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Deep ultraviolet (DUV) nonlinear optical (NLO) crystals have important applications in lithography, microfabrication, and high-resolution photoelectric spectrometer. In recent years, significant progress has been made in both the theoretical design and experimental research of DUV NLO crystals. This review aims to offer a comprehensive perspective on exploring a new generation of DUV NLO materials. First, we summarize various computer-aided strategies for crystal structure design and emphasize their significant role in advancing the discovery of DUV NLO materials. Then, we outline several representative DUV crystals of experimental synthesis. Finally, we discuss the future prospects for exploring new generations of DUV NLO materials. We believe that employing data-driven, computer-aided methods to explore DUV NLO materials will help address the current challenges in the field of DUV NLO materials research. The close integration of calculation and experimentation will unlock new opportunities. [ABSTRACT FROM AUTHOR]

Since the study of short‐wave ultraviolet nonlinear optical crystals based on the quasi‐phase‐matching principle will effectively expand the development field of nonlinear optical materials, it is urgent to achieve their directional design. Here, it is proposed and implemented on account of the hexagonal wurtzite structure. Thus, the LiRbSeO4 crystal is obtained with [SeO4] groups as the main tetrahedra and [LiO4] as the separation. It exhibits a wide transparency window (0.22–5.57 µm) and an excellent nonlinear optical effect (≈1.7 × KH2PO4), which originates from the synergy of distorted tetrahedral groups and their uniform arrangement. More importantly, the LiRbSeO4 crystal is confirmed to be a ferroelectric with a typical polarization‐electric field hysteresis loop (a coercive field of 27.71 kV cm−1, a remanent polarization of 8.60 µC cm−2 at 373 K). Its ferroelectricity is further identified by the domain engineering studies, involving piezoelectricity and the chirality. Besides, ferroelectricity is attributed to the ordered arrangement of tetrahedra with the polarization reversal involving atomic displacements. Therefore, the LiRbSeO4 crystal is a promising short‐wave ultraviolet nonlinear optical candidate based on the quasi‐phase‐matching principle. This work also provides a new horizon for the design of nonlinear optical crystals. [ABSTRACT FROM AUTHOR]

Oriented synthesis of functional materials is a focus of attention in material science. As one of the most important function materials, infrared nonlinear optical materials with large second harmonic generation effects and broad optical band gap are in urgent need. In this work, directed by the theoretical structure prediction, the first series of non‐centrosymmetric (NCS) alkali‐alkaline earth metal [PS4]‐based thiophosphates LiCaPS4 (Ama2), NaCaPS4 (P21), KCaPS4 (Pna21), RbCaPS4 (Pna21), CsCaPS4 (Pna21) were successfully synthesized. Comprehensive characterizations reveal that ACaPS4 could be regarded as promising IR NLO materials, exhibiting wide band gap (3.77–3.86 eV), moderate birefringence (0.027–0.064 at 1064 nm), high laser‐induced damage threshold (LIDT, ~10×AGS), and suitable phase‐matching second harmonic generation responses (0.4–0.6×AGS). Structure‐properties analyses illustrate that the Ca−S bonds show non‐ignorable covalent feature, and [PS4] together with [CaSn] units play dominant roles to determine the band gap and SHG response. This work indicates that Li‐, Na‐ and K‐ analogs may be promising infrared nonlinear optical material candidates, and this is the first successful case of "prediction to synthesis" involving infrared (IR) nonlinear optical (NLO) crystals in the thiophosphate system and may provide a new avenue to the design and oriented synthesis of high‐performance function materials in the future. [ABSTRACT FROM AUTHOR]

Hg-based compounds show abundant structural diversity and distinguished properties. Herein, a new phase transition compound CsHg2I5 was reported. The high-temperature phase β-CsHg2I5 with rare [Hg2I5] dimers was synthesized by the flux method at 573 K, and it shows a reversible phase transition at a low temperature of ∼100 K to form the low-temperature phase α-CsHg2I5. The two phases crystallize in the same P21/c space group, with different crystal structures. β-CsHg2I5 is composed of rare [Hg2I5] dimers and [CsI11] polyhedral units, while α-CsHg2I5 is composed of [Hg4I11] and [CsI10] units. The experimental band gap of β-CsHg2I5 was found to be 2.58 eV. Owing to the presence of [Hg2I5]∞ pseudo-layers, β-CsHg2I5 exhibits large optical anisotropy with a calculated birefringence of 0.132@1064 nm. Meanwhile, β-CsHg2I5 is a congruent compound and the congruent point is ∼481 K. Theoretical calculations indicate that the rare [Hg2I5] dimer is a nonlinear active unit, which can be used as a new fundamental building block for the design of advanced nonlinear optical materials. Moreover, a CsI–HgI2 pseudo-binary diagram was drawn. The results enrich the structural diversity of Hg-based halides and give some insights into the development of new functional materials based on rare [Hg2I5] dimers. [ABSTRACT FROM AUTHOR]

