Real-space Atom-cutting Method Research Articles

Size effect and identified superior functional units enhancing second harmonic generation responses on the II-IV-V2 type nonlinear optical crystals

The II-IV-V2 type nonlinear optical (NLO) materials with chalcopyrite structures have wide applications in infrared (IR) frequency conversion, optical parametric oscillation, and terahertz wave producing, which can be as references to explore IR NLO systems. The template effect, mainly the size effect of five II-IV-V2 type compounds (ZnGeP2, ZnSiP2, ZnSnP2, CdGeP2 and CdGeAs2) on the optical properties is analyzed in terms of microscopic structures and electronic structures. The mechanism for significant differences in the second harmonic generation (SHG) intensities from 2.8 to 15 × AgGaS2 (the IR benchmark) is clarified via the density functional theory, the real space atom-cutting method and SHG-weight density analysis. It is found that the size effect of ions in this family of crystals is mainly responsible for the decreases of band gaps, then leads to the enhancement of SHG intensities. Moreover, the real space atom-cutting calculations show that the [SiP4], [GeP4], [SnP4] and [GeAs4] dominate the contributions to SHG response, which are identified as the superior functional units and are conducive to enhancing the SHG effects. Therefore, one can select those NLO superior functional units that enlarge the cell size and SHG responses to obtain new IR NLO materials.

Intriguing Structural Transition Inducing Variable Birefringences in ABa2MS4Cl (A = Rb, Cs; M = Ge, Sn).

A suite of quinary thiohalides, ABa2MS4Cl (A = Rb, Cs; M = Ge, Sn), have been successfully synthesized. ABa2MS4Cl (A = Rb, Cs; M = Ge, Sn) undergoes an interesting structural transition from P21/ c (α phase) to I4/ mcm (β phase), which also leads to obvious disparities on the birefringence from 0.041 (α phase) to 0.094 (β phase). Moreover, the result of deploying a real-space atom-cutting method on ABa2MS4Cl reveals that their (MS4)4- groups may take the major effect in terms of the anisotropy by a structural transition.

Theoretical study on Pb2VO2F5: large birefringence derived from optical anisotropies of VO2F4 groups

The Pb2VO2F5 crystal was synthesized by hydrothermal method. The crystal structure consists of tetranuclear [Pb4O6F22] and twinborn V2O4F6 units, which share the O and F atoms forming a three-dimensional framework. The birefringence of the compound was investigated using the first principles. The calculations results show that the compound has a large birefringence of 0.336 at 1064 nm. Researches indicate that its large birefringence originates mainly from the distorted VO2F4 groups based on the real-space atom-cutting method and structure–property relationships analysis. Furthermore, the IR spectrum, thermal behaviors, and UV–Vis–IR diffuse reflectance spectroscopy of the compound are also presented.

First-principles study lone-pair effects of Sb (III)-S chromophore influence on SHG response in quaternary potassium containing silver antimony sulfides

First-principles studies of the geometric structures, electronic structures and optical properties of non-centrosymmetrical (NCS) KAg2SbS4, K2AgSbS4 and K2Ag3Sb3S7 and centrosymmetrical (CS) KAg2SbS3 and K3Ag9Sb4S12 have been performed by means of density functional theory. It is indicated that although they have similarity in their anionic groups and, the electronic structures, prominent features of the optical anisotropy or second-order nonlinear optical (NLO) susceptibilities are apparently different. The calculated birefringences are 0.0537, 0.0343, 0.1324, 0.2217 and 0.0604 which are attributed to the different anionic groups [SbS3] and triangles [AgS3]. Also, the calculated NLO responses are about 0.5×, 1.0× and 2.0× times than that commercial AgGaS2 (AGS, d36=11pm/V) for K2AgSbS4, KAg2SbS4 and K2Ag3Sb3S7, respectively. In comparison with the absolute magnitude of second harmonic generation (SHG) coefficients, the order K2Ag3Sb3S7>KAg2SbS4>K2AgSbS4 is clearly established in the SHG response. Further analysis based on the real-space atom-cutting method reveals that the main sources of the SHG properties of these compounds are from the Sb-S group, especially K2Ag3Sb3S7 is mainly attributed to the lone pair stereochemical activity of Sb (III)-S group.

