Absorption and photoconductivity spectra of Ag2GeS3 crystal: experiment and theory.

Spectral features of polycrystalline Ag(2)GeS(3) samples synthesized from high-purity elements (at least 99.99 wt.% purity) in quartz ampoules evacuated to 0.1 Pa were explored. The band energy gap of Ag(2)GeS(3) crystals estimated from the fundamental absorption edge and photoconductivity spectra were found to be equal to 1.98 eV and 2.16 eV, respectively. Simultaneously we have performed calculations of the band structure, total and partial density of states and the electron charge density using the ab initio FP-LAPW method. All the calculations were performed with four different exchange-correlation (xc) potentials. It was found that the effect of using different xc is very marginal on the valence band maximum (VBM) while it is dramatically shifted the conduction band minimum (CBM) towards higher energies with respect to the Fermi energy position. Our theoretical results have given a band energy gap equal to 0.40 eV (for LDA), 0.42 eV (GGA), 1.03 eV (EVGGA) and 1.30 eV (mBJ) xc potentials. Thus the underestimation of the energy gap in LDA and GGA was partially corrected in EVGGA and mBJ model. As a remarkable fact mBJ did not bring the energy band gap very close to the experimental once. We have discovered that the Ag-s states have only a small effect on the conduction bands shifts whereas Ge-s states have a strong effect in extending of the gap, while remaining the valence bands unchanged.

Second harmonic generation and hyperpolarizabilities of the double-cubane compound [Sb7S8Br2](AlCl4)3: chalcogenide in ionic liquids.

Because noncentrosymmetric [Sb7S8Br2](AlCl4)3 single crystals possess a wide optical transparency region, it is a promising material for nonlinear optical applications. We have calculated the dispersion of linear and nonlinear optical susceptibilities including optical second harmonic generation (SHG) using a relaxed geometry. We find that the fundamental optical absorption edge situated at about 2.03 eV is in excellent agreement with the experimental data. Calculations of ε(2)(xx)(ω), ε(2)(yy)(ω), and ε(2)(zz)(ω) tensor components of the frequency-dependent dielectric function are presented. The single crystal possesses a considerable anisotropy of linear optical susceptibilities, which usually favors an enhanced phase matching conditions necessary to observe SHG and optical parametric oscillator (OPO) effects. Our calculations show that, in [Sb7S8Br2](AlCl4)3, |χ123(2)(ω)| is the principal tensor component having the highest value of SHG at zero frequency limit as well as at 1.165 eV (λ = 1064 nm) laser wavelength generation. The microscopic second-order hyperpolarizability, ß123, of the dominant SHG component is calculated at the static limit and at λ = 1064 nm.

Density functional calculations, electronic structure, and optical properties of molybdenum bimetallic nitrides Pt2Mo3N and Pd2Mo3N.

The electronic band structure, origin of chemical bonds, and dispersion of linear optical susceptibilities for Pt(2)Mo(3)N and Pd(2)Mo(3)N have been investigated within the framework of density functional theory (DFT). The atomic positions of Pt(2)Mo(3)N and Pd(2)Mo(3)N crystalline compounds taken from the X-ray diffraction data (El-Himri, A.; Marrero-Lopez, D.; Nunez, P. J. Solid State Chem. 2004, 177, 3219) were optimized by minimization of the forces acting on the atoms using a full potential linear augmented plane wave (FLAPW) method. We employed the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE). The DFT calculations show that these compounds have metallic origin with strong orbital hybridization near the Fermi energy level (E(F)). The calculated density of states (DOS) at the Fermi energy (E(F)) is about 1.83 and 1.02 states/Ry cell, and the bare linear low-temperature electronic specific heat coefficient (γ) is found to be 0.32 and 0.18 mJ/mol-K(2) for Pt(2)Mo(3)N and Pd(2)Mo(3)N, respectively. The Fermi surface of Pt(2)Mo(3)N (Pd(2)Mo(3)N) is composed of three (five) sheets. The bonding features of the compounds are analyzed using the electronic charge density contour in the (110) crystallographic plane. The linear optical properties are calculated with and without the Drude term.

