Powder Second Harmonic Generation Research Articles

Giant Optical Anisotropy in a Covalent Molybdenum Tellurite via Oxyanion Polymerization.

Large birefringence is a crucial but hard-to-achieve optical parameter that is a necessity for birefringent crystals in practical applications involving modulation of the polarization of light in modern opto-electronic areas. Herein, an oxyanion polymerization strategy that involves the combination of two different types of second-order Jahn-Teller distorted units is employed to realize giant anisotropy in a covalent molybdenum tellurite. Mo(H2O)Te2O7 (MTO) exhibits a record birefringence value for an inorganic UV-transparent oxide crystalline material of 0.528 @ 546nm, which is also significantly larger than those of all commercial birefringent crystals. MTO has a UV absorption edge of 366nm and displays a strong powder second-harmonic generation response of 5.4 times that of KH2PO4. The dominant roles of the condensed polytellurite oxyanions [Te8O20]8- in combination with the [MoO6]6- polyhedra in achieving the giant birefringence in MTO are clarified by structural analysis and first-principles calculations. The results suggest that polymerization of polarizability-anisotropic oxyanions may unlock the promise of birefringent crystals with exceptional birefringence.

Chromium iodate: the structure and origin of optical second harmonic generation.

Iodate crystal materials are some of the important candidates for mid-infrared nonlinear optical (NLO) crystals. Almost all the structures and NLO properties of monometallic iodates have been studied, except for a few difficult to synthesize crystalline phases. In this work, crystalline Cr(IO3)3 was synthesized and its optical properties were studied. Cr(IO3)3 has a strong powder second-harmonic generation (SHG) effect (1.3 × AgGaS2) and a large birefringence (0.24) at 2100 nm. Theoretical calculations indicate that the 3d orbital of Cr3+ is strongly involved in the SHG process, and the contribution of the metal cation centered unit CrO6 to the SHG response is much greater than that of the non-metal cation centered unit IO3. This study provides a new case among the few NLO materials in which the SHG response is dominated by metal cation centered groups.

Syntheses, Crystal Structure, and Second Harmonic Generation Response of Four Novel Noncentrosymmetric Tellurites of Quaternary A/M/Te/O (A = Ba, Sr, Ca; M = V, Nb, Ta) System: BaTa4Te3O17, BaNb4Te3O17, SrTa4Te3O17, and CaV2TeO8.

Four new noncentrosymmetric tellurites of quaternary A/M/Te/O (A = Ba, Sr, Ca; M = V, Nb, Ta) system, namely, BaTa4Te3O17 (1), BaNb4Te3O17 (2), SrTa4Te3O17 (3), and CaV2TeO8 (4), were synthesized and characterized by single-crystal X-ray diffraction, thermal analysis, diffuse reflectance spectroscopy, and powder second harmonic generation (SHG) response measurements. The isostructural compounds 1-3 crystallize in the P212121 space group and have a three-dimensional (Ta4Te3O17)2-/(Nb4Te3O17)2- anionic framework, whereas the layered compound 4 crystallizes in the Ccc2 space group and has a [V2TeO8]2- anionic layer. The phase-matching compounds 1, 2, and 3 and the non-phase-matching compound 4 show powder SHG responses equivalent to ∼27, ∼13, ∼24, and ∼68% of that of LiNbO3, respectively.

Giant Mid‐Infrared Second‐Harmonic Generation Response in a Densely‐Stacked Van Der Waals Transition‐Metal Oxychloride

AbstractSecond‐harmonic generation (SHG) is a fundamental optical property of nonlinear optical (NLO) crystals. Thus far, it has proved difficult to engineer large SHG responses, particularly in the mid‐infrared region, owing to the difficulty in simultaneously controlling the arrangement and density of functional NLO‐active units. Herein, a new assembly strategy employing functional modules only, and aimed at maximizing the density and optimizing the spatial arrangement of highly efficient functional modules, has been applied to the preparation of NLO crystals, affording the van der Waals crystal MoO2Cl2. This exhibits the strongest powder SHG response (2.1×KTiOPO4 (KTP) @ 2100 nm) for a transition‐metal oxyhalide, a wide optical transparency window, and a sufficient birefringence. MoO2Cl2 is the first SHG‐active transition‐metal oxyhalide effective in the infrared region. Theoretical studies and crystal structure analysis suggest that the densely packed, optimally‐aligned [MoO4Cl2] modules within the two‐dimensional van der Waals layers are responsible for the giant SHG response.

