SHG Response Research Articles

ChemInform Abstract: K2Au(IO3)5 and β‐KAu(IO3)4: Polar Materials with Strong SHG Responses Originating from Synergistic Effect of AuO4 and IO3 Units.

AbstractThe new polar compounds (IV) and (V) are characterized by single crystal XRD, IR, UV/VIS/NIR diffuse reflectance spectroscopy, and DFT electronic band structure calculations.

NaBa4(GaB4O9)2X3 (X = Cl, Br) with NLO-Active GaO4 Tetrahedral Unit: Experimental and ab Initio Studies

Two gallium borates, NaBa4(GaB4O9)2X3 (X = Cl, Br), were synthesized by high temperature solution method. The title compounds crystallize in the same space group P42nm. Their structures feature a 3D [GaB4O9]∞ framework composed of GaO4 tetrahedra and B4O9 groups. NaBa4(GaB4O9)2X3 (X = Cl, Br) compounds show the largest second harmonic generation responses among alkali metal and alkaline-earth metal gallium borates that are 1.5 and 1.1 times that of KH2PO4 (KDP), respectively, and are type I phase-matchable. Combined electronic structure and SHG density calculations as well as local dipole moment analysis have revealed that B–O groups and GaO4 tetrahedron play major roles in SHG response.

A pair of novel 4-connected homochiral coordination polymers based on proline-tetrazole ligand

A pair of novel 2D homochiral coordination polymers based on proline-tetrazole ligand, namely [Cd((S)-PTZ)2]n (1-L) and [Zn((S)-PTZ)2]n (2-L), were obtained and characterized by elemental analysis, IR spectroscopy, thermogravimetric analysis, solid-state CD spectra, single-crystal X-ray crystallography and powder X-ray diffraction. The compounds 1-L and 2-L display 4-connected net and SHG response.

K2 Au(IO3)5 and β-KAu(IO3)4: Polar Materials with Strong SHG Responses Originating from Synergistic Effect of AuO4 and IO3 Units.

Two new polar potassium gold iodates, namely, K2 Au(IO3)5 (Cmc21) and β-KAu(IO3)4 (C2), have been synthesized and structurally characterized. Both compounds feature zero-dimensional polar [Au(IO3)4](-) units composed of an AuO4 square-planar unit coordinated by four IO3(-) ions in a monodentate fashion. In β-KAu(IO3)4, isolated [Au(IO3)4](-) ions are separated by K(+) ions, whereas in K2 Au(IO3)5, isolated [Au(IO3)4](-) ions and non-coordinated IO3(-) units are separated by K(+) ions. Both compounds are thermally stable up to 400 °C and exhibit high transmittance in the NIR region (λ=800-2500 nm) with measured optical band gaps of 2.65 eV for K2 Au(IO3 )5 and 2.75 eV for β-KAu(IO3)4. Powder second-harmonic generation measurements by using λ=2.05 μm laser radiation indicate that K2 Au(IO3)5 and β-KAu(IO3)4 are both phase-matchable materials with strong SHG responses of approximately 1.0 and 1.3 times that of KTiOPO4, respectively. Theoretical calculations based on DFT methods confirm that such strong SHG responses originate from a synergistic effect of the AuO4 and IO3 units.

Influence of original and simulated microscopic units on SHG response in semiorganic NLO materials

The SHG effect increase was explored by combining the electronic structure, SHG density, molecular orbital analysis and hyperpolarizability density.

Active performance of tetrahedral groups to SHG response: theoretical interpretations of Ge/Si-containing borate crystals.

As potential candidates for deep-UV nonlinear optical (NLO) crystals, borosilicates and borogermanates, which contain NLO-active groups such as B-O, Si-O and Ge-O, have fascinated many scientists. The crystal structures, electronic structures and optical properties of seven borates in different B/R (R = Si, Ge) ratios have been studied using DFT methods. Through the SHG-density, we find that besides the recognized contribution of the π-conjugation configuration of BO3 to second harmonic generation (SHG), the tetrahedra have a non-negligible influence. This is because the non-bonding p orbitals of the bridging oxygen in the tetrahedra are observably closer to the Fermi level than those in BO3, which is observed in the PDOS of Rb4Ge3B6O17 and RbGeB3O7. This conclusion would be very meaningful in the understanding of the relationship between the crystal structure and nonlinear optical properties.

Rb2SeOCl4·H2O: a polar material among the alkali metal selenite halides with a strong SHG response.

