Energy Level Model Research Articles

The enhanced nonlinear absorption, nonlinear refraction and optical limiting properties of Bi2WO6/ZnO composites

The Bi2WO6/ZnO (BWO/ZnO) composites were synthesized by using a hydrothermal method. Then, the BWO/ZnO composites were poured into methyl methacrylate through polymerization to make the BWO/ZnO/PMMA organic glasses (OGs) with different amounts. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to investigate the crystal structures and morphology size of BWO/ZnO composites. The nonlinear absorption (NLA), nonlinear refraction (NLR) and optical limiting (OL) properties were systematically investigated by using the Z-scan technique at 532 nm. The BWO/PMMA and BWO/ZnO/PMMA OGs may transform from saturable absorption (SA) to reverse saturable absorption (RSA) by varying the input energy. The NLA, NLR and OL performances of the BWO/ZnO/PMMA OGs are superior to those of the BWO/PMMA OGs and other bismuth-based metal oxides. The synergistic interaction of BWO and ZnO plays a crucial role in the BWO/ZnO/PMMA OGs. The NLA mechanisms were analyzed using an energy-level model.

A model explaining the anomalous fading effect in thermoluminescence (TL)

An energy-level model consisting of an electron trap, a hole trap and a hole recombination center is proposed to explain the anomalous-fading effect of thermoluminescence which has been observed in several materials. The present model is related to a thermoluminescence glow-curve consisting of two peaks. The relevant set of coupled differential equations is considered and a set of the relevant parameters is chosen. The equations are solved both by making plausible analytical approximations and numerically by using a Matlab solver; the results of the two approaches are in very good agreement. The simulated sample is excited at 100K and then held at room temperature or lower for different lengths of time before simulating the heating stage. It is found that as expected, the low-temperature peak at ∼400K decays very quickly and quite anomalously, the high-temperature peak at ∼570K is also fading much faster than expected for a peak occurring at this temperature. The dependence on the fading temperature, an effect which has been found in some materials is also demonstrated in the simulations. The numerical simulations and the analytical approximations can explain these results and show why this decoupling between the peak temperature and the fading takes place.

A model explaining the inability of exciting thermoluminescence (TL) peaks in certain low temperature ranges

In some instances reported in the literature, a thermoluminescence peak that was expected to be excited by certain irradiation was not excitable at a certain temperature range below that of the peak although it was excitable at different temperature ranges. Two specific cases of this kind are calcium tungstate and semiconducting diamonds excited by UV light. The resemblance between these two different materials is quite surprising. In both cases, when the sample is UV irradiated at ~80 K and heated up, a glow curve consisting of two peaks is measured, one at ~150 K and the other at ~260 K. However, if the sample is held at temperatures between 150 and 260 K, the efficiency of excitation of the 260 K peak decreases very significantly with temperature so that it can hardly be excited above 200 K. In this work we present a possible energy-level model, previously used to explain the anomalous heating-rate effect, which can account for this rather anomalous effect. The model includes an electron trap, a hole trap and a hole recombination center. The transitions taking place during excitation, relaxation and heating are followed by using the appropriate sets of simultaneous differential equations. Simulation of the process by using a certain set of parameters is reported. Also, a theoretical account with approximations is utilized and both yield practically the same results. The effect of inability of excitation of the second peak at the temperature range between the two peaks is demonstrated.

Saturable and reverse saturable absorption in molybdenum disulfide dispersion and film by defect engineering

Understanding and controlling defect in two-dimensional materials is important for both linear and nonlinear optoelectronic devices, especially in terms of tuning nonlinear optical absorption. Taking advantage of an atomic defect formed easily by smaller size, molybdenum disulfide nanosheet is prepared successfully with a different size by gradient centrifugation. Interestingly, size-dependent sulfur vacancies are observed by high-resolution X-ray photoelectron spectroscopy, atomic force microscopy, and transmission electron microscopy. The defect effect on nonlinear absorption is investigated by Z-scan measurement at the wavelength of 800 nm. The results suggest the transition from saturable absorption to reverse saturable absorption can be observed in both dispersions and films. First principle calculations suggest that sulfur vacancies act as the trap state to capture the excited electrons. Moreover, an energy-level model with the trap state is put forward to explain the role of the sulfur vacancy defect in nonlinear optical absorption. The results suggest that saturable absorption and reverse saturable absorption originate from the competition between the excited, defect state and ground state absorption. Our finding provides a way to tune the nonlinear optical performance of optoelectronic devices by defect engineering.