Investigation of the linear and nonlinear optical susceptibilities of KTiOPO4 single crystals: theory and experiment.

Experimental and theoretical studies of the linear and nonlinear optical susceptibilities for single crystals of potassium titanyl phosphate KTiOPO4 are reported. The state-of-the-art full potential linear augmented plane wave method, based on the density functional theory, was applied for the theoretical investigation. The calculated direct energy band gap at Γ, using the Engel-Vosko exchange correlation functional, is found to be 3.1 eV. This is in excellent agreement with the band gap obtained from the experimental optical absorption spectra (3.2 eV). We have calculated the complex dielectric susceptibility ε(ω)dispersion, its zero-frequency limit ε1(0) and the birefringence of KTiOPO4. The calculated birefringence at the zero-frequency limit Δn(0) is equal to about 0.07 and Δn(ω) at 1.165 eV (λ = 1064 nm) is 0.074. We also report calculations of the complex second-order optical susceptibility dispersions for the principal tensor components: χ113((2))(ω), χ232((2))(ω), χ311((2))(ω), χ322((2))(ω), and χ333((2))(ω). The intra- and interband contributions to these susceptibilities are evaluated. The calculated total second order susceptibility tensor components |χ(ijk)((2))(ω)| at λ = 1064 nm for all the five tensor components are compared with those obtained from our measurements performed by nanosecond Nd:YAG laser at the fundamental wavelength (λ = 1064 nm). Our calculations show reasonably good agreement with our experimental nonlinear optical data and the results obtained by other authors. The calculated the microscopic second order hyperpolarizability, ß333, vector component along the dipole moment direction for the dominant component χ333((2))(ω) is found to be 31.6 × 10⁻³° esu, at λ = 1064 nm.

Effect of U on the electronic properties of neodymium gallate (NdGaO3): theoretical and experimental studies.

We have performed a density functional calculation for the centrosymmetric neodymium gallate using a full-potential linear augmented plane wave method with the LDA and LDA+U exchange correlation. In particular, we explored the influence of U on the band dispersion and optical transitions. Our calculations show that U = 0.55 Ry gives the best agreement with our ellipsometry data taken in the VUV spectral range with a synchrotron source. Our LDA+U (U = 0.55) calculation shows that the valence band maximum (VBM) is located at T and the conduction band minimum (CBM) is located at the center of the Brillouin zone, resulting in a wide indirect energy band gap of about 3.8 eV in excellent agreement with our experiment. The partial density of states show that the upper valence band originates predominantly from Nd-f and O-p states, with a small admixture of Nd-s/p and Ga-p B-p states, while the lower conduction band prevailingly originates from the Nd-f and Nd-d terms with a small contribution of O-p-Ga-s/p states. The Nd-f states in the upper valence band and lower conduction band have a significant influence on the energy band gap dispersion which is illustrated by our calculations. The calculated frequency dependent optical properties show a small positive uniaxial anisotropy.

Density functional calculation for Li2CuSn as an electrode material for rechargeable batteries.

The all electron full potential linearized augmented plane wave method has been used for an ab initio theoretical study of the band structure, density of states and electron charge density, and the spectral features of the linear and nonlinear optical susceptibilities for the host CuSn and Li(2)CuSn compounds. We have calculated the density of states at Fermi energy and the electronic specific heat coefficient (gamma). The total charge densities in the (100) and (110) planes were calculated. We noticed that inserting Li into CuSn leads to give two structures in the spectral features of the linear optical susceptibilities while the host compound gives only one structure. Insertion of Li into CuSn leads to breaking the symmetry resulting in noncentrosymmetric material. We have calculated the complex second-order nonlinear optical susceptibility tensor for the intercalated compound.

Ab initio calculation of the electronic band structure, density of states and optical properties of alpha-2-methyl-1-nitroisothiourea.