An alkali metal selenite chloride, Rb2SeOCl4·H2O, has been hydrothermally synthesized and structurally characterized. It is the first example of an alkali metal selenite halide in the literature. The compound crystallizes in the noncentrosymmetric (NCS) space group, Cmc21(36), of the orthorhombic system with a = 10.342(3) Å, b = 10.124(3) Å, c = 9.158(3) Å, and α = β = γ = 90°. The anionic [SeOCl4]2- groups are arranged in the crystal in nearly the same direction, giving rise to a relatively large macroscopic dipole moment, causing the compound to display second harmonic generation (SHG) eight times as strong as that of KDP, measured using the Kurtz-Perry method on powders. First-principle density functional theory (DFT) calculations were carried out to interpret the relationship between the crystal structure and properties.

A series of s-block (Ca, Sr and Ba) metal–organic frameworks: synthesis and structure–property correlation

A series of metal–organic frameworks based on alkaline earth metal ions (Ca, Sr and Ba) and 1,3,5-benzenetribenzoic acid (BTB) have been synthesized and characterized. These are [H2N(CH3)2][Ca7(BTB)5(H2O)8(DMF)4]·4H2O (1), [H2N(CH3)2]2[Sr5(H2O)6(BTB)4] (2) and [H2N(CH3)2][Ba(H2O)(BTB)] (3). All the structures are three-dimensional in nature with different secondary building units. Compound 1 contains one-dimensional Ca–O–Ca zigzag chains while compound 2 contains Sr5O28 pentameric clusters and compound 3 contains one-dimensional Ba–O–Ba chains. Both compounds 1 and 3 form (3,6)-net connectivity while compound 2 forms (3,12)-net connectivity. Optical band gap energy measurements show that compound 1 (2.65 eV) has low band gap energy compared to 2 (3.22 eV) and 3 (3.32 eV). This variation in band gap energy may be due to the difference in structural arrangement. Compound 3 crystallizes in a non-centrosymmetric space group (Pna21), which belongs to the polar point group C2v. This compound displays a strong SHG response and good ferroelectric and piezoelectric properties.

A series of new silver selenites with d0-TM cations

Four silver selenites containing d0-TM cations have been reported, one of them displays a moderately strong SHG response.

Ferrocene-quinoxaline Y-shaped chromophores as fascinating second-order NLO building blocks for long lasting highly active SHG polymeric films.

The first example of a Y-shaped ferrocene quinoxaline derivative with a surprisingly high and stable second harmonic generation (SHG) response in composite polymeric films is reported. The interesting quadratic hyperpolarizability values of different substituted Y-shaped chromophores are also investigated in solution by the EFISH technique.

The mechanism of large second harmonic generation enhancement activated by Zn2+ substitution.

Substitution is an effective method to create new nonlinear optical (NLO) materials with an enhanced second harmonic generation (SHG) response. However, the large SHG enhancement caused by the substitution of Mg2+ for Zn2+ in an isostructural system is rare and has not been investigated yet. Here we elucidate the contribution of Zn2+ with d10 electronic configuration to the SHG response based on two isostructural silicates Sr2MSi2O7 (M = Zn and Mg). The powder SHG measurements show that the SHG response of Sr2ZnSi2O7 is approximately 35 times that of α-SiO2 and 7 times higher than its isostructural compound Sr2MgSi2O7. The calculated band structures reveal that the difference in band gaps is minute and the sp hybridization between 4s-orbitals of Zn and 2p-orbitals of O is stronger than that between 3s-orbitals of Mg and 2p-orbitals of O. The significantly enhanced SHG response of Sr2ZnSi2O7 can be interpreted by the enhanced inter-band dipole resulting from strong sp hybridization between Zn and O. This provides a route to design new materials with enhanced NLO properties in the isostructural system.

Explorations of New SHG Materials in the Alkali-Metal-Nb(5+)-Selenite System.

