Designing Silicates as Deep-UV Nonlinear Optical (NLO) Materials using Edge-Sharing Tetrahedra

Discovering new deep-ultraviolet (DUV) nonlinear optical (NLO) materials is currently a great challenge. The reported DUV NLO materials are almost exclusively borates or phosphates. Silicates—the largest constituent of the earth's crust—are excluded owing to their weak second harmonic generation (SHG) response. We report a silicate, Li₂BaSiO₄, with edge-sharing LiO₄–SiO₄ tetrahedra that achieves the balance between a short UV absorption edge, below 190 nm, and a large SHG response, 2.8×KDP. The SHG intensity is the largest for silicates without second-order Jahn–Teller cations, and exceeds that of non-isomorphic Li₂SrSiO₄ by more than an order of magnitude. As such Li₂BaSiO₄ may be seen as a promising DUV-UV NLO material. This research indicates that edge-sharing tetrahedra is a new design parameter for discovering new DUV NLO materials.     

Cation‐Tuned Synthesis of Fluorooxoborates: Towards Optimal Deep‐Ultraviolet Nonlinear Optical Materials

The development of new nonlinear optical (NLO) materials for deep‐ultraviolet (DUV) applications is in great demand. However, the synthesis of an ideal DUV NLO crystal is a serious challenge. Herein, three new alkali‐metal fluorooxoborates, AB₄O₆F (A=K, Rb, and Cs, and a mixed cation between two of them), were successfully synthesized by cation regulation. It is found that all reported compounds exhibit short UV absorption edges (<190 nm), and show second harmonic generation (SHG) responses ranging from 0.8 to 1.9 KH₂PO₄ (KDP). Interestingly, by judicious selection of the A‐site alkali‐metal cations, the arrangement of NLO‐active structural units is fine‐tuned to an optimal configuration, which contributes to large SHG responses.     

NH4Be2BO3F2 and γ‐Be2BO3F: Overcoming the Layering Habit in KBe2BO3F2 for the Next‐Generation Deep‐Ultraviolet Nonlinear Optical Materials

KBe₂BO₃F₂ (KBBF) is still the only practically usable crystal that can generate deep‐ultraviolet (DUV) coherent light by direct second harmonic generation (SHG). However, applications are hindered by layering, leading to difficulty in the growth of thick crystals and compromised mechanical integrity. Despite efforts, it is still a great challenge to discover new nonlinear optical (NLO) materials that overcome the layering while keeping the DUV SHG available. Now, two new DUV NLO beryllium borates have been successfully designed and synthesized, NH₄Be₂BO₃F₂ (ABBF) and γ‐Be₂BO₃F (γ‐BBF), which not only overcome the layering but also can be used as next‐generation DUV NLO materials with the shortest type I phase‐matching second‐harmonic wavelength down to 173.9 nm and 146 nm, respectively. Significantly, γ‐BBF is superior to KBBF in all metrics and would be the most outstanding DUV NLO crystal.     

Fluorooxoborates: Beryllium‐Free Deep‐Ultraviolet Nonlinear Optical Materials without Layered Growth

Deep‐ultraviolet nonlinear optical (DUV NLO) crystals are the key materials to extend the output range of solid‐state lasers to below 200 nm. The only practical material KBe₂BO₃F₂ suffers high toxicity through beryllium and strong layered growth. Herein, we propose a beryllium‐free material design and synthesis strategy for DUV NLO materials. Introducing the (BO₃F)⁴⁻, (BO₂F₂)³⁻, and (BOF₃)²⁻ groups in borates could break through the fixed 3D B–O network that would produce a larger birefringence without layering and simultaneously keep a short cutoff edge down to DUV. The theoretical and experimental studies on a series of fluorooxoborates confirm this strategy. Li₂B₆O₉F₂ is identified as a DUV NLO material with a large second harmonic generation efficiency (0.9×KDP) and a large predicted birefringence (0.07) without layering. This study provides a feasible way to break down the DUV wall for NLO materials.     