The electronic and optical properties of alpha-2-methyl-1-nitroisothiourea have been studied using a full potential linear augmented plane wave method within density-functional theory along with the Engel-Vosko exchange correlation function. The structural data obtained by Vasil'ev et al. [Vasil'ev, A. D.; Astakhov, A. M.; Gelemurzina, I. V.; Stepanov, R. S. Dokl. Chem. 2001, 379, (4-6), 232-235; translated from Dokl. Akad. Nauk 2001, 379 (6), 781-784] from X-ray diffraction was used. Our calculations show that the valence band maximum (VBM) is located at the T and the conduction band minimum (CBM) is located at the R point of the Brillouin zone, resulting in an indirect energy gap of 3.1 eV. The calculated partial density of states shows that the upper valence band and the lower conduction band arise predominantly from the O-p, N-p and S-p states. The compound has a large uniaxial dielectric anisotropy and a large negative birefringence.

Band structure, density of states, and optical susceptibilities of a novel lithium indium orthoborate Li3InB2O6.

By use of the structural parameters of the single crystal lithium indium orthoborate obtained by Penin et al. (Solid State Sci. 2001, 3, 461-468), from X- ray diffraction data, we present a first-principle study of the electronic structure and the linear optical properties for the novel lithium indium orthoborate Li3InB2O6. A full-potential linear augmented plane wave method within density functional theory with the Engel-Vosko exchange correlation was used. This compound has a wide direct energy band gap of about 3.8 eV with both the valence band maximum and conduction band minimum located at the center of the Brillouin zone. Our calculations of the partial density of states shows that the upper valence band originates predominantly from the O-p, B- p, and In-p states, and the lower conduction band is dominated by the O-s/p, In-p, and B-p states. Thus the O-p states in the upper valence band and lower conduction band has a significant effect on the energy band gap dispersion. The uniaxial anisotropy [deltaepsilon=(epsilon0zz-epsilon0xx)/epsilon0tot] is about -0.041.

X-ray diffraction, X-ray photoelectron spectra, crystal structure, and optical properties of centrosymmetric strontium borate Sr2B16O26.

We report results of X-ray diffraction (XRD) and valence band X- ray photoelectron (VB-XPS) spectra for strontium borate Sr(2)B(16)O(26). The X-ray structural analysis shows that the single crystals of Sr(2)B(16)O(26) crystallize in the monoclinic space group P2(1)/c with a = 8.408(1) A, b = 16.672(1) A, c = 13.901(2) A, beta = 106.33(1) degrees , and Z = 4. The crystal structure consists of a 3D network of the complex borate anion [B(16)O(20)O(12/2)](4-), formed by 12 BO(3) triangles and four BO(4) tetrahedra, which can be viewed as three linked [B(3)O(3)O(4/2)](-) triborate groups bonded to one pentaborate [B(5)O(6)O(4/2)](-) group and two BO(3) triangles. Using this structure, we have performed theoretical calculations using the all-electron full potential linearized augmented plane wave (FP-LAPW) method for the band structure, density of states, electron charge density, and the frequency-dependent optical properties. Our experimental VB-XPS of Sr(2)B(16)O(26) is compared with results of our FP-LAPW calculations. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Gamma of the Brillouin zone (BZ) resulting in a direct energy gap of about 5.31 eV. Our measured VB-XPS show reasonable agreement with our calculated total density of states for the valence band that is attributed to the use of the full potential method.

Enhancing the resolution of the forward second harmonic imaging using the two-photon laser scanning microscope.

UNLABELLED: The effect of increasing the numerical aperture and changing the immersion imaging medium of the objective in the forward second harmonic imaging of the grana and intergranal parts of chloroplasts in the leaves of shade moss Plagiomnium affine plant were demonstrated. Also the enhancement of the forward second harmonic generation images and simultaneously records the forward and backward second harmonic signals were demonstrated. The second harmonic signals in the forward and backward directions are generated by a tightly focused linearly polarized femtosecond pulses laser. CONCLUSIONS: Forward second harmonic signals were successfully enhanced by capturing higher order of diffraction rays from the samples using high numerical aperture water immersion objective.

