Range Of Excitation Intensities Research Articles

Photoluminescence of CaGa2S4: Nd, Yb compound at room temperature

In this paper, CaGa2S4:3%Nd,5%Yb; CaGa2S4:5%Yb; CaGa2S4:3%Nd compounds were synthesized, and their photoluminescence (PL) spectra, PL excitation (PLE) spectra and PL decay properties were studied at room temperature under one-photon and multi-photon excitation in a wide range of excitation intensities. The synthesis of the CaGa2S4 compound was carried out by a solid-phase reaction of a stoichiometric mixture of CaS and Ga2S3 powders in a quartz ampoule. Yb and Nd doping was carried out during the synthesis process using YbF3 and NdF3. PLE spectra were studied in the wavelength range from 250 to 850 nm, and PL spectra in the range of 400–1500 nm. It has been shown that after introducing Yb3+ ions into thiogallate crystals with Nd3+ ions, a new band near 488 nm is formed and the PL intensity increases by 1–2 orders of magnitude. The anti-Stokes PL spectra of CaGa2S4: Nd, Yb crystals in the visible region of the spectrum (400–700 nm) were studied when excited by continuous wave laser radiation with a wavelength of 975 nm. The kinetics of PL decay of all types of compounds at different wavelengths of single- and multi-photon excitation were determined. Depending on the PLE conditions, wavelength and intensity of the exciting radiation, monoexponential and non-monoexponential decay kinetics were observed with decay times in the range of 20–90 μs. Possible mechanisms of PLE and PL of CaGa2S4 crystals doped with neodymium and ytterbium ions are discussed.

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.

Data Driven prediction of forced nonlinear vibrations using stabilised Autoregressive Neural Networks

AbstractIn this work, we propose a novel approach to the data‐driven prediction of vibration responses of nonlinear systems. The main idea is based on Autoregressive Neural Networks (ARNN) to model the nonlinear transfer behaviour between an external excitation and the system response. We propose an autoregressive network architecture with embedded symmetry using bias‐free tanh activation and guarantee Input‐to‐State‐Stability (ISS) by enforcing a special penalty term to the weights. The resulting training procedure is analysed for the example of a DUFFING oscillator with white noise excitation. In a BAYESian optimisation, it is found that beyond enforcing input‐to‐state‐stability, the stabilising penalty term also decreases sensitivity with respect to other training parameters compared to other classical techniques. Furthermore, we show that the stabilised ARNN is able to give excellent approximations of the nonlinear response of the DUFFING oscillator for a wide range of excitation intensities. In contrast, linear models, such as autoregressive models with exogenous input (ARX) in time domain or linear transfer functions in frequency domain, will only find some linear approximation. In particular, by construction, they will not be able to capture nonlinear effects for arbitrary amplitudes and excitation levels.

Construction of motional phase maps for granular dampers

Harmonically vibrated granular media exhibit a variety of motional behaviours depending on amplitude, frequency, and vibration-to-gravity directional orientation. Motional behaviour defines the physical interactions of particles in the granular media and therefore the energy dissipation performance. A "phase map" that describes motional behaviour over broad ranges of frequency and amplitude is therefore a very useful tool in damper design. However, at present, identification of the operating motional conditions within the granular media has only been conducted by visual observation of the particles following a particle-level simulation or a specifically designed experiment. Because of this, design optimisation over a broad range of amplitude and frequency becomes costly. This paper aims to help reduce this cost through the development of approximate phase maps based on expected dissipative interactions of particles. Three-dimensional discrete element method simulations are conducted over a wide range of excitation intensities under two different vibration-to-gravity directional orientations (i.e., perpendicular, and parallel to the standard gravity direction) to allow the observation of as many motional phases as possible. The effect of particle size, volume filling ratio and particle shape on granular energy dissipation sources are also investigated.

Design of robust particle dampers using inner structures and coated container walls

Classical particle dampers suffer from their non-robust damping behavior, i.e. they can only be efficiently applied to a specific frequency range and amplitude range. The reason for that is that particle motion, also called motion mode, and damper efficiency show a strong correlation. By changing particle or container properties the motion modes are shifted to other excitation conditions but their efficient range is not much affected. To increase the damping performance and robustness of particle dampers, two approaches are presented here by introducing new motion modes. Therefore, the particle dampers are analyzed experimentally using a shaker setup and numerically using the discrete element method. The first design approach uses inner structures inside the particle damper, manufactured by a 3D printer. The inner structures consist of different numbers of beams, placed perpendicular to the container moving direction. They lead to a much more robust damper as the transition between the motion modes gets smoother. For the second approach, the container walls are equipped with different soft polymers. In this way a new motion mode at low excitation intensities is observed, leading to a high efficiency possibly on a large excitation intensity range. For an easy calculation of the necessary wall’s Young’s modulus an analytical formula based on Hertz impact theory is derived.

