Recent advances in rational structure design for nonlinear optical crystals: leveraging advantageous templates

Nonlinear optical (NLO) crystals that can expand the spectral range of laser outputs have attracted significant attention for their optoelectronic applications. The research progress from the discovery of new single crystal structures to the realization of final device applications involves many key steps and is very time consuming and challenging. Consequently, exploring efficient design strategies to shorten the research period and accelerate the rational design of novel NLO materials has become imperative to address the pressing demand for advanced materials. The recent shift in paradigm toward exploring new NLO crystals involves significant progress from extensive "trial and error" methodologies to strategic approaches. This review proposes the concept of rational structure design for nonlinear optical crystals leveraging advantageous templates. It further discusses their optical characteristics, promising applications as second-order NLO materials, and the relationship between their structure and performance, and highlights urgent issues that need to be addressed in the field of NLO crystals in the future. The review aims to provide ideas and driving impetus to encourage researchers to achieve new breakthroughs in the next generation of NLO 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.

Accurate X-ray diffraction data required for proper evaluation of bond valence sums and global instability indexes: redetermination of the crystal structures of diamond-like Cu2CdSiS4 and Cu2HgSnS4 as a case study

Our calculations of the global instability index (G) values for some diamond-like materials with the general formula I2-II-IV-VI4 have indicated that the structures may be unstable or incorrectly determined. To compute the G value of a given compound, the bond valence sums (BVSs) must first be calculated using a crystal structure. Two examples of compounds with high G values, based on data from the literature, are the wurtz-stannite-type dicopper cadmium silicon tetrasulfide (Cu2CdSiS4) and the stannite-type dicopper mercury tin tetrasulfide (Cu2HgSnS4), which were first reported in 1967 and 1965, respectively. In the present study, Cu2CdSiS4 and Cu2HgSnS4 were prepared by solid-state synthesis at 1000 and 900 °C, respectively. The phase purity was assessed by powder X-ray diffraction. Optical diffuse reflectance UV/Vis/NIR spectroscopy was used to estimate the optical bandgaps of 2.52 and 0.83 eV for Cu2CdSiS4 and Cu2HgSnS4, respectively. The structures were solved and refined using single-crystal X-ray diffraction data. The structure type of Cu2CdSiS4 was confirmed, where Cd2+, Si4+ and two of the three crystallographically unique S2- ions lie on a mirror plane. The structure type of Cu2HgSnS4 was also verified, where all ions lie on special positions. The S2- ion resides on a mirror plane, the Cu+ ion is situated on a fourfold rotary inversion axis and both the Hg2+ and the Sn4+ ions are located on the intersection of a fourfold rotary inversion axis, a mirror plane and a twofold rotation axis. Using the crystal structures solved and refined here, the G values were reassessed and found to be in the range that indicates reasonable strain for a stable crystal structure. This work, together with some examples gathered from the literature, shows that accurate data collected on modern instrumentation should be used to reliably calculate BVSs and G values.

KHg4Ga3S9: A Hg-Based Sulfide with Nonlinear-Optical Activity in the A-MII-MIII-Q (A = Alkali Metal; MII = d10 Metal; MIII = Ga, In; Q = S, Se) System

The search for new high-performance infrared (IR) nonlinear-optical (NLO) materials is a hot topic in the fields of laser chemistry and inorganic solid-state chemistry. Here, a new Hg-based sulfide KHg4Ga3S9 in the family of A-MII-MIII-Q (A = alkali metal; MII = d10 metal; MIII = Ga, In; Q = S, Se) was synthesized. It crystallizes in the orthogonal system of the C2221 structure, which is rare for IR NLO chalcogenides. Its anionic framework {[Hg4Ga3S9]-}∞ is constructed by two types of interconnected helical chains, viz., the inner layer ({[Hg6Ga2S29/3]4/3-}∞) and the outer layer ({[Hg2Ga4S25/3]2/3-}∞). It exhibits a moderate NLO response and a high laser-induced damage threshold. Theoretical calculations indicate that the HgS4 unit accounts for its much larger NLO response compared to RbCd4Ga3S9. The influence of alkali metals and d10 metals on the initial phase-matching wavelength is also discussed. This work provides inspiration for improving the properties of NLO materials' properties.