Giant Mid-Infrared Second-Harmonic Generation Response in a Densely-Stacked Van Der Waals Transition-Metal Oxychloride.

Second-harmonic generation (SHG) is a fundamental optical property of nonlinear optical (NLO) crystals. Thus far, it has proved difficult to engineer large SHG responses, particularly in the mid-infrared region, owing to the difficulty in simultaneously controlling the arrangement and density of functional NLO-active units. Herein, a new assembly strategy employing functional modules only, and aimed at maximizing the density and optimizing the spatial arrangement of highly efficient functional modules, has been applied to the preparation of NLO crystals, affording the van der Waals crystal MoO2 Cl2 . This exhibits the strongest powder SHG response (2.1×KTiOPO4 (KTP) @ 2100 nm) for a transition-metal oxyhalide, a wide optical transparency window, and a sufficient birefringence. MoO2 Cl2 is the first SHG-active transition-metal oxyhalide effective in the infrared region. Theoretical studies and crystal structure analysis suggest that the densely packed, optimally-aligned [MoO4 Cl2 ] modules within the two-dimensional van der Waals layers are responsible for the giant SHG response.

Quadruple-Bidentate Nitrate-Ligated A2 Hg(NO3 )4 (A=K, Rb): Strong Second-Harmonic Generation and Sufficient Birefringence.

The design of efficient nonlinear optical (NLO) crystals continues to pose significant challenges due to the difficulty of assembling polar NLO-active modules in an optimal additive fashion. We report herein the first NLO-active mercuric nitrates A2 Hg(NO3 )4 (A=(KHNO), Rb (RHNO)), for which assembly is induced by ionic polarization of the d10 cations. The two new crystalline compounds are isostructural, featuring interesting pseudo-diamond-like structures with parallel [Hg(NO3 )4 ] modules, and leading to strong powder second-harmonic generation (SHG) responses of 9.2 (KHNO) and 8.8 (RHNO) times that of KH2 PO4 . In combination with the simple solution preparation of centimeter-scale crystals, sufficient birefringence, and short ultraviolet (UV) cutoff edges, these attributes make KHNO and RHNO promising candidates for UV NLO materials. Theoretical calculations and single-crystal structure analysis reveal that the newly-developed highly condensed and distorted [Hg(NO3 )4 ] module, with an Hg2+ cation that is quadruply bidentate nitrate-ligated, is crucial for the significant SHG responses. This work highlights the potential importance of modules with multiple bidentate ligands for the development of high-performing next-generation NLO materials.

Syntheses and characterization of quaternary selenites and tellurite of antimony, NaSbSe2O7, AgSbSe2O7 and Na2Sb4Te2O14

NaSbSe2O7, AgSbSe2O7 and Na2Sb4Te2O14 compounds have been synthesized by solid state reactions and characterized by single crystal X-ray diffraction, thermal analysis, reflectance spectroscopy, and second harmonic generation (SHG) response studies. The isostructural NaSbSe2O7 and AgSbSe2O7 selenites crystallize in centrosymmetric Cmcm space group with respective orthorhombic unit cell parameter values of a = 10.0704(4) and 10.0449(5) Å, b = 8.3074(4) and 8.3825(5) Å and c = 7.1449(3) and 7.1438(5) Å. Their one-dimensional structure consists of Na+ or Ag+ cations and anionic [SbSe2O7]- chains made up of SbO6 octahedra and SeO3 trigonal pyramids. Na2Sb4Te2O14 is a mixed-valent compound, which crystallizes in noncentrosymmetric P3121 space group with hexagonal unit cell parameter values of a = 7.32670(10) Å and c = 18.5150(5) Å. It has a three-dimensional hexagonal-tungsten-oxide (HTO) related anionic [Sb4Te2O14]2- structural framework and shows powder SHG response equivalent to 0.4 × KDP.