Pendulum vibration absorber with a nonlinear viscous damping mechanism

SummarySimple pendulum vibration absorbers, as one of the most common models of absorbers, experience nonlinear behaviors in the large amplitude of oscillations, and under such conditions, they are not effective enough and are more prone to lose their stability. To overcome some of these drawbacks, a complementary damping mechanism including two nonlinear elements is introduced. Alongside the ordinary linear viscous damper, a damping mechanism is also utilized to control the oscillations of the dynamic structure exposed to the excitation forces more intense than the ones considered in the optimization. In other words, they make this system more robust and stable in the face of excitations stronger than the presupposed conditions. The complementary elements of this mechanism are proposed to offer an appropriate approximation of various likely models which can be used as practical nonlinear damping mechanisms. The steady‐state solutions of the nonlinear governing equations are achieved with pinpoint precision with the help of the harmonic balance method and more precise approximations of trigonometric functions. To enhance the impacts of this mechanism, optimization is performed for a compound objective function in addition to the conventional one. This study explains how such mechanisms are capable of contributing to the higher robustness and stability of pendulum vibration absorbers under a wider range of excitation intensities. The performance of this absorber is also studied for an N‐story shear structure as a multidegree of freedom (MDOF) system.

Nonlinear optical characteristics of non-covalently functionalised graphene-pyrene-conjugated phthalocyanine hybrids

Improving the non-linear absorption properties of graphene materials is valuable for optical limiting applications. We report the non-linear optical characteristics of new hybrid materials formed by the non-covalent functionalisation of graphene nanoplatelets (GNs) with pyrene-conjugated Zn(II) phthalocyanines and pyrene-conjugated Zn(II) azaphthalocyanines. The GN–pyrene-conjugated Zn(II) phthalocyanine hybrid (GPc) shows a promising change from the two-photon absorption behaviour of GNs to three-photon absorption, the first such example in a graphene based hybrid system under nanosecond pulse excitation. Fluorescence and transient absorption spectroscopies are employed to describe the non-linear optical phenomena of the prepared hybrid materials. The GPc hybrid exhibits a low optical limiting threshold induced by three-photon absorption and a favourable linear transmittance (above 90%). The GN–pyrene-conjugated azaphthalocyanine hybrid (GAzaPc) also shows promising non-linear optical coefficients. A unique optical limiting performance (80% reduction in intensity) is achieved throughout the experimental range of excitation intensities. Our research offers a way to design hybrid material structures with a large effective three-photon absorption coefficient and nearly constant optical limitation across a wide intensity range with 92% linear transmittance, which would be highly useful in non-linear photonic applications.

Charge-carrier photogeneration and extraction dynamics of polymer solar cells probed by a transient photocurrent nearby the regime of the space charge-limited current

To understand the complex behaviors of photogenerated charge carriers within polymer-based bulk-heterojunction-type solar cells, the charge-carrier photogeneration and extraction dynamics are simultaneously estimated using a transient photocurrent technique under various external-bias voltages, and a wide range of excitation intensities are analyzed. For this purpose, conventional devices with 80 nm thick active layers consisting of a blend of representative P3HT and PTB7 electron-donating polymers and proper electron-accepting fullerene derivatives were used. After the correction for the saturation behavior at a high excitation-intensity range nearby the regime of the space charge-limited current, the incident-photon-density-dependent maximum photocurrent densities at the initial peaks are discussed as the proportional measures of the charge-carrier-photogeneration facility. By comparing the total number of the extracted charge carriers to the total number of the incident photons and the number of the initially photogenerated charge carriers, the external quantum efficiencies as well as the extraction quantum efficiencies of the charge-carrier collection during a laser-pulse-induced transient photocurrent process were obtained. Subsequently, the charge-carrier concentration-dependent mobility values were obtained, and they are discussed in consideration of the additional influences of the charge-carrier losses from the device during the charge-carrier extraction that also affects the photocurrent-trace shape.