Spin-polarized structural, electronic, and magnetic properties of diluted magnetic semiconductors Cd(1-x)Mn(x)S and Cd(1-x)Mn(x)Se in zinc blende phase.

We studied the structural, spin-polarized electronic band structures, density of states, and magnetic properties of the diluted magnetic semiconductors (DMSs) Cd(1-x)Mn(x)S and Cd(1-x)Mn(x)Se in zinc blende phase (B3) with 25% Mn by using the ab initio method. The calculations were performed by using the full potential linearized augmented plane wave plus local orbitals (FP-L/APW+lo) method within the spin-polarized density functional theory and the local spin density approximation (LSDA). Calculated electronic band structures and the density of states of these DMSs are discussed in terms of the contribution of Mn 3d(5)4s(2), Cd 4d(10)5s(2), S 3s(2)3p(4), and Se 4s(2)4p(4) partial density of states and we also compute the local magnetic moments. We estimated the spin-exchange splitting energies, Delta(x)(d) and Delta(x)(p-d), produced by the Mn 3d states, and we found that the effective potential for the minority spin is more attractive than that for the majority spin. We determine the s-d exchange constant N(0)alpha and p-d exchange constant N(0)beta, which resembles a typical magneto-optical experiment. The calculated total magnetic moment is found to be 5.0020 and 5.00013 mu(B) for Cd(1-x)Mn(x)S and Cd(1-x)Mn(x)Se, respectively. These values indicate that every Mn impurity adds no hole carriers to the perfect CdS and CdSe crystals. Moreover, we found that p-d hybridization reduces the local magnetic moment of Mn from its free space charge value of 5.0micro(B) and produces small local magnetic moments on the nonmagnetic Cd and S sites.

X-ray diffraction, crystal structure, and spectral features of the optical susceptibilities of single crystals of the ternary borate oxide lead bismuth tetraoxide, PbBiBO4.

The all-electron full-potential linearized augmented plane-wave method has been used for an ab initio theoretical study of the band structure, the spectral features of the optical susceptibilities, the density of states, and the electron charge density for PbBiBO4. Our calculations show that the valence-band maximum (VBM) and conduction-band minimum (CBM) are located at the center of the Brillouin zone, resulting in a direct energy gap of about 3.2 eV. We have synthesized the PbBiBO4 crystal by employing a conventional solid-state reaction method. The theoretical calculations in this work are based on the structure built from our measured atomic parameters. We should emphasize that the observed experimental X-ray diffraction (XRD) pattern is in good agreement with the theoretical one, confirming that our structural model is valid. Our calculated bond lengths show excellent agreement with the experimental data. This agreement is attributed to our use of full-potential calculations. The spectral features of the optical susceptibilities show a small positive uniaxial anisotropy.

X-ray photoelectron spectrum and electronic properties of a noncentrosymmetric chalcopyrite compound HgGa(2)S(4): LDA, GGA, and EV-GGA.

An all electron full potential linearized augmented plane wave method has been applied for a theoretical study of the band structure, density of states, and electron charge density of a noncentrosymmetric chalcopyrite compound HgGa(2)S(4) using three different approximations for the exchange correlation potential. Our calculations show that the valence band maximum (VBM) and conduction band minimum (CBM) are located at Gamma resulting in a direct energy gap of about 2.0, 2.2, and 2.8 eV for local density approximation (LDA), generalized gradient approximation (GGA), and Engel-Vosko (EVGGA) compared to the experimental value of 2.84 eV. We notice that EVGGA shows excellent agreement with the experimental data. This agreement is attributed to the fact that the Engel-Vosko GGA formalism optimizes the corresponding potential for band structure calculations. We make a detailed comparison of the density of states deduced from the X-ray photoelectron spectra with our calculations. We find that there is a strong covalent bond between the Hg and S atoms and Ga and S atoms. The Hg-Hg, Ga-Ga, and S-S bonds are found to be weaker than the Hg-S and Ga-S bonds showing that a covalent bond exists between Hg and S atoms and Ga and S atoms.