Ag4SnGe2S7: A Noncentrosymmetric Chalcogenide in I4-II-IV2-VI7 System with Non-Diamond-Like Structure Featuring 1D ∞[SnGe2S8]6- Infinite Chain

The I₄-II-IV₂-VI₇ metal chalcogenide system has become an attractive research system because of its excellent physical and chemical properties. Here, we report the discovery of a new Sn^II-based quaternary chalcogenide in the I₄-II-IV₂-VI₇ system, Ag₄SnGe₂S₇, with a non-diamond-like structure and crystallizing in the Cc space group. The compound is characterized by isolated pyramid-like [SnS₃] units and one-dimensional _∞[SnGe₂S₈]^6- infinite chains with two orientations formed by the corner-sharing connected [SnGe₂S₈]^6- units. It has a band gap of 2.40 eV and is insensitive to air and moisture.

Crystal structure, electronic structure, and optical properties of the novel Li4CdGe2S7, a wide-bandgap quaternary sulfide with a polar structure derived from lonsdaleite

The novel quaternary thiogermanate Li4CdGe2S7 (tetralithium cadmium digermanium heptasulfide) was discovered from a solid-state reaction at 750 °C. Single-crystal X-ray diffraction data were collected and used to solve and refine the structure. Li4CdGe2S7 is a member of the small, but growing, class of I4–II–IV2–VI7 diamond-like materials. The compound adopts the Cu5Si2S7 structure type, which is a derivative of lonsdaleite. Crystallizing in the polar space group Cc, Li4CdGe2S7 contains 14 crystallographically unique ions, all residing on general positions. Like all diamond-like structures, the compound is built of corner-sharing tetrahedral units that create a relatively dense three-dimensional assembly. The title compound is the major phase of the reaction product, as evidenced by powder X-ray diffraction and optical diffuse reflectance spectroscopy. While the compound exhibits a second-harmonic generation (SHG) response comparable to that of the AgGaS2 (AGS) reference material in the IR region, its laser-induced damage threshold (LIDT) is over an order of magnitude greater than AGS for λ = 1.064 µm and τ = 30 ps. Bond valence sums, global instability index, minimum bounding ellipsoid (MBE) analysis, and electronic structure calculations using density functional theory (DFT) were used to further evaluate the crystal structure and electronic structure of the compound and provide a comparison with the analogous I2–II–IV–VI4 diamond-like compound Li2CdGeS4. Li4CdGe2S7 appears to be a better IR nonlinear optical (NLO) candidate than Li2CdGeS4 and one of the most promising contenders to date. The exceptional LIDT is likely due, at least in part, to the wider optical bandgap of ∼3.6 eV.

Ba2HgTe5: a Hg-based telluride with giant birefringence induced by linear [HgTe2] units

Ba2HgTe5, the first Hg-based telluride birefringent material, was successfully synthesized. The analysis of the response electron distribution anisotropy illustrates that the large birefringence of Ba2HgTe5 originates from the linear [HgTe2] unit.

Na4Ga8S14: A Ga-riched wide band gap ternary alkali-metal sulfide with unique [Ga12S42] 12-menbered rings

A Ga-riched ternary alkali-metal sulfide Na4Ga8S14 has been synthesized by high temperature solid-state reaction. It crystallizes in the centrosymmetric Pbca (no. 61) space group with cell parameters a = 13.5260(4)...

Cu4MnGe2S7 and Cu2MnGeS4: two polar thiogermanates exhibiting second harmonic generation in the infrared and structures derived from hexagonal diamond

