Wide 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.

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.

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.

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.

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.

Evidence for Multiple Trapping Mechanisms in Single CdSe/ZnS Quantum Dots from Fluorescence Intermittency Measurements over a Wide Range of Excitation Intensities

Fluorescence intermittency of single CdSe/ZnS core/shell quantum dot particles is investigated over a wide range of excitation intensities and at two excitation wavelengths. Deviation from previously observed power law behavior in both the on- and off-duration probability distributions is observed at both excitation wavelengths, one near the band gap and one 350 meV above the band gap. Increasing the excitation intensity modifies the off-duration probability distribution such that the probability of long off-time events decreases, an effect that is observed to saturate at an average number of excitons created per pulse of one, ⟨N⟩ ≈ 1. The on-duration probability distribution is well-described by a power law for short on-time events, crossing over to an exponential distribution for long on-time events. Increasing the excitation intensity induces this crossover to occur at earlier times, saturating at ⟨N ⟩ ≈ 2. The different intensity-dependent trends for the on- and off-duration probability distributions ...

Correlation effect of Rabi oscillations of excitons in quantum dots

We performed a transient four-wave mixing experiment on a strain-compensated InAs quantum dot (QD) ensemble over a wide range of excitation intensities. Under the resonant excitation of an exciton ground state, an extremely long dephasing time of 1 ns was found. By increasing the areas of the excitation pulses, Rabi oscillations of excitonic polarizations were clearly observed. The corresponding Rabi frequency is three orders of magnitude higher than the measured dephasing rate. For larger pulse areas, we found that the deviation of experimental data from two-level predictions became significant. The deviations cannot be explained by taking into account, as has been suggested in other research, excitation density-dependent dephasing or Hartree–Fock-type Coulomb interactions between excitons.

Singlet exciton diffusion in MEH-PPV films studied by exciton–exciton annihilation

We report time-resolved photoluminescence studies in MEH-PPV films for a wide range of excitation intensities. The results fit well to a diffusion-limited exciton–exciton annihilation model with an annihilation constant γ = (2.8 ± 0.5) × 10 −8 cm 3 s −1 . This enables us to estimate the exciton diffusion coefficient to be D = (3 ± 1) × 10 −3 cm 2 s −1 . This corresponds to a diffusion length between 5 and 8 nm for one-dimensional diffusion.

Role of alloy fluctuations in photoluminescence dynamics of AlGaN epilayers

The near-band-edge (NBE) photoluminescence (PL) of AlGaN layers with different Al content was analyzed in a wide range of excitation intensities and temperatures. The PL peculiarities indicated that tails of density of states are formed in AlGaN alloys due to the fluctuation of the alloy composition. The model involving recombination through one type of nonradiative center is proposed. The dependence of NBE PL integrated intensity on excitation power for AlGaN is weaker than that for GaN, which is attributed to carrier localization in alloys with compositional fluctuations and, thus, reduction of probability of nonradiative recombination.

Nonequilibrium Occupancy of Dangling Bond Defects in Undoped Amorphous Silicon Studied by Subgap-Light-Induced Electron Spin Resonance

A detailed investigation of the nonequilibrium occupancy of band tail states and dangling bond (DB) states in undoped amorphous silicon has been performed using the subgap-light-induced electron spin resonance (subgap LESR) technique. Measurements performed over a wide range of excitation intensities and temperatures reveal a systematic change of the LESR line-shape. Lowering the excitation intensity transforms the superposition of band-tail electron and hole absorption lines, corresponding to the creation of these carriers, into a reversed single DB line, corresponding to the annihilation of neutral DB states. The LESR behavior is reasonably well interpreted in terms of the charge neutrality requirement for band-tail carriers and charged DBs under illumination. A careful inspection of the experimental results on the basis of numerical calculation as well as analytical consideration proves that neutral DBs predominantly occur in thermal equilibrium, and that the ratio of charged to neutral defect density is not more than 10%.

Room-temperature optically pumped laser emission from a-plane GaN with high optical gain characteristics

Photoluminescence (PL) and optical gain (OG) spectra of a-plane GaN layers have been analyzed over a wide range of excitation intensities. The samples were fully coalesced layers grown by metalorganic chemical vapor deposition over r-plane sapphire substrates using epitaxial layer overgrowth (ELOG) and selective area lateral epitaxy (SALE) procedures. ELOG and SALE a-plane samples showed a strong stimulated emission line in backscattering-geometry PL spectra along with extremely high OG coefficient values (in SALE samples more than 2000 cm−1). Structures prepared with natural cleaved facet cavities based on these films were used to demonstrate optically pumped room-temperature lasing.

Ultrafast polarization modulation induced by the “virtual excitation” of spin-polarized excitons in quantum wells: Application to all-optical switching

The operation of an all-optical coherent polarization switch that makes use of spin-polarized virtual excitons in unstrained quantum wells is thoroughly investigated experimentally over a wide range of excitation intensities. The device is shown to exhibit a 415 fs switching time and a contrast ratio of >300:1 at ∼100 K in a thin (40 well) sample. The switching mechanisms are discussed in terms of the circular optical selection rules and the virtual excitation is studied by performing differential measurements for various input polarizations. The polarization state changes induced by the spin-polarized virtual population are measured and their contributions to the switch signal separated using a combination of time-averaged, time-resolved, and spectrally resolved ellipsometric techniques.

