Balanced infrared nonlinear optical performance achieved by modulating the covalency and ionicity distributions in the electron localization function map

Emergent Mid-Infrared Nonlinear Optical Candidates With Targeted Balance Performances

Infrared nonlinear optical (NLO) crystal materials exert a crucial role in laser technology, which is extensively utilized in the fields of medical laser, long-distance laser communication, infrared laser guidance, etc. Currently, the commercially available infrared NLO crystals are diamond-like structural crystals AgGaQ2 (Q = S, Se) and ZnGeP2. However, their applications are significantly limited owing to their inherent drawbacks, such as low laser damage thresholds and narrow band gaps. Therefore, exploring novel infrared NLO materials with excellent performances is urgent. At present, candidate systems for exploring infrared NLO materials mainly are chalcogenides, pnictides, metal halides for popular systems, and chalcohalides, oxyhalides, heavy metal oxides, oxychalcogenides, nitrides for emergent systems. Notably, among them, pnictides generally exhibited a stronger NLO performance than other systems, but a narrower band gap. Accordingly, after the detailed literature survey, to the best knowledge, ≈139 compounds achieve balanced performances (Eg ≥ 3.0 eV, dij ≥ 0.5 × AgGaS2) in the remaining systems, in which there are 2 metal halides, 9 oxyhalides, 10 heavy metal oxides, 17 nitrides, 19 oxychalcogenides, 22 chalcohalides, and 60 chalcogenides. Thus, the structure-property survey of these compounds produces the practical design strategy to explore emergent infrared NLO crystal materials with balanced properties.

Fully tricoordinated assembly unveils a pioneering nonlinear optical crystal (SbTeO3)(NO3)

Balancing the critical property requirements is key to surmounting the obstacles in the application of nonlinear optical (NLO) crystals. Tricoordinated units, characterized by nearly the lowest coordination number, are common in inorganic NLO-active oxides; however, crystals solely composed of such units are rare. Herein, by assembling three distinct tricoordinated units (SbO3, TeO3, and NO3) into a single crystal, a pioneering fully tricoordinated NLO material, (SbTeO3)(NO3), was synthesized via a facile volatilization method. As the first reported tellurite-antimonite NLO crystal, (SbTeO3)(NO3) exhibits well-balanced properties: a strong phase-matched second-harmonic generation (SHG) effect (2.2 × KDP), short UV cutoff edge (253 nm) and moderate birefringence (0.081@546 nm). Unlike most deliquescent nitrates, (SbTeO3)(NO3) demonstrates exceptional water resistance (>30 days), attributed to its unique hydrophobic layers and stereochemically active lone pair (SCALP) electrons in the Sb3+ and Te4+ cations. Theoretical calculations reveal that the optical bandgap and SHG effect of (SbTeO3)(NO3) are collectively governed by the three tricoordinated motifs, with individual SHG contributions of 20.92%, 23.88%, and 55.12% from [SbO3], [TeO3] and [NO3], respectively. This breakthrough underscores the efficacy of the fully tricoordinated assembly strategy in engineering NLO materials with optimally balanced properties.

Assembly of π-Conjugated [B3O6] Units by Mer-Isomer [YO3F3] Octahedra to Design a UV Nonlinear Optical Material, Cs2YB3O6F2

Achieving the extreme balance of the key performance requirements is the crucial to breakthrough the application bottleneck for nonlinear optical (NLO) materials. Herein, by assembly of the π-conjugated [B3O6] functional species with the aid of structure-directing property of mer-isomer [YO3F3] octahedra, a new ultraviolet (UV) NLO material, Cs2YB3O6F2 with aligned arrangement of coplanar [B3O6] groups has been synthesized. The polar material exhibits the rare coexistence of the largest second harmonic generation response of 5.6×KDP, the largest birefringence of 0.091 at 532 nm, the shortest Type I phase-matching down to 200.5 nm and deep-ultraviolet transparency among reported acentric rare-earth borates with [B3O6] groups. Remarkably, benefiting from the enhanced bonding force among functional units [B3O6], a firm three-dimensional framework is constructed, which facilitates the growth of large crystals. This can be proved by a block shape crystal with dimensional of 6×5×4 mm3, indicating that it was a promising UV NLO crystal. This work provides a powerful strategy to design UV NLO materials with good performances.