Energy transfer, tunable photoluminescence of Sr3Lu(PO4)3:Tb3+, Eu3+ phosphors

Series of Tb3+ or Eu3+ singly doped and Tb3+/Eu3+ co-doped Sr3Lu(PO4)3 phosphors were prepared by the high temperature solid reaction method. Compared with the excitation spectra of Tb3+ singly doped samples, the effective excitation range of Tb3+/Eu3+ co-doped samples is extended in the near UV region, and the excitation intensity in range 330–380 nm is enhanced. The existence of energy transfer from Tb3+ to Eu3+ ions was confirmed by photoluminescence spectra and lifetime measurements. Under the excitation of 377 nm, the characteristic emission peaks of Tb3+ and Eu3+ ions were both observed, and the ratio of green and red emission components was changed by increasing the concentration of Eu3+. Hence, the emission color of samples Sr3Lu1-x(PO4)3:0.08Tb3+, xEu3+ (x = 0, 0.02, 0.04, 0.06, 0.08 and 0.10) can be turned from green to red region.

Strong Plasmon Enhancement of the Saturation Photon Count Rate of Single Molecules.

Plasmon resonances have appeared as a promising method to boost the fluorescence intensity of single emitters. However, because research has focused on the enhancement at low excitation intensity, little is known about plasmon–fluorophore coupling near the point where the dye saturates. Here we study plasmon-enhanced fluorescence at a broad range of excitation intensities up to saturation. We adopt a novel DNA-mediated approach wherein dynamic single-molecule binding provides a controlled particle–fluorophore spacing, and dynamic rebinding circumvents artifacts due to photobleaching. We find that near saturation the maximum photon count rate is enhanced by more than 2 orders of magnitude at the optimal particle–fluorophore spacing, even for a dye with a high intrinsic quantum yield. We compare our results to a numerical model taking into account dye saturation. These experiments provide design rules to maximize the photon output of single emitters, which will open the door to studying fast dynamics in real time using single-molecule fluorescence.

Lifetime spectroscopy with high spatial resolution based on temperature- and injection dependent photoluminescence imaging

In this paper we present a method for performing temperature- and injection dependent lifetime spectroscopy (TIDLS) with high spatial resolution based on steady state photoluminescence (PL) images taken at a range of excitation intensities and temperatures. The PL lifetime images are calibrated based on temperature dependent photoconductance measurements, thus requiring a minimum of assumptions regarding the temperature dependency of the luminescence signal and detection system. PL image acquisition at varying conditions and subsequent data analysis is automated, allowing for investigations of different samples without the need for excessive operator time. We demonstrate the method by presenting lifetime data and TIDLS analysis from commercial HPMC-Si samples, highlighting the local differences in the recombination properties for high lifetime in-grain areas and lower lifetime areas dominated by dislocation clusters. These regions have been investigated for two etched and passivated neighboring wafers, one in the as-cut state and one where the bulk lifetime had been improved through key solar cell process steps. For the as-cut wafer we find two dominant defects in the low lifetime area, one of which is identified as FeB pairs, with Et = 0.90 ± 0.01 (above the valence band) and k = 0.4 ± 0.1. In the in-grain region of this wafer we also observe two dominating defects, one with an energy level of either 0.28 ± 0.01 eV or 0.74 ± 0.01 eV above the valence band and a k value of 25 ± 1. The other defect is shallow, within ~0.27 eV of the band edge, and could not be further identified with the chosen temperature range. For the wafer that had passed through solar cell processing, we find a clear, overall improvement in the lifetime and a change in the type of dominating defects: We observe that a single, deep defect with 0.35 eV < Et < 0.7 eV and a capture cross section ratio k = 10 ± 1 can fully describe the observed lifetime data in the in-grain area after solar cell processing, whereas two defects are needed to describe the behavior of the lower lifetime regions. One of these defects is also a deep defect with 0.34 eV < Et < 0.69 eV but with a higher k of 25 ± 2, and is thus similar to one of the defects observed in the in-grain region of the as-cut wafer. The other is a shallow defect with Et within 0.21 eV of the band edge. Finally, we show how the method can be used to show interesting new properties of the lifetime maps, including maps of the temperature coefficient of the lifetime, maps of the injection dependence of the lifetime and maps at a constant injection level.