Optical materials with non‐π‐conjugated units usually can exhibit large bandgaps, which are conducive to a wide ultraviolet (UV) transparency window. However, the relatively small structural anisotropy hinders the application of these materials as nonlinear optical (NLO) and birefringent materials. Herein, a universal strategy of dissimilating non‐π‐conjugated units is proposed to enhance birefringence, optimize phase‐matching (PM) wavelength, and maintain a large bandgap. The functionality of the series of hetero‐anionic units is distinguished in terms of optimizing birefringence and PM wavelength. In particular, the dimeric hetero‐anionic unit N(SO2F)2− with the dual‐heteroatom dissimilation is found to be a novel superior optical hereto‐gene. In addition, AN(SO2F)2 (A = Li, Na‐I, K‐I/II, H) and Ba(SO3CF3)2 are identified with large birefringence of approximately 0.051–0.106 at 546 nm, which are much larger than those of corresponding sulfates. HN(SO2F)2 exhibits a short PM wavelength (208 nm), a large NLO coefficient (≈ 2 × KDP), and an extremely short UV cutoff edge (149 nm). Experimental polarizing microscope measurement confirms that the birefringence of Pbca‐KN(SO2F)2 is significantly enhanced to 0.06 at 546.1 nm, which is 12 times higher than that of K2SO4. Finally, this strategy offers direction for the design and synthesis of novel NLO or birefringent materials in the future. [ABSTRACT FROM AUTHOR]

The development of anti‐laser damage infrared (IR) nonlinear optical (NLO) materials is an urgent need but challenging due to the contradiction between wide bandgap and large NLO coefficient. Herein, by introducing a halogen into the NLO‐active tetrahedral unit, two halogenated metal chalcogenides Cs3Ga8S13Cl and Cs9Ga8Se12Cl9 with unprecedented [GaQ3Cl] (Q = S/Se) tetrahedral units are rationally designed and fabricated in experiments by the flux method, enriching the chemical and structural diversity of chalcogenides. Cs3Ga8S13Cl shows a wide bandgap of ≈3.76 eV, resulting in a high laser‐induced damage threshold of ≈8 × AgGaS2 (AGS) and a moderate NLO response (≈0.6 × AGS) with phase‐matching behavior. The results imply that Cs3Ga8S13Cl is a promising IR NLO material for the high powder laser application and demonstrate the feasibility of the halogenated tetrahedron strategy in regulating the optical properties of chalcogenides. [ABSTRACT FROM AUTHOR]

The approach of synergistically combining mixed chromophores has garnered significant attention attributed to its ability to leverage the advantages of both groups and design novel nonlinear optical (NLO) materials. Herein, Sr(NO3)(NH2SO3)·H2O, a noncentrosymmetric compound, which represents the first reported nitrate sulfamate compound, is successfully designed and synthesized. The compound features zigzag [SrNO10(H2O)]∞ infinite layers, forming a honeycomb‐like graphene topology in its layered structure. The unique arrangement results in an excellent second harmonic generation response, 5.1 times that of KH2PO4, and an optimized birefringence of 0.06650 at 532 nm. In addition, the presence of interlayered covalent [NH2SO3] groups facilitates improved crystal growth habits, resulting in a large single crystal measuring 19.0 × 17.0 × 3.5 mm3. In this discovery, a new class of NLO materials is introduced and the scope of structural chemistry is significantly expanded. [ABSTRACT FROM AUTHOR]

The exploration of antilaser damage wide bandgap infrared (IR) nonlinear optical (NLO) materials is urgent but challenging. Herein, by introducing the idea of fluorination into chalcogenides, a wide bandgap IR NLO material Na3SiS3F with unprecedented [SiS3F] tetrahedra is designed and synthesized. Na3SiS3F shows a wide bandgap of 4.75 eV (the largest one in the reported quaternary metal chalcogenides), resulting in a high laser damage induced threshold of ≈5 × AgGaS2 (AGS). Meanwhile, the compound has a moderate NLO response (≈0.3 ×AGS) with phase‐matching behavior, large birefringence (0.15@1064 nm), and wide IR transparent region. The introduction of fluorine breaks the structural symmetry and broadens the highest occupied molecular orbital‐lowest unoccupied molecular orbital (HOMO‐LUMO) gap, polarizability anisotropy, and hyperpolarizability of the Si–S tetrahedral unit. The results indicate that Na3SiS3F is a promising IR NLO material for the high‐power laser application and open an avenue for the design of new wide bandgap IR NLO materials based on NLO‐active [SiSxF4−x] (x = 1, 2, 3) mixed anionic tetrahedral group. [ABSTRACT FROM AUTHOR]

NaSrBS3, the first noncentrosymmetric alkali and alkaline earth metal mixed thioborate with the isolated [BS3] as the building units, has been synthesized successfully under the guidance of theoretical prediction. Comprehensive characterizations reveal that NaSrBS3 exhibits wide bandgap (3.94 eV), large birefringence (0.185 at 1064 nm), a high laser‐induced damage threshold (9 × AGS), and a suitable phase‐matching second‐harmonic generation (SHG) response (0.3 × AGS). Structure‐properties analyses illustrate that the [BS3] units determine the bandgap and contribute the most to the birefringence and SHG response. This work is the first successful case of "prediction to synthesis" involving infrared (IR) nonlinear optical (NLO) crystals in the thioborate system and provides an avenue to SHG materials design. [ABSTRACT FROM AUTHOR]

Traditional infrared (IR) nonlinear optical (NLO) materials such as AgGaS2 are crucial to key devices for solid‐state lasers, however, low laser damage thresholds intrinsically hinder their practical application. Here, a robust strategy is proposed for unbiased high‐throughput screening of more than 140 000 materials to explore novel IR NLO materials with high thermal conductivity and wide band gap which are crucial to intrinsic laser damage threshold. Via our strategy, 106 compounds with desired band gaps, NLO coefficients and thermal conductivity are screened out, including 8 nitrides, 68 chalcogenides, in which Sr2SnS4 is synthesized to verify the reliability of our process. Remarkably, thermal conductivity of nitrides is much higher than that of chalcogenides, e.g. 5×AgGaS2 (5.13 W/m K) for ZrZnN2, indicating that nitrides could be a long‐neglected system for IR NLO materials. This strategy provides a powerful tool for searching NLO compounds with high thermal conductivity. [ABSTRACT FROM AUTHOR]

