W.-D. Cheng

Density functional theoretical determinations of electronic and optical properties of nanowires and bulks for CdS and CdSe

The authors present density functionaltheoretical results of electronic and optical properties of wurtzite-type CdS and CdSesemiconductornanowires and bulks. The results show that quantum confinement will increase the energy gap, decrease the dielectric function, change the sign of birefringence, and increase the absolute value of birefringence at low-frequency region, as the dimensions of CdS and CdSe are reduced from the bulk to a nanowire with diameter of several nanometers. The optical spectral peaks of CdSenanowires show redshifts as compared with the corresponding ones of CdSnanowires.

A theoretical investigation of hyperpolarizability for small GanAsm (n+m=4–10) clusters

In this paper, the second and third order polarizabilities of small Ga(n)As(m) (n + m=4-10) clusters are systematically investigated using the time dependent density functional theory (TDDFT)6-311+G* combined with the sum-over-states method (SOSTDDFT6-311+G*). For the static second order polarizabilities, the two-level term (beta(vec.2)) makes a significant contribution to the beta(vec) for all considered Ga(n)As(m) clusters except for the Ga3As4 cluster. And, for the static third order polarizabilities, the positive channel (gamma(II)) makes a larger contribution to gamma(tot) than the negative channel (gamma(I)). Similar to the cubic GaAs bulk materials, the small Ga(n)As(m) cluster assembled materials exhibit large second order (1 x 10(-6) esu) and third order susceptibilities (5 x 10(-11) esu). The dynamic behavior of beta(-2omega; omega, omega) and gamma(-3omega; omega, omega, omega) show that the small Ga(n)As(m) cluster will be a good candidate of nonlinear optical materials due to the avoidance of linear resonance photoabsorption.

TDHF-SOS treatments on linear and nonlinear optical properties of III-V semiconductor clusters (Ga3As3, Ga3Sb3, In3P3, In3As3, In3Sb3)

Abstract We employ an ab initio time-dependent Hartree–Fock (TDHF) formalism combined with sum-over-states (SOS) method to calculate the linear and nonlinear optical properties of direct semiconductor clusters. The obtained electronic absorption spectra predict a variety of the spectral shapes and peak positions and an increase of the second-order nonlinear optical response with an increase of V-group ionic radius in the title clusters. Charge transfers from the π bonding to π antibonding orbitals between III and V group atoms make a significant contribution to the second-order polarizability.

First-principles study: size-dependent optical properties for semiconducting silicon carbide nanotubes

Using first-principles calculations, we investigate the effect of tube size on optical properties of the zigzag, armchair, and chiral SiC nanotubes. The results indicate that the optical spectra of SiC nanotubes are dependent on the diameter and chirality, and that optical anisotropy is observed for different light polarizations. For a given chirality of SiCNTs, redshifts or blueshifts of the peaks in the dielectric function and energy loss function with increasing tube diameter are possible due to the competition between the size effect and pi orbitals overlapping, and the shifts become smaller as the tube diameter increases. The unusual optical properties of semiconducting SiC nanotubes present an opportunity for applications in electro-optical devices.

A periodic density functional theory study on the effects of halides encapsulated in SiC nanotubes

In this paper we present results of density functional theory calculations on the configurations, band structures, and optical properties of halides MCl (M=K,Ag) intercalated single-wall SiC nanotubes. The results show that the M-Cl distances perpendicular to the tube axis are slightly smaller than the ones parallel to the tube axis, which could be due to the axial strain of MCl. The electronic and optical properties of the resulting MCl@SiCNT composite are modified with respect to both the bulk halide and the empty nanotube. It is shown that AgCl affects the structures and properties of SiC nanotubes more significantly than KCl, and that the interaction between the nanotube and the encapsulated halide is stronger for narrower SiC nanotube. The AgCl encapsulation into SiCNTs results in band gap narrowing of AgCl@SiCNTs.

First-principles calculations of band structures and dynamic optical properties of CsCdBr3 and RbCdI3∙H2O crystals

We report the band structures as well as frequency-dependent (dynamic) linear and nonlinear optical properties of CsCdBr3 and RbCdI3∙H2O crystals calculated within density functional theory with a nonlocal gradient correction, including the dielectric function, linear and nonlinear refractive indices, frequency doubling, and self-focusing of light. The electronic origin of the optical susceptibilities is described and the critical power of light self-focusing is estimated for these two crystals.

Ab initio study of ferromagnetic semiconductor Ge1−xMnxTe

First-principles calculations are made on the Ge1−xMnxTe diluted magnetic semiconductors with different compositions. The origin of their ferromagnetism is investigated and the carrier-induced ferromagnetism is developed for these diluted magnetic semiconductors. The Mn 3d states localized with divalent character are deep below the Fermi level, and the carriers introduced by defects are supported in our calculation. The p states of Ge also play an important role in the occurrence of ferromagnetism especially at high temperature. It is thought that Ge1−xMnxTe with a moderate composition (x = 0.51) of Mn atoms is a good candidate for DMS materials.

Magnetodielectric and magnetoelastic coupling in Co4Nb2O9 below Néel temperature

The dielectric constant of Co4Nb2O9 shows an obvious upturn below Neel temperature (TN), and the magnetic field enhances this anomaly. Thus the magnetodielectric effect exists in the entire temperature region below TN, not only near TN as reported previously. At the same time, the strain exhibits a drop below TN, and the magnetic field suppresses this anomaly, demonstrating magnetoelastic coupling in Co4Nb2O9. The origin of dielectric anomaly and magnetodielectric effect below TN may be related to the phonon mode shifts induced by magnetoelastic coupling.

Ab initio characterization of the electronic and optical properties of a new IR nonlinear optical crystal: K3V5O14

We present detailed investigations of the electronic and optical properties of K3V5O14, including the band structure, density of states (DOS), population analysis, dielectric function, refractive index and second-order nonlinear susceptibilities. The calculations are performed using the ab initio pseudopotential density functional method combined with an anharmonic oscillator model, in which we employ the Perdew–Burke–Eruzerhof form of the generalized gradient approximation together with plane-wave basis sets for expanding the periodic electron density. From the band calculation, K3V5O14 is predicted to be an indirect band gap semiconductor. From the DOS and population analysis, we find that the bonding between K+ and V5O14 layers is mainly ionic while that between V and O is covalent. It is indicated that the hybridization of V-3d with the O-2p states is very important for the optical properties of K3V5O14. The calculated birefringence is large enough to achieve phase-matchable conditions, and our calculated second-harmonic-generation (SHG) coefficients agree with experimental results.

Modeling of configurations and third-order nonlinear optical properties of methyl silsesquioxanes

Configuration optimizations, excited state properties, and the frequency-dependent third-order nonlinear optical polarizabilities have been investigated on a series of methyl-silsesquioxane (MeT) cages [CH(3)SiO(1.5)](n) (n=4, 6, 8, and 10) using ab initio quantum mechanical methods coupled with the sum-over-states methods. The obtained electronic absorption spectra show a redshift as the cage size increases, and the absorption spectra are assigned as charge transfers from oxygen p type to silicon s type atomic orbitals. The calculated average third-order polarizabilities of in the three optical physical processes (third-harmonic generation, the electric-field-induced second-harmonic generation, and degenerate four-wave mixing) have wide nonresonance regions. For all the considered species, the values of gamma decrease in the order of [MeT](4)>[MeT](8)(C(2v))=[MeT](10)>[MeT](6)>[MeT](8)(O(h)).