REI 5 O 14 (RE=Y and Gd): Promising SHG Materials Featuring the Semicircle‐Shaped I 5 O 14 3− Polyiodate Anion

Chiral cadmium-amine complexes for stimulating non-linear optical activity and photoluminescence in solids based on aurophilic stacks

The design of high-performance optical materials can be realized using coordination polymers (CPs) often supported by non-covalent interactions, such as metallophilicity. The challenge is to control two or more optical effects, e.g., non-linear optics (NLO) and photoluminescence (PL). We present a new strategy for the combination of the NLO effect of second-harmonic generation (SHG) and the visible PL achieved by linking dicyanidoaurate(i) ions, which form luminescent metallophilic stacks, with cadmium(ii) complexes bearing chiral amine ligands, used to break the crystal's symmetry. We report a family of NLO- and PL-active materials based on heterometallic Cd(ii)-Au(i) coordination systems incorporating enantiopure propane-1,2-diamine (pda) ligands (1-S, 1-R), their racemate (2), and enantiopure trans-cyclopentane-1,2-diamine (cpda) ligands (3-S, 3-R). Due to acentric space groups, they exhibit the SHG signal, tunable within the range of 11-24% of the KDP reference, which was correlated with the dipole moments of Cd(ii) units. They show efficient blue PL whose energy and quantum yield, the latter ranging from 0.40 to 0.83, are controlled by Cd(ii) complexes affecting the Au-Au distances and vibrational modes. We prove that chiral Cd(ii)-amine complexes play the role of molecular agents for the stimulation of both the NLO and PL of the materials based on aurophilic stacks.

Eu2 P2 S6 : The First Rare-Earth Chalcogenophosphate Exhibiting Large Second-Harmonic Generation Response and High Laser-Induced Damage Threshold

Metal chalcogenophosphates are receiving increasing interest, specifically as promising infrared nonlinear optical (NLO) candidates. Here, a rare-earth chalcogenophosphate Eu₂ P₂ S₆ crystallizing in the monoclinic noncentrosymmetric space group Pn was synthesized using a high-temperature solid-state method. Its structure features isolated [P₂ S₆ ]^4- dimer, and two types of EuS₈ bicapped triangular prisms. Eu₂ P₂ S₆ exhibits a phase-matchable second-harmonic generation (SHG) response ≈0.9×AgGaS₂ @2.1 μm, and high laser-induced damage threshold of 3.4×AgGaS₂ , representing the first rare-earth NLO chalcogenophosphate. The theoretical calculation result suggests that the SHG response is ascribed to the synergetic contribution of [P₂ S₆ ]^4- dimers and EuS₈ bicapped triangular prisms. This work provides not only a promising high-performance infrared NLO material, but also opens the avenue for exploring rare-earth chalcogenophosphates as potential IR NLO materials.

An Effective Strategy for Designing Nonlinear Optical Crystals by Combining the Structure-Directing Property of Oxyfluorides with Chemical Substitution

Rationally designing a high-performance nonlinear optical (NLO) crystal remains a great ongoing challenge. It involves not only the design of noncentrosymmetric structures but also property optimization. In this communication, a new strategy for effectively designing the NLO crystal has been put forward, that is, using the structure-directing property of oxyfluoride anions to obtain a noncentrosymmetric and polar structure, and then by substitution of IO₃ ^- for isovalent F^- anions to further enhance the SHG response. With this strategy, a new iodate fluoride, Ba₂ [MoO₃ F(IO₃ )](MoO₃ F₂ ) has been successfully designed from the Ba₂ (MoO₃ F₂ )F₂ with the cis-directing [MoO₄ F₂ ]^4- groups. It exhibits a large SHG response (≈8×KDP), a wide transparent region (0.30-10.92 μm), a high laser-induced damage threshold (LDT) (88.53 MW cm^-2 ), and a large birefringence (≈0.264@532 nm). These indicate Ba₂ [MoO₃ F(IO₃ )](MoO₃ F₂ ) would be a promising NLO crystal.

A Congruent-Melting Mid-Infrared Nonlinear Optical Vanadate Exhibiting Strong Second-Harmonic Generation

Study of mid-infrared (mid-IR) nonlinear optical (NLO) materials is hindered by the competing requirements of optimized second-harmonic generation (SHG) coefficient d_ij and laser-induced damage threshold (LIDT) as well as the harsh synthetic conditions. Herein, we report facile hydrothermal synthesis of a polar NLO vanadate Cs₄ V₈ O₂₂ (CVO) featuring a quasi-rigid honeycomb-layered structure with [VO₄ ] and [VO₅ ] polyhedra aligned parallel. CVO possesses a wide IR-transparent window, high LIDT, and congruent-melting behavior. It has very strong phase-matchable SHG intensities in metal vanadate family (12.0 × KDP @ 1064 nm and 2.2 × AGS @ 2100 nm). First-principles calculations suggest that the exceptional SHG responses of CVO largely originate from virtual electronic transitions within [V₄ O₁₁ ]_∞ layer; the excellent optical transmittance of CVO arises from the special characteristics of vibrational phonons resulting from the layered structure.