The new, quaternary diamond-like semiconductor (DLS) Cu₄MnGe₂S₇ was prepared at high-temperature from a stoichiometric reaction of the elements under vacuum. Single crystal X-ray diffraction data were used to solve and refine the structure in the polar space group Cc. Cu₄MnGe₂S₇ features [Ge₂S₇]^6- units and adopts the Cu₅Si₂S₇ structure type that can be considered a derivative of the hexagonal diamond structure. The DLS Cu₂MnGeS₄ with the wurtz-stannite structure was similarly prepared at a lower temperature. The achievement of relatively phase-pure samples, confirmed by X-ray powder diffraction data, was nontrival as differential thermal analysis shows an incongruent melting behaviour for both compounds at relatively high temperature. The dark red Cu₂MnGeS₄ and Cu₄MnGe₂S₇ compounds exhibit direct optical bandgaps of 2.21 and 1.98 eV, respectively. The infrared (IR) spectra indicate potentially wide windows of optical transparency up to 25 μm for both materials. Using the Kurtz-Perry powder method, the second-order nonlinear optical susceptibility, χ⁽²⁾, values for Cu₂MnGeS₄ and Cu₄MnGe₂S₇ were estimated to be 16.9 ± 2.0 pm V^-1 and 2.33 ± 0.86 pm V^-1, respectively, by comparing with an optical-quality standard reference material, AgGaSe₂ (AGSe). Cu₂MnGeS₄ was found to be phase matchable at λ = 3100 nm, whereas Cu₄MnGe₂S₇ was determined to be non-phase matchable at λ = 1600 nm. The weak SHG response of Cu₄MnGe₂S₇ precluded phase-matching studies at longer wavelengths. The laser-induced damage threshold (LIDT) for Cu₂MnGeS₄ was estimated to be ∼0.1 GW cm^-2 at λ = 1064 nm (pulse width: τ = 30 ps), while the LIDT for Cu₄MnGe₂S₇ could not be ascertained due to its weak response. The significant variance in NLO properties can be reasoned using the results from electronic structure calculations.

Investigation into Structural Variation from 3D to 1D and Strong Second Harmonic Generation of the AHgPS4 (A+ = Na+, K+, Rb+, Cs+) Family

The continuous exploration of multinary chalcogenide semiconductors has provided a variety of new functional materials. In this paper, four new quaternary chalcogenides AHgPS₄ (A⁺ = Na⁺, K⁺, Rb⁺, Cs⁺) have been prepared by solid-state syntheses. These findings complement the lack of research on this quaternary system. Influenced by the size effect of cations and the coordination mode of Hg, the four compounds crystallize in four different space groups [NaHgPS₄, P4̅n2; KHgPS₄, Pnn2; RbHgPS₄, P2₁/n; CsHgPS₄, P2₁2₁2₁] and show an interesting evolution from a 3D framework structure to a 1D chain structure. Moreover, all of these compounds feature noncentrosymmetric (NCS) structures except for RbHgPS₄. The materials exhibit wide band gaps of 2.7 eV < E_g < 3.0 eV. The NCS- related second-harmonic-generation (SHG) property of NaHgPS₄ and KHgPS₄ was also studied. They display strong powder SHG responses (3.14 × AgGaS₂ for NaHgPS₄; 4.15 × AgGaS₂ for KHgPS₄), which indicate their intriguing potential as IR nonlinear-optical materials. Moreover, first-principles theoretical calculations were performed to understand the structure-property relationships of these materials.

Syntheses, Structures and Properties of Alkali and Alkaline Earth Metal Diamond-Like Compounds Li2MgMSe4 (M = Ge, Sn)

Two new diamond-like (DL) chalcogenides, Li2MgGeSe4 and Li2MgSnSe4, have been successfully synthesized using a conventional high-temperature solid-state method. The two compounds crystallize in the non-centrosymmetric space group Pmn21 with a = 8.402 (14) Å, b = 7.181 (12) Å, c = 6.728 (11) Å, Z = 2 for Li2MgSnSe4, and a = 8.2961 (7) Å, b = 7.0069 (5) Å, c = 6.6116 (6) Å, Z = 2 for Li2MgGeSe4. The calculated results show that the second harmonic generation (SHG) coefficients of Li2MgSnSe4 (d33 = 12.19 pm/v) and Li2MgGeSe4 (d33 = -14.77 pm/v), mainly deriving from the [MSe4] (M = Ge, Sn) tetrahedral units, are close to the one in the benchmark AgGaS2 (d14 = 13.7 pm/V). The calculated band gaps for Li2MgSnSe4 and Li2MgGeSe4 are 2.42 and 2.44 eV, respectively. Moreover, the two compounds are the first series of alkali and alkaline-earth metal DL compounds in the I2-II-IV-VI4 family, enriching the structural diversity of DL compounds.