Ultraviolet (UV) and Deep‐UV (DUV, λ<200 nm) nonlinear optical (NLO) materials play a significant role to convert the wavelength of laser for producing UV/DUV coherent light in the laser industry. The Al [Ne]3s23p1 atom with empty d orbitals can form sp3, sp3d, and sp3d2 hybrid orbitals when bonding with O/F atoms, which leads to the construction of [AlOmFn] (m+n=4, 5, 6) units. Therein, aluminoborates have received intensive attention due to the effective structural and functional adjustment effects of the diverse Al−O/F units. Accordingly, numerous aluminoborates as candidates for the next generation of UV/DUV NLO materials were discovered. In this review, recently discovered aluminoborates are overviewed and analyzed, including their syntheses, crystal structure features, and second‐order NLO performances. We aim to provide the latest progress and outlook on the crystal chemistry of aluminoborates and boost the finding of the next NLO candidates with high performances. [ABSTRACT FROM AUTHOR]

A long‐lasting challenge in ultraviolet (UV) nonlinear optical (NLO) materials design is to combine conflicting and harsh properties into one structure, including wide UV transparency, strong NLO efficiency, and moderate birefringence to realize phase‐matching at short wavelength. A fluorine‐directed material design strategy is adopted to accelerate the process of finding NLO materials with optimal performances. In this work, taking LiB3O5 (LBO) as a maternal structure, one O atom with two F atoms is substituted and LiNaB6O9F2 which possesses novel double interpenetrating 3[B6O9F2]∞ networks and fluorine‐directed optimal performances including enhanced birefringence and phase‐matching capacity is successfully obtained. The perfect balance among deep‐UV cutoff edge, large second‐harmonic generation (SHG) response (1.1 × KDP), and moderate birefringence (0.067 at 1064 nm) of LiNaB6O9F2 makes it a promising UV NLO crystal. [ABSTRACT FROM AUTHOR]

Design and exploratory synthesis of new mid‐infrared (mid‐IR) nonlinear optical (NLO) materials are urgently needed for modern laser science and technology because the widely used IR NLO crystals still suffer from their inextricable drawbacks. Herein, a multi‐level data‐driven approach to realize fast and efficient structure prediction for the exploration of promising mid‐IR NLO materials is proposed. Techniques based on machine learning, crystal structure prediction, high‐throughput calculation and screening, database building, and experimental verification are tightly combined for creating pathways from chemical compositions, crystal structures to rational synthesis. Through this data‐driven approach, not only are all known structures successfully predicted but also five thermodynamically stable and 50 metastable new selenides in AIBIIISe2 systems (AI = Li, Na, K, Rb, and Cs; BIII = Al and Ga) are found, among which eight outstanding compounds with wide bandgaps (> 2.70 eV) and large SHG responses (>10 pm V−1) are suggested. Moreover, the predicted compounds I4¯2d‐LiGaSe2 and I4/mcm‐KAlSe2 are successfully obtained experimentally. In particular, LiGaSe2 exhibits a robust SHG response (≈2 × AGS) and long IR absorption edge that can cover two atmospheric windows (3–5, 8–12 µm). Simultaneously, this new research paradigm is also applicative for discovering new materials in other fields. [ABSTRACT FROM AUTHOR]

Design and fabrication of new infrared (IR) nonlinear optical (NLO) materials with balanced properties are urgently needed since commercial chalcopyrite‐like (CL) NLO crystals are suffering from their intrinsic drawbacks. Herein, the first defect‐CL (DCL) alkali‐earth metal (AEM) selenide IR NLO material, DCL‐MgGa2Se4, has been rationally designed and fabricated by a structure prediction and experiment combined strategy. The introduction of AEM tetrahedral unit MgSe4 effectively widens the band gap of DCL compounds. The title compound exhibits a wide band gap of 2.96 eV, resulting in a high laser induced damage threshold (LIDT) of ≈3.0 × AgGaS2 (AGS). Furthermore, the compound shows a suitable second harmonic generation (SHG) response (≈0.9 × AGS) with a type‐I phase‐matching (PM) behavior and a wide IR transparent range. The results indicate that DCL‐MgGa2Se4 is a promising mid‐to‐far IR NLO material and give some insights into the design of new CL compound with outstanding IR NLO properties based on the AEM tetrahedra and the structure predication and experiment combined strategy. [ABSTRACT FROM AUTHOR]

Discovery of new efficient nonlinear optical (NLO) materials with large second‐order nonlinearity for the short‐wave ultraviolet spectral region (λPM≤266 nm, PM=phase‐matching) is still very challenging. Herein, a new beryllium‐free borate CaZn2(BO3)2 with Sr2Be2B2O7 (SBBO) double‐layered like configuration was rationally designed, which not only preserves the structural merits but also eliminates the limitations of the SBBO crystal. CaZn2(BO3)2 shows a large PM second harmonic generation (SHG) reponse of 3.8×KDP, which is 38 times higher than that of its barium analogue. This enhancement mainly originates from the 1[Zn2O6]∞ polar chains with a large net dipole moment and [BO3] units with a high NLO active density. Our findings show the great significance of the [ZnO4] tetrahedra introduced strategy to design beryllium‐free SBBO‐type NLO crystals and also verify the feasibility of using simple non‐isomorphic substitution to induce giant second‐order nonlinearity enhancement. [ABSTRACT FROM AUTHOR]

