ doping

Luminescent thermometry has been irreplaceable role in non-contact temperature sensing. However, the relative sensitivity (Sr) in some rare earths doped fluoride up-conversion materials still limited below 1% K−1. Herein, Eu3+ doping LiYGeO4 (LYGO) phosphors enabled ratio-metric thermometer based on thermal enhancement of Eu−O charge transfer band (CTB) and thermal quenching of 4f-4f transition. The Sr was achieved 0.64% K−1 in LYGO: Eu3+ at 313 K. Furthermore, we presented a temperature-responsive phosphor through Bi3+/Eu3+ co-doped LYGO, the excitation intensities of Bi3+ ions decreased significantly with rising temperature, which realized more than twice of magnitude changes in the Sr (1.76% K−1). In addition, LYGO: Eu3+ phosphors also featured excellent red emission and high thermal stability. All the results indicated that LYGO: Eu3+ and LYGO: Eu3+, Bi3+ phosphors can be potentially candidate for white light-emitting diode (WLED) and temperature sensing.

Sensitive Luminescence Thermometry through Excitation Intensity Ratio in Eu-Doped BaTiO3.

Optical ratiometric thermometry techniques have gained much attention in recent years due to their reliable and noncontact temperature sensing capability for industrial and biorelated applications. Herein, we exploited the temperature dependence of the absorption band of BaTiO3 (BTO) for novel excitation intensity ratio (EIR) thermometry. Photoluminescence and excitation properties of Eu3+-doped BTO powders were studied as a function of Eu3+ doping concentration. The excitation peak intensities at 397 and 468 nm, corresponding to the 7F0 → 5L6 and 5D2 transitions of Eu3+, were used as EIR parameters. The temperature dependence of the EIR can be explained by the competitive absorption between Eu3+ and the BTO host. The EIR properties were studied in relation to the doping concentration, registering a maximum relative sensitivity (Sr) of 4.89% K-1 in BTO:Eu3+ (0.5%) at 303 K. An amphoteric Eu3+ occupation mode at both Ba2+ and Ti4+ sites was found to interpret the doping concentration dependence of the Sr. The reduced Ba2+ site occupation ratio proved to be responsible for the low Sr values at high Eu3+ doping concentrations. Accordingly, an Eu3+/Ti3+ codoping method was further proposed to improve the Sr by increasing the Ba2+ site occupation ratio. Our result showed that BTO:Eu3+ (0.5%) demonstrated an enhancement of Sr from 4.89 to 6.42% K-1 at 303 K after 2% Ti3+ codoping.

Indexes for evaluation of dynamic characteristics of pressure-sensitive paint based on pressure sensitivity and frequency response

Two evaluation indexes were proposed to select the optimal PSP for unsteady pressure measurement from various PSPs with different pressure sensitivity and frequency response. An effective sensitivity coefficient calculated by pressure sensitivity and gain attenuation due to the response delay was proposed. Furthermore, an effective amount of intensity change was recommended, which takes into account the emission intensity and the effective sensitivity coefficient, because the magnitude of the intensity of a PSP is also important for unsteady-pressure measurement with high-speed sampling. A total of five types of PC-PSPs developed in previous studies were compared: two types of fast-response PC-PSPs using Pt(II) meso-tetra (pentafluorophenyl) porphine (PtTFPP) with poly(isobutyl methacrylate) (poly(IBM)) and ruthenium complex with RTV silicone, respectively, and three types of PC-PSPs using PtTFPP with poly[1-trimethylsilyl)-1-propyne] (poly(TMSP)). A comparison was made using the proposed evaluation indexes under various pressure ranges. The results shows that poly(TMSP)-based PC-PSP has a high effective sensitivity coefficient at pressures less than 20 kPa. On the other hand, poly(IBM)-based PC-PSP has the highest effective sensitivity coefficient at a pressure of 100 kPa. The effective amount of intensity change of poly(TMSP)-based PC-PSP is the highest at 2 kPa, but that of poly(IBM)-based PC-PSP is the highest at a pressure higher than 5 kPa among the evaluated PC-PSPs due to its high luminescence intensity. A PSP with high emission intensity will provide high performance in terms of fluctuation of emission intensity detected by the photodetector when the excitation intensity and the exposure time are limited due to limitations of optical equipment or high-speed sampling.