Two charge states of the CN acceptor in GaN: Evidence from photoluminescence

We have found a photoluminescence (PL) band with unusual properties in GaN. The blue band, termed as the ${\mathrm{BL}}_{C}$ band, has a maximum at about 2.9 eV and an extremely short lifetime (shorter than 1 ns for a free-electron concentration of about ${10}^{18}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$). The electron- and hole-capture coefficients for this defect-related band are estimated as ${10}^{\ensuremath{-}9}$ and ${10}^{\ensuremath{-}10}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{3}/\mathrm{s}$, respectively. The ${\mathrm{BL}}_{C}$ band is observed only in GaN samples with relatively high concentration of carbon impurity, where the yellow luminescence (the YL1 band) with a maximum at 2.2 eV is the dominant defect-related PL. Both the YL1 and ${\mathrm{BL}}_{C}$ bands likely originate from the ${\mathrm{C}}_{\mathrm{N}}$ defect, namely, from electron transitions via the $\ensuremath{-}/0$ and $0/+$ thermodynamic transition levels of the ${\mathrm{C}}_{\mathrm{N}}$. The ${\mathrm{BL}}_{C}$ band appears only at high excitation intensities in $n$-type GaN samples codoped with Si and C, and it can be found in a wide range of excitation intensities in semi-insulating (presumably $p$-type) GaN samples doped with C. The properties and behavior of the YL1 and ${\mathrm{BL}}_{C}$ bands can be explained using phenomenological models and first-principles calculations.

Interlayer charge transfer in n-modulation doped Al1−xGaxAs–GaAs single heterostructures

We studied the interlayer charge transfer in n-modulation doped Al1−xGaxAs–GaAs single heterostructures by photoluminescence and magnetotransport. Photoluminescence contributions from a high-mobility quasi-twodimensional electron system at the interface and GaAs bulk type excitons were analyzed, covering an excitation intensity range of nearly three orders of magnitude. The experiment was compared with selfconsistent band structure calculations that allowed to follow the charge redistribution in the sample. After sample cool down in the dark a parallel conducting quasi-twodimensional channel of low electron mobility appeared in the Al1−xGaxAs-layer, that gained in density up to a saturation value. There was only little interaction between the Al1−xGaxAs and GaAs sides in the persistent regime and a reduction of the depletion charge in the GaAs could largely account for the interface channel density enhancement. Contrary, the negative photoeffect present during continuous illumination strongly depends on the interaction between the interface and the parallel channel, which serves as a temporary sink for excess charges from the GaAs side. When the illumination is switched off the parallel channel acts as a source and the sample essentially returns to the prior illumination state of saturation.

Excitation Intensity Dependence of Photoluminescence Blinking in CsPbBr3 Perovskite Nanocrystals

Perovskite semiconductors have emerged as a promising class of materials for optoelectronic applications. Their favorable device performances can be partly justified by the defect tolerance that originates from their electronic structure. The effect of this inherent defect tolerance, namely the absence of deep trap states, on the photoluminescence (PL) of perovskite nanocrystals (NCs) is currently not well understood. The PL emission of NCs fluctuates in time according to power law kinetics (PL intermittency, or blinking), a phenomenon that has been explored over the past two decades in a vast array of nanocrystal (NC) materials. The kinetics of the blinking process in perovskite NCs have not been widely explored. Here, PL trajectories of individual orthorhombic cesium lead bromide (CsPbBr3) perovskite NCs are measured using a range of excitation intensities. The power law kinetics of the bright NC state are observed to truncate exponentially at long durations, with a truncation time that decreases with i...

Harvesting performance of quad-stable piezoelectric energy harvester: Modeling and experiment

Abstract To overcome the defects of bi-stable energy harvester (BEH) and improve the energy conversion ability under weak stochastic excitation, we developed a novel quad-stable energy harvester (QEH). This configuration is composed of a piezoelectric cantilever beam with a tip magnet and three external fixed magnets. By adjusting positions of the fixed magnets and altering the distances between the tip and the fixed magnets, four stable equilibrium positions (SEPs) can be realized in the static state of QEH. The diagram of potential energy illustrates that QEH has shallower and wider potential wells than BEH at the same separation distance, implying that it can execute jumping across the potential barrier easily. Validation experiment was carried out and the experimental results showed that QEH could create larger deflection and generate higher output voltage than BEH nearly over the whole range of excitation intensity. Furthermore, by controlling the gap and separation distances, the QEH can be optimized to reach the maximum output power for a certain excitation intensity.

