Three-photon-excited upconversion luminescence of YVO4 single crystal by infrared femtosecond laser irradiation

We report on the upconversion luminescence of a pure YVO4 single crystal excited by an infrared femtosecond laser. The luminescent spectra show that the upconversion luminescence comes from the transitions from the lowest excited states T-3(1), T-3(2) to the ground state (1)A(1) of the VO43-. The dependence of the fluorescence intensity on the pump power density of laser indicates that the conversion of infrared irradiation to visible emission is dominated by three-photon excitation process. We suggest that the simultaneous absorption of three infrared photons promotes the VO43- to excited states, which quickly cascade down to lowest excited states, and radiatively relax to ground states, resulting in the broad characteristic fluorescence of VO43-. (c) 2005 Optical Society of America.

Laser intensity dependence of high-order harmonic generation from aligned CO2 molecules

We investigate experimentally the high-order harmonic generation from aligned CO2 molecules and demonstrate that the modulation inversion of the harmonic yield with respect to molecular alignment can be altered dramatically by fine-tuning the intensity of the driving laser pulse for harmonic generation. The results can be modeled by employing the strong field approximation including a ground state depletion factor. The laser intensity is thus proved to be a parameter that can control the high-harmonic emission from aligned molecules.

Sideband manipulation of population inversion in a three-level Λ atomic system

Sideband manipulation of population inversion in a three-level A atomic configuration is investigated theoretically. Compared with the case of a nearly monochromatic field, a population inversion between an excited state and a ground state has been found in a wide sideband intensity range by increasing the difference in frequency between three components. Furthermore, the population inversion can be controlled by the sum of the relative phases of the sideband components of the trichromatic pump field with respective to the phase of the central component. Changing the sum phase from 0 to pi, the population inversion between the excited state and the ground state can increase within nearly half of the sideband intensity range. At the same time, the sideband intensity range that corresponds to the system exhibiting inversion rho(00) > rho 11 also becomes wider evidently.

The dispersive properties of an excited-doublet four-level atomic system

We have investigated the dispersive properties of excited-doublet four-level atoms interacting with a weak probe field and an intense coupling laser field. We have derived an analytical expression of the dispersion relation for a general excited-doublet four-level atomic system subject to a one-photon detuning. The numerical results demonstrate that for a typical rubidium D1 line configuration, due to the unequal dipole moments for the transitions of each ground state to double excited states, generally there exists no exact dark state in the system. Close to the two-photon resonance, the probe light can be absorbed orgained and propagate in the so-called superluminal form. This system may be used as an optical switch.

Enhancement of three-photon absorption cross-sections in a novel class of symmetrical charge transfer fluorene-based molecules

The three-photon absorption effect (3PA) of two novel symmetrical charge transfer fluorene-based molecules (abbreviated as BASF and BMOSF) has been determined by using a Q-switched Nd:YAG laser pumped with 38 ps pulses at 1064 nm in DMF. The measured 3PA cross-sections are 84 x 10(-78) and 114 x 10(-78) cm(6) s(2), respectively. The geometries and electronic excitations of these two molecules are systematically studied by PM3 and ZINDO/S methods. The relationships between 3PA cross-sections and intramolecular charge transfer are discussed micromechanically. The experimental and theoretical results have shown that the larger intramolecular charge transfer, which was characterized by the charge density difference between the ground state (SO) and the first excited state (S-I), the greater enhancement of the 3PA cross-sections. (c) 2005 Elsevier B.V. All rights reserved.

Efficient diode-pumped Yb : Gd2SiO5 laser

An efficient diode-pumped laser was demonstrated by using an ytterbium-doped laser crystal, Yb:Gd2SiO5 (Yb:GSO), wherein Yb3+ ions exhibit the largest ground-state splitting among all the ytterbium-doped crystals. The Yb:GSO laser can be operated at a low pumping threshold, and the most efficient laser occurs around 1088 nm since the corresponding emission band has the largest emission cross section and the lowest thermal population. A slope efficiency of 75% was demonstrated for a continuous-wave Yb:GSO laser at 1094 nm, and self-pulsed lasers were achieved within the tunable range of 1091-1105 nm, which are the longest laser wavelengths achieved for Yb3+ lasers. (c) 2006 American Institute of Physics.

Codoping Na+ to modulate the spectroscopy and photoluminescence properties of Yb3+ in CaF2 laser crystal

kinds of Yb3+- and Na+-codoped CaF2 laser crystal with different Na:Yb ratios of 0, 1.5, and 10 are grown by the temperature gradient technique. Room-temperature absorption, photoluminescence spectra, and fluorescence lifetimes belonging to the transitions between ground state F-2(7/2) and excited state F-2(5/2) of Yb3+ ions in the three crystals are measured to study the effect of Na+. Experimental results show that codoping Na+ ions in different Na:Yb ratios can modulate the spectroscopy and photoluminescence properties of Yb3+ ions in a CaF2 lattice in a large scope. (c) 2005 Optical Society of America

Studies of magnetic interactions in Mn-doped β-Ga2O3 from first-principles calculations

The magnetic behavior of Mn-doped beta-Ga2O3 is Studied from first-principles calculations within the generalized gradient approximation method. Calculations show that ferromagnetic ordering is always favorable for configurations in which two Mn ions substitute either tetrahedral or octahedral sites, and the ferromagnetic ground state is also sometimes favorable for configurations where one Mn ion substitutes a tetrahedral site and another Mn ion substitutes an octahedral site. However, the configurations of the latter case are less stable than those of the former. (c) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Origins of magnetism in transition metal doped Cul

Cupric iodide is a p-type semiconductor and has a large band gap. Doping of Mn, Co, and Ni are found to make gamma-CuI ferromagnetic ground state, while Cr-doped and Fe-doped CuI systems are stabilized in antiferromagnetic configurations. The origins of the magnetic ordering are demonstrated successfully by the phenomenological band coupling model based on d-d level repulsions between the dopant ions. Furthermore, using a molecular-orbital bonding model, the electronic structures of the doped CuI are well understood. According to Heisenberg model, high-T-C may be expected for CuI:Mn and CuI:Ni if there are no native defects or other impurities.