Design and synthesis of Ba3SiSe5 with suitable birefringence modulated via MIV atoms in the Ba-MIV-Q (MIV = Si, Ge; Q = S, Se) system

A new ternary Ba-based selenide, Ba₃SiSe₅, was synthesized by a high-temperature solid-state method. It crystallizes in the centrosymmetric space group Pnma (no. 62) of the orthorhombic system. The structure of the title compound consists of unique Se(4)Ba layers and discrete SiSe₄ tetrahedra. The structure and computational properties of Ba₃SiSe₅ are systematically studied together with those of the Ba-M^IV-Q (M^IV = Si, Ge; Q = S, Se) system, and show an interesting difference in dimensions formed by one of the crystallographic Ba atoms and M^IVQ₄ tetrahedra, as well as optical property transformations modulated by M^IV atoms. First principles methods were employed to obtain a better understanding of the relationship between structures and properties. Ba₃SiSe₅ maintains a moderate birefringence of 0.044@1064 nm and the real space atom cutting method indicates that the SiSe₄ tetrahedra make the major contribution to its birefringence.

Highly Distorted [HgS4] Motif-Driven Structural Symmetry Degradation and Strengthened Second-Harmonic Generation Response in the Defect Diamond-Like Chalcogenide Hg3P2S8

Chalcogenides with diamond-like (DL) structures are a treasury of infrared nonlinear optical (NLO) materials. Here, a ternary Hg-based chalcogenide with a defect DL structure, Hg₃P₂S₈, is synthesized by solid-state reaction. Driven by the highly distorted [HgS₄] tetrahedra, this compound displays an interesting structural symmetry degradation from tetragonal to orthorhombic compared with its analogue Zn₃P₂S₈. Meanwhile, the overall performances of Hg₃P₂S₈ are quite remarkable, including a very strong phase-matchable second-harmonic generation (SHG) response (4.2 × AgGaS₂), large band gap (2.77 eV), wide IR transparent range (0.45-16.7 μm), and high laser-induced damage threshold (4 × AGS). Furthermore, the theoretical analysis and local dipole moment calculations elucidate that the highly distorted [HgS₄] tetrahedra contribute a lot to the enhancement of the SHG effect. This discovery will motivate the exploration of other DL Hg-based chalcogenides serving as high-performing mid-IR NLO materials.

Li4 MgGe2 S7 : The First Alkali and Alkaline-Earth Diamond-Like Infrared Nonlinear Optical Material with Exceptional Large Band Gap

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 Li₄ MgGe₂ S₇ , 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, Li₄ MgGe₂ S₇ displays a suitable SHG response (0.7 × AgGaS₂ ) with a type I phase-matching behavior. The results indicate that Li₄ MgGe₂ S₇ 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.

A series of M3PS4 (M = Ag, Cu and Ag/Cu) thiophosphates with diamond-like structures exhibiting large second harmonic generation responses and moderate ion conductivities

Diamond-like thiophosphates exhibiting large second harmonic generation responses and moderate ion conductivities were systematically studied.

EuHgGeSe4 and EuHgSnS4: Two Quaternary Eu-Based Infrared Nonlinear Optical Materials with Strong Second-Harmonic-Generation Responses

Metal chalcogenides play a critical role in the infrared (IR) nonlinear optical (NLO) field. However, Eu-based chalcogenide-type IR NLO materials are still scarce up to now. In this paper, two new quaternary Eu-based chalcogenides, EuHgGeSe₄ and EuHgSnS₄, containing the "NLO active groups" [HgQ₄]^6- (Q = S, Se) and [GeSe₄]^4-/[SnS₄]^4- were synthesized through traditional high-temperature solid-state reactions. They possess noncentrosymmetric structures, crystallizing in the Ama2 space group, and exhibit strong phase-matchable second-harmonic-generation (SHG) responses (3.1× and 1.77× that of AgGaS₂ for EuHgGeSe₄ and EuHgSnS₄, respectively). Meanwhile, the optical band gaps of EuHgGeSe₄ (1.97 eV) and EuHgSnS₄ (2.14 eV) were determined from UV-vis-NIR diffuse reflectance spectra. Differential scanning calorimetry (DSC) analyses reveal the congruent-melting behavior of EuHgGeSe₄. Furthermore, structural analysis and theoretical calculations verify the critical driving effects of [HgQ₄]^6- tetrahedra on the strong SHG activity. The overall results demonstrate that EuHgGeSe₄ and EuHgSnS₄ are potential IR NLO materials.