Giant Parametric Amplification of the Inverse Cotton–Mouton Effect in Antiferromagnetic Crystals

The interaction of light with spins in a Heisenberg antiferromagnet with a negligibly low magnetic anisotropy as in RbMnF3 has been theoretically analyzed. In particular, an optical pump–probe experiment, where coherent terahertz magnons are excited by short pump laser pulses in the antiferromagnet and are then detected by a probe pulse delayed with respect to the pump pulse, has been simulated. It has been shown that the giant increase in the intensity of excitation of terahertz magnons under the influence of the inverse Cotton–Mouton effect occurs near parametric resonance, i.e., in the parametric instability region.

Hysteresis measurement and generic modeling of magnetostrictive materials under high-frequency excitation and high-intensity bias field

The applied DC (direct-current) bias fields can cause the changes of hysteresis and loss properties of magnetostrictive alloys materials, which further affect the output characteristics of electromagnetic devices. This study investigates the impact of high-frequency AC (alternating-current) excitation and high intensity DC bias magnetic field on hysteresis characterization for different magnetostrictive materials. To obtain the hysteresis properties of materials, a generic dynamic magnetic measured system is established, which can test the low-permeability magnetic materials with a DC bias strength range of 0–20000 A/m at AC excitation frequency range of 1–9 kHz. The tested results show that the Terfenol-D, Fe83Ga17, and Fe30Co70 exhibit different DC bias sensitivities, which cause the largest deviation (31.68 %) of losses compared with unapplied DC bias field. To better predict these changes, a generic high-frequency dynamic Jiles-Atherton (J-A) model with the coupling AC excitation and DC bias field is proposed. The AC excitation and DC bias related terms are introduced to characterize distortion and ellipticity phenomenon for these hysteresis loops. A combined method of Simulated Annealing Particle Swarm Optimization-Linear Decreasing Inertia Weight (SA-PSO-LDIW) algorithm and Neural Network (NN) are proposed to identify the introduced relevant parameters. This paper solves the problem of high-frequency hysteresis measurement and loss calculation for low permeability and high electrical impedance magnetostrictive rods-shaped material which can help guide the high frequency application for magnetostrictive materials.

Flexible Organic Chiral Crystals with Thermal and Excitation Modulation of the Emission for Information Transmission, Writing, and Storage

AbstractOrganic single crystals quickly emerge as dense yet light and nearly defect‐free media for emissive elements. Integration of functionalities and control over the emissive properties is currently being explored for a wide range of these materials to benchmark their performance against organic emissive materials diluted in powders or films. Here, we report mechanically flexible emissive chiral organic crystals capable of an unprecedented combination of fast, reversible, and low‐fatigue responses. UV‐excited single crystals of both enantiomers of the material, 4‐chloro‐2‐(((1‐phenylidene)imino)methyl)phenol, exhibit a drastic yet reversible change in the emission color from green to orange‐yellow within a second and can be cycled at least 2000 times. The photoresponse was found to depend strongly on the excitation intensity and temperature. Combining chirality, mechanical compliance, rapid emission switching, multiple responses, and writability by UV light, this material provides a unique and versatile platform for developing organic crystal‐based materials for on‐demand signal transfer, information storage, and cryptography.

Flexible Organic Chiral Crystals with Thermal and Excitation Modulation of the Emission for Information Transmission, Writing, and Storage.