The mechanism for the nonlinear optical properties in La9Na3B8O27, La2Na3B3O9 and La2CaB10O19: ab initio studies

The nonlinear optical (NLO) properties in La9Na3B8O27, La2Na3B3O9 and La2CaB10O19 are investigated using a plane-wave pseudopotential method. Based on the first-principles electronic band structures, the birefringence and second harmonic generation (SHG) coefficients for the three crystals are determined. The calculated values match well with the experimental results. In order to elucidate the microscopic origins of the NLO performances in these crystals, a real-space atom-cutting method is adopted to calculate the contribution from the respective ions and groups. It is revealed that the La cations have covalent interactions with the neighbor O ions so as to form the La–O groups. The La–O bond covalency, combined with the distortion of La–O groups, makes the large contribution to the overall SHG coefficients in addition to the B–O groups, as also intuitively demonstrated by a SHG-density analysis.

The interaction between cations and anionic groups inducing SHG enhancement in a series of apatite-like crystals: A first-principles study

It is an interesting topic to reveal the origin of the SHG intensity enhancement after substitution from alkali and alkali-earth metal atoms to cadmium in a series of apatite-like borates KSr4(BO3)3, Ca5(BO3)3F, Cd5(BO3)3F. Combined with the first-principles method, SHG-density method and real-space atom-cutting method, the electronic structure, the optical properties and the contribution of respective ion and ion groups have been investigated. Second harmonic generation (SHG) responses are mainly attributed to BO3 groups with π conjugated configuration and their alignment framework. The contributions of A site are more important in CaBOF and CdBOF compounds than in KSrBO. It is also demonstrated that the strong covalent interactions between the boron–oxygen groups and the cadmium atoms contribute the enhancement of SHG responses after substitution from alkali and alkali-earth metal atoms.

Experimental and Theoretical Studies on the Linear and Nonlinear Optical Properties of Bi2ZnOB2O6

A large single crystal of a nonlinear optical material Bi2ZnOB2O6 (BZB) has been grown by the Czochralski method. The experiments of linear and nonlinear optical property were performed. On the basis of the density functional theory (DFT), the first-principles calculations have been employed to study the structural and electronic properties of BZB successfully. The calculated results are essentially consistent with the corresponding experimental results. In addition, to gain further insight into the structure–property relationship, a real-space atom-cutting method was adopted to analyze the origin of the nonlinear optical response of BZB for the first time. The results indicate that the contributions of the (BiO6)9– and (B2O5)4– groups are dominant in the BZB crystal for producing large microscopic second-order susceptibilities.

Ab initio studies on the optical effects in the deep ultraviolet nonlinear optical crystals of the KBe2BO3F2 family

Electronic structures of the deep ultraviolet nonlinear optical crystals of the KBe2BO3F2 (KBBF) family, including KBBF, RbBe2BO3F2 and CsBe2BO3F2, have been investigated based on a plane-wave pseudopotential method. Their linear and nonlinear optical coefficients are also calculated, and are in good agreement with the experimental results. A real-space atom-cutting method is adopted to analyze the respective contributions of the alkali metal cations and anionic groups to optical response. The results show that the contributions of anionic groups to the nonlinear optical anisotropic responses are dominant, but the influence of the A-site alkali metal cations becomes slightly more pronounced with the increase of their radius. Moreover, the birefringence difference among these crystals strongly depends on the volume effect, i.e., the spatial density of the (BO3)3− anionic groups.