Discovery of new efficient nonlinear optical (NLO) materials with large second‐order nonlinearity for the short‐wave ultraviolet spectral region (λPM≤266 nm, PM=phase‐matching) is still very challenging. Herein, a new beryllium‐free borate CaZn2(BO3)2 with Sr2Be2B2O7 (SBBO) double‐layered like configuration was rationally designed, which not only preserves the structural merits but also eliminates the limitations of the SBBO crystal. CaZn2(BO3)2 shows a large PM second harmonic generation (SHG) reponse of 3.8×KDP, which is 38 times higher than that of its barium analogue. This enhancement mainly originates from the 1[Zn2O6]∞ polar chains with a large net dipole moment and [BO3] units with a high NLO active density. Our findings show the great significance of the [ZnO4] tetrahedra introduced strategy to design beryllium‐free SBBO‐type NLO crystals and also verify the feasibility of using simple non‐isomorphic substitution to induce giant second‐order nonlinearity enhancement. [ABSTRACT FROM AUTHOR]

Exploring significant ultraviolet/deep‐ultraviolet nonlinear optical (NLO) materials is hindered by rigorous and contradictory requirements, especially, possessing a moderate optical birefringence to meet phase‐matching (PM). Except for suitable birefringence, small chromatic dispersion is also crucial to blue‐shift the PM wavelength. Here, the introduction of a fluorinated tetrahedral boron‐centred chromophore strategy was proposed to optimize the chromatic dispersion. Herein, the [BF4]− unit with a large HOMO–LUMO band gap was introduced to the Na−B−O−F system and Na4B8O9F10 was designed and synthesized successfully for the first time. Na4B8O9F10 with an optimized chromatic dispersion can achieve a short second harmonic generation PM wavelength of 240 nm with a relatively small birefringence (0.036@1064 nm). Notably, Na4B8O9F10 is the first acentric crystal with [BF4]− units among the reported metal–fluorooxoborate systems, involving isolated [BF4]− and novel [B7O10F6]5− fundamental building blocks. [ABSTRACT FROM AUTHOR]

Large band gap and strong nonlinear optical (NLO) effect are two valuable but contradictory parameters, which are difficult to balance in one infrared (IR) NLO material. Herein, the first alkali and alkaline‐earth metal diamond‐like (DL) IR NLO material Li4MgGe2S7, presenting a honeycomb‐like 3D framework constructed by 6‐membered LiS4 rings and GeMgS6 zigzag chains, was rationally designed and synthesized. The introduction of rigid alkali metal and alkaline‐earth metal LiS4 and MgS4 tetrahedra effectively broadens the band gap of DL compound to 4.12 eV (the largest one in the reported quaternary metal chalcogenides), generating a high laser damage threshold of 7 × AgGaS2 at 1064 nm. Furthermore, Li4MgGe2S7 displays a suitable SHG response (0.7 × AgGaS2) with a type I phase‐matching behavior. The results indicate that Li4MgGe2S7 is a promising IR NLO material for the high‐power laser application and it provides an insight into the design of new DL compound with outstanding IR NLO performances. [ABSTRACT FROM AUTHOR]

A new congruent-melting lead barium borate, Ba2.5Pb1.5B12O22, was synthesized via a high-temperature solid-state reaction under atmospheric pressure. It crystallizes in the noncentrosymmetric space group Cmc21 with a unit cell of a = 19.051(8) Å, b = 10.726(3) Å, c = 8.612(3) Å, and Z = 4. It features a new functional building block (FBB), [B12O26]16−, that is made up of five [B3O8]7− and one [BO3]3− connected by sharing vertex O atoms. It is derived by introducing Pb into Ba2B6O11, which leads to a structural transformation from a centrosymmetric to a noncentrosymmetric space group. Its second harmonic generation response (SHG) is approximately 0.9 times that of KDP at the fundamental wavelength of 1064 nm and is also phase-matchable. It has a wide transparent region with a UV cutoff edge of about 250 nm according to the reflectance spectra and high thermal stability, which illustrates that it is a potential candidate for ultraviolet nonlinear optical materials. [ABSTRACT FROM AUTHOR]

The generated light can be tuned to cover almost the entire spectral range from deep‐ultraviolet to terahertz wavelengths by utilizing the nonlinear optical crystals with a simple frequency doubling process. Among them, the discovery of novel candidates for the production of deep‐ultraviolet light is by extension a great challenge toward realizing the vast potential. Actually, the availability for this process mainly depends on whether the critical performance can be well coexisted in one practical crystal. Herein, the first magnesium fluorooxoborate MgB5O7F3 was synthesized as a new competitive candidate for deep‐ultraviolet nonlinear optical application. It has a sufficiently large nonlinearity and a deep‐ultraviolet phase matching wavelength, indicating that it holds great potential for the production of coherent light below 200 nm. The critical performance enhancement of MgB5O7F3 when compared with its isomorphic phases was demonstrated and discussed. More importantly, we proposed that fluorooxoborate system with the general formula of MB5O7F3 (M=divalent metal) possesses stable fluorine terminated framework, which makes them tend to retain their crystallized space groups unchanged. [ABSTRACT FROM AUTHOR]

Deep‐ultraviolet nonlinear optical (DUV NLO) materials are attracting increasing attention because of their structural diversity and complexity. Using the two‐dimensional (2D) crystal structure prediction method combined with the first‐principles calculations, here we propose layered 18‐membered‐ring (18MR) boron oxide B2O3 polymorphs as high‐performance NLO materials. 18MR‐B2O3 with the AA and AB stackings are potential DUV NLO materials. The superior performing 18MR‐B2O3AB has an unprecedentedly high second harmonic generation coefficient of 1.63 pm V−1, the largest among the DUV NLO materials, three times larger than that of the advanced DUV NLO material KBe2BO3F2 and comparable to that of β‐BaB2O4. Its unusually large birefringence of 0.196 at 400 nm guarantees the phase‐matching wavelength λPM to reach this material's extreme absorption edge of ≈154 nm. [ABSTRACT FROM AUTHOR]