Organic single crystals quickly emerge as dense yet light and nearly defect-free media for emissive elements. Integration of functionalities and control over the emissive properties is currently being explored for a wide range of these materials to benchmark their performance against organic emissive materials diluted in powders or films. Here, we report mechanically flexible emissive chiral organic crystals capable of an unprecedented combination of fast, reversible, and low-fatigue responses. UV-excited single crystals of both enantiomers of the material, 4-chloro-2-(((1-phenylidene)imino)methyl)phenol, exhibit a drastic yet reversible change in the emission color from green to orange-yellow within a second and can be cycled at least 2000 times. The photoresponse was found to depend strongly on the excitation intensity and temperature. Combining chirality, mechanical compliance, rapid emission switching, multiple responses, and writability by UV light, this material provides a unique and versatile platform for developing organic crystal-based materials for on-demand signal transfer, information storage, and cryptography.

Evaluating the Impact of the Tamm-Dancoff Approximation on X-ray Spectrum Calculations.

The impact of the Tamm-Dancoff approximation (TDA) for time-dependent density functional theory (TDDFT) calculations of X-ray absorption and X-ray emission spectra (XAS and XES) is investigated, showing small discrepancies in the excitation energies and intensities. Through explicit diagonalization of the TDDFT Hessian, XES was considered by using full TDDFT with a core-hole reference state. This has previously not been possible with most TDDFT implementations as a result of the presence of negative eigenvalues. Furthermore, a core-valence separation (CVS) scheme for XES is presented, in which only elements including the core-hole are considered, resulting in a small Hessian with the dimension of the number of remaining occupied orbitals of the same spin as the core-hole (CH). The resulting spectra are in surprisingly good agreement with the full-space counterpart, illustrating the weak coupling between the valence-valence and valence-CH transitions. Complications resulting from contributions from the discretized continuum are discussed, which can occur for TDDFT calculations of XAS and XES and for TDA calculations of XAS. In conclusion, we recommend that TDA be used when calculating X-ray emission spectra, and either CVS-TDA or CVS-TDDFT can be used for X-ray absorption spectra.

Structured Excitation Energy Transfer: Tracking Exciton Diffusion below Sunlight Intensity.

With the increasing demand for new materials for light-harvesting applications, spatiotemporal microscopy techniques are receiving increasing attention as they allow direct observation of the nanoscale diffusion of excitons. However, the use of pulsed and tightly focused laser beams generates light intensities far above those expected under sunlight illumination, leading to photodamage and nonlinear effects that seriously limit the accuracy and applicability of these techniques, especially in biological or atomically thin materials. In this work, we present a novel spatiotemporal microscopy technique that exploits structured excitation in order to dramatically decrease the excitation intensity, up to 10,000-fold when compared with previously reported spatiotemporal photoluminescence microscopy experiments. We tested our method in two different systems, reporting the first exciton diffusion measurement at illumination conditions below sunlight, both considering average power and peak exciton densities in an organic photovoltaic sample (Y6), where we tracked the excitons for up to five recombination lifetimes. Next, nanometer-scale energy transport was directly observed for the first time in both space and time in a printed monolayer of the light-harvesting complex 2 from purple bacteria.

Composition dependence of electronic defects in CuGaS2

AbstractCuGaS2 films grown by physical vapor deposition were studied by photoluminescence (PL) spectroscopy, using excitation intensity and temperature‐dependent analyses. We observed free and bound exciton recombinations, three donor‐to‐acceptor (DA) transitions, and deep‐level transitions. The DA transitions at ~2.41, 2.398, and ~2.29 eV are attributed to a common donor level ~35 meV and two shallow acceptors at ~75 and ~90 meV and a deeper acceptor at 210 meV above the valence band. This electronic structure is similar to those of other chalcopyrite materials. The observed DA transitions are accompanied by several phonon replicas. The Cu‐rich and near‐stoichiometric CuGaS2 films are dominated by transitions involving the acceptor at 210 meV. All films show deep‐level transitions at ~2.15 and 1.85 eV due to broad deep defect bands. The slightly Cu‐deficient films were dominated by intense transitions at ~2.45 eV, which were attributed to excitonic transitions, and a broad defect transition at 2.15 eV.

