Three-level Energy Research Articles

Pulsewidth Modulation of Neutral-Point-Clamped Indirect Matrix Converter

An indirect matrix converter is an alternative ldquoall-semiconductorrdquo energy processor proposed recently for converting an ac source with fixed magnitude and frequency to a variable voltage and frequency supply that can meet the requirements of a particular industry application. In principle, an indirect matrix converter is constructed by connecting a front-end bidirectional rectification stage to a conventional two-level inversion stage with no bulky capacitive or inductive energy storage connected to their intermediate dc link. Through the proper modulation and compensation of the resulting converter, sinusoidal input current and output voltage can be achieved with minimized rectification switching loss, rendering the indirect matrix converter as a competitive choice for interfacing the utility grid to, e.g., defense facilities that require a different frequency supply. As an improvement, an indirect matrix converter assembled using a three-level energy processing stage has briefly been mentioned in the literature, but has neither been tested in simulation nor experimentally because its operational principles, pulsewidth-modulation (PWM) control, and index compensation have not yet been discussed in the existing literature. Addressing the previously described issues, this paper focuses on the operational mode analysis of a three-level indirect matrix converter implemented using a neutral-point-clamped inversion stage and the design of a number of PWM and modulation ratio compensation schemes for controlling the converter with improved waveform quality. The performances and practicalities of the designed schemes are verified in simulation and experimentally using an implemented laboratory prototype with some representative results captured and presented in this paper.

Theoretical study of one-photon and two-photon absorption properties for 2,1,3-benzothiadiazole-based red-fluorescent dyes

We have theoretically investigated a series of D-π-A-π-D type 2,1,3-benzothiadiazoles (BTD)-based fluorescent dyes by using DFT(B3LYP/6-31G(d)) and ZINDO/SOS methods. The geometrical and electronic structures, one-photon absorption (OPA), and two-photon absorption (TPA) properties have been studied in detail. The calculated results indicate that conjugation length, π-electron bridges and terminal electron-donating groups (D) are three important factors for influencing OPA and TPA properties. Both oscillator strength ( f) of one-photon absorption and two-photon absorption cross-section values increase by degrees with the strength of terminal electron donors from molecule 1a to 1e. When increasing π-conjugation length between the BTD unit and the amino groups, two-photon absorption cross-section values of molecules 2–6 are enhanced about 1.5–2.6 times relative to 1c. Thereinto, the molecule 5 with additional ethyne π-conjugated bridge shows a larger two-photon absorption cross-section (270 GM) in comparison with ethene, benzene, thiophene and phenylethene π-conjugated bridge. The possible reason is discussed in detail by three-level energy model. Finally, we design a molecule 5′ with additional ethyne π-conjugated bridge and asymmetrical electron-donating groups, its two-photon absorption cross-section (336 GM) is larger than any other molecules in this system because the asymmetry makes Δ μ 0 n increased obviously. The calculations also show low-energy transition arising from charge-transfer makes one-photon and two-photon absorption of these BTD derivatives locate in longer wavelength regions (420–550 nm for OPA and 690–880 nm for TPA). So our theoretical findings demonstrate that D-π-A-π-D type BTD derivatives are promising molecules that can be assembled to a series of materials with large TPA cross-section and emitting spectra in the red region.

Magneto-optical probing of weak disorder in a two-dimensional hole gas

In two-beam magneto-photoluminescence spectra of a two-dimensional valence hole gas we identify the three-level energy spectrum of a free positive trion with a field-induced singlet-triplet transition. The recombination spectrum of acceptor-bound trions is also detected, including a cyclotron replica corresponding to the hole shake-up process. The emergence of a shake-up peak at low temperature is shown to be a sensitive probe of the presence of a small number of impurities inside the high-mobility quantum well, and its relative position is directly related to the hole cyclotron mass.

Contribution of two-photon and excited state absorption in ‘axial-bonding’ type hybrid porphyrin arrays under resonant electronic excitation

Two-photon absorption (2PA) and excited state absorption (ESA) properties of ‘axial-bonding’ type hybrid porphyrin arrays are investigated in the picosecond regime under single-photon resonant excitation condition. A crossover from reverse saturable absorption (RSA) to saturable absorption (SA) and back to RSA is observed with the increase of excitation intensity. In the corresponding intensity ranges, third- and fifth-order phase conjugate signals from degenerate four wave mixing are observed. These observations are explained using rate equations for population densities in a three-level energy scheme. At higher intensities, 2PA is found to be dominant contributor to nonlinear absorption compared to ESA.

Two-photon absorption and excited state absorption properties of an organic dye PSPI

The reason why there is a 90 nm red-shift for the highest upconversion wavelength compared with the strongest nonlinear absorption wavelength is given. A three-level energy model is proposed for an organic dye, trans-4-[p-(pyrrolidinyl)styryl]-N-methylpyridinium iodide (abbreviated as PSPI). It is the excited state absorption (ESA) that leads to the reduction of the upconversion efficient at wavelength shorter than 950 nm. The three-level energy model is proved to be right by using the femtosecond pumped two-photon excited spectrum. The TPA cross-sections and the ESA coefficients at various wavelengths are calculated using the measured nonlinear transmittance and upconverison efficiencies. The largest TPA cross-section σ2′ is 14.1×10−48 cm4s/photon at 960 nm. The experimental results show that the electrons are excited to the same excited states for both the TPA and one-photon absorption processes.