Two charge states of dominant acceptor in unintentionally doped GaN: Evidence from photoluminescence study

Photoluminescence of the dominant deep-level acceptor in high-purity freestanding GaN is studied over a wide range of excitation intensities. A yellow luminescence (YL) band at about 2.2 eV saturates with increasing excitation intensity, whereas a green luminescence (GL) band at about 2.5 eV increases as a square of the excitation intensity. The YL and GL bands are attributed to two charge states of the same defect, presumably a gallium vacancy-oxygen complex.

Universal distribution of optically excited carriers in tetrahedral amorphous semiconductors

The decay of optically excited electrons and holes in tetrahedrally coordinated amorphous semiconductors is a universal property of these amorphous solids [Phys. Rev. Lett. 84 (2000) 4180]. The experimental measurements employ a seldom-used electron spin resonance (ESR) detection technique that surpasses the sensitivity of previous measurements by several orders of magnitude. This technique employs second harmonic detection (SH-ESR) of the ESR. Measurements have been performed over a wide range of excitation intensities (nW/cm 2 to W/cm 2) on hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous germanium (a-Ge:H). Using the SH-ESR technique, the kinetics can be studied down to saturated carrier densities as low as approximately 10 14 cm −3 . In addition to the long-time decay curves, both the saturated densities of carriers after long-time irradiation and the most probable recombination lifetimes for the optically excited carriers in a-Si:H agree well with models for the universal diffusion and recombination of carriers at low temperatures. Similar results are obtained for samples of a-Ge:H with germanium dangling-bond densities ≲10 17 cm −3 . A comparison of the ESR lineshapes for the electrons and holes trapped in the conduction and valence band tails, respectively, with electrons and holes trapped in a divacancy in crystalline silicon shows that the asymmetries and localizations of the wavefunctions are similar in both cases.

Magnetic Resonance Probes Of Band Tail States And Defects In Tetrahedrally Coordinated Amorphous Semiconductors

AbstractRecent electron spin resonance (ESR) results relating to (1) recombination processes for optically excited electrons and holes in tetrahedrally coordinated amorphous semiconductors and (2) kinetics of metastable defects (dangling bonds associated with the Staebler-Wronski effect) in hydrogenated amorphous silicon (a-Si:H). With regard to recombination processes, ESR measurements have been performed over a wide range of excitation intensities (nW/cm2 to W/cm2) on hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous germanium (a-Ge:H). The kinetics can be studied down to carrier densities as low as 1014 cm-3. The longtime decay curves show that at large carrier separation (1) the random distribution of optically excited electrons and holes is subject to the condition of charge neutrality, and (2) the decays are universal and independent of the densities of localized, band-tail states. With regard to the metastable defects in a-Si:H, the kinetics of the production and thermal annealing of silicon dangling bonds have been measured at temperatures between 25 and 480 K using ESR. Below about 150 K the measurement of the dangling bonds is masked by long-lived, band tail carriers that accumulate with time. The production rate for silicon dangling bonds decreases with decreasing temperature and is nearly temperature independent below approximately 100 K. Defects created by 10 hours of irradiation below 100 K anneal almost completely at 300 K. In a- Ge:H, the first measurements of optically induced, metastable germanium dangling bonds have been made.

Excitation Intensity Dependence of Light-Induced Electron Spin Resonance in Hydrogenated Amorphous Silicon Films

ABSTRACTLight-induced electron spin resonance in a-Si:H films was measured using second harmonic detection over a wide range of excitation intensities. Second harmonic detection results in an undifferentiated absorption lineshape that saturates much less easily with increasing microwave power. The ratio of the spin densities of the broad line to the narrow line is approximately 3 at a generation rate of 1019 cm−3s−1, and this ratio increases only slightly with decreasing excitation intensity. The kinetics of the LESR after the light is turned on or off show dispersive behavior. The dispersive response of the LESR is related to the dispersive transport of carriers at low temperature. A model that assumes a bimolecular recombination fits the experimental results reasonably well.

Fundamental Study of Structural Response Control by Friction Controllable Active Brace Using Piezoelectric Actuator.

Active/passive mass dampers for buildings are mainly used to improve habitability or human comfort in the buildings in the event of strong winds, and energy dissipation devices are mainly used to enhance the safety of the buildings in the event of strong earthquakes. However very few systems are useful for both of these purposes. As a system which can serve both purposes, an active brace system was proposed, which is installed in stories of a building and can vary the friction force acting between the upper and lower floors of the story using piezoelectric actuators. Thus the active brace works as a friction controllable damper or a variable stiffness system for semiactive response control of the building. Excitation tests were carried out for a single-story building model with a mass of 1 150 kg supported by 4 columns. The active brace model equipped with two piezoelectric actuators produced a friction force of up to 2.5 kN. Through the test, it was shown that the response control by the active brace exhibited good performance throughout a wide range of excitation intensity.

Mass Damper Using Friction-Dissipating Devices

The paper proposes and studies a nonlinear mass damper that uses friction dampers acting transversely to the direction of the motion of the mass damper as a means for energy dissipation. The resistant scheme of this mass damper shows a hysteretic damping mechanism in which the dissipation of energy is quadratic in the amplitude of deformation. The effectiveness of the mass damper in reducing vibrations of the main structural system to which it is attached is investigated. The statistical linearization technique is used to estimate the mean square response of the system subjected to random excitation. The optimum parameters of this mass damper are defined as those that minimize the mean square deformation of the main structural system subjected to random excitation. The study confirms that the efficacy of this nonlinear tuned mass damper (TMD) system is comparable to that of a linear TMD system for a wide range of excitation intensity.