CsB4O6F: A Congruent‐Melting Deep‐Ultraviolet Nonlinear Optical Material by Combining Superior Functional Units

The discovery of new nonlinear optical (NLO) materials for coherent light generation in the deep‐ultraviolet (DUV, wavelength below 200 nm) region is essential for the development of laser technologies. Herein, we report a new material CsB₄O₆F (CBF), which combines the superior structural properties of two well‐known NLO materials, β‐BaB₂O₄ (BBO) and KBe₂BO₃F₂ (KBBF). CBF exhibits excellent DUV optical properties including a short cutoff edge (155 nm), a large SHG response (≈1.9×KDP), and a suitable birefringence that enables frequency doubling down to 171.6 nm. Remarkably, CBF melts congruently and shows an improved growth habit. In addition, our rational design strategy will contribute to the discovery of DUV NLO materials.     

Beryllium‐Free β‐Rb2Al2B2O7 as a Possible Deep‐Ultraviolet Nonlinear Optical Material Replacement for KBe2BO3F2

A new beryllium‐free deep‐ultraviolet (DUV) nonlinear optical (NLO) material, β‐Rb₂Al₂B₂O₇ (β‐RABO), has been synthesized and characterized. The chiral nonpolar acentric material shows second‐harmonic generation (SHG) activity at both 1064 and 532 nm with efficiencies of 2×KH₂PO₄ and 0.4×β‐BaB₂O₄, respectively, and exhibits a short absorption edge below 200 nm. β‐Rb₂Al₂B₂O₇ has a three‐dimensional structure of corner‐shared Al(BO₃)₃O polyhedra. The discovery of β‐RABO shows that through careful synthesis and characterization, replacement of KBe₂BO₃F₂ (KBBF) by a Be‐free DUV NLO material is possible.     

Deep‐Ultraviolet Nonlinear Optics in a Borate Framework with 21‐Ring Channels

A new borate LiBa₃(OH)[B₉O₁₆][B(OH)₄], which combines the uniform porosity of open‐frameworks with the extraordinary NLO properties of borates, has been obtained under hydrothermal conditions by using mixed lithium and barium ions as templates. The framework displays an acs‐type net with large 21‐ring channels. The second harmonic generation (SHG) measurement shows that it is a type I phase‐matchable material with a strong SHG signal intensity about 3.1 times that of KDP (KH₂PO₄). UV/Vis–NIR diffuse reflectance analysis indicates that the compound has a wide transparency range with the short‐wavelength absorption edge below 200 nm. These characteristics reveal that the compound is a promising deep‐UV nonlinear optical material.     

Poly(difluorophosphazene) as the First Deep‐Ultraviolet Nonlinear Optical Polymer: A First‐Principles Prediction

A novel concept to obtain the deep‐ultraviolet (DUV) nonlinear optical (NLO) materials is proposed based on the assembling of one‐dimensional (1D) polar motifs into quasi‐1D polymer patterns. Based on the first‐principles calculations, we have successfully discovered an excellent DUV NLO polymer, i.e., poly(difluorophosphazene), with the chemical formula of (PNF₂)_n. Calculations reveal that PNF₂ has a larger band gap, a stronger second harmonic generation effect, a larger birefringence, and a shorter phase‐matching cutoff than KBe₂BO₃F₂. These findings not only demonstrate that the PNF₂ is the first reported DUV NLO polymer, but also could open a new direction to discover novel DUV NLO materials in polymer systems.     

Rational Design of the Metal‐Free KBe2BO3F2⋅(KBBF) Family Member C(NH2)3SO3F with Ultraviolet Optical Nonlinearity

The first fluorosulfonic ultraviolet (UV) nonlinear optical (NLO) material, C(NH₂)₃SO₃F, is rationally designed by taking KBe₂BO₃F₂ (KBBF) as the parent compound. C(NH₂)₃SO₃F features similar topological layers as KBBF by replacing inorganic (BO₃)^3? with organic C(NH₂)₃⁺ trigonal units and BeO₃F with SO₃F^? tetrahedra. Therefore, C(NH₂)₃SO₃F is a metal‐free UV NLO crystal. Benefiting from the coplanar configuration of the C(NH₂)₃⁺ cationic groups, it possesses a large SHG response of 5×KDP and moderate birefringence of 0.133@1064 nm. Besides, it has a short UV cutoff edge of 200 nm. The calculated results reveal the shortest SHG phase‐matching wavelengths can reach 200 nm. These findings highlight the exploration of metal‐free compounds as nontoxic and low‐cost UV NLO materials as a new research area.     