Four sodium‐based ternary IR nonlinear optical (NLO) materials, Na6MQ4 (M=Zn, Cd; Q=S, Se), were prepared through a high‐temperature flux method. The crystal structure of the compounds is built up of isolated [MQ4] tetrahedra and a 3D framework formed by the NaQn (n=4, 5) units. The two selenides, Na6MSe4 (M=Zn, Cd), as promising IR NLO materials, show moderate second‐harmonic generation (SHG) responses (0.9 and 0.5×AgGaS2) with good phase‐matching behavior, as well as high laser damage thresholds (2 and 1.9×AgGaS2). The two sulfides, Na6MS4 (M=Zn, Cd), exhibit higher laser damage thresholds (13 and 4×AgGaS2), but smaller SHG responses (0.3 and 0.2×AgGaS2). Theoretical calculations and statistical analyses indicate that the SHG effect and band gap in the compounds originate mainly from the distorted NaQ4 NLO‐active units with a short Na−S bond length, which provides a new insight into the design of novel IR NLO materials. [ABSTRACT FROM AUTHOR]

Infrared (IR) nonlinear optical (NLO) materials are the key devices for generating tunable infrared output between ∼3 and ∼20 μm by laser frequency conversion techniques. Based on the study of structure and properties, chalcogenides, pnictides and oxides, have been demonstrated as the most promising systems for the exploration of new IR NLO materials with excellent optical performances. Over the past decades, many state-of-the-art IR NLO materials have been discovered in these systems. In this work, the synthesis, characterization and performance of the new developed promising IR NLO materials are summarized and analyzed. The typical IR NLO materials with large-size single crystals are selected as the representatives for the detailed discussions. Moreover, the discrepancies in optical properties of single crystal, polycrystalline powders, and the corresponding calculated results are discussed, aiming to provide suggestions for the exploration of next generation IR NLO material in these systems. [ABSTRACT FROM AUTHOR]

The first tin (II) borate bromide, Sn2B5O9Br (SBOB), as an outstanding bifunctional material with excellent linear and nonlinear optical (NLO) performance is synthesized successfully. As far as it is known, it exhibits the largest birefringence among the reported borates (0.439 at 546 nm), even over famous birefringent materials YVO4 and TiO2. Meanwhile, it also owns a strong second harmonic generation (SHG) response of 2.4 times that of KH2PO4. Theoretical calculations display that its incredible optical properties mainly originate from the extremely intensive stereochemical activity lone pair (SCALP) of [SnO7Br2] polyhedra, which results in the enhancement of birefringence and SHG response, increasing to 2.6‐ and 4.8‐fold, respectively, from Sn2B5O9Cl (SBOC) to SBOB. These perceptions will guide and accelerate the exploration of excellent birefringence and NLO materials in tin (II)‐based compound. [ABSTRACT FROM AUTHOR]

Advances in the generation of coherent lasers at wavelengths below 200 nm as well as the associated equipment and technologies have greatly spurred the development of nonlinear optical (NLO) materials that are capable of phase‐matching in the deep‐ultraviolet (deep‐UV) spectral regions. However, few crystals can meet the technical demands, and studies to date have focused mainly on finding the next competitive candidates. Consequently, several crystals have emerged that hold great potential for the production of deep‐UV harmonic coherent light, which benefit from newly developed systems with novel NLO‐active units and chromophores. This Review provides a comprehensive overview of the developments of emergent candidates in the field of deep‐UV NLO materials, discussing the recent efforts and active advances in the search for superior systems and in understanding their structure–property relationships. The intention is to provide a clear perspective on the current emergent deep‐UV NLO candidates, and to accelerate the discovery of next candidates with high performances to meet the urgent demands. [ABSTRACT FROM AUTHOR]

Inorganic infrared (IR) second-order nonlinear optical (NLO) crystals have become increasingly important for fulfilling the demands of modern laser technology through frequency conversion via optical parametric oscillation (OPO) and optical parameter amplification (OPA) technology. The A I2 BIICIVD VI4 family contains a large number of compounds (to our best knowledge, 102 formula) and shows abundant structural diversity; it could be regarded as a potential source of IR second-order NLO materials with tunable structures and properties. This article summarizes the authors' contributions to the A I2 BIICIVD VI4 family, together with other reported related results where the NLO properties have been provided (a total of 38 compounds). In the title family: (1) the AI site can be occupied by the univalent coinage metals Cu and Ag or the alkali metals Li and Na; (2) the BII site involves divalent metals, including the alkaline earth metals (AEM) Sr and Ba or the d10 metals Zn, Cd, and Hg; (3) the CIV site is prevalently IVA group semi-conducting Si and Ge, or Sn; and (4) the DVI site is S or Se. The structures and optical properties of these compounds are summarized and the influence of substitution at each site on the structures and properties is systematically analyzed. [ABSTRACT FROM AUTHOR]