A selenite containing unusual [Ga2F(Se2O5)4(HSeO3)2]5- clusters with the polarization ‘lighted’ by F− anions

Rationally designing inorganic compounds with the specific functional property is still a great challenge. Herein, the first metal selenite Ga2(Se2O5)2(HSeO3)(H2SeO3)F containing three different kinds of Se–O groups and Ga(Se2O5)(HSeO3) have been successfully synthesized by the hydrothermal method. In Ga(Se2O5)(HSeO3), the polar GaO6 octahedra and Se–O groups arrange in opposite directions, resulting in its centrosymmetric (CS) crystal structure, whereas in Ga2(Se2O5)2(HSeO3)(H2SeO3)F, by combining the chemical tailoring with the structure-directing properties of F− anions, unprecedented lantern-like polar structural clusters [Ga2F(Se2O5)4(HSeO3)2]5- are constructed, and these clusters adopt well-ordered alignments along the c axis promoting its noncentrosymmetric (NCS) and polar structure. Structural analysis indicates that the bifunctional F− anion plays the importantly polar ‘lighted’ roles for designing and synthesizing NCS and polar material of Ga2(Se2O5)2(HSeO3)(H2SeO3)F. The electron densities of states (DOS) calculations suggest that the interaction between oxygen and selenite determines their band gaps and the introduction of large electronegative F− anions obviously widens the UV cut-off edge of Ga2(Se2O5)2(HSeO3)(H2SeO3)F. In addition, performance measurements including powder second-harmonic generation (SHG), thermal analysis and infrared and UV-vis-NIR diffuse reflectance spectroscopy were performed on the title compounds.

A Polar Tetragonal Tungsten Bronze with Colossal Second-Harmonic Generation.

A polar tetragonal tungsten bronze, Pb1.91 K3.22 □0.85 Li2.96 Nb10 O30 (□: vacancies), has been successfully synthesized by a high temperature solid-state reaction. Single crystal and powder X-ray diffraction indicate that the structure of Pb1.91 K3.22 □0.85 Li2.96 Nb10 O30 crystallizing in the noncentrosymmetric (NCS) space group, P4bm, consists of 3D framework with highly distorted NbO6 , LiO9 , PbO12 , and (Pb/K)O15 polyhedra. While NCS Pb1.91 K3.22 □0.85 Li2.96 Nb10 O30 undergoes a reversible phase transition between polar (P4bm) and nonpolar (P4/mbm) structure at around 460 °C, the material decomposes to centrosymmetric Pb1.45 K3.56 Li3.54 Nb10 O30 (P4/mbm) once heated to 1200 °C. Powder second-harmonic generation (SHG) measurements with 1064nm radiation indicate that Pb1.91 K3.22 □0.85 Li2.96 Nb10 O30 exhibits a giant phase-matchable SHG intensity of ≈71.5 times that of KH2 PO4 , which is the strongest intensity in the visible range among all nonlinear optical materials reported to date. The observed colossal SHG should be attributable to the synergistic effect of dipole moments from the well-aligned NbO6 octahedra, the constituting distortive channels with vacancies, and highly polarizable cations.

ε-La(IO3)3: A Polar Iodate with High Thermal Stability and a Large Second-Harmonic-Generation Response Obtained by a Supercritical Hydrothermal Method.

The synthesis conditions at high temperature and high pressure are favorable for exploring new compounds with novel structures and properties. In this work, a new polar iodate, ε-La(IO3)3, is obtained by a supercritical hydrothermal method with high temperature and high pressure (T = 400 °C and P ≈ 25 MPa). Different from the known phases, ε-La(IO3)3 crystallizes in the chiral space group P21, which features a three-dimensional framework with multiple IO3- groups stacked along different directions around the LaOx polyhedra. ε-La(IO3)3 possesses high thermal stability up to 525 °C and exhibits a wide band gap of about 4.05 eV. Attributed to its noncentrosymmetric arrangement, ε-La(IO3)3 is second-harmonic-generation (SHG)-active and the powder SHG response is measured to be 11.1 × KH2PO4 at 1064 nm in the 26-50 μm particle size range. This work has enriched the structural diversity of iodates and would further promote the materials' exploration under a supercritical hydrothermal method.