CsZn2BO3X2 (X2=F2, Cl2, and FCl): A Series of Beryllium-Free Deep-Ultraviolet Nonlinear-Optical Crystals with Excellent Properties

Designing deep-ultraviolet (DUV) nonlinear-optical (NLO) crystals is one of the major current research interests, but it faces a great challenge. In order to overcome the problem of crystal growth and the toxicity of BeO raw materials in KBe₂BO₃F₂ (KBBF), the only applicable DUV NLO crystal so far, we substitute Be²⁺ cations with Zn²⁺ in the KBBF structure and modify the halogen anions, by which three new Zn-containing KBBF-like compounds, CsZn₂BO₃X₂ (X₂=F₂, Cl₂, and FCl), have been successfully synthesized. They all exhibit excellent NLO properties, including improved SHG responses (2.8–3.5×KDP) and short UV cut-off edges (<190 nm). In comparison with KBBF, CsZn₂BO₃X₂ (X₂=F₂, Cl₂, and FCl) are all chemically benign and have better growth habits, so they are all promising as DUV NLO crystals. Further study on structure–property relationships indicates that the mixing of halogen anions is a feasible strategy to enhance the SHG responses of the KBBF family.     

Li7Cd4.5Ge4Se16 and Li6.4Cd4.8Sn4Se16: Strong Nonlinear Optical Response in Quaternary Diamond‐Like Selenide Networks

Two new selenides with diamond‐like structures, Li₇Cd_4.5Ge₄Se₁₆ and Li_6.4Cd_4.8Sn₄Se₁₆, were synthesized by using a conventional high‐temperature solid‐state reaction method. They crystallize in the space group Pna2₁ (no. 33) of the orthorhombic system. Their three‐dimensional frameworks consist of corner‐sharing LiSe₄, CdSe₄, and MSe₄ (M=Ge, Sn) tetrahedra. These two compounds exhibit strong powder second‐harmonic generation responses that are about 1.2 and 2.5 times that of the benchmark AgGaS₂ at a laser wavelength of λ=2.09 μm, and also demonstrate type I phase‐matchable behavior. The optical bandgaps were determined to be 2.18 and 1.95 eV for Li₇Cd_4.5Ge₄Se₁₆ and Li_6.4Cd_4.8Sn₄Se₁₆, respectively. Furthermore, these two materials exhibit congruent melting behavior at rather low temperatures of 985 and 1060 K, respectively, which makes bulk single crystal growth by using the Bridgman–Stockbarger method possible. Our study indicates that these two materials show advantages over the traditional IR NLO material CdSe and are promising for practical applications.     

Rb3VO(O2)2CO3: A Four‐in‐One Carbonatoperoxovanadate Exhibiting an Extremely Strong Second‐Harmonic Generation Response

A nonlinear optical (NLO) carbonatoperoxovanadate, Rb₃VO(O₂)₂CO₃, was synthesized through a simple solution‐evaporation method in phase‐pure form. Single‐crystal X‐ray diffraction revealed that the structure of Rb₃VO(O₂)₂CO₃ consists of important noncentrosymmetric (NCS) chromophores, that is, π‐delocalized (CO₃)^2? groups, a second‐order Jahn–Teller (SOJT) distortive V⁵⁺ cation, and π‐localized distorted O₂^2? groups, as well as charge‐balancing polarizable Rb⁺ ions. The powder second‐harmonic generation (SHG) measurements indicated that Rb₃VO(O₂)₂CO₃ is phase‐matchable (Type I) and exhibits a remarkably strong SHG response circa 21.0 times that of potassium dihydrogen phosphate (KDP), which is the largest efficiency observed among carbonate NLO materials. First‐principles calculation analysis suggests that the extremely large SHG response of Rb₃VO(O₂)₂CO₃ is attributed to the synergistic effect of the cooperation of all the constituting NCS chromophores.     

