The influence of oxygen vacancies on the linear and nonlinear optical properties of Pb7O(OH)(3)(CO3)(3)(BO3)

We have investigated the influence of the oxygen vacancy on the linear and nonlinear optical properties and the microscopic first hyperpol of asymmetric Pb7O(OH)(3)(CO3)(3)(BO3). The O-vacancy reduces the energy gap and changes the energy band gap from indirect to direct. The calculated indirect energy band gap of Pb7O(OH)(3)(CO3)(3)(BO3) (I) of 3.56 eV is in good agreement with the experimental gap of 3.65 eV. The direct gap in O-deficient Pb7O(OH)(3)(CO3)(3)(BO3)) (II) is 1.61 eV. The oxygen vacancy results in a red-shifted energy band gap, making the material useful in the visible region. Calculations show that I exhibits a negative uniaxial anisotropy and birefringence, whereas II exhibits positive uniaxial anisotropy and birefringence. This shows that the O-vacancy has a significant influence on the uniaxial anisotropy and birefringence. We have calculated the second harmonic generation (SHG) for I and II at zero energy limit and at wavelength lambda = 1064 nm. The calculated SHG for I at lambda = 1064 nm is close to the experimental value of the well known compound KTiOPO4 (KTP), whereas for II it is about a quarter of the experimental value of the KTP. In addition, the microscopic first hyperpolarizability for I and II is calculated at the static limit and at wavelength 1064 nm

Optical second harmonic generation in Yttrium Aluminum Borate single crystals (theoretical simulation and experiment)

Experimental measurements of the second order susceptibilities for the second harmonic generation are reported for YAl3(BO3)4 (YAB) single crystals for the two principal tensor components xyz and yyy. First principles calculation of the linear and nonlinear optical susceptibilities for Yttrium Aluminum Borate YAl3(BO3)4 (YAB) crystal have been carried out within a framework of the full-potential linear augmented plane wave (FP-LAPW) method. Our calculations show a large anisotropy of the linear and nonlinear optical susceptibilities. The observed dependences of the second order susceptibilities for the static frequency limit and for the frequency may be a consequence of different contribution of electron-phonon interactions. The imaginary parts of the second order SHG susceptibility chi_{123}^{(2)}(omega), chi_{112}^{(2)}(omega), chi_{222}^{(2)}(omega), and chi_{213}^{(2)}(omega) are evaluated. We find that the 2(omega) inter-band and intra-band contributions to the real and imaginary parts of chi_{ijk}^{(2)}\l(omega) show opposite signs. The calculated second order susceptibilities are in reasonably good agreement with the experimental measurements.Comment: 16 pages, 11 figure

Electronic structure, linear, nonlinear optical susceptibilities and birefringence of CuInX2 (X = S, Se, Te) chalcopyrite-structure compounds

The electronic structure, linear and nonlinear optical properties have been calculated for CuInX2 (X=S, Se, Te) chalcopyrite-structure single crystals using the state-of-the-art full potential linear augmented plane wave (FP-LAPW) method. We present results for band structure, density of states, and imaginary part of the frequency-dependent linear and nonlinear optical susceptibilities. We find that these crystals are semiconductors with direct band gaps. We have calculated the birefringence of these crystals. The birefringence is negative for CuInS2 and CuInSe2 while it is positive for CuInTe2 in agreement with the experimental data. Calculations are reported for the frequency-dependent complex second-order non-linear optical susceptibilities . The intra-band and inter-band contributions to the second harmonic generation increase when we replace S by Se and decrease when we replace Se by Te. We find that smaller energy band gap compounds have larger values of in agreement with the experimental data and previous theoretical calculations.Comment: 17 pages, 6 figure

Electronic and optical properties of chair-like and boat-like graphane

We have studied two favorable conformations of graphane, the chair-like graphane and boat-like graphane. In the chair-like, the H atoms attached to the C atoms alternate on both sides of the sheet while in the boat-like, the C bonded H atoms alternate in pair. Both conformations of graphane have a 2D puckered honeycomb like structure with one hydrogen atom bonded covalently ( sp(3)) to each carbon atom. The chair-like belongs to the P (3) over bar m1 (164) space group and the boat-like belongs to the Pmmn (59) space group. We have used the state-of-the-art full potential linear augmented plane wave (FPLAPW) method with different possible approximations for the exchange-correlation (XC) potential. The XC potential was described by the local density approximation (LDA) of Ceperley-Alder (CA), the generalized gradient approximation (GGA) of Perdew-Becke-Ernzerhof (PBE) and the Engel-Vosko generalized gradient approximation (EVGGA). The calculated partial density of states for both configurations, show that there exists a strong hybridization between C and H orbitals which confirms the existence of the covalent bonds. The electronic charge density distribution of both configurations (chair-like and boat-like) has been calculated, the charge accumulates along C-C and C-H bonds. According to the electronegativity values of C (2.55) and H (2.1), it is clear that there is strong covalent bonding between C and H atoms. The linear optical properties give a deep insight into the electronic structure. The calculated values of the energy gap and the bond lengths show good agreement with previous results