The Kurtz-Perry Powder Technique Revisited: A Study of the Effect of Reference Selection on Powder Second-Harmonic Generation Response.

The accurate evaluation of nonlinear optical (NLO) coefficient, the main parameter affecting light conversion efficiency, plays a crucial role in the development of NLO materials. The Kurtz-Perry powder technique can evaluate second-harmonic generation (SHG) intensity in pristine powder form, saving a significant amount of time and energy in the preliminary screening of materials. However, the Kurtz-Perry method has recently been subject to some controversy due to the limitations of the Kurtz-Perry theory and the oversimplified experimental operation. Therefore, it is very meaningful to revisit and develop the Kurtz-Perry technique. In this work, on the basis of introducing the light scattering effect into the original Kurtz-Perry theory, the theoretical expression of second-harmonic generation intensity with respect to band gap and refractive index are analyzed. In addition, the reference-dependent SHG measurements were carried out on polycrystalline LiB3O5 (LBO), AgGaQ2 (Q = S, Se), BaGa4Q7 (Q = S, Se), and ZnGeP2 (ZGP), and the results of SHG response emphasize the importance of using appropriate references to the Kurtz-Perry method. In order to obtain reliable values of nonlinear coefficients, two criteria for selecting a reference compound were proposed: (1) it should possess a band gap close to that of the sample to be measured and (2) it should possess a refractive index close to that of the sample to be measured. This work might shed light on improvements in accuracy that can be made for effective NLO coefficients obtained using the Kurtz-Perry method.

K2CdGe3S8: A New Infrared Nonlinear Optical Sulfide

A quaternary metal chalcogenide, namely K2CdGe3S8 (I), is obtained through a high-temperature solid-state approach. Compound I crystallizes with the non-centrosymmetric space group P212121. It features a 2D layer structure with [CdGe3S8] layers consisting of tetrahedral GeS4 and CdS4 units, and counter K+ embedded between the layers. The compound exhibits a powder second-harmonic generation (SHG) response of ~0.1 times that of KH2PO4 (KDP) with phase-matchable character at the laser wavelength of 1064 nm. Remarkably, it has a wide band gap (3.20 eV), which corresponds to a favorable high laser-induced damage threshold of 6.7 times that of AgGaS2. In addition, the calculated birefringence (Δn) is 0.039 at the wavelength of 1064 nm, which satisfies the Δn criteria for a promising infrared NLO material.

Two Acentric Aluminoborates Incorporated d10 Cations: Syntheses, Structures, and Nonlinear Optical Properties.

Two acentric aluminoborates (ABOs), [Zn(en)2Al{B5O9(OH)}{BO(OH)2}] (1) and [Cd(en)2AlB5O10]·H2O (2) (en = ethylenediamine), have been solvothermally made. 1 includes a two-dimensional (2D) wavy ABO layer using B5O9(OH) clusters and AlO3{BO(OH)2} groups, in which both units can be regarded as three-connected nodes, and simplifying the ABO layer to a hcb-type network. 2 features an acentric three-dimensional (3D) porous framework with a unique unc-type network constituted by strictly alternating connected B5O10 clusters and AlO4 units. The structural transformation from a 2D layer 1 to a 3D framework 2 was achieved with the elimination of the terminal hydroxyls in layer 1 by adjusting synthetic conditions in the same solvent system. Metal-amine complexes Zn(en)2/Cd(en)2 bond to the inorganic walls and are located in the cavity of frameworks 1 and 2, respectively. Compounds 1 and 2 exhibit large second-harmonic generation (SHG) responses that are 2.2 and 2.7 times those of KH2PO4 (KDP), respectively, which are among the largest powder SHG responses for all deep-ultraviolet (deep-UV) ABOs. The UV-vis diffuse reflectance spectra of 1 and 2 show a wide transparency window below 190 nm. Density functional theory (DFT) calculations indicate that the B-O units and the introduced distorted d10 metal polyhedra played a decisive role in the optical properties of both compounds.