Standard high-temperature solid-state reactions of NaCl, Nb2O5, and SeO2 resulted in two new sodium selenites containing a second-order Jahn-Teller (SOJT) distorted Nb(5+) cation, namely, Na2Nb4O7(SeO3)4 (P1̅; 1) and NaNbO(SeO3)2 (Cmc21; 2). Compound 1 exhibits an unusual 3D [Nb4O7(SeO3)4](2-) anionic network composed of 2D [Nb4O11(SeO3)2](6-) layers which are further bridged by additional SeO3(2-) anions via corner sharing; the 2D [Nb4O11(SeO3)2](6-) layer is formed by unusual quadruple [Nb4O17](14-) niobium oxide chains of corner-sharing NbO6 octahedra being further interconnected by selenite anions via Nb-O-Se bridges. The polar compound 2 features a 1D [NbO(SeO3)2](-) anionic chain in which two neighboring Nb(5+) cations are bridged by one oxo and two selenite anions. The alignments of the polarizations from the NbO6 octahedra in 2 led to a strong SHG response of ∼7.8 × KDP (∼360 × α-SiO2), which is the largest among all phases found in metal-Nb(5+)-Se(4+)/metal-Nb(5+)-Te(4+)-O systems. Furthermore, the material is also type I phase matchable. The above experimental results are consistent with those based on DFT theoretical calculations. Thermal stabilities and optical properties for both compounds are also reported.

Effect of the tetrahedral groups on the optical properties of LaBRO5 (R = Si and Ge): A first-principles study

As potential candidates for deep-UV nonlinear optical (NLO) crystals, borosilicates and borogermanates, which contain NLO-active groups such as B-O, Si-O, and Ge-O groups, have fascinated many material scientists' research enthusiasm. In this paper, the electronic structures and optical properties of two isostructural noncentrosymmetric crystals LaBRO5 (R = Si and Ge) have been studied by the first-principles method. Combined with the analyses of the SHG-density and the localized electron-density difference, contributions of the constituent tetrahedra to the total NLO responses are investigated. Eventually, BO4 and GeO4 groups give nearly equal contributions to the SHG effect of LaBGeO5, but for LaBSiO5, SiO4 groups express stronger SHG response than that of BO4. Such interesting conclusion is consistent with the distortion index analyses and dipole moment.

Experimental and theoretical studies of optical and nonlinear optical properties for MnTeMoO6 crystal

MnTeMoO6 crystal was obtained by top-seeded solution growth method. Optical and nonlinear optical properties of MnTeMoO6 crystal were measured. Band structures, DOS, optical properties of MnTeMoO6 were calculated. Both calculated and experiment results of the optical properties show that a transitional absorption band is exhibited in MnTeMoO6 crystal. Second harmonic generation measurements show that the SHG response with fundamental wavelength 1.06 μm decreases to a quarter of that with fundamental wavelength 2.05 μm. The transitional absorption band has a great effect on the nonlinear optical property of MnTeMoO6 crystal. The formation of the transitional absorption band in MnTeMoO6 crystal would be attributed to the internal transitions of transition-meta ions with incomplete electron shells (Mn2+ 3d34s2). In addition, the transitional absorption band plays an important role in the coloration of MnTeMoO6 crystal, and the crystal can be made into selective optical oscillators and laser wavelength recognizing device by utilizing the characteristic of the transitional absorption band.

Series of SHG Materials Based on Lanthanide Borate-Acetate Mixed Anion Compounds.

The first examples of lanthanide borate-acetate mixed anion compounds, namely, Ln2(CH3CO2)2[B5O9(OH)]·H2O (Ln = La 1; Ce 2; Pr 3), were synthesized under hydrothermal conditions. These compounds are isostructural and crystallize in polar space group Cc. They display a unique three-dimensional (3D) framework built by a 3D network of lanthanide borate further decorated by acetate anions. The borate anion exhibits a 2D layer in the ac plane with large 9-member rings (MRs) which are filled by lanthanide(III) ions into a {Ln[B5O9(OH)]}(-) 2D layer. Adjacent {Ln[B5O9(OH)]}(-) layers are bridged by remaining lanthanide (III) ions to form a 3D network of lanthanide borate. It is noteworthy that Ln2(CH3CO2)2[B5O9(OH)]·H2O (Ln = La 1; Ce 2; Pr 3) can be changed into Ln2(CH3CO2)2[B5O9(OH)] (Ln = La 4; Ce 5; Pr 6) under heating at 500 K. Compounds 1-4 display moderate SHG signals of about 2.0, 1.0, 1.4, and 2.5 times that of KH2PO4, respectively, and they are phase matchable. Their SHG responses mainly arise from the synergistic polarization effects of both asymmetric borate clusters and π-conjugated CH3COO(-) anions.