Influence of most reactive inorganic cation in the optical and biological activities of L-Lysine monohydrochloride crystal

Slow evaporation method was used to grow the pure and K⁺ ion doped L-Lysine monohydrochloride (L-LMHCL) crystals which has optical and antibiotic applications. The space group, structure and slight shifting of peaks are confirmed using single crystal XRD and the powder XRD. The FTIR analysis also shows that the K⁺ doped L-LMHCL has a slight shifting in the spectrum which indicates the functional group of L-LMHCL and the interaction between the K⁺ ions. The existence of K⁺ ion in the doped crystal is assured by the presence of potassium in the EDAX spectrum. The wide optical band gap was found for pure and K⁺ doped crystal using UV spectra and these are utilized in optoelectronic and nonlinear applications. The Kurtz Perry technique specified the NLO property of grown crystals. The dielectric property crystals was studied by varying the temperature. As a result, the highest dielectric constant is observed in doped crystal. An antibacterial activity against certain bacteria like E-coli, pseudomonas aeruginosa and staphylococcus aureus are provided by mm range for the grown crystals.

Syntheses and crystal structures of the quaternary thio-germanates Cu4FeGe2S7 and Cu4CoGe2S7

The quaternary thio-germanates Cu4FeGe2S7 (tetra-copper iron digermanium hepta-sulfide) and Cu4CoGe2S7 (tetra-copper cobalt digermanium hepta-sulfide) were prepared in evacuated fused-silica ampoules via high-temperature, solid-state synthesis using stoichiometric amounts of the elements at 1273 K. These isostructural compounds crystallize in the Cu4NiSi2S7 structure type, which can be considered as a superstructure of cubic diamond or sphalerite. The monovalent (Cu+), divalent (Fe2+ or Co2+) and tetra-valent (Ge4+) cations adopt tetra-hedral geometries, each being surrounded by four S2- anions. The divalent cation and one of the sulfide ions lie on crystallographic twofold axes. These tetra-hedra share corners to create a three-dimensional framework structure. All of the tetra-hedra align along the same crystallographic direction, rendering the structure non-centrosymmetric and polar (space group C2). Analysis of X-ray powder diffraction data revealed that the structures are the major phase of the reaction products. Thermal analysis indicated relatively high melting temperatures, near 1273 K.

Biosensors Based on Advanced Sulfur-Containing Nanomaterials

In recent years, sulfur-containing nanomaterials and their derivatives/composites have attracted much attention because of their important role in the field of biosensor, biolabeling, drug delivery and diagnostic imaging technology, which inspires us to compile this review. To focus on the relationships between advanced biomaterials and biosensors, this review describes the applications of various types of sulfur-containing nanomaterials in biosensors. We bring two types of sulfur-containing nanomaterials including metallic sulfide nanomaterials and sulfur-containing quantum dots, to discuss and summarize the possibility and application as biosensors based on the sulfur-containing nanomaterials. Finally, future perspective and challenges of biosensors based on sulfur-containing nanomaterials are briefly rendered.

Two Covalent Ultraviolet Nonlinear Optical Crystals

Nonlinear optical (NLO) crystals are the vital components of laser science and technology, as they can convert lasers in common wavelengths into new wavelength bands for ultraviolet (UV), IR, and even terahertz laser output. Known UV NLO crystals mainly focus on crystals containing cations, but covalent crystals have rarely been reported. Here we report two covalent NLO crystals, B₂ O₃ I and B₂ O₃ II. According to the first-principles calculations, B₂ O₃ I and II have extremely short absorption edges of about 134 nm and 141 nm, large NLO coefficients of d₂₂ =1.38 pm/V and d₂₄ =0.702 pm/V, as well as sufficient birefringences of 0.037 and 0.031, respectively. Notably, the absorption edges are almost the shortest among NLO crystals. Meanwhile, the NLO coefficients are evidently larger than that of another well-known covalent NLO crystal α-SiO₂ and are comparable to those of the commercial UV NLO crystal LiBO₃ with Li⁺ cation. Furthermore, the birefringences are significantly larger than that of α-SiO₂ , which are favorable to the phase matching for both crystals. These results reveal that B₂ O₃ I and B₂ O₃ II are excellent candidates for UV NLO applications. In-depth calculations are carried out to reveal the origin of excellent NLO properties. These covalent crystals provide a new direction for the research of UV NLO crystals.