Cooling-excited infrared thermography for enhancing the detection of concrete filled steel tube interfacial debonding at concrete hydration

Interfacial Debonding occurring in concrete-filled steel Tube (CFST) arch bridges during construction is a critical issue, which can reduce the CFST carrying capacity and thus degrade the operational lifespan of the bridge. Timely detection of this type of defect during bridge construction can be highly cost-effective but rare sensing technology was reported for detection at this stage. Infrared thermography has been recognized as a potential detection method but still faces the challenge of low thermal contrast developed from concrete hydration. This research investigates the feasibility of using hydration heat as an internal heat source and proposes water-spray cooling as an external excitation to improve infrared debonding detection. The experimental study is carried out to investigate the detectability enhancement before and after the cooling excitation in terms of different debonding sizes and thicknesses. An image enhancement method is then proposed for debonding visualization based on the temperature difference matrix. In addition, the numerical simulation is conducted to analyze the cooling effect regarding the excitation intensity variation. The findings reveal that the thermal contrast of debonding ranges from 0.1 to 0.35℃ before cooling excitation and is enhanced by 2–3 times thereafter. In addition, the developed maximum thermal contrast of debonding can be characterized through a linear relationship to the cooling excitation intensity based on numerical analysis. The proposed method shows significant feasibility for early detection of debonding in CFST during arch bridge construction, which enables a new potential for structural inspection.

Formation of hollow silver nanoparticles under irradiation with ultrashort laser pulses

Abstract We have studied the formation of cavities in spherical silver nanoparticles embedded in silica, irradiated with fs laser pulses that produce an intense electronic excitation. Experimentally determined aspect ratio, i.e. the ratio between the cavity and nanoparticle size, for hollow structures formed under different irradiation conditions shows a very good agreement with values obtained by means of atomistic simulations. According to the predictions of the atomistic model, one can produce at will hollow silver nanoparticles with cavities of tailored dimensions, having an accurate control. Hence, laser irradiation can be used to control and design the optical response by tuning the localized surface plasmon resonances of the hollow nanoparticles.

Probing cell metabolism using the two-photon excitation autofluorescence lifetime of tryptophan.

Compared to intensity detection, fluorescence lifetime has the advantage of being unaffected by variations in excitation intensity, fluorophore concentration, or attenuation due to biological absorption and scattering. In this Letter, to the best of our knowledge, we present the use of the two-photon excitation autofluorescence lifetime imaging of tryptophan (TRP) to probe cell metabolism for the first time. Tests of pure chemical samples showed that the fluorescence lifetime of TRP was highly sensitive to changes in molecular conformation and the environment. In in vitro cell experiments, we successfully utilized the fluorescence lifetime of TRP to distinguish tumor cells from healthy cells, track the therapeutic effect of the tumor immunotherapy drug 1-MT for HeLa cells, and monitor cells in response to carbonyl cyanide 3-chlorophenylhydrazone (CCCP)-induced cell apoptosis. These results reveal that the two-photon excitation autofluorescence lifetime of TRP could be a sensitive natural probe of cell metabolism in living cells.

Synthesis, crystal structure and photoluminescence properties of Mn4+ activated K2WO2F4·H2O