Fluorooxoborates as potential deep ultraviolet (DUV) nonlinear optical (NLO) materials have exhibited diverse structures and NLO properties with metal cationic changes. Herein, the general mechanisms of metal cations on band gaps and optical properties in a series of typical fluorooxoborates have been clarified. It reveals that the framework of the emblematic 18‐membered ring oxyfluorides has the flexibility of being able to contain different cations spanning from alkali to d10 metals by investigating the stability of the artificial CdB5O7F3 structure. Besides, introducing d10 metal cations can enhance the second harmonic generation (3.1×KH2PO4 (KDP), d36=0.39 pm V−1) and also keep a DUV spectral transparency (Eg>6.2 eV). Thus, the d10‐containing fluorooxoborate exhibits a great potential to be a new DUV optical material for nonlinear light‐matter interactions. [ABSTRACT FROM AUTHOR]

Three mixed-metal phosphates, namely, centrosymmetric LiSrP3O9 and LiCsP2O6 as well as noncentrosymmetric K2SrP4O12 have been synthesized by the high temperature solution method. These structures were characterized by single-crystal X-ray diffraction and their P–O units are shown to feature distinct polyanionic configurations from [PO3]∞ chains in LiSrP3O9 to [P4O12]4− rings in LiCsP2O6 and K2SrP4O12. Besides, K2SrP4O12 shows a moderate second harmonic generation response about 0.6 times that of KH2PO4 and can be applied as a potential nonlinear optical material for the ultraviolet spectral region. Thermal behaviour analyses indicate that the compounds LiSrP3O9 and K2SrP4O12 remain stable up to 659 and 698 °C, respectively. In addition, the crystal structures, thermal stabilities and infrared spectra of the title compounds have been examined. [ABSTRACT FROM AUTHOR]

Fluorooxoborates have inspired investigations of deep‐ultraviolet (DUV) nonlinear optical (NLO) materials that can meet the multiple criteria. Herein, five stable structures with the composition of BaB2O3F2 (I–V) are discovered using the ab initio evolutionary algorithm. Among them, BaB2O3F2‐I has been synthesized experimentally and confirms the reliability of the method. All of the predicted structures possess extremely wide band gaps (8.1–9.0 eV). Moreover, four new structures exhibit giant second harmonic generation (SHG) coefficients (>3×KDP, d36=0.39 pm V−1). A novel type of the [BOF] layer with BO3:BO3F ratio of [1:1] is found in BaB2O3F2‐II and BaB2O3F2‐III. While BaB2O3F2‐IV and BaB2O3F2‐V are solely composed of the BO3F group and have colossal SHG coefficients (ca. 4×KDP). It gives the direct evidence that the BO3F group could generate strong SHG effect. Most importantly, the influences of BO3:BO3F ratio and their number density on band gap, birefringence and SHG effects are investigated. [ABSTRACT FROM AUTHOR]

Herein, a promising infrared nonlinear optical (IR NLO) material, Li2ZnGeS4, was successfully synthesized in the typical quaternary diamond-like semiconductor (DLS) system. This material exhibits excellent performances including a wide band gap (3.49 eV), a high laser-damage threshold (LDT, 8 × benchmark AgGaS2), and a good NLO effect (0.7 × AgGaS2) with phase-matching behavior. Furthermore, based on first principles calculations, the theoretical results are in agreement with the experimental results. [ABSTRACT FROM AUTHOR]

Metal‐centered MS4 tetrahedra are fundamentally important building blocks for the structural construction of infrared (IR) nonlinear optical (NLO) mateirals, because they have a large effect on properties like second‐harmonic generation, birefringence, band gap, etc. Therefore, rational design and selection of these tetrahedra could effectively help synthesize new materials and give satisfactory properties. In this work, we reported a new NLO‐active [Ge2S5(S2)]4− unit composed of two corner‐shared GeS4 tetrahedra with a nonpolar covalent S−S bond, discovered in a new compound of Li2Cs4Ge2S5(S2)Cl2. To the best of our knowledge, this [Ge2S5(S2)]4− unit has never been discovered. [Ge2S5(S2)]4− units have intrinsic asymmetric features and lead to the whole structure being noncentrosymmetric (NCS). Further, real‐space atom‐cutting methods reveal that they make almost all the contribution to the second‐harmonic generation response (SHG) for Li2Cs4Ge2S5(S2)Cl2. All of the above indicate that the [Ge2S5(S2)]4− unit is a new functional unit and a good choice for designing new IR NLO materials. Nonlinear optical materials: A fundamental building unit, [Ge2S5(S2)]4− dimer, discovered in thiogermanate Li2Cs4Ge2S5(S2)Cl2, makes almost all the contribution to the SHG response of Li2Cs4Ge2S5(S2)Cl2 (see figure). [ABSTRACT FROM AUTHOR]