Synthesis and characterization of a new rare earth borate nonlinear optical crystal K7PbLu2B15O30.

A new complex rare earth borate K7PbLu2B15O30 was prepared by the spontaneous crystallization method. K7PbLu2B15O30 is crystallized in the chiral trigonal space group R32 with cell parameters a = b = 13.0893(3) Å, c = 15.2379(6) Å, α = β = 90°, γ = 120°, and Z = 3. The basic structure of the crystal can be seen as composed of B5O10 groups and LuO6 polyhedra sharing oxygen atoms, while K+ and Pb2+ fill the space to balance the charge. The UV transmission cut-off edge of K7PbLu2B15O30 was less than 300 nm, and the powder SHG response was roughly 1.1 times that of KDP. Furthermore, a first-principles analysis was performed to see more about the relationship between the crystal structure and optical characteristics.

Syntheses, structures, and optical properties of n = 3 layered Dion–Jacobson perovskites, RbEu2-xBixTi2NbO10 (0 ≤ x ≤ 2)

A series of n = 3 Dion–Jacobson type perovskites, RbEu2-xBixTi2NbO10 (0 ≤ x ≤ 2) have been synthesized by high temperature solid-state reactions. The Rietveld refinements using powder X-ray diffraction data indicate that the reported materials crystallizing in the noncentrosymmetric (NCS) polar orthorhombic space group, Ima2 (No. 46) or centrosymmetric (CS) tetragonal space group, P4/mmm (No. 123) reveal solid-solution behaviors depending on the amount of Eu3+ and Bi3+ cations. Detailed structural analyses indicate that once more than 30% of A-site cations are occupied by Eu3+ cations in RbEu2-xBixTi2NbO10, the centricity of the solid solutions changes from NCS to CS. Powder second-harmonic generation (SHG) measurements suggest while RbBi2Ti2NbO10 reveals strong SHG (2.5 × KDP), the SHG intensity abruptly decreases as small amount of Eu3+ cations occupy the site of lone pair cation, Bi3+. The origin of the large SHG response of polar RbBi2Ti2NbO10 and the abrupt decrease of SHG for the Eu3+-doped RbEu2-xBixTi2NbO10 solid solutions are explained by a detailed structural analysis and dipole moment calculations along with the electron localization function diagrams. Detailed characterizations including infrared and ultraviolet–visible spectroscopy, thermogravimetric analysis, density functional theory calculations, energy dispersive X-ray analysis, and photoluminescence measurements are provided.

Growth, Structure, and Dielectric and Optics Properties of a Novel Optical Crystal with High Optical Nonlinearity

In this study, a new class of organic salt nonlinear optical (NLO) crystals, 2-HDSAI and HDSAI, were synthesized and grown by designing and adjusting the methyl position on the receptor. Single-crystal XRD analysis confirms that the crystal belongs to monoclinic group P21. Among them, the HDSAI crystal has second harmonic generation (SHG) activity, and the maximum powder SHG is about 0.7 times that of OH1. The maximum nonlinear optical coefficient of the crystal is d11 = 62 pm/V @1064 nm by theoretical calculation. The HDSAI single crystal was successfully grown by the slow cooling method. Through the study of the optical and dielectric properties of the crystal, the results showed that the newly designed crystal has high transmittance (>75%) and low dielectric constant (εr ≈ 2). The thermal property test shows that the crystal has high thermal stability.

Synthesis, Crystal Structure, and Characterizations of Two Tantalum Phosphates A3TaP2O9 (A=K, Na).