Oxyfluorides activated with Mn4+ have great potential in phosphors converted LED. Phosphors with intensive zero phonon line (ZPL) emission are expected to improve the performance of the light sources. Microcrystalline powders of oxyfluoride K2WO2F4·H2O activated with Mn4+ were synthesized by using a coprecipitation method. The structural parameters and atomic coordinations of K2WO2F4·H2O were determined by both refinement and calculation. K2WO2F4·H2O crystallizes in the monoclinic crystal system with C2/m group space. The unit cell could be considered as a stack of KWO2F4 slices with another K+ ion and H2O molecule in the interstitial. The morphology and photoluminescence properties of K2WO2F4·H2O: Mn4+ were studied as a function of Mn4+ concentration. It is revealed that the microcrystalline shows highly orientational preferred growth. Mn4+ shows bright red emission upon the excitation of blue light. The emission is due to the 2Eg→4A2g transition. Furthermore, the photoluminescence features intensive ZPL emission and short excitation state lifetime, which is ascribed to the highly distorted coordination environment. The effects of octahedron distortion and the second coordination sphere on the ZPL emission intensity are discussed.

Hot luminescence of two-dimensional electron hole systems in modulation-doped silicon

Modulation doping of silicon has great potential for miniaturization, surface passivation, and third generation photovoltaics (PV). At a modulation-doped silicon surface, we observe the formation of a 2D hole layer at the silicon surface at low temperatures by means of photoluminescence (PL) measurements. A line shape analysis of band–band and hot luminescence reveals the hole density (which is equal to the modulation-doped acceptor density). A high excitation intensity leads to a Fermi edge singularity of the band–band and hot PL emission. While the 2D layer can be characterized by the observed luminescence, the spectral region of twice the bandgap is fully dominated by emission from this surface layer, impeding the measurement of bulk hot luminescence, e.g., from Auger electrons or from nonthermalized carriers in a hot carrier PV device.

An All‐Colloidal and Eco‐Friendly Quantum‐Dot Laser

AbstractColloidal zinc‐chalcogenide quantum dots (QDs) are emerging as the promising heavy‐metal‐free light–emitters, however, the development of light amplification and lasing therein remains challenging. Here, corroborated by comprehensive in situ and transient spectroscopy, it is unraveled that the ultrafast photooxidization‐induced trapping process (≈2 ps) and the sub‐bandgap photoinduced absorption under intense excitation are the main hindrances to light amplification in ZnSeTe QDs. Upon tackling the hurdle by heterostructure engineering, this study demonstrates that the ZnSeTe QDs exhibit intrinsically superior gain performance, including the high gain cross‐section (5.6 × 10−16 cm2), long gain lifetime (615 ps), low pump threshold (≈25.9 µJ cm−2) and high fatigue resistance. On this basis, a microfluidic channel is designed to mass‐produce the micellar cavities whose surfaces are treated to combine with the ZnSeTe QDs. The active QD‐micelle hybrids show state‐of‐the‐art lasing performance and allow the fiber‐based micro‐manipulation toward the single‐mode lasers. These findings not only provide essential knowledge on the gain physics in ZnSeTe QDs, but also open a new avenue to develop high‐performance eco‐friendly lasers.

Dynamic Characteristics of Bistable Vibration Energy Harvester Incorporating Electromagnetic Generator

Although various vibration energy harvesters have been designed over the past few decades, efforts to develop efficient, broadband energy harvesters continue. This work provides a detailed insight into a bistable vibration harvester subjected to correlated Gaussian white noise, with the friction between the rack and pinion described by the slip-stick model. Using the harmonic balance method, the frequency response curve of the amplitude under different mass ratios is discussed. The system response will be enhanced with an increased mass ratio for sinusoidal excitation, but not in the case of random excitation. By employing the stochastic average of the energy envelope, the dynamical governing equation of the harvester is solved, and the probability density functions (PDFs) under different damping coefficients, nonlinear stiffness of the restoring force, and excitation intensities are derived. The results are compared with those from Monte Carlo simulations (MCS) and show good accuracy. The results reveal the presence of P-bifurcations. When the nonlinear stiffness and damping coefficient vary, the number of peaks in the PDFs of system displacement and velocity changes. By adjusting the system parameters, the motion of the system can be significantly enhanced.