Data-driven prediction of diamond-like infrared nonlinear optical crystals with targeting performances

Combining high-throughput screening and machine learning models is a rapidly developed direction for the exploration of novel optoelectronic functional materials. Here, we employ random forests regression (RFR) model to investigate the second harmonic generation (SHG) coefficients of nonlinear optical crystals with distinct diamond-like (DL) structures. 61 DL structures in Inorganic Crystallographic Structure Database (ICSD) are selected, and four distinctive descriptors, including band gap, electronegativity, group volume and bond flexibility, are used to model and predict second-order nonlinearity. It is demonstrated that the RFR model has reached the first-principles calculation accuracy, and gives validated predictions for a variety of representative DL crystals. Additionally, this model shows promising applications to explore new crystal materials of quaternary DL system with superior mid-IR NLO performances. Two new potential NLO crystals, Li2CuPS4 with ultrawide bandgap and Cu2CdSnTe4 with giant SHG response, are identified by this model.

First-Principles Design and Simulations Promote the Development of Nonlinear Optical Crystals

A hot topic in materials science is to search for nonlinear optical (NLO) crystals, which are indispensable in current laser technology, future optical information, and precision measurements. In the period of the 1980s and 1990s, the anionic group theory proposed by Prof. Chuangtian Chen has greatly promoted the inventions of BaB₂O₄ (BBO), LiB₃O₅ (LBO), and KBe₂BO₃F₂ (KBBF) which are widely applied in the ultraviolet (UV) spectral region today. From the beginning of this century, the rapid development of laser science and technology urgently demands new NLO crystals for wider application ranges. However, commercial NLO crystals in deep-UV and mid-infrared (mid-IR) regions are scarce. The challenge arises from the stringent criteria at various wavelengths and inefficient exploration strategy. As such, more comprehensive and quantitative theoretical guidance is necessary to improve and supplement the NLO structure-property understandings. Benefiting from high-performance computing resources, first-principles design and simulations came into being, which is more applicable to the understanding of mid-IR NLO mechanism and suitable for the efficient design of new NLO structures for current needs. In the past decade, a complete set of computational research programs based on first-principles simulations have been developed, which have promoted the development of NLO crystals in the deep-UV and mid-IR regions, and guided the subsequent and further experimental explorations. Based on our developed first-principles materials design system, the discoveries of NLO materials have ranged from basic theoretical design to rapid-prototyping and final experimental synthesis. In this Account, we will concisely summarize our ab initio guided and forward-looking studies on NLO crystals, which are our original contributions to this field and can be consulted by other material fields. First, we will review the development of NLO crystals and the important features of NLO materials. Second, we will summarize the important role of computer-aided design in advancing the NLO material field and our developed NLO material design system based on the first-principles simulations. Third, we will introduce the first-principles design for new deep-UV NLO crystals using two novel design proposals, i.e., interlayer cationic replacement and intralayer anionic substitution. Meanwhile, we will illustrate the hierarchical molecular engineering optimizations for mid-IR NLO crystals by illustrating an extended mid-IR NLO family pedigree, from which many promising mid-IR NLO systems were predicted theoretically and confirmed experimentally. Finally, we will give an outlook to explore new functional NLO crystals guided by our first-principles design and simulations. We believe that the computer-assisted exploration for new functional NLO materials is useful for understanding structure-property relationships and can provide researchers with a new approach to cost-effective and data-driven materials design.

A review of the AI2BIICIVDVI4 family as infrared nonlinear optical materials: the effect of each site on the structure and optical properties

The AI2B^IIC^IVDVI4 family as promising infrared NLO materials is summarized. The influence of each site substitutions on the structures and properties is systematically analyzed.

Artificial Second-Order Nonlinear Optics in a Centrosymmetric Optical Material BiVO4: Breaking the Prerequisite for Nonlinear Optical Materials

Second-order nonlinear optics (NLO) is the foundation of frequency conversion for the generation of coherent light at frequencies where lasers have no emissions or operate poorly. The prerequisite for NLO materials is noncentrosymmetric symmetry that can generate an effectively non-counterbalanced spontaneous electronic polarization. Here, we propose that this material restriction can be broadened by controlling the electron distribution with a local internal electrostatic field (IEF), and we demonstrate artificially created and manipulated second harmonic generation (SHG) in a centrosymmetric optical material, a superimposed Co²⁺- and Mo⁶⁺-doped BiVO₄ thin film with 2/m point group symmetry, where a homojunction producing tunable effective polarization is formed. The SHG was characterized and tuned by IEF. This work breaks the structural symmetry constraint on NLO materials. Besides, the phase-matching-like condition was realized for the further improvement of the efficient frequency conversion. Because polarization is also a prerequisite for many other functions besides SHG, we believe that this work should provide some inspiration for the further development of optoelectronic, photonic, and electronic materials.

