Pushing Nonlinear Optical Oxides into the Mid-Infrared Spectral Region Beyond 10 μm: Design, Synthesis, and Characterization of La3SnGa5O14

La3Ga5M0.5Sn0.5O14, (M = Ge, Si): Design and Synthesis of Two Langasite Nonlinear Optical Materials with Large Second Harmonic Generation and Birefringence Induced by Distorted (Sn/M)O6 Octahedra

Nonlinear optical (NLO) coherent light sources are widely applied in many areas of science and technology. As the core medium, the NLO material is required to have a wide transparent range, a large NLO response, and a high laser damaged threshold (LDT). It is common knowledge that langasite (La3Ga5SiO14, LGS) crystal has an underdeveloped second-harmonic generation (SHG) coefficient and a small birefringence, which seriously restrict its application in the NLO field, despite that it has a broad transmittance spectrum and a moderate LDT. Herein, we have successfully obtained novel langasite NLO crystals LGSS (La3Ga5Si0.5Sn0.5O14) and LGGS (La3Ga5Ge0.5Sn0.5O14), with short UV absorption edges of 209 and 212 nm, respectively. Incorporating heavy ions Sn4+ into the structure, a distorted BO6 octahedron was adjusted by the radius difference between Sn4+ and Si4+/Ge4+, which caused the strong SHG responses in LGSS (∼10.77 × KDP) and LGGS (∼9.23 × KDP) and increased birefringences of 0.034 and 0.025, respectively. Besides, they also had large energy band gaps (4.95 eV for LGSS, and 4.93 eV for LGGS), which allowed high LDTs with LGSS of 1.3 GW/cm2 and LGGS of 813 MW/cm2. This work demonstrates a new strategy to enhance SHG responses and birefringence for existing NLO materials and enriches langasite family crystals.

Excellent Nonlinear Optical M[M4 Cl][Ga11 S20 ] (M = A/Ba, A = K, Rb) Achieved by Unusual Cationic Substitution Strategy

The typical chalcopyrite AgGaQ2 (Q = S, Se) are commercial infrared (IR) second-order nonlinear optical (NLO) materials; however, they suffer from unexpected laser-induced damage thresholds (LIDTs) primairy due to their narrow band gaps. Herein, what sets this apart from previously reported chemical substitutions is the utilization of an unusual cationic substitution strategy, represented by [[SZn4 ]S12 + [S4 Zn13 ]S24 + 11ZnS4 ⇒ MS12 + [M4 Cl]S24 + 11GaS4 ], in which the covalent Sx Zny units in the diamond-like sphalerite ZnS are synergistically replaced by cationic Mx Cly units, resulting in two novel salt-inclusion sulfides, M[M4 Cl][Ga11 S20 ] (M = A/Ba, A = K, 1; Rb, 2). As expected, the introduction of mixed cations in the GaS4 anionic frameworks of 1 and 2 leads to wide band gaps (3.04 and 3.01 eV), which exceeds the value of AgGaS2 , facilitating the improvement of high LIDTs (9.4 and 10.3 × AgGaS2 @1.06 µm, respectively). Furthermore, compounds 1 and 2 exhibit moderate second-harmonic generation intensities (0.84 and 0.78 × AgGaS2 @2.9 µm, respectively), mainly originating from the orderly packing tetrahedral GaS4 units. Importantly, this study demonstrates the successful application of the cationic substitution strategy based on diamond-like structures to provide a feasible chemical design insight for constructing high-performance NLO materials.

Preparation and Optical Properties of Nd:Bi2Ti2O7 Laser Crystal with Disordered Structure and Attractive Multiwavelength Emission Characteristics

Laser crystals with multiwavelength emission characteristics are potential light sources for terahertz radiation. Herein, the pure and Nd-doped Bi2Ti2O7 (BTO) laser crystals with sizes up to 16 × 13 × 5 mm3 were successfully grown using the flux method in the KF-B2O3-CaBi4Ti4O15 growth system. The crystal structure, ideal morphology, chemical, mechanical, and thermal properties, optical transmission and Raman spectra, refractive index, absorption, and fluorescence spectra, as well as fluorescence lifetimes, were systematically studied. Besides, the spectral parameters of Nd3+ ions in the BTO crystal were systematically calculated based on the Judd-Ofelt theory. The Nd:BTO crystal has a wide transmittance range (0.44-7.30 μm), a small coefficient of thermal expansion (5.80 × 10-6 K-1), and a large absorption full width at half-maximum (fwhm) (31.2 nm) at around ∼804 nm, making it more potential for use in high-power laser systems. Moreover, fluorescence spectra show four emission peaks at 1054, 1062, 1104, and 1112 nm. The strong multiwavelength emission property makes Nd:BTO a promising laser crystal, serving as a potential light source for terahertz radiation.

Quadruple-Bidentate Nitrate-Ligated A2 Hg(NO3 )4 (A=K, Rb): Strong Second-Harmonic Generation and Sufficient Birefringence

The design of efficient nonlinear optical (