Rare-earth-based chalcogenides and their derivatives: an encouraging IR nonlinear optical material candidate

With the continuous development of laser technology and the increasing demand for lasers of different frequencies in the infrared (IR) spectrum, research on infrared nonlinear optical (NLO) crystals has garnered growing attention. Currently, the three main commercially available types of borate materials each have their drawbacks, which limit their applications in various areas. Rare-earth (RE)-based chalcogenide compounds, characterized by the unique f-electron configuration, strong positive charges, and high coordination numbers of RE cations, often exhibit distinctive optical responses. In the field of IR-NLO crystals, they have a research history spanning several decades, with increasing interest. However, there is currently no comprehensive review summarizing and analyzing these promising compounds. In this review, we categorize 85 representative examples out of more than 400 non-centrosymmetric (NCS) compounds into four classes based on the connection of different asymmetric building motifs: (1) RE-based chalcogenides containing tetrahedral motifs; (2) RE-based chalcogenides containing lone-pair-electron motifs; (3) RE-based chalcogenides containing [BS3] and [P2Q6] motifs; and (4) RE-based chalcohalides and oxychalcogenides. We provide detailed discussions on their synthesis methods, structures, optical properties, and structure-performance relationships. Finally, we present several favorable suggestions to further explore RE-based chalcogenide compounds. These suggestions aim to approach these compounds from a new perspective in the field of structural chemistry and potentially uncover hidden treasures within the extensive accumulation of previous research.

A UV non-hydrogen pure selenite nonlinear optical material for achieving balanced properties through framework-optimized structural transformation

For non-centrosymmetric (NCS) oxides intended for ultraviolet (UV) nonlinear optical (NLO) applications, achieving a wide band gap, large second harmonic generation (SHG) intensity, and sufficient birefringence to satisfy phase matching is a significant challenge due to their inherent incompatibility. To address this issue, this study proposes a strategy called framework-optimized structural transformation. Building upon centrosymmetric (CS) NaGa(SeO3)2 as a foundation, an original UV selenite NLO material, NaLu(SeO3)2, was successfully synthesized. The derived NaLu(SeO3)2 exhibits a balanced comprehensive performance, including a band gap (5.3 eV), an SHG response (2.7 × KDP), a UV cut-off edge (210 nm), a laser-induced damage threshold (LIDT) (151.69 MW cm-2), birefringence (Cal: 0.138@546 nm, Exp: 0.153@546 nm), thermal stability (∼575 °C) and environmental stability. Notably, its SHG effect, band gap, LIDT, and birefringence are all the largest among UV non-hydrogen pure selenite materials. Such progress can be attributed to the successful arrangement of the SeO3 groups by optimizing the cations on the framework of the parent compound.

Hg3Se(SeO3)(SO4): A Mixed-Valent Selenium Compound with Mid-Infrared Transmittance Obtained by In Situ Reaction

Exploring new material systems is a highly significant task in the field of inorganic chemistry. A new mixed-valent selenium compound, Hg3Se(SeO3)(SO4), was successfully synthesized through in situ reactions. This compound exhibits a novel three-dimensional structure composed of Hg3Se(SO4) layers bridged by SeO3 trigonal pyramids. It is the first structure containing (SeO3)2-, (SO4)2-, and Se2- simultaneously. In addition, Hg3Se(SeO3)(SO4) possesses a wide bandgap (3.5 eV), moderate birefringence (Cal:0.064@546 nm, Exp:0.069@546 nm), a high laser-induced damage threshold (23.35 MW cm-2), and a wide transmittance window (0.28-6.6 μm). Our work demonstrates that mixed-valent (+4, -2) selenite selenide can be potential optical materials for the mid-infrared region.