Ab initio studies on the mechanism for linear and nonlinear optical effects in YAl3(BO3)4

First-principles studies of the linear and nonlinear optical properties for YAl3(BO3)4 (YAB) are presented. Based upon the electronic band structure, the optical refractive indices, birefringence, and second harmonic generation (SHG) coefficients of YAB are calculated, which are in good agreement with experimental values. In addition, the SHG-weighted electron density analysis and the real-space atom-cutting method are adopted to elucidate the origin of the linear and nonlinear optical effects in YAB. The results show that the anionic (BO3) groups have dominant contributions to the birefringence. The contribution of the Al cations to the optical effects is negligibly small. However, the Y cations bond to the neighbor O anions and form the deformed (YO6) octahedra, which results in the large SHG effects in YAB.

The mechanism of linear and nonlinear optical effects in fluoride crystals

Electronic structure calculations of Na2SbF5, BaMgF4, and BaZnF4 are performed from first principles based on a plane-wave pseudopotential method. The linear optical properties and the static second-harmonic generation coefficients are also calculated. Furthermore, a real-space atom-cutting method is employed to analyze the respective contributions of the anionic groups and cations in Na2SbF5 to the optical response. The results show that the anionic groups (SbF5)2− in the crystal are quite favorable for producing larger microscopic second-order susceptibilities.

The development of new borate-based UV nonlinear optical crystals

The developments of nonlinear optical (NLO) crystals in the last decade are reviewed from two aspects. In the theoretical part, on the ab initio plane-wave pseudopotential energy method, calculation of the optical responses, including the second harmonic generation coefficients, a computer-program package, and a real-space atom-cutting method are described and employed to evaluate the degree of approximation of the anionic group theory for nonlinear optical crystals. The ab initio method combined with the anionic group theory gives much more insight into the understanding of the relationship between the optical responses and microscopic structures of NLO crystals, borate-based NLO crystals in particular. In the experimental part, we describe how to use the anionic group theory method and an experimental evaluation system to develop a series of borate-based UV-NLO crystals such as KBe2BO3F2 (KBBF) and K2Al2B2O7 (KABO). The capability of these new UV-NLO crystals in producing deep-UV harmonic output is evaluated and determined. Finally, recent advances in the generation of deep- and vacuum-UV harmonic output with these new NLO crystals are reported.

Mechanism for linear and nonlinear optical effects in crystals of the Sr2Be2B2O7 family

Electronic structure calculations of Sr2Be2B2O7 (SBBO) family crystals including Sr2Be2B2O7, BaAl2B2O7, and K2Al2B2O7 have been performed based on a plane-wave pseudopotential method. Moreover, the linear optical coefficients and the static second-harmonic generation (SHG) coefficients of BaAl2B2O7 and K2Al2B2O7 are calculated at the independent-particle level. The calculated refractive indices and SHG coefficients are in good agreement with experimental values. In addition, a real-space atom-cutting method is adopted to analyze the respective contributions of the cations and anionic group to optical response. The results show that in these crystals, when the radii of the cations increase, their contributions to the SHG effect become slightly more pronounced, however, the contributions to the SHG coefficients from the (BO3)3− and (AlO4)5− anionic groups are dominant and comparable.

Mechanism for linear and nonlinear optical effects in KBe 2BO 3F 2 (KBBF) crystal

Electronic structure calculations of KBe 2BO 3F 2 crystal from first principles have been performed based on a plane-wave pseudopotential method for the first time. Its optical linear and second harmonic generation (SHG) coefficients are also calculated. The SHG coefficient measured early was much higher than that of our calculation, however, recent experiment confirms our calculated results. Moreover, a real-space atom-cutting method is adopted to analyze the respective contributions of the cation and anionic group to optical response. The results show that the contributions to the SHG coefficients from the (BO 3) 3− and (BeFO 3) 5− groups are dominant and comparable.