Be2 (BO3 )(IO3 ): The First Anion-mixed Van der Waals Member in the KBe2 BO3 F2 Family with a Very Strong Second Harmonic Generation Response

To obtain new nonlinear optical (NLO) materials with a large second harmonic generation (SHG) effect has always been a great challenge. We have synthesized the first metal borate-iodate NLO crystal, Be₂ (BO₃ )(IO₃ ) (BBIO), by multicomponent modification of KBe₂ BO₃ F₂ (KBBF), in which the structural features of KBBF were maintained and (IO₃ )^- groups were connected to honeycomb [Be₂ BO₃ O₂ ]_∞ layers. As the first KBBF family member with mixed anionic groups, BBIO benefited from the synergistic effect of (IO₃ )^- , (BO₃ )^3- and (BeO₄ )^6- groups, and exhibited a very strong SHG response of ≈7.2×KH₂ PO₄ (KDP, @1064 nm) and a large birefringence (Δn) of 0.172 at 1064 nm. BBIO may, unexpectedly, optimize growth habits via van der Waals forces. This study presents borate-iodate as a new NLO material and it demonstrates opportunities in KBBF structural engineering.

[o-C5 H4 NHOH]2 [I7 O18 (OH)]⋅3 H2 O: An Organic-Inorganic Hybrid SHG Material Featuring an [I7 O18 (OH)] ∞ 2 - Branched Polyiodate Chain

An organic-inorganic hybrid polyiodate, namely, [o-C₅ H₄ NHOH]₂ [I₇ O₁₈ (OH)]⋅3 H₂ O (I), featuring a novel branched polyiodate chain has been obtained by evaporation method. [o-C₅ H₄ NHOH]₂ [I₇ O₁₈ (OH)]⋅3 H₂ O (I) crystalizes in the polar space group Ia and features an [I₇ O₁₈ (OH)] ∞ 2 - branched polyiodate chain in which [I₃ O₉ ]^3- trimers are grafted on the same side of the one-dimensional (1D) chain based on [I₄ O₁₁ (OH)]^3- tetramers. The asymmetric organic amine groups are beneficial to the polymerization of iodate groups and inducing the formation of the non-centrosymmetric (NCS) structure. Compound I exhibits a rather large Second-Harmonic- Generation (SHG) signal of 8.5×KH₂ PO₄ (KDP) upon 1064 nm laser radiation, a moderate band gap of 3.90 eV and a high laser-induced-damage-threshold (LIDT) of 182.34 MW cm^-2 , hence it is a promising new SHG material. The relationship between the structures of the organic amine groups and the overall structures has been also analyzed.

(NH4 )Bi2 (IO3 )2 F5 : An Unusual Ammonium-Containing Metal Iodate Fluoride Showing Strong Second Harmonic Generation Response and Thermochromic Behavior

An ammonium-containing metal iodate fluoride compound, (NH₄ )Bi₂ (IO₃ )₂ F₅ , featuring a two-dimensional double-layered framework constructed by [BiO₂ F₅ ]^6- and [BiO₄ F₄ ]^9- polyhedra, as well as [IO₃ ]^- groups, was successfully synthesized. The well-ordered alignment of these SHG-active units leads to an extraordinary strong SHG response of 9.2 times that of KDP. Moreover, this compound possesses a large birefringence (Δn=0.0690 at 589.3 nm), a wide energy band gap (E_g =3.88 eV), and a high laser damage threshold (LDT; 40.2×AgGaS₂ ). In particular, thermochromic behavior was observed for the first time in this type of compound. Such multifunctional crystals will expand the application of nonlinear optical materials.

LiMII (IO3 )3 (MII =Zn and Cd): Two Promising Nonlinear Optical Crystals Derived from a Tunable Structure Model of α-LiIO3

Excellent nonlinear optical materials simultaneously meet the requirements of large SHG response, phase-matching capability, wide transparency windows, considerable energy band-gap, good thermal stability and structure stability. Herein, two new promising nonlinear optical (NLO) crystals LiM^II (IO₃ )₃ (M^II =Zn and Cd) are rationally designed by the aliovalent substitution strategy from the commercialized α-LiIO₃ with the perfect parallel alignment of IO₃ groups. Compared with parent α-LiIO₃ and related A^I ₂ M^IV (IO₃ )₆ , the title compounds exhibit more stable covalent 3D structure, and overcome the racemic twinning problem of A^I ₂ M^IV (IO₃ )₆ . More importantly, both compounds inherit NLO-favorable structure merits of α-LiIO₃ and show larger SHG response (≈14× and ≈12×KDP), shorter absorption edge (294 and 297 nm) with wider energy band-gap (4.21 and 4.18 eV), good thermal stability (460 and 430 °C), phase-matching behaviors, wider optical transparency window and good structure stability, achieving an excellent balance of NLO properties.