Two new compounds K3TaP2O9 and Na3TaP2O9 were obtained by spontaneous crystallization with KH2PO4 and K2HPO4 or NaH2PO4 and Na2HPO4 as fluxes, respectively. K3TaP2O9 belongs to the P21/m space group of the monoclinic system, with cell parameters of a = 9.8912(7) Å, b = 7.1861(5) Å, c = 13.6457(9) Å, β = 94.047(2)°, and Z = 4. In contrast, Na3TaP2O9 pertains to the P212121 space group of the orthorhombic system, with cell parameters of a = 7.1241(7) Å, b = 9.3071(10) Å, c = 12.3752(13) Å, and Z = 4. Both of the structures feature the existence of the zigzag (TaP2O9)∞ chains with the -(O4Ta)-O-(TaO4)- connection. Also, two compounds have high transparency of about 90% in the 400-2500 nm range. Theoretical calculations and powder second-harmonic generation test show that the compound Na3TaP2O9 has a large band gap of 5.5 eV, a moderate powder second-harmonic generation response of 0.6 × KDP, and a large birefringence of 0.11.

Second-Harmonic-Generation Effect and Giant Optical Birefringence in the Weyl Material CaAgAs.

Nonlinear-optical (NLO) materials suitable for the "8-14 μm" atmospheric transparent window are in urgent need. Arsenide is one of the most promising material systems for such an application. However, the second-harmonic-generation (SHG) effect of arsenide is difficult to characterize using the conventional powder SHG technique with a 2 μm fundamental laser. To overcome this problem, we focused on a novel arsenide Weyl material, CaAgAs, with a zero band gap and proposed a modified powder SHG measurement method for narrow-band-gap materials. We successfully observed the SHG signal of CaAgAs, which was approximately 0.7 times that of CdGeAs2. Moreover, on the basis of first-principles calculations, the largest SHG coefficient for CaAgAs is equal to 236 pm/V, and the ionic bonds in the [Ca3As13] motif play an indispensable role for the SHG effect because of the distorted Kagome lattice pattern. CaAgAs is predicted to have strong optical anisotropy with a giant optical birefringence of 0.52.

Ultrawide Bandgap and Outstanding Second-Harmonic Generation Response by a Fluorine-Enrichment Strategy at a Transition-Metal Oxyfluoride Nonlinear Optical Material.

The development of nonlinear optical (NLO) materials has been hindered by competing microstructure requirements: the need to simultaneously engineer a large hyperpolarizability (a large second-harmonic generation (SHG)) and a wide HOMO-LUMO gap (a wide band gap). Herein, a non-centrosymmetric transition-metal (TM) oxyfluoride K5 (NbOF4 )(NbF7 )2 (KNOF) with an extremely high F/O ratio is constructed in high yield. KNOF exhibits an extremely wide band gap (5.88 eV) and a strong powder SHG response (4.0×KH2 PO4 )-both being the largest values for TM-centered oxyfluorides-as well as a birefringence sufficient for applications. The dominant roles of the partially fluorinated [NbO2 F4 ] and totally fluorinated [NbF7 ] groups in achieving the enlarged band gap in KNOF have been clarified by first-principles calculations. Our results suggest that maximizing the fluorine content of oxyfluorides may unlock the promise of short-wavelength-transparent materials with exceptional NLO performance.

Ultrawide Bandgap and Outstanding Second‐Harmonic Generation Response by a Fluorine‐Enrichment Strategy at a Transition‐Metal Oxyfluoride Nonlinear Optical Material

AbstractThe development of nonlinear optical (NLO) materials has been hindered by competing microstructure requirements: the need to simultaneously engineer a large hyperpolarizability (a large second‐harmonic generation (SHG)) and a wide HOMO–LUMO gap (a wide band gap). Herein, a non‐centrosymmetric transition‐metal (TM) oxyfluoride K5(NbOF4)(NbF7)2 (KNOF) with an extremely high F/O ratio is constructed in high yield. KNOF exhibits an extremely wide band gap (5.88 eV) and a strong powder SHG response (4.0×KH2PO4)—both being the largest values for TM‐centered oxyfluorides—as well as a birefringence sufficient for applications. The dominant roles of the partially fluorinated [NbO2F4] and totally fluorinated [NbF7] groups in achieving the enlarged band gap in KNOF have been clarified by first‐principles calculations. Our results suggest that maximizing the fluorine content of oxyfluorides may unlock the promise of short‐wavelength‐transparent materials with exceptional NLO performance.