The introduction of d10-metal cations is of importance in the design of infrared nonlinear optical materials. The role of d10-metal cations on the band gap and the second-harmonic generation (SHG) effect as well as the structure–property relationship were investigated in the Li2MGeS4 (M = Cd, Hg) and AB2S4 (A = Cd, Hg; B = Al, Ga) systems by using the first-principles calculations. The results show that the decreased band gap is related to a higher valence band maximum (VBM) caused by the larger dp repulsion from the Hg-5d orbitals and a downshift in the conduction band minimum (CBM) due to the lower energy of the Hg-6s orbitals. In addition, the relatively enhanced SHG response can be attributed to the decreased charge-transfer energy and the enhanced hybridization between the S-3p orbitals and the Hg-5d orbitals. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Special review on the IR NLO materials with balanced properties is systematically reported. • Relationship between microscopic groups and material performances is analyzed. • Efficient strategies toward design of new promising IR NLO materials are provided. Abstract Infrared (IR) lasers have been widely applied in the many military and civil fields. Nonlinear optical (NLO) crystals have been developed as the critical materials to achieve the efficient IR laser output by the typical frequency-conversion technology. However, commercial IR NLO materials are seriously limited in the IR region because of their low laser-damage thresholds (LDTs) or harmful two-photon absorption (TPA). Therefore, the discovery of new IR NLO materials with optimal key performances (concurrently large NLO coefficient (d ij) and wide optical bandgap (E g)) has become imperative. Recently, metal chalcogenides have become a rich source for exploration of IR NLO materials and hundreds of them have been discovered. Based on the good balance between experimental E g (≥3.0 eV) and d ij (≥0.5 × AgGaS 2) required for one IR NLO material, to our best knowledge, about 49 chalcogenides satisfy the above condition while limiting the scope to the equal or greater than ternary system. We focus on the structure-performance relationship for 49 compounds and the result shows that alkali or/and alkaline earth metals must be the preferred cations to maintain the wide E g. Besides, their critical anionic groups are summarized and shown as follows: (i) typical tetrahedral MIIIQ 4 (MIII = Ga, In) or/and MIVQ 4 (MIV = Si, Ge; Q = S, Se) or PS 4 units; (ii) d 10 elements-centered (MIIQ 4 : MII = Zn, Cd) and typical MIIIQ 4 or MIVQ 4 tetrahedra; (iii) halogen-centered polyhedral ligands. So the combination of above anionic groups and alkali or/and alkaline earth metals into crystal structures produces the feasible design strategy to explore new IR NLO materials with excellent performances. [ABSTRACT FROM AUTHOR]

Borates are one of the most important classes of functional materials, and several hundreds of artificial borates have been synthesized. The substitution of oxygen by fluorine leads to manifold classes of borates. Fluorooxoborates (also known as fluoroborates), in which the F atoms covalently connect with the B atoms, show additional compositional and structural diversity compared to classic borates. Recently, owing to the large polarizability anisotropy, large HOMO–LUMO gaps, and high hyperpolarizability of the oxyfluoride BOxF4−x building blocks, fluorooxoborates have received unprecedented attention in the search for new ultraviolet (UV) and deep‐UV (DUV) nonlinear optical (NLO) materials. Specifically, some compounds have excellent NLO properties that are comparable or superior to KBe2BO3F2, which is the only usable crystal that generates coherent light below 200 nm through a direct second harmonic generation (SHG) process. This Minireview illustrates recent progress on the synthesis, crystal structures, structure‐properties relationships and applications of fluorooxoborates. This paper concludes by highlighting the outstanding opportunities offered by NLO fluorooxoborate crystals as an innovative avenue for DUV all solid‐state coherent light generation. In at the deep end: The deep ultraviolet nonlinear optical materials with excellent comprehensive performance are the key for the development of deep ultraviolet all solid‐state lasers. This Minireview focuses on the design strategies, synthetic approaches, crystal chemistry, and structure‐properties relationships of recent research highlight fluorooxoborates. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

The search of new borates with improved functional properties has attracted considerable attention. Herein, a new polar fluorooxoborate, NaB4O6F (NBF) was prepared by high-temperature solid-state reaction. NBF belongs to the AB4O6F family (A=alkali metal or ammonium), a series of compounds that undergoes significant cation-dependent structural changes. NBF is of particular interest owing to the special cation position. Temperature-dependent ionic conductivity measurements show that NBF is a solid ionic conductor, and it has the lowest active energy of 32.5 kJmol@1 of fluorooxoborates. NBF also shows a second-harmonic generation (SHG) response of 0.9 XKH2PO4 and 0.2 X β-BaB2O4, at 1064 and 532 nm, respectively, and it has a short UV cutoff edge below 180 nm. Based on bond valence (BV) concepts, symmetry analysis, and the first principles calculation, the unique [B4O6F]∞ layer can be regarded as the "multifunctional unit", which is responsible for the observed properties of NBF. [ABSTRACT FROM AUTHOR]

Abstract: Fluorooxoborates, benefiting from the large optical band gap, high anisotropy, and ever‐greater possibility to form non‐centrosymmetric structures activated by the large polarization of [BOxF4−x](x+1)− building blocks, have been considered as the new fertile fields for searching the ultraviolet (UV) and deep‐UV nonlinear optical (NLO) materials. Herein, we report the first asymmetric alkaline‐earth metal fluorooxoborate SrB5O7F3, which is rationally designed by taking the classic Sr2Be2B2O7 (SBBO) as a maternal structure. Its [B5O9F3]6− fundamental building block with near‐planar configuration composed by two edge‐sharing [B3O6F2]5− rings in SrB5O7F3 has not been reported in any other borates. Solid state 19F and 11B magic‐angle spinning NMR spectroscopy verifies the presence of covalent B−F bonds in SrB5O7F3. Property characterizations reveal that SrB5O7F3 possesses the optical properties required for deep‐UV NLO applications, which make SrB5O7F3 a promising crystal that could produce deep‐UV coherent light by the direct SHG process. [ABSTRACT FROM AUTHOR]

Abstract: 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Metal borate halides refer to the metal borates containing additional halogen anions ( i.e. , F, Cl, Br or I). In recent years, motivated by the well-known high performance nonlinear optical (NLO) material, KBe 2 BO 3 F 2 (KBBF), metal borate halides have been arousing intensive interest. The combination of halogen anions with borate groups affords a large number of metal borate halides with anionic structures ranging from 0D clusters, 1D chains, 2D layers to 3D networks. In addition, the introduction of halogen anions into borates is proven to be an effective strategy to design new noncentrosymmetric (NCS) crystals, which is required for NLO materials. In this review, recently discovered metal borate halides are summarized, including their synthesis, crystal structure features, properties and second-order NLO performances. This review also presents the structure–properties relationship studies focusing on the positive influence of introducing halogen anions on the optical properties. We hope that this work will obtain a clear perspective on crystal chemistry of metal borate halides and facilitate the discovery of new second-order NLO materials for urgent demands in laser sciences. [ABSTRACT FROM AUTHOR]