SnI4⋅(S8)2: A Novel Adduct‐Type Infrared Second‐Order Nonlinear Optical Crystal

A simple adduct from tin tetraiodide SnI₄ and octasulfur S₈, SnI₄?(S₈)₂ ( 1 ), is obtained employing a facile reaction. The combination of Sn⁴⁺ ions with d¹⁰ electron configuration, acentric SnI₄ tetrahedra, and lone‐pair effects of S₈, makes 1 a phase‐matchable infrared NLO crystal with a moderate second‐harmonic generation (SHG) response and a very high laser‐induced damage threshold (LIDT), which is well confirmed by the DFT calculations.     

High‐pressure syntheses of α‐RE2B4O9 (RE = Sm,Ho), with a structure type displaying edge‐sharing BO4 tetrahedra

The compounds α‐RE₂B₄O₉, with RE = Sm (disamarium tetraborate) and Ho (diholmium tetraborate), were synthesized under conditions of high pressure and high temperature in a Walker‐type multianvil apparatus, at 7.5 GPa and 1323 K for α‐Sm₂B₄O₉ and at 10 GPa and 1323 K for α‐Ho₂B₄O₉. The crystal structures were determined from single‐crystal X‐ray diffraction data collected at room temperature. The structures are isotypic with the already known α‐RE₂B₄O₉ (RE = Eu–Dy) phases, displaying the new structural motif of edge‐sharing BO₄ tetrahedra next to the known motif of corner‐sharing BO₄ tetrahedra. As the end members of this isotypic series, the two title compounds mark the borders of the stability field of the appearance of edge‐sharing BO₄ tetrahedra.     

Rb4Li2TiOGe4O12: A Titanyl Nonlinear Optical Material with the Widest Transparency Range

Practical mid‐infrared (MIR) coherent light beams generated by frequency conversion in nonlinear optical (NLO) crystals are indispensable in time‐resolved infrared vibrational spectroscopy, remote light detection and ranging, and free‐space communications. Herein, a new titanyl germanate Rb₄Li₂TiOGe₄O₁₂ (RLTG) MIR NLO crystal was obtained by heavier element substitution. It features a complicated structure network composed of compressed TiO₅ square pyramids and distorted GeO₄ tetrahedra, separated by Rb⁺ and Li⁺ cations. More importantly, RLTG exhibits concurrently short ultraviolet (0.28 μm) and long IR (5.58 μm) transmittance cutoffs, fully covering the atmospheric transparent window of 3–5 μm. Related to the short UV cutoff, it shows a higher laser‐induced damage threshold in comparison to commercial MIR NLO crystals, about twice that of KTiOPO₄ (KTP) and 50 times that of AgGaS₂ (AGS).     

Lu2SiO5 by single‐crystal X‐ray and neutron diffraction

The structure of dilutetium silicon pentaoxide, Lu₂SiO₅, has isolated ionic SiO₄ tetrahedral units and non‐Si‐bonded O atoms in distorted OLu₄ tetrahedra. The OLu₄ tetrahedra form edge‐sharing infinite chains and double O₂Lu₆ tetrahedra along the c axis. The edge‐sharing chains are connected to the O₂Lu₆ double tetrahedra by isolated SiO₄ units. The structure has been determined by neutron diffraction.     

K2Na3[{B6O10(OH)}{B3O4(OH)3}] ⋅ H2O: A Layered Borate Built by Mixed Oxoboron Clusters with Nonlinear-Optical Property

A new acentric borate, K₂Na₃[{B₆O₁₀(OH)}-{B₃O₄(OH)₃}] ⋅ H₂O ( 1 ) has been made under solvothermal condition. 1 has layered structure made by B₆O₁₃(OH)-based chains and B₃O₅(OH)₃-bridging clusters. Second-harmonic generation (SHG) measurements reveal that 1 is a phase-matchable nonlinear optical (NLO) material, showing the SHG signal intensity of 1.8 times that of KDP (KH₂PO₄). Besides, UV-Vis diffuse reflectance spectrum shows that 1 has the short deep UV (DUV) absorption cutoff edge of 198 nm. Thermogravimetric analysis reveals it has good thermal stability. Also 1 represents firstly mixed oxoboron clusters-made 2D layered borate with NLO property.     