Hydrogen motion and stretched-exponential relaxation in a-Si:H

Relaxation phenomena involving the motion of hydrogen in hydrogenated amorphous silicon, that have traditionally been described with a distribution of relaxation times and a stretched-exponential functional, were recently analysed by Van de Walle (Van de Walle C G 1996 Phys. Rev. B 53 11 292) in terms of retrapping in a three-level energy diagram. We show that the expression derived by Van de Walle can also be obtained on the basis of hydrogen transport via interstitial positions which are in thermal equilibrium with a set of hydrogen traps located about 1 eV below the hydrogen chemical potential. However, the models based on hydrogen retrapping only give a good account of the relaxations for temperatures above room temperature. Dispersive behaviour is assumed to be responsible for the discrepancy at lower temperatures.

Shock wave induced phase transition in $\alpha$ -FePO $_4$

Shock wave induced response of the berlinite form of FePO\(_4\) has been investigated up to 8.5 GPa. The X-ray diffraction measurements on the shock recovered samples reveal transition to the mixture of an amorphous phase and an orthorhombic phase around 5 GPa. The proportion of the amorphous material in the recovered sample is found to decrease at higher pressure. The results are interpreted in terms of a three-level free energy diagram for the crystal to amorphous transitions.

Relaxation Of Non-Equilibrium Hydrogen Distributions In a-Si:H

ABSTRACTIt is shown that a reduction of the three-level energy diagram proposed by Van de Walle (Phys. Rev. B 53, 11292, 1996) to describe the relaxation of non-equilibrium hydrogen distributions, to just the interstitial transport level and a distribution of traps, allows an essentially equivalent formulation of the hydrogen relaxation kinetics. The modified formulation offers the possibility of accounting for dispersive diffusion while preserving the essential multiple retrapping aspect of the original proposal.

The Spectrum of Ground-State Reversed Transition of CH3OH Far-Infrared Laser

Using the CH3OH molecular energy levels data base management program, we deduced that when a CH3OH laser pumped by CO2-9P(16) line, there should be a FIR laser line of 918μm wavelength, which corresponding to a transition in ground-state reversed three-level energy system. For the first time, the spectra of ground-state reversed three-level system transitions were calculated by solving the density matrix equations, and the spectrum characteristics of 918μm line were studied. Experimentally, the CH3OH FIR laser line of 918μm wavelength was obtained by pumping with a TEA-CO2 laser. The experimental results were in good agreement with the theoretical calculations.

Decay kinetics of exciton luminescence from two coexisting off-center configurations in KCl:I

Short-lived components of the ns time range were found in the luminescence decays of two coexisting off-center excitons (types II and III) in KCl:I. Temperature dependence of the intensities and lifetimes were analyzed by means of a three-level energy diagram for the adiabatic potential energy surfaces (APES's) of the lowest singlet and triplet states. We found that the potential barrier between the two configurations is higher on the singlet APES than on the triplet APES. An exchange energy between the singlet and the triplet was estimated to be ∼92 meV at the type II configuration while 32 meV at the type III configuration. This provides a direct evidence for the anticipation that the exchange energy must be appreciably enhanced at a weakly off-centered configuration [9].

Magnetic susceptibilities of V3+ in corundum: Magnetic anisotropy at high fields.

We theoretically investigate the behavior of the ${\mathrm{V}}^{3+}$ ion as an impurity in ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$ under high magnetic fields, up to 20 T. In particular, we investigate the introduction of magnetic anisotropy that is lower than the trigonal symmetry of the host crystal. Two approaches are used for the calculations. First, fourth-order perturbation theory is used to develop quartic terms plus one sextic term in the susceptibility tensor that are good for fields up to 4 T. Then, the three-level energy matrix is reduced exactly to obtain the anisotropy at higher fields. It is found that the dominant contributions to the magnetic-induced anisotropy arise from the ${\mathrm{\ensuremath{\chi}}}_{\mathit{x}\mathit{x}\mathit{x}\mathit{x}}$ term, while the ${\mathrm{\ensuremath{\chi}}}_{\mathit{x}\mathit{x}\mathit{z}\mathit{z}}$=${\mathrm{\ensuremath{\chi}}}_{\mathit{z}\mathit{x}\mathit{x}\mathit{z}}$, ${\mathrm{\ensuremath{\chi}}}_{\mathrm{zzzz}}$, and the ${\mathrm{\ensuremath{\chi}}}_{\mathit{x}\mathit{x}\mathit{x}\mathit{x}\mathit{x}\mathit{x}}$ terms give a much lower contribution. Temperature-dependent effects are reported. There is a very small dependence of the magnetization upon the zero-field splitting.