Photoluminescence in Organometal Halide Perovskites: Free Carrier Versus Exciton

The photoexcitation dynamics in CH3NH3PbI3 thin films has been studied by nanosecond time-resolved photoluminescence (PL) and transient photoinduced absorption. Under excitation intensity from ∼0.1 to ∼100 μJ/cm2, free carrier is the dominant photoexcitation species. However, in the intermediate excitation intensity from ∼1 to ∼10 μJ/cm2, a small fraction of photoexcitations are excitons, which, however, are responsible for PL within this excitation intensity range. The branching ratio between excitons and free carriers at 2 μJ/cm2 was estimated to be 17%. Furthermore, the spectral narrowing PL was found at the low temperature after the formation of excitons, indicating that laser action could be due to excitonic emission. Our findings provide fundamental insights into the optical properties of organometal halide perovskites with direct implications for optoelectronics.

High photoluminescence stability of CaGa4O7:Eu3+ red phosphor in wide excitation intensity interval

The photoluminescence (PL) of CaGa4O7 compound doped with Eu3+ ions is studied. It is shown that room temperature emission spectrum of CaGa4O7 compound consists of groups of lines in the red spectral region, caused by electronic transitions 5D0→7Fj(j = 1,2,3,4) in Eu3+ ions with the most intensive lines at 612nm and 615nm. It is evaluated that low local symmetry of Eu3+ ion appears in the presence of several lines for each transitions. The PL decay constants were found to be in the range of 1.1–1.3ms. Non-exponential rise was observed in the beginning of PL kinetics and assumed with energy transfer between Eu3+ ions in different sites in host lattice. Weak thermal quenching is shown in the temperature range of 10–300K. Extreme stability of PL spectra and efficiency in a wide excitation intensity range of 104–108W/cm2 were achieved. A slight emission efficiency reversible droop by only 25% is found to be at the excitation intensities from 2·107W/cm2 to 108W/cm2.

Bright single-mode random laser from a concentrated solution of π-conjugated polymers.

Using thin stripe excitation of a 10 ns pulsed laser, we observed robust and bright random laser (RL) emission in high concentrated solutions of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) and poly[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT). In particular, within the proper excitation intensity range, single line RL emission is observed in both solutions, with full width at half-maximum of 0.17 nm and 0.4 nm for MEH-PPV and PCPDTBT, respectively. The reason for the random laser is that the refractive index fluctuation due to the aggregation in concentrated solution results in spatially extended random lasing modes, which are amplified through traveling light along the interface between solution and cuvette. Our work provides a simple but effective method to achieve bright single-mode RLs, with conversion efficiency on the order of 10%.

Mechanism and dynamics of biexciton formation from a long-lived dark exciton in a CdTe quantum dot

We study the biexciton formation process in a single CdTe/ZnTe quantum dot. Consistently with previous studies, we find subquadratic dependence of the biexciton photoluminescence intensity on the excitation power. We quantitatively explain this dependence in terms of the interplay between two alternative biexciton formation mechanisms, including the formation from a long-lived dark exciton residing in the quantum dot. This mechanism is demonstrated to be a dominant one for a wide range of excitation intensities. Our study is complemented by determination of a characteristic biexciton formation time, which is shown to be governed by the spin relaxation dynamics of an excited electron pair.

Deep-level emission in ZnO nanowires and bulk crystals: Excitation-intensity dependence versus crystalline quality

The excitation-intensity dependence of the excitonic near-band-edge emission (NBE) and deep-level related emission (DLE) bands in ZnO nanowires and bulk crystals is studied, which show distinctly different power laws. The behavior can be well explained with a rate-equation model taking into account deep donor and acceptor levels with certain capture cross sections for electrons from the conduction band and different radiative lifetimes. In addition, a further crucial ingredient of this model is the background n-type doping concentration inherent in almost all ZnO single crystals. The interplay of the deep defects and the background free-electron concentration in the conduction band at room temperature reproduces the experimental results well over a wide range of excitation intensities (almost five orders of magnitude). The results demonstrate that for many ZnO bulk samples and nanostructures, the relative intensity R = INBE/IDLE can be adjusted over a wide range by varying the excitation intensity, thus, showing that R should not be taken as an indicator for the crystalline quality of ZnO samples unless absolute photoluminescence intensities under calibrated excitation conditions are compared. On the other hand, the results establish an all-optical technique to determine the relative doping levels in different ZnO samples by measuring the excitation-intensity dependence of the UV and visible luminescence bands.