Effect of surface oxidation on nonlinear optical absorption in WS2 nanosheets

Controlling nanosheet size and understanding size-dependent surface oxidation are of quite importance for the nonlinear optical properties of two-dimensional materials. Herein, size-separation of WS2 nanosheets was prepared successfully using liquid phase exfoliation and a gradient centrifugation method. We confirmed the higher surface oxidation with the decrease of nanosheet size by both high-resolution X-ray photoelectron spectroscopy and Raman spectroscopy. To investigate the effect of size-dependent surface oxidation on nonlinear optical properties in WS2, Z-scan technique equipped with 800 nm femtosecond laser was used. We observed the conversion between saturable absorption and reverse saturable absorption in WS2 dispersions as well as the decrease of saturable absorption in WS2 films due to the increase of reverse saturable absorption contribution of WO3. Energy-level model based on type II WS2/WO3 heterostructure was put forth to understand nonlinear optical absorption of the oxidized WS2. The results show that charge transportation from WS2 to WO3 increases the reverse saturable absorption contribution in WO3 while decreases the saturable absorption in WS2. The results pave the way to design desirable nonlinear optical devices by controlling size with different level of surface oxidation.

Morphological State Transition Dynamics in EGF-Induced Epithelial to Mesenchymal Transition.

Epithelial to Mesenchymal Transition (EMT) is a multi-state process. Here, we investigated phenotypic state transition dynamics of Epidermal Growth Factor (EGF)-induced EMT in a breast cancer cell line MDA-MB-468. We have defined phenotypic states of these cells in terms of their morphologies and have shown that these cells have three distinct morphological states—cobble, spindle, and circular. The spindle and circular states are the migratory phenotypes. Using quantitative image analysis and mathematical modeling, we have deciphered state transition trajectories in different experimental conditions. This analysis shows that the phenotypic state transition during EGF-induced EMT in these cells is reversible, and depends upon the dose of EGF and level of phosphorylation of the EGF receptor (EGFR). The dominant reversible state transition trajectory in this system was cobble to circular to spindle to cobble. We have observed that there exists an ultrasensitive on/off switch involving phospho-EGFR that decides the transition of cells in and out of the circular state. In general, our observations can be explained by the conventional quasi-potential landscape model for phenotypic state transition. As an alternative to this model, we have proposed a simpler discretized energy-level model to explain the observed state transition dynamics.

Transition from saturable absorption to reverse saturable absorption in MoTe2 nano-films with thickness and pump intensity

Nonlinear optical effect such as saturable absorption (SA) and reverse saturable absorption (RSA) play a key role for all-optical logic gates, in which transition between them is a challenging issue for the optoelectronic applications. Herein, we find MoTe2 nano-films fabricated with liquid-phase exfoliation and a vacuum filtration method which demonstrate both SA and RSA with different thicknesses and pump intensity. The SA nonlinear optical susceptibility for thick film (100 nm) is Imχ(3) ∼ 10−11 esu. However, the thin film (30 nm) exhibited RSA with the Imχ(3) as high as ∼9.96 × 10−11 esu. The different nonlinear optical response with the thickness is due to the interlayer coupling effect. Under high pump intensity, the thick MoTe2 film also demonstrates transition from SA to RSA. The four energy-level model is employed to analyze the competition between the ground-state and excited-state absorption. We found that the conversion of SA to RSA depends not only on pump intensity but also on the absorption cross-section and transition probability between energy level. The transition properties of SA and RSA for MoTe2 films can be used in all-optical logic gates, fast optical switch, optical limiter, mode storage and so on.

Electron-hole symmetry and solutions of Richardson pairing model

Richardson approach provides an exact solution of the pairing Hamiltonian. This Hamiltonian is characterized by the electron-hole pairing symmetry, which is however hidden in Richardson equations. By analyzing this symmetry and using an additional conjecture, fulfilled in solvable limits, we suggest a simple expression of the ground state energy for an equally-spaced energy-level model, which is applicable along the whole crossover from the superconducting state to the pairing fluctuation regime. Solving Richardson equations numerically, we demonstrate a good accuracy of our expression.

Indirect optical transitions in hybrid spheres with alternating layers of titania and graphene oxide nanosheets

In this report, we studied the optical properties of hybrid spherical structures consisting of alternating nanosheets of titania (TiO(2)) and graphene oxide (GO) prepared by a layer-by-layer self-assembly technique. Compared to samples with only TiO(2) spheres or GO nanosheets, a blue-to-red light emission band emerges and persists in this novel composite material even after it was further reduced through microwave irradiation. From detailed time-resolved measurements and energy-level structure modeling, this unexpected fluorescent feature was attributed to the indirect optical transitions between TiO(2) and the localized sp(2) domains of GO in a charge-separated configuration.