Second harmonic generation from thick leaves using the two-photon laser scanning microscope.

A laser-scanning microscope using second harmonic generation (SHG) as a probe is shown to produce high-resolution images of duckweed Lemna minuta leaves. These leaves are multi-cell layer thick. Second harmonic light is generated by a tightly focused short pulse laser beam and is collected by two objectives yielding forward and backward second harmonic digital images. This measurement shows that the signal of the second harmonic imaging in the forward and backward directions depends on the thickness of the chloroplast and that the forward-SH image was brighter than the backward-SH image. The image intensity also depended on the orientation of the chloroplast in relation to the illuminating polarization direction. Their light-induced re-orientation which was affected by the intensity of the illumination could be observed during the experiments. The novelty of this work is to establish new compact technique in which one can use the SH imaging to investigate the true architecture of the sensitive samples, the unknown samples and the samples which is not producing auto-fluorescence. Moreover, investigation of new or unknown samples needs a long time for looking at details of the sample. Thereby the sample will be exposed for long time to the laser radiation that will cause photobleaching and photodamage. Since the SHG does not undergo photobleaching and photodamage this will be the promising technique for investigating the sensitive and new unknown samples. Then one can move to acquire fluorescence images after complete investigation of the true architecture of the sample. The other advantage of SHG is that it has the ability to image highly ordered structural proteins without any exogenous labels. The SHG is an intrinsic and a coherent process. Imaging of intrinsic compounds avoids the complications of slicing and labeling, and samples can be investigated under physiological conditions.

Second harmonic imaging of chloroplasts using the two-photon laser scanning microscope.

BACKGROUND: Second harmonic information reveals information about the structure of spatially oriented structures with an asymmetry. We study the second harmonic images of the grana and intergranal parts of chloroplasts in the leaves of the shade moss Plagiomnium affine. RESULTS: A two-photon microscope with blocking filters to suppress fluorescence generated both forward and backward second harmonic illumination to be captured. In the starch free chloroplasts of the single cell layered laminae strong second harmonic generation (SHG) from the granal regions was emitted. Upon illumination the chloroplasts changed their orientation affecting the SHG signal. Chloroplast is reoriented upon illumination. CONCLUSIONS: SHG signals were successfully obtained from the grana without any possible confusion from SHG starch grains due to their absence.

Electronic band structures of AV(2) (A = Ta, Ti, Hf and Nb) Laves phase compounds.

First-principles density functional calculations, using the all-electron full potential linearized augmented plane wave method, have been performed in order to investigate the structural and electronic properties for Laves phase AV(2) (A = Ta, Ti, Hf and Nb) compounds. The generalized gradient approximation and the Engel-Vosko-generalized gradient approximation were used. Our calculations show that these compounds are metallic with more bands cutting the Fermi energy (E(F)) as we move from Nb to Ta, Hf and Ti, consistent with the increase in the values of the density of states at the Fermi level N(E(F)). N(E(F)) is controlled by the overlapping of V-p/d, A-d and A-p states around the Fermi energy. The ground state properties of these compounds, such as equilibrium lattice constant, are calculated and compared with the available literature. There is a strong/weak hybridization between the states, V-s states are strongly hybridized with A-s states below and above E(F). Around the Fermi energy we notice that V-p shows strong hybridization with A-p states.

Synthesis, IR, UV-vis spectra, x-ray diffraction and band structure of a non-centrosymmetric borate single-crystal CaBiGaB(2)O(7).