Li2ZnGeS4: a promising diamond-like infrared nonlinear optical material with high laser damage threshold and outstanding second-harmonic generation response

A promising IR nonlinear optical material Li₂ZnGeS₄ with a diamond-like configuration and high optical performance is reported.

Investigation on two new antimony-based quaternary chalcogenides: Na6CdSb4S10 and Na3CdSbSe4

Two new Sb-based chalcogenides exhibiting interesting structural features and optical properties were successfully synthesized for the first time.

ABaSbQ3 (A = Li, Na; Q = S, Se): diverse arrangement modes of isolated SbQ3 ligands regulating the magnitudes of birefringences

The interrelation of arrangement modes of isolated SbQ₃ ligands on structures and birefringences has been systematically studied in series of chalcogenides.

Structural modulation induced by MIIIA metals in Ba3MQ4X (M = Al, Ga, In; Q = S, Se; X = Cl, Br): an experimental and computational analysis

The optical properties of Ba₃MQ₄X (M = Al, Ga, In; Q = S, Se; X = Cl, Br) have been studied and their structures have been compared.

Ba3Mg3(BO3)3F3 polymorphs with reversible phase transition and high performances as ultraviolet nonlinear optical materials

Nonlinear optical (NLO) materials are the vital components of future photoelectric technologies as they can broaden the tunable wavelength range supplied by common laser sources. However, the necessary prerequisites for a practical NLO material are rather strict. Accordingly, considerable efforts have been focused on finding potential NLO materials. Here we report two asymmetric beryllium-free borates Pna2₁- and P\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\bar 6$$\end{document}6¯2m-Ba₃Mg₃(BO₃)₃F₃ featuring NLO-favorable ²_∞[Mg₃O₂F₃(BO₃)₂] layered structures. The reversible phase transition among two polymorphs was demonstrated by multiple experimental tests. The optical measurements reveal that Pna2₁-Ba₃Mg₃(BO₃)₃F₃ possesses the optical properties required for ultraviolet NLO applications. Remarkably, Pna2₁-Ba₃Mg₃(BO₃)₃F₃ has a large laser damage threshold, a deep-ultraviolet cutoff edge, a favorable anisotropic thermal expansion as well as the capacity of insolubility in water. These optical properties can be comparable or superior to that of commercial NLO material β-BaB₂O₄, which make Pna2₁-Ba₃Mg₃(BO₃)₃F₃ a promising ultraviolet NLO material.

Nonlinear optical crystals suitable for the UV spectral region could simplify short-wavelength generation and make it more efficient. Here, the authors design and demonstrate that one of two asymmetric borate polymorphs exhibits promising optical and mechanical properties for generating UV light.

New strategy for designing promising mid-infrared nonlinear optical materials: narrowing the band gap for large nonlinear optical efficiencies and reducing the thermal effect for a high laser-induced damage threshold

To circumvent the incompatibility between large nonlinear optical (NLO) efficiencies and high laser-induced damage thresholds (LIDTs) in mid-infrared NLO materials, a new strategy for designing materials with both excellent properties is proposed. This strategy involves narrowing the band gap for large NLO efficiencies and reducing the thermal effect for a high LIDT. To support these proposals, a series of isostructural chalcogenides with various tetrahedral center cations, Na2Ga2MQ6 (M = Ge, Sn; Q = S, Se), were synthesized and studied in detail. Compared with the benchmark AGS, these chalcogenides exhibit significantly narrower band gaps (1.56-1.73 eV, AGS: 2.62 eV) and high NLO efficiencies (1.6-3.9 times that of AGS at 1910 nm), and also outstanding LIDTs of 8.5-13.3 × those of AGS for potential high-power applications, which are contrary to the conventional band gap view but can be attributed to their small thermal expansion anisotropy, surmounting the NLO-LIDT incompatibility. These results shed light on the search for practical IR NLO materials with excellent performance not restricted by NLO-LIDT incompatibility.

Structural insights into T2-cluster-containing chalcogenides with vertex-, edge- and face-sharing connection modes of NaQ6 ligands: Na3ZnMIIIQ4 (MIII = In, Ga; Q = S, Se)

New series of T₂-cluster-containing chalcogenides exhibiting the novel connection mode of NaS₆ ligands and torsional adjacent T₂-clusters were reported.