Li2MnSnSe4: A New Quaternary Diamond‐Like Semiconductor with Nonlinear Optical Response and Antiferromagnetic Property

A new selenide with a diamond‐like structure, Li₂MnSnSe₄, was synthesized for the first time by using a conventional high‐temperature solid‐state reaction method. Li₂MnSnSe₄ crystallizes in the space group Pmn 2₁ (no. 31) of the orthorhombic system. Its three‐dimensional framework is constructed by corner‐sharing LiSe₄, MnSe₄, and SnSe₄ tetrahedra. The title compound has been discovered to have both type I phase‐matchable behavior and to exhibit moderate powder second‐harmonic generation intensity, about 0.5 times that of commercial AgGaS₂ in the particle size of 200–250 μm at a laser radiation of 2.09 μm. In addition, Li₂MnSnSe₄ exhibits congruent melting behavior, which makes the bulk single‐crystal growth by the Bridgman–Stockbarger method possible. The temperature‐dependent susceptibility measurement indicates an antiferromagnetic interaction with a Néel temperature (T _N) of 8.6 K for this compound.     

Synthesis and characterization of main‐chain NLO oligomers and polymer that contain 4‐dialkylamino‐ 4′‐(alkylsulfonyl)azobenzene chromophores

An ω‐amino carboxylic acid monomer that contained a nonlinear optical (NLO) chromophore was prepared by a convergent synthesis. Strategies for selective protection/deprotection of the amino and carboxylic acid functionalities were developed. The protected monomer, 4‐[N‐(4‐benzyloxycarbonyl)butyl‐N‐methylamino]‐4′‐[2″,5″‐bis(decyloxy)‐4″‐(phthalimidomethyl)benzylsulfonyl]azobenzene, could be deprotected selectively or sequentially to give HOOC‐monomer‐N‐phthaloyl, benzyl‐OOC‐monomer‐NH₂, or HOOC‐monomer‐NH₂. Sequential synthesis was performed to yield main‐chain NLO dimers and tetramers. This was accomplished by selective deprotection and dicyclohexylcarbodiimide coupling. The HOOC‐monomer‐NH₂ was polymerized by treatment with diphenylphosphoryl azide to give a main‐chain NLO polyamide. The monomer, dimer, tetramer, and polymer NLO materials were characterized by ¹H, ¹³C, IR, and UV–visible spectroscopy as well as by gel permeation chromatography, differential scanning calorimetry, and elemental analysis. The NLO properties of these materials were measured. Thin films of the oligomers and polymer were prepared by spin casting on indium‐tin oxide coated glass. The second‐order NLO properties of the oligomers and polymer thin films were studied by in situ corona poling/second‐harmonic generation and attenuated total reflection methods. The optimal poling temperatures were significantly lower than the melting temperatures or glass‐transition temperatures of the oligomers and polymer. The poling efficiency increased in the following order: monomer, oligomers, and polymer. An electro‐optic coefficient of 4 pm/V at 1.06 μm was obtained for the polymer. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 546–559, 2000     

Large Second Harmonic Generation (SHG) Effect and High Laser‐Induced Damage Threshold (LIDT) Observed Coexisting in Gallium Selenide

A big challenge for nonlinear optical (NLO) materials is the application in high power lasers, which needs the simultaneous occurrence of large second harmonic generation (SHG) and high laser induced damage threshold (LIDT). Herein we report the preparation of a new Ga₂Se₃ phase, which shows the SHG intensities of around 2.3 times and the LIDT of around 16.7 times those of AgGaS₂ (AGS), respectively. In addition, its IR transparent window ca. 0.59–25 μm is also significantly wider than that of AGS (ca. 0.48–≈11.4 μm). The occurrence of the strong SHG responses and good phase‐matching indicate that the structure of the new Ga₂Se₃ phase can only be non‐centrosymmetric and have a lower symmetry than the cubic γ‐phase. The observed excellent SHG and phase‐matching properties are consistent with our diffraction experiments and can be well explained by using the orthorhombic models obtained through our high throughput simulations.