A non-centrosymmetric borate, CaBiGaB(2)O(7), has been grown by a solid-state reaction method at a temperature below 700 °C. The single-crystal x-ray structural analysis has shown that it crystallizes in the tetragonal space group [Formula: see text] with a = 0.7457(1) nm, c = 0.4834(1) nm, Z = 2. It has a three-dimensional (3D) structure in which [B(2)O(7)](8-) groups are bridged by [GaO(4)](5-) tetrahedra through shared O atoms to form 2D [Formula: see text] layers that are further linked by Bi(3+)/Ca(2+) cations giving rise to the final 3D framework. The IR spectrum confirms the presence of [BO(4)](5-) groups and the UV-vis diffuse reflectance spectrum shows that the optical band gap is about 2.9 eV. This value is compared with our band structure calculations using the full potential linearized augmented plane wave approach within the framework of the Engel-Vosko GGA formalism.

Reduction of the pulse duration of the ultrafast laser pulses of the Two-Photon Laser Scanning Microscopy (2PLSM).

BACKGROUND: We provide an update of our two-photon laser scanning microscope by compressing or reducing the broadening of the pulse width of ultrafast laser pulses for dispersion precompensation, to enable the pulses to penetrate deeply inside the sample. FINDINGS: The broadening comes as the pulses pass through the optical elements. We enhanced and modified the quality and the sharpness of images by enhancing the resolution using special polarizer namely Glan Laser polarizer GL10. This polarizer consists of two prisms separated by air space. This air separation between the two prisms uses to delay the red wavelength when the light leaves the first prism to the air then to second prism. We note a considerable enhancing with using the GL polarizer, and we can see the details of the leaf structure in early stages when we trying to get focus through z-stacks of images in comparison to exactly the same measurements without using GL polarizer. Hence, with this modification we able to reduce the time of exposure the sample to the laser radiation thereby we will reduce the probability of photobleaching and phototoxicity. When the pulse width reduced, the average power of the laser pulses maintained at a constant level. Significant enhancement is found between the two kinds of images of the Two-Photon Excitation Fluorescence (TPEF). CONCLUSION: In summary reduction the laser pulse width allowed to collect more diffraction orders which will used to form the images. The more diffraction orders the higher resolution images.

Electronic, linear, and nonlinear optical properties of III-V indium compound semiconductors.

We have made an extensive theoretical study of the electronic, linear, and nonlinear optical properties of the III-V indium compound semiconductors InX (X=P, As, and Sb) with the use of full potential linear augmented plane wave method. The results for the band structure, density of states, and the frequency-dependent linear and nonlinear optical responses are presented here and compared with available experimental data. Good agreement is found. Our calculations show that these compounds have similar electronic structures. The valence band maximum and the conduction band minimum are located at Gamma resulting in a direct energy gap. The energy band gap of these compounds decreases when P is replaced by As and As by Sb. This can be attributed to the increase in bandwidth of the conduction bands. The linear and nonlinear optical spectra are analyzed and the origin of some of the peaks in the spectra is discussed in terms of the calculated electronic structure. The calculated linear optical properties show very good agreement with the available experimental data. We find that the intra-and interband contributions of the second-harmonic generation increase when moving from P to As to Sb. The smaller energy band gap compounds have larger values of chi(123) ((2))(0) in agreement with the experimental measurements and other theoretical calculations.

Theoretical investigation of the electronic properties, and first and second harmonic generation for cadmium chalcogenide.

We present the results of the ab initio theoretical study of the electronic properties, and first and second harmonic generation for CdX compounds with zinc-blende structure performed using the full potential linearized augmented plane wave method. Our calculations show that these compounds have similar structures. The valence band maximum and the conduction band minimum are located at Gamma, resulting in a direct energy gap. The energy gap of these compounds decreases when S is replaced by Se and Se by Te, in agreement with the experimental data and previous theoretical work. This can be attributed to the increase in the bandwidth of the conduction bands. The optical spectra are analyzed and the origin of some of the peaks in the spectra is discussed in terms of the calculated electronic structure. Our calculations for the linear optical properties show excellent agreement with the available experimental data.