Multiphoton photocurrent in wide bandgap semiconductors for nonlinear optoelectronics: Comparison of GaP, GaN/InGaN, and SiC

Wide bandgap semiconductors are ideally suited for nonlinear optoelectronics. Because their bandgaps are larger than 2 eV, simultaneous absorption of two or more near-infrared photons is necessary to excite the electrons from the valence to the conduction band. Understanding of the processes that affect multiphoton absorption is important in the design and fabrication of optoelectronic devices. Here, we present an overview of the photocurrent response in photodetectors made from GaP, GaN, InGaN, and SiC when they are excited by photons at 1.2 eV. Recent measurements have shown that sub-bandgap absorptions contribute to photocurrent in GaP, and, thus, it is not a good material for nonlinear optoelectronics. Similarly, the response of GaN is affected by long-lived trapped charges. Photocurrents in InGaN and SiC are predominantly from three- and four-photon absorption, respectively. Moreover, these materials can withstand excitation intensities higher than 1011 W cm−2, making them appropriate platforms for nonlinear optoelectronics.

Giant enhancement of third harmonic generation in an array of graphene ribbons using amplification of surface plasmon polaritons by optical gain

In this paper, we theoretically study the enhancement of third-harmonic generation in a plasmonic structure composed of an array of trilayer graphene ribbons sandwiched between two CaF2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$CaF_{2}$$\\end{document} layers. In fact, we suggest a new method for more enhancement of nonlinearity in plasmonic structures using incorporation of optical gain into graphene ribbons. As the pump intensity increases, the maximum output intensity of third harmonic generated (THG) wave versus fundamental frequency is blue-shifted while its value enhances. Our analysis indicates that the enhancement factor of THG in our proposed structure is 1.1 × 107 without occurring an electric breakdown compared to case at which an optically pumped trilayer graphene sheet sandwiched between two CaF2 layers. Therefore, only presence of optical gain is not sufficient for significant enhancement of output intensity of THG wave and excitation of SPPs through the structure is also essential. On the other hand, our results demonstrate that the output intensity of THG wave from the proposed structure under optical pumping enhances by 105\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^{5}$$\\end{document} times compared to the plasmonic structure without optical gain which confirms the role of optical gain for THG enhancement in the plasmonic structure. This is because the gain in graphene ribbons amplifies the SPPs waves leading to the more field enhancement along the graphene ribbons which results in significant enhancement of THG wave in the plasmonic structure in comparison with one without gain. Therefore, we reveal that both SPPs and optical gain contribute to the strong output intensity of THG in our proposed structure compared to the trilayer graphene sheet inserted between two CaF2 layers.

Ligand Desorption and Fragmentation in Oleate-Capped CdSe Nanocrystals under High-Intensity Photoexcitation.

Semiconductor nanocrystals (NCs) offer prospective use as active optical elements in photovoltaics, light-emitting diodes, lasers, and photocatalysts due to their tunable optical absorption and emission properties, high stability, and scalable solution processing, as well as compatibility with additive manufacturing routes. Over the course of experiments, during device fabrication, or while in use commercially, these materials are often subjected to intense or prolonged electronic excitation and high carrier densities. The influence of such conditions on ligand integrity and binding remains underexplored. Here, we expose CdSe NCs to laser excitation and monitor changes in oleate that is covalently attached to the NC surface using nuclear magnetic resonance as a function of time and laser intensity. Higher photon doses cause increased rates of ligand loss from the particles, with upward of 50% total ligand desorption measured for the longest, most intense excitation. Surprisingly, for a range of excitation intensities, fragmentation of the oleate is detected and occurs concomitantly with formation of aldehydes, terminal alkenes, H2, and water. After illumination, NC size, shape, and bandgap remain constant although low-energy absorption features (Urbach tails) develop in some samples, indicating formation of substantial trap states. The observed reaction chemistry, which here occurs with low photon to chemical conversion efficiency, suggests that ligand reactivity may require examination for improved NC dispersion stability but can also be manipulated to yield desired photocatalytically accessed chemical species.