Rb8Nb10Ge6O41: a new niobium-germanate crystal featuring unique one-dimensional [Nb7O30]∞ chains and wide mid-IR transparency

Germanate oxides have garnered considerable interest owing to their diverse structural configuration and intriguing properties. Herein, we present a novel niobium germanate crystal, Rb8Nb10Ge6O41, extracted through the process of spontaneous crystallization. It showcases a unique three-dimensional (3D) structural framework composed of one-dimensional (1D) twisted [Nb7O30]∞ chains and isolated [Ge3O9] rings, arising from the divergent polymerized manifestations of [NbO6] and [GeO4] basic building blocks, respectively, marking the first instance of such a topography in germanate materials. Notably, the title compound exhibits exceptional thermal stability up to 1250 °C with a good congruent melting nature. Moreover, it achieves a short ultraviolet edge at 306 nm and a favorable infrared edge cutoff exceeding 6.2 μm, thus indicating a wide transparency window. Additionally, this study elucidates the microscopic birefringence of Rb8Nb10Ge6O41 and clarifies the intricate relationship between its structure and properties. Our findings suggest that the polymerization of distinct structural motifs within a single compound is an effective strategy for exploring novel inorganic materials.

Nonlinear Optical Phosphide CuInSi2P4: The Inaugural Member of Diamond-Like Family I-III-IV2-V4 Inspired by ZnGeP2

Noncentrosymmetric phosphides have garnered significant attention as promising systems of infrared (IR) nonlinear optical (NLO) materials. Herein, a new quaternary diamond-like phosphide family I-III-IV2-V4 and its inaugural member, namely, CuInSi2P4 (CISP), were successfully fabricated by isovalent and aliovalent substitution based on ZnGeP2. First-principles calculations revealed that CISP has a large NLO coefficient (d14 = 110.8 pm/V), which can be attributed to the well-aligned tetrahedral [CuP4], [InP4], and [SiP4] units. Remarkably, the extremely small thermal expansion anisotropy (0.09) of CISP enables it to exhibit a considerable laser-induced damage threshold (LIDT, 5.0 × AgGaS2@1.06 μm) despite the relatively narrow band gap (0.81 eV). This work improves the chemical diversity of inorganic phosphide and promotes the development of phosphide systems, which may provide valuable perspectives for future exploration of IR NLO materials.

Nonlinear Optical Sulfides LiMGa8 S14 (M = Rb/Ba, Cs/Ba) Created by Li+ Driven 2D Centrosymmetric to 3D Noncentrosymmetric Transformation

Cations that can regulate the configuration of anion group are greatly important but regularly unheeded. Herein, the structural transformation from 2D CS to 3D noncentrosymmetric (NCS, which is the prerequisite for second-order NLO effect) is rationally designed to newly afford two sulfides LiMGa8 S14 (M = Rb/Ba, 1; Cs/Ba, 2) by introducing the smallest alkali metal Li+ cation into the interlamination of 2D centrosymmetric (CS) RbGaS2 . The unusual frameworks of 1 and 2 are constructed from C2 -type [Ga4 S11 ] supertetrahedrons in a highly parallel arrangement. 1 and 2 display distinguished NLO performances, including strong phase-matchable second-harmonic generation (SHG) intensities (0.8 and 0.9 × AgGaS2 at 1910 nm), wide optical band gaps (3.24 and 3.32 eV), and low coefficient of thermal expansion for favorable laser-induced damage thresholds (LIDTs, 4.7, and 7.6 × AgGaS2 at 1064 nm), which fulfill the criteria of superior NLO candidates (SHG intensity >0.5 × AGS and band gap >3.0 eV). Remarkably, 1 and 2 melt congruently at 873.8 and 870.5 °C, respectively, which endows them with the potential of growing bulk crystals by the Bridgeman-Stockbarge method. This investigated system provides a new avenue for the structural evolution from layered CS to 3D NCS of NLO materials.

