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[en] In this paper, a theoretical model is used to investigate the lasing characteristics of InGaAs/GaAs quantum-dot lasers. The rate equations for InGaAs/GaAs are numerically solved using the fourth order Runge-Kutta method. Self-assembled quantum-dot lasers are studied considering the homogeneous and inhomogeneous broadening of the optical gain, as well as the nonlinear gain and thermal carrier escape from quantum dots. For different carrier relaxation times, the current-output power characteristics and light emissions of the quantum-dot laser have been calculated. The effect of the coverage factor is studied, and an optimized value is introduced for each carrier relaxation time. Moreover, the modulation responses for various inhomogeneous broadening and relaxation times are presented. Simulated results show that increasing the relaxation time decreases the modulation bandwidth. Furthermore, an increase in inhomogeneous broadening decreases the modulation bandwidth.
[en] Eu3+-activated La2W3O12 phosphors have been synthesized by using the traditional solid-state reaction. X-ray diffraction, photoluminescence (PL) spectra at room temperature and high temperature, and luminescence decay kinetics have been used to characterize the synthesized samples. For the PL emission spectra, the electric dipole transition 5D0 - 7F2 of Eu3+ is the dominant one. The chromaticity coordinate is (0.665, 0.334), which is close to the National Television Standard Committee standard value for red phosphor (x = 0.67, y = 0.33). For the excitation spectrum, the La2W3O12 :Eu3+ phosphor shows an intense absorption near 400 nm, which is available for near-UV-excited white LEDs. An analysis based on the Inokuti-Hirayama model shows that the interaction between Eu3+ ions occurs via a dipole-quadrupole type interaction.
[en] Inherently soft zinc-selenides have been hardened through beryllium doping. High-quality stoichiometric ternaries of BexZn1-xSe have been synthesized by using the Bridgeman technique. State-of-art X-ray absorption spectroscopy is performed by varying the concentration of the cationic dopant, Be, from 6% to 55% in the host ZnSe. Extended X-ray absorption fine structure analyses are carried out to study the next-neighbor and next nearest neighbor atomic positions, nature of the substitutional doping, extent of bond length homogeneity, the presence of involuntary contrast among path distances, and the crossover from a soft to a hard character of the ternary with increasing Be concentration. Our results indicate the presence of a non-regular impulsive hardening in the ternary with a disparity at the lower and the higher Be-doping levels, which are discussed vis-a-vis self-accommodation of substitutional dopants in the host lattice.
[en] Using the full potential linearized augmented plane wave (FLAPW) method, we have explored the magnetic properties of one monolayer (ML) of MgN and MgC films on MgO(001) surfaces. In addition, the influence of a MgO capping layer on the magnetic states of MgC and MgN films has been investigated. We observed that both MgC/MgO(001) and MgN/MgO(001) display ferromagnetic (FM) state. Interestingly, we find that both free standing MgN film and MgN/MgO(001) systems display half metallic feature. The calculated magnetic moments of C and N in MgC/MgO(001) and MgN/MgO(001) are 0.3 μB and 0.57 μB, respectively. The essential magnetic properties of both systems are unchanged even in the presence of a MgO capping layer. In particular, the half metallic feature in MgN/MgO(001) is still maintained in both the MgN/MgO(001) and the MgO(1 ML)/MgN/MgO(001) systems. The adsorption energy calculations indicate that the MgC layer is physisorbed on MgO(001), while the MgN layer is chemisorbed. Due to the rigid half metallic feature and the character of adsorption, the MgN/MgO(001) can be utilized for potential spintronics purposes.
[en] Panoramic radiography with which only structures within a certain image layer are in focus and others out of focus on the panoramic image has become a popular imaging technique especially in dentistry. However, the major drawback to the technique is a mismatch between the structures to be focused and the predefined image layer mainly due to the various shapes and sizes of dental arches and/or to malpositioning of the patient. These result in image quality typically inferior to that obtained using intraoral radiographic techniques. In this paper, to overcome these difficulties, we suggest a new panoramic reconstruction algorithm, the so-called adaptive panoramic tomography (APT), capable of reconstructing multifocal image layers with no additional exposure. In order to verify the effectiveness of the proposed algorithm, we performed systematic simulation studies with a circular rotational movement and investigated the image performance.
[en] Multifragmentation of carbon-induced fission is considered following the statistical scission model. The fragment products in the exit channel are two different deformed nuclei, as well as some nucleons and / or alpha particles. The angular distributions and variances of this multifragmentation process, including two deformed heavy fragments, have been numerically calculated. The present calculations show the strong dependence of these values on the nuclear temperature. The present calculated differential cross-sections are in good agreements with the experimental data for an incident carbon energy of 84 A MeV.
[en] In this paper, we studied the accretion of dark energy onto a wormhole in the Friedmann-Robertson-Walker (FRW) cosmological model. Contrary to the black hole case, in the phantom case, the accretion gives rise to an increase in the wormhole's throat size and will stop at special regions.
[en] The phase retrieval problem is a problem of reconstruction of a signal from the magnitude of its Fourier transform. In this paper, we consider the problem of reconstructing an unknown one dimensional signal from the magnitude of its Fourier transform and the magnitude of the Fourier transform of another signal that is made by the addition of a linear phase signal at the center of the original signal. Since there are two kinds of linear phase signals, i.e., symmetric and antisymmetric, we consider the two cases separately. After showing that there are exactly two solution signals that satisfy the given condition, we develop a closed-form algorithm that may reconstruct the two solution signals.
[en] We prepared polycrystalline silver indium selenide thin films by vacuum evaporation on Si (100) substrates at a high temperature using a stochiometric powder. The films were characterized by X-ray diffraction (XRD) and UV-VIS-NIR spectroscopy. For the fabrication of densely distributed one-dimensional nanostructures of silver indium selenide on Si substrates, thermally evaporated films of AIS on Si (100) substrates were irradiated by incident 200-MeV Ag+ ions at a fluence of 5 x 1011 ion/cm2. At elevated substrate temperatures, silver indium selenide (AIS) exhibited a nanorod -like structure. The optical and the structural properties of the irradiated films were studied using UV-VIS-NIR Reflection spectroscopy, atomic force microscopy (AFM), field emission scanning electron microscopy (FESEM), and XRD. The controlled fabrication of such densely distributed one-dimensional nanorods on a Si substrate by using the ion beam technique, we believe, will open a variety of applications, such as nanoelectronics and optoelectronics devices.
[en] The electronic structure and the optical properties of MnCo2O4 thin films were investigated by using X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and spectroscopic ellipsometry (SE) and were compared with those of Co3O4. Polycrystalline films of the spinel oxides were prepared on Al2O3(0001) substrates by using a sol-gel method. XPS investigation indicated that the Mn ions in MnCo2O4 had a valence of +3 and occupied the octahedral sites of the spinel lattice. XRD results revealed that the inverse-spinel MnCo2O4 sample exhibited distortion of the lattice to a tetragonal structure while the Co3O4 sample had a cubic structure. The lattice distortion implies a high-spin t32ge1g d-electron configuration of the octahedral Mn3+ ions, which is susceptible to the Jahn-Teller effect. The dielectric function spectra of MnCo2O4 and Co3O4 measured by using SE in the visible-ultraviolet (1.5 - 8.5 eV) range exhibited a number of optical absorption structures that could be explained in terms of electronic charge-transfer (CT) transitions between the ions located at different lattice sites. The valence-electronic states involved in the CT transitions were identified from the electronic structure predicted by the XPS analyses.