Intensity analysis and energy-level modeling of Nd3+ in Nd3+:Y2O3 nanocrystals in polymeric hosts

Optical absorption and emission intensities are investigated for Nd3+ in nanocrystalline Nd3+:Y2O3. Room temperature absorption intensities of Nd3+(4f3) transitions in synthesized Nd3+:Y2O3 nanocrystals have been analyzed using the Judd–Ofelt (J-O) approach to obtain the phenomenological intensity parameters. The J-O intensity parameters are used to calculate the spontaneous emission probabilities, radiative lifetimes, and branching ratios of the Nd3+ transitions from the upper multiplet manifolds to the corresponding lower-lying multiplet manifolds L2S+1J of Nd3+(4f3). The emission cross sections and room temperature fluorescence lifetimes of the important intermanifold F43/2→I4J (J=9/2,11/2,13/2,15/2) transitions have been determined. We also compare the spectra of the Nd3+:Y2O3 nanocrystals to those of the nanocrystals embedded in polymeric matrices of epoxy and chitosan, and we find similarities in terms of the detailed Stark energy levels of the Nd3+ ion in the Y2O3 nanocrystalline host. The 300 K spectra are analyzed for the energy (Stark) level transitions between the L2S+1J multiplet manifolds of Nd3+(4f3). The results of this study are also compared with a crystal-field splitting analysis reported earlier for single-crystal Nd3+:Y2O3 grown by a modified flame fusion method. We find that the spectroscopic properties of our nanocrystals embedded in polymeric hosts compare favorably with other ceramic and single-crystal forms of Nd3+:Y2O3 currently available.

Higher-order triplet interaction in energy-level modeling of excited-state absorption for an expanded porphyrin cadmium complex

Recent measurements of transmission versus fluence for a methanol-solvated asymmetric pentaazadentate porphyrin-like (APPC) cadmium complex, [(C6H4-APPC)Cd]Cl, showed the limitations of current energy-level models in predicting the transmission behavior of organic reverse saturable absorbers at fluences greater than 1 J/cm². A new model has been developed that incorporates higher-order triplet processes and accurately fits both nanosecond and picosecond transmission-versus-fluence data. This model has provided the first known determination of a higher triplet excited-state absorption cross section and lifetime for an APPC complex and also described a previously unreported feature in the transmission-versus-fluence data. The intersystem crossing rate and the previously neglected higher triplet excited-state absorption cross section are shown to govern the excited-state population dynamics of methanol-solvated [(C6H4-APPC)Cd]Cl most strongly at more-practical device energies.

Thermally activated self-alignment of exchange coupling in NiO/NiFe bilayers

Long-term and temperature stability of the exchange coupling are important conditions for the practical application of spin-valve devices. Here we present magnetic measurements on the properties of sputtered NiO/NiFe bilayers. After deposition a temperature dependent magnetic self-alignment process was observed where the exchange bias field Heb increased according to a ln(t) law. The temperature dependence of this aftereffect indicates that a thermally activated process takes place which can be explained by a two energy-level relaxation model. In addition the long-term stability of the exchange coupling during a forced antiparallel alignment of the NiFe layer was investigated. A significant improvement of the coupling stability can be reached by cooling the samples through the Neél temperature in a magnetic field. Finally, the temperature dependence of Heb was determined for differently treated samples.

Dynamical instabilities andI−Vcharacteristics in resonant tunneling through double-barrier quantum well systems

Based on our time-dependent numerical simulation results of a resonant tunneling structure, a resonant tunneling theory for double-barrier quantum well systems (DBQWS's) is presented. The origin of intrinsic high-frequency current oscillation in DBQWS's, a long-time unsolved device physics problem, is explained, in terms of a time-dependent energy-level coupling model (TDELCM) as the result of the coupling between the emitter quantum well and the main quantum well and the wave-corpuscle duality of electrons. The origin of the intrinsic high-frequency current oscillation in DBQWS's and that of the hyteresis and plateaulike structure in $I\ensuremath{-}V$ curves are two different aspects of the problem. A qualitative analysis of the creation of the hyteresis and plateaulike structure in $I\ensuremath{-}V$ curves is also given. The TDELCM sets the foundation of the time-independent energy-level coupling model that was presented in our recent paper [P. Zhao et al., J. Appl. Phys. 87, 1337 (2000)]. It presents insight into the whole process of resonant tunneling through a DBQWS.