Oxychalcogenides: A Promising Class of Materials for Nonlinear Optical Crystals with Mixed-Anion Groups

Infrared nonlinear optical (IR NLO) materials are of great significance in the development of IR laser technology. But rationally designing high-performance IR NLO materials remains a huge challenge due to the conflict between the necessary properties required for NLO materials. Notably, oxychalcogenides with mixed-anion groups have drawn extensive interest as a family of important IR NLO candidates because they integrate the property advantages of oxides and chalcogenides by chemical substitution engineering. In this review, we provide a survey of reported oxychalcogenides and aim to present the development of NLO oxychalcogenides from the perspective of rational design of their structural chemistry. Furthermore, we focus on the relationships between partial substitution and structural symmetry as well as optical properties. These provide some helpful guidance for the further exploration and design of novel oxychalcogenide materials with excellent NLO performance in the future.

Enhancing Third-Order Nonlinear Optical Property by Regulating Interaction between Zr4(embonate)6 Cage and N, N-Chelated Transition-Metal Cation

Herein, the combination of anionic Zr₄L₆ (L = embonate) cages and N, N-chelated transition-metal cations leads to a series of new cage-based architectures, including ion pair structures (PTC-355 and PTC-356), dimer (PTC-357), and 3D frameworks (PTC-358 and PTC-359). Structural analyses show that PTC-358 exhibits a 2-fold interpenetrating framework with a 3,4-connected topology, and PTC-359 shows a 2-fold interpenetrating framework with a 4-connected dia network. Both PTC-358 and PTC-359 can be stable in air and other common solvents at room temperature. The investigations of third-order nonlinear optical (NLO) properties indicate that these materials show different degrees of optical limiting effects. It is surprising that increasing coordination interactions between anion and cation moieties can effectively enhance their third-order NLO properties, which can be attributed to the formation of coordination bonds that facilitate charge transfer. In addition, the phase purity, UV-vis spectra, and photocurrent properties of these materials were also studied. This work provides new ideas for the construction of third-order NLO materials.

Rb2CdSi4S10: novel [Si4S10] T2-supertetrahedra-contained infrared nonlinear optical material with large band gap

Infrared nonlinear optical (IR-NLO) materials with wide band gaps are important for generating high-power laser light for modern laser technologies. Herein, a wide band gap IR-NLO material, Rb₂CdSi₄S₁₀, was rationally designed and fabricated by introducing a NLO-active [Si₄S₁₀] T2-supertetrahedron (ST) into the quaternary sulfide system. The Rb₂CdSi₄S₁₀ shows the largest band gap (4.23 eV) among the quaternary chalcogenide IR-NLO materials reported, which results in a high laser-induced damage threshold (LIDT) of ∼5 × AgGaS₂ (AGS) at 1064 nm. At the same time, it has a moderate second-harmonic generation (SHG) response (0.6 × AGS). Based on statistical analyses, the Rb₂CdSi₄S₁₀ is the first compound to be discovered in the AI2B^IICIV4QVI10 family, and also the first Si-rich sulfide IR-NLO material with a [Si₄S₁₀] T2-supertetrahedra. The results indicate that Rb₂CdSi₄S₁₀ is a promising new IR-NLO material, and the NLO-active [Si₄S₁₀] T2-ST unit could be used for the exploration of IR-NLO material with excellent performances.

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.

Alkali metal partial substitution-induced improved second-harmonic generation and enhanced laser-induced damage threshold for Ag-based sulfides

Two new pentanary sulfides LiAgIn2GeS6 and NaAgIn2GeS6 were derived from parent Ag2In2GeS6via Li and Na partial substitution of Ag element, and their SHG efficiencies and LIDT intensities enhanced concurrently.

Achieving a strong second harmonic generation response and a wide band gap in a Hg-based material

The property-oriented structural design strategy is employed in Hg-based chalcogenide to synthesize a new IR NLO crystal, [Ba4Cl2][HgGa4S10], which exhibits well-balanced NLO properties, including large SHG response and wide band gap.