Highlights: [•] Introduction of strong electron acceptors has remarkably enhanced the non-linearity. [•] The present calculations give a theoretical framework of NLO materials. [•] The proposed compounds should be targeted for further synthetic investigations. [ABSTRACT FROM AUTHOR]

The organic-inorganic hybrid nonlinear optical (NLO) material NH4B( d-(+)-C4H4O5)2 ·H2O (NBC) was synthesized in a borate-carboxylic acid system. Its structure was determined by single crystal X-ray diffraction. It crystallizes in the orthorhombic system, space group Pna21 (No. 33), with cell parameters a = 11.484(6) Å, b = 5.354(3) Å, c = 21.079(12) Å, V = 1296.0(12), Z = 4. It exhibits a three-dimensional pseudo tunnel structure consisting of fundamental building block [B( d-(+)-C4H4O5)2]- anions. The small cavities are occupied by the H2O molecules and NH4+ cations, which stabilize the whole structure by O-H ···O and N-H ···O hydrogen bonds. The powder X-ray diffraction (PXRD) of the crystal was also recorded. Elemental analyses, FT-IR and FT-Raman spectra analyses, thermal analysis, and diffuse-reflectance spectra for the compound are also presented, as are band structures and density of states calculation. Nonlinear optical measurements indicate that the material has second harmonic generation (SHG) properties and is phase-matchable. [ABSTRACT FROM AUTHOR]

Abstract: Pb4O(BO3)2 has a layered-type arrangement with optimally aligned BO3 triangles. The optical band gap is 3.317eV obtained via the extrapolation method from the UV–vis–IR optical diffuse reflectance spectrum, consequently the absorption edge is about 374nm. Density functional calculations using a generalized gradient approximation were utilized to investigate the electronic structures and optical properties of Pb4O(BO3)2. The calculated band structures show a direct gap of 2.608eV, which is in agreement with the experimental optical band gap. A delocalized bonding of BO3 triangles and the stereo-effect of the lone pair 6s 2 of lead cations are studied in electron densities. The birefringence is about 0.039–0.061 with the wavelength larger than about 375nm. The calculated second-order susceptibility d 24=3.5 d 36 (KDP) which is well consistent with the powder SHG intensity. [Copyright &y& Elsevier]

Infrared nonlinear optical (IR NLO) materials with excellent performances are particularly important in laser technology. However, to design and synthesize an efficient IR NLO material with a balance between the optical band gap and the NLO coefficient is still a huge challenge. With this in mind, four new IR NLO materials Na2BaSnS4, Na2BaSnSe4, Na2BaGeS4, and Na2BaGeSe4 were successfully designed and synthesized. The compounds exhibit excellent properties with a suitable balance of band gap and NLO coefficient measured for Na2BaSnS4 (3.27 eV and about 17×KDP, that is, about 17 times that of KH2PO4 (KDP)) and Na2BaGeS4 (3.7 eV and about 10×KDP), demonstrating that the systems satisfy the key requirements as promising IR NLO candidates. Remarkably, the new compounds also undergo a novel structural transformation from tetragonal to trigonal systems, the first time that this has been reported for quaternary metal chalcogenides. [ABSTRACT FROM AUTHOR]

A noncentrosymmetric chiral Strandberg-type polyoxometalate based material, Cs4Mo5P2O22, that contains unusual one dimensional chains constructed from [Mo5P2O23]6− units has been reported. Cs4Mo5P2O22 shows a moderate SHG response, and is phase-matchable. [ABSTRACT FROM AUTHOR]

A new congruent-melting lead barium borate, Ba2.5Pb1.5B12O22, was synthesized via a high-temperature solid-state reaction under atmospheric pressure. It crystallizes in the noncentrosymmetric space group Cmc21 with a unit cell of a = 19.051(8) Å, b = 10.726(3) Å, c = 8.612(3) Å, and Z = 4. It features a new functional building block (FBB), [B12O26]16−, that is made up of five [B3O8]7− and one [BO3]3− connected by sharing vertex O atoms. It is derived by introducing Pb into Ba2B6O11, which leads to a structural transformation from a centrosymmetric to a noncentrosymmetric space group. Its second harmonic generation response (SHG) is approximately 0.9 times that of KDP at the fundamental wavelength of 1064 nm and is also phase-matchable. It has a wide transparent region with a UV cutoff edge of about 250 nm according to the reflectance spectra and high thermal stability, which illustrates that it is a potential candidate for ultraviolet nonlinear optical materials. [ABSTRACT FROM AUTHOR]

The conflict between beneficial layered structure for performances and layered growth habits in KBe2BO3F2 (KBBF) always restricts its practical applications. A beryllium‐free KBBF family member, Zn2BO3(OH), is explored to feature the same topological layer with KBBF by replacing [BeO3F]5− with [ZnO3(OH)]5− and excellent UV performances. It exhibits a second harmonic generation response of about 1.5 × KH2PO4 with the UV cutoff edge of 204 nm. The birefringence of Zn2BO3(OH) in the visible region is about 0.067, which is larger than those of commercial UV crystals LiB3O5, CsB3O5, and CsLiB6O10. Additionally, it has excellent thermal and water‐resistant stabilities. Owing to the removal of interlayer cations, Zn2BO3(OH) shows better growth habits than KBBF while achieving the balance between beneficial layered structure and layer tendency. [ABSTRACT FROM AUTHOR]