Numerical studies of the isomer selectivity of two- and three-step resonance ionization of niobium

Through numerical simulations in the density- matrix and amplitude formalisms, we study the isomer se- lectivity of a double- and triple-resonance ionization scheme of niobium (Nb). The energy-level model comprises three or four hyperfine doublets with the magnetic substates in- corporated. The model parameters (hyperfine splittings, level decay rates) are chosen to imitate accurately a particular ex- citation scheme of Nb. The doublets are coupled by pulsed multimode fields. The field fluctuations are modeled through random, abrupt jumps in the values of the field parame- ters (phases, amplitudes, frequencies). The isomer selectiv- ity is computed in different noise scenarios, and the effect of small ( 300 MHz) isotope shifts on the selectivity is studied.

Intra-cavity frequency-doubling of a Nd:YAG laser passively Q-switched with GaAs

A simple and compact all-solid-state green-pulse laser source is described. A diode-pumped Nd:YAG laser is Q-switched by a GaAs saturable absorber and intra-cavity frequency-doubled by a KTP crystal. At 3.7 W of pump power the device produces high-quality 20.5-μJ, 25-ns pulses at a pulse repetition rate of 12 kHz. The dynamics of the pulse formation is described by rate equations and an energy-level model which accounts for the various energy-transfer processes in GaAs.

Transient bleaching/induced absorption in reduced SrTiO_3 under picosecond excitation

Optical transient induced absorption and bleaching as a function of wavelength for reduced SrTiO3 with a picosecond pump–probe technique have been studied. An energy-level model of the reduced SrTiO3 is proposed to explain the experimental results. The reduced SrTiO3 crystal examined has two absorption bands centered at 1.7 eV and 2.4 eV, which are assigned to transitions from levels lying in the band gap approximately 1.5 eV and 0.8 eV above the valence band, respectively, to the conduction band. The relaxation times of electrons from the conduction band to the level at 1.5 eV and from the level at 1.5 eV to the level at 0.8 eV are determined to be ∼40 ps and much larger than 500 ps, respectively.

Multilevel rate-equation analysis to explain the recent observations of limitations to optical limiting dyes.

A theoretical rate-equation analysis is presented to investigate the molecular properties that are important for achieving high-fluence optical limiting. Critical conditions for achieving induced absorption are derived in terms of the material and laser parameters by employing a range of hierarchical energy-level models. The influences of stimulated emission and saturation of the excited-state absorption are seen to induce the regime of optical limiting to one of increasing transmittance. This is in direct agreement with recent experimental measurements. \textcopyright{} 1996 The American Physical Society.

FRAGMENTATION PROBABILITIES OF EXCITED ALKALI CLUSTERS

Employing transient resonant two-color two-photon ionization spectroscopy in the picosecond and femtosecond regime, we have investigated the fragmentation dynamics of several excited electronic states of alkali clusters. In the case of the Na 3 C-state a simple energy-level model well describes the single exponential decay found for the temporal evolution of the transient spectra. This allowed to estimate fragmentation probabilities of the C-state vibrational bands as well as the onset of predissociation. For larger clusters, Na n as well as K n with n≥3, the ultrafast transients could be measured. By means of an improved fragmentation model the fragmentation probabilities are determined as a function of the cluster size. Significant differences between. Na n and K n clusters are discussed.

Excited-state nonlinear absorption and its application in photonic technology

The main achievements in our research on the physical phenomena of the excited-state nonlinear absorption in a molecular system and its applications in photonic technology are described. In the first part of this paper, some energy-level models and rate-equations are used to explain various mechanisms of the excited-state nonlinear absorption under different conditions, such as reverse saturable absorption caused by the triplet and/or the singlet excited-state absorption; saturable absorption due to the first and/or the second singlet excited-state absorption; and the excited-state nonlinear absorption induced by two-photon absorption. The experimental results for metal-organic and C60 materials irradiated by ps and ns laser pulses are consistent with the simulated curves of the transmittance versus fluences. In the second part, the applications of excited-state nonlinear absorption in photonic techniques including optical bistability, optical switching, optical limiting, optical modulation, optical logic and optical storage are introduced. The working principles of the photonic devices based on the excited-state nonlinear absorption are presented. The experimental characteristic curves are found in good agreement with the theoretical simulations for these devices.

Energy-Level Model for Isothermal Seed Germination

We have been successful in building an energy-level model that describes seed germination. We used the autocatalytic reaction rate equation to fit the germination rate for seed germination. The two parameters [A] 0 and [F] 0 were found by fitting the integrated germination rate equation to the data. The values of [A] 0 and [F] 0 obey the Arrhenius equation energies for the second and third stage of the four-compartmental model of seed germination