Classical two-site fidelity and quantum phase transitions in the Ising model and the extended Hubbard model

In this Letter, the classical two-site-ground-state fidelity (CTGF) is exploited to identify quantum phase transitions (QPTs) for the transverse field Ising model (TFIM) and the one-dimensional extended Hubbard model (EHM). Our results show that the CTGF exhibits an abrupt change around the regions of criticality and can be used to identify QPTs in spin and fermionic systems. The method is especially convenient when it is connected with the density-matrix renormalization group (DMRG) algorithm. (C) 2008 Elsevier B.V. All rights reserved.

Study of valence intersubband absorption in tensile strained Si/SiGe quantum wells

The hole subband structures and effective masses of tensile strained Si/Si1-yGey quantum wells are calculated by using the 6x6 k.p method. The results show that when the tensile strain is induced in the quantum well, the light-hole state becomes the ground state, and the light hole effective masses in the growth direction are strongly reduced while the in-plane effective masses are considerable. Quantitative calculation of the valence intersubband transition between two light hole states in a 7nm tensile strained Si/Si0.55Ge0.45 quantum well grown on a relaxed Si0.5Ge0.5 (100) substrates shows a large absorption coefficient of 8400 cm(-1).

Thermodynamic properties of Cu-3(CO3)(2)(OH)(2) and the spin-1/2 distorted diamond Heisenberg chain

The thermodynamic properties of the spin-1/2 diamond quantum Heisenberg chain model have been investigated by means of the transfer matrix renormalization group (TMRG) method. Considering different crystal structures, by changing the interactions among different spins and the external magnetic fields, we first investigate the magnetic susceptibility, magnetization, and specific heat of the distorted diamond chain as a model of ferrimagnetic spin systems. The susceptibility and the specific heat show different features for different ferromagnetic (F) and antiferromagnetic (AF) interactions and different magnetic fields. A 1/3 magnetization plateau is observed at low temperature in a magnetization curve. Then, we discuss the theoretical mechanism of the double-peak structure of the magnetic susceptibility and the three-peak structure of the specific heat of the compound Cu-3(CO3)(2)(OH)(2), on which an elegant measurement was performed by Kikuchi [Phys. Rev. Lett. 94, 227201 (2005)]. Our computed results are consistent with the main characteristics of the experimental data. Meanwhile, we find that the double-peak structure of susceptibility can be found in several different kinds of spin interactions in the diamond chain. Moreover, a three-peak behavior is observed in the TMRG results of magnetic susceptibility. In addition, we perform calculations relevant for some experiments and explain the characteristics of these materials. (c) 2007 American Institute of Physics.

Spin states in InAs/AlSb/GaSb semiconductor quantum wells

We investigate theoretically the spin states in InAs/AlSb/GaSb broken-gap quantum wells by solving the Kane model and the Poisson equation self-consistently. The spin states in InAs/AlSb/GaSb quantum wells are quite different from those obtained by the single-band Rashba model due to the electron-hole hybridization. The Rashba spin splitting of the lowest conduction subband shows an oscillating behavior. The D'yakonov-Perel' spin-relaxation time shows several peaks with increasing the Fermi wave vector. By inserting an AlSb barrier between the InAs and GaSb layers, the hybridization can be greatly reduced. Consequently, the spin orientation, the spin splitting, and the D'yakonov-Perel' spin-relaxation time can be tuned significantly by changing the thickness of the AlSb barrier.

Quantum critical phenomena in the XY spin chain with the Dzyaloshinski-Moriya interaction (Retracted article. See vol 80, artn 019903, 2009)

We study the effects of the Dzyaloshinski-Moriya (DM) anisotropic interaction on the ground-state properties of the Heisenberg XY spin chain by means of the fidelity susceptibility, order parameter, and entanglement entropy. Our results show that the DM interaction could influence the distribution of the regions of quantum phase transitions and cause different critical regions in the XY spin model. Meanwhile, the DM interaction has effective influence on the degree of entanglement of the system and could be used to increase the entanglement of the spin system.

The electronic structure of strained ZnO/MgxZn1-xO superlattices and the influence of polarization

Based on the effective-mass model, the lower energies of the electron and the hole of ZnO/MgxZn1-xO superlattices are calculated. Because of the mismatch of the lattice constant between the ZnO well and the MgxZn1-xO barrier, piezoelectric and spontaneous polarization exist in ZnO/MgxZn1-xO superlattices and a macroscopical internal electric held is found when well width L-w >4 nm and Mg concentration x > 0.2. The parameters of ZnO/MgxZn1-xO superlattices such as lattice constant, band offset, etc. are also proposed. Through calculations, we found the internal electric field can change the lowest energies of the electron and hole to 105.4 and 85.1 meV when well width L-w up to 70 angstrom, which will influence the electronic and optical properties of ZnO/MgxZn1-xO superlattices greatly, while the Rashba effect from the internal electric field is so small that it can be neglected. The ground state exciton energies with different Mg concentration x are also calculated by variational method, our results are very close to the experimental results when Mg concentration x <= 0.3. (C) 2008 Elsevier B.V. All rights reserved.