Mechanism for linear and nonlinear optical effects in SrBe3O4 crystal

Electronic and band structure calculations of SrBe3O4 crystal from first principles are performed based on a plane-wave pseudopotential method for the first time. The linear refractive indices and the static second-harmonic generation (SHG) coefficients are also calculated by the SHG formula improved by our group. The calculated values are in good agreement with the experimental values. A real-space atom-cutting method is adopted to analyze the respective contributions of the cation and anionic groups to the optical response. The results show that the contribution of the (SrO9) group to the SHG coefficients is more pronounced than that of (BeO3)4− and (BeO4)6− groups. Because the plane BeO3 and tetrahedral BeO4 groups have a little covalent bonding, they only give a tenth of the contribution of the SrO9 group.

Mechanism for linear and nonlinear optical effects in monoclinic bismuth borate (BiB3O6) crystal

Electronic structure calculations of BiB3O6 crystal from first principles are performed based on a plane-wave pseudopotential method. The linear refractive indices and the static second-harmonic generation (SHG) coefficients are also calculated by the SHG formula improved by our group. The calculated values are in good agreement with the experimental values. A real-space atom-cutting method is adopted to analyze the respective contributions of the cation and anionic groups to the optical response. The results show that the contribution of the (BiO4)5− anionic group to the SHG coefficients is more pronounced than that of the (BO3)3− and (BO4)5− groups.

CALCULATIONS FOR THE LINEAR AND NONLINEAR OPTICAL COEFFICIENTS OF NaNO2 CRYSTAL

Based on a plane-wave pseudopotential method,the electronic structure calculations of NaNO2 from first principles are perforined.The linear refractive indices and the static second-harmonic generation (SHG) coefficients are also calculated by the SHG formula improved by us.The calculated values are in good agreement with the experimental values.A real-space atom-cutting method is adopted to analyze the respective contributions of the cation and anionic groups to optical response.The results show that the contribution of the (NO2)- anionic group to the birefringence and the SHG coefficients is dominated in NaNO2 crystal,which is in agreement with the result obtained by the anionic group model.

Mechanism for linear and nonlinear optical effects inLiB3O5,CsB3O5,andCsLiB6O10crystals

Electronic structure calculations of ${\mathrm{LiB}}_{3}{\mathrm{O}}_{5},$ ${\mathrm{CsB}}_{3}{\mathrm{O}}_{5},$ and ${\mathrm{CsLiB}}_{6}{\mathrm{O}}_{10}$ crystals from first principles are performed based on a plane-wave pseudopotential method. The static second-harmonic generation (SHG) coefficients are calculated at the independent-particle level with a formalism improved by our group and co-workers [Phys. Rev. B 60, 13 380 (1999)]. A real-space atom-cutting method is adopted to analyze the respective contributions of the cation and anionic groups to optical response. The calculated refractive indices and SHG coefficients are in good agreement with the experimental values. On the basis of these calculations, the influence of the cations on the band gaps and the optical responses is evaluated. The results show that with the increase of their radius their contributions to SHG become slightly more pronounced.

Mechanism for linear and nonlinear optical effects inβ−BaB2O4crystals

Electronic structure calculations of $\ensuremath{\beta}\ensuremath{-}{\mathrm{BaB}}_{2}{\mathrm{O}}_{4}$ from first principles are performed based on a plane-wave pseudopotential method, and the linear optical properties are then obtained. The static second-harmonic generation (SHG) coefficients are calculated at the independent-particle level with a formalism originally given by Aversa and Sipe [Phys. Rev. B 52, 14 636 (1995)] and later rearranged by Rashkeev et al. [Phys. Rev. B 57, 3905 (1998)] to explicitly show Kleinman's symmetry. The formalism is improved to be more efficient in reducing the $k$ points necessary for convergence. A real-space atom-cutting method is suggested to analyze the respective contributions of various transitions among ions and ion groups to optical response. The contribution of the cation Ba to SHG effects is found to be not important but non-negligible, while its contribution to birefringence is negligible.