The lone-pairs enhanced birefringence and SHG response: A DFT investigation on M2B5O9Cl (M = Sr, Ba, and Pb)

It is important to investigate the electronic structures and optical properties of the UV NLO compound containing lone-pairs electrons, aiming to make a subtle balance among relative large SHG response, big birefringence and deep bandgap. In this paper, the electronic structures and optical properties of M2B5O9Cl (M=Sr, Ba, and Pb) have been investigated using the DFT method. The results show that the enhanced birefringence and SHG response are found in Pb2B5O9Cl comparison with that of Sr2B5O9Cl and Ba2B5O9Cl. And the enhanced birefringence and SHG response in Pb2B5O9Cl originate from the covalent interaction between the oxygen and lead atoms.

Nonlinear Optical Properties of Thiolate-Protected Gold Clusters

Thiolate-protected gold clusters are promising candidates for imaging applications due to their interesting, size-dependent properties. Their high stability and the ability to functionalize the clusters with biocompatible ligands render the clusters interesting for various imaging techniques such as fluorescence microscopy or second-harmonic generation microscopy. The latter nonlinear optical effect has not yet been observed on this type of ultrasmall nanoparticle. We hereby present second- and third-harmonic generation and multiphoton fluorescence of two thiolate-protected gold clusters: Au25(SCH2CH2Ph)18 and Au38(SCH2CH2Ph)24. At a fundamental wavelength of 800 nm, the Au38(SCH2CH2Ph)24 cluster is active. In contrast, Au25(SCH2CH2Ph)18 does not yield significant SHG signal. We ascribe this to the center of inversion in the Au25 cluster. Measurements on chiral Au25(capt)18 (capt: captopril) gave an SHG response, supporting this interpretation. We also observed third-harmonic generation at a fundamental wavelength of 1200 nm. At 800 and 1100 nm, the clusters decompose after short illumination time but are stable at illumination at 1200 nm. This may be exploited in combined deep tissue imaging and photothermal heating for theranostics applications.

A facile synthetic route to a new SHG material with two types of parallel π-conjugated planar triangular units.

A new SHG material, namely, Pb2(BO3)(NO3), which contains parallel π-conjugated nitrate and borate anions, was obtained through a facile hydrothermal reaction by using Pb(NO3)2 and Mg(BO2)2⋅H2O as starting materials. Its structure contains honeycomb [Pb2(BO3)]∞ layers with noncoordination [NO3](-) anions located at the interlayer space. Pb2(BO3)(NO3) shows a remarkable strong SHG response of approximately 9.0 times that of potassium dihydrogen phosphate (KDP) and the material is also phase-matchable. The large SHG coefficient of Pb2(BO3)(NO3) arises from the synergistic effect of the stereoactive lone pairs on Pb(2+) cations and parallel alignment of π-conjugated BO3 and NO3 units. Based on its unique properties, Pb2(BO3)(NO3) may have great potential as a high performance NLO material in photonic applications.

Structures and multiple properties of two polar metal-organic frameworks based on achiral N,O-coordinated ligands: toward multifunctional materials.

Two novel metal-organic frameworks (MOFs) [Cd3(padc)(Hpadc)(H2padc)(H2O)]n·nH2O (1, H3padc = pyrazole-3,5-dicarboxylic acid) and [Co4(pidc)2(Hpidc)4(H2O)3]n·12nH2O (2, H2pidc = pyridine-2,5-dicarboxylic acid), that both crystallize in polar space groups, were solvothermally synthesized by using achiral N,O-coordinated ligands. Compound 1 consists of trinuclear Cd(II)-based units that are further bridged by the backbone of H3padc ligands to form a three-dimensional (3-D) (4,6)-connected fsc topology network, while compound 2 features two types of double-helical tubes with different chiralities connecting with each other alternatively to construct a typical 2-D (3,6)-connected kgd topology network. Importantly, 1 exhibits combined properties of photoluminescence (PL) and second harmonic generation (SHG), and represents the first noncentrosymmetric H3padc-based MOF that was obtained without any ancillary ligands. While 2 shows strong antiferromagnetic interactions between paramagnetic Co(II) centers, and the aqueous solution of 2 exhibits effective homogeneous photocatalysis properties under visible irradiation. Further, the mechanisms of the physical properties of 1 and 2 are discussed in detail.

Morphology and polarization-dependent second harmonic generation in single hexagonal sodium niobate micro/nano-crystals

The SHG responses from single hexagonal NaNbO3 micro/nano-crystals with different morphologies and sizes were studied in detail. It was demonstrated that the same crystal structure displays a similar SHG response.