Adsorbing H2S onto a single graphene sheet: A possible gas sensor

The electronic structure of pristine graphene sheet and the resulting structure of adsorbing a single molecule of H2S on pristine graphene in three different sites (bridge, top, and hollow) are studied using the full potential linearized augmented plane wave method. Our calculations show that the adsorption of H2S molecule on the bridge site opens up a small direct energy gap of about 0.1 eV at symmetry point M, while adsorption of H2S on top site opens a gap of 0.3 eV around the symmetry point K. We find that adsorbed H2S onto the hollow site of pristine graphene sheet causes to push the conduction band minimum and the valence band maximum towards Fermi level resulting in a metallic behavior. Comparing the angular momentum decomposition of the atoms projected electronic density of states of pristine graphene sheet with that of H2S-graphene for three different cases, we find a significant influence of the location of the H2S molecule on the electronic properties especially the strong hybridization between H2S molecule and graphene sheet

Dispersion of the linear and nonlinear optical susceptibilities of the CuAl(S1-xSex)(2) mixed chaclcopyrite compounds

Based on the electronic band structure, we have calculated the dispersion of the linear and nonlinear optical susceptibilities for the mixed CuAl(S1-xSex)(2) chaclcopyrite compounds with x = 0.0, 0.25, 0.5, 0.75, and 1.0. Calculations are performed within the Perdew-Becke-Ernzerhof general gradient approximation. The investigated compounds possess a direct band gap of about 2.2 eV (CuAlS2), 1.9 eV (CuAl(S0.75Se0.25)(2)), 1.7 eV (CuAl(S0.5Se0.5)(2)), 1.5 eV (CuAl(S0.25Se0.75)(2)), and 1.4 eV (CuAlSe2) which tuned to make them optically active for the optoelectronics and photovoltaic applications. These results confirm that substituting S by Se causes significant band gaps' reduction. The optical function's dispersion epsilon(xx)(2) (omega) and epsilon(zz)(2) (omega)/epsilon(xx)(2) (omega), epsilon(yy)(2) (omega) and epsilon(zz)(2) (omega) was calculated and discussed in detail. To demonstrate the effect of substituting S by Se on the complex second-order nonlinear optical susceptibility tensors, we performed detailed calculations for the complex second-order nonlinear optical susceptibility tensors, which show that the neat parents compounds CuAlS2 and CuAlSe2 exhibit vertical bar chi((2))(123)(-2 omega;omega;omega)vertical bar as the dominant component, while the mixed alloys exhibit vertical bar chi((2))(111)(-2 omega;omega;omega)vertical bar as the dominant component. The features of vertical bar chi((2))(123)(-2 omega omega;omega;omega)vertical bar and vertical bar chi((2))(111)(-2 omega omega;omega;omega)vertical bar spectra were analyzed on the basis of the absorptive part of the corresponding dielectric function epsilon(2) (omega) as a function of both omega/2 and omega

Thermoelectric properties of a single graphene sheet and its derivatives

The thermoelectric properties of pristine graphene and H2S adsorbed onto bridge, hollow and top sites of a graphene sheet are investigated using the semi-classical Boltzmann transport theory. The average values of electrical conductivity, thermal conductivity, Seebeck coefficient, figure of merit (ZT) and the average value of the power factor (P-av) are reported and discussed in detail. While pristine graphene is a zero band gap semiconductor, adsorption of H2S onto the bridge site opens up a direct energy gap of about 0.04 eV, adsorption of a H2S molecule onto the top site opens up a gap of 0.3 eV, and adsorption of H2S onto the hollow site makes it metallic. The investigation of ZT and power factor values suggests that a top-site configuration could be a potential candidate for thermoelectric applications in the range 300-600 K

Linear and nonlinear optical properties for AA and AB stacking of carbon nitride polymorph (C3N4)

The linear and nonlinear optical susceptibilities of AA and AB stacking of the carbon nitride polymorph were calculated using the all electron full potential linear augmented plane wave method based on density functional theory. The complex part of the dielectric function is calculated using the recently modified Becke and Johnson (mBJ) approximation which gives a better optical gap in comparison to the Ceperley-Alder (CA) local density approximation, the Perdew-Burke-Ernzerhof generalized gradient approximation, and the Engel-Vosko generalized gradient approximation. The complex dielectric function and other optical constants like refractive index, absorption coefficient, reflectivity and energy loss function are calculated and discussed in detail. The calculated uniaxial anisotropy (-1.06 and -1.04) gives a maximum value of birefringence (-0.89 and -0.87) which increases the suitability of both AA and AB stacking for a large second harmonic generation. The calculated second order susceptibility tensor components vertical bar chi((2))(333)(omega)vertical bar at the static limit are 19.4 pm V-1 and 59.6 pm V-1 for AA and AB stacking which increases to 34.2 pm V-1 and 106.7 pm V-1 at lambda = 1064 nm. The first hyperpolarizability beta(333)(omega) for AA and AB stacking C3N4 for the dominant component vertical bar chi((2))(333)(omega)vertical bar at the static limit are calculated to be (1.6 x 10(-30) esu and 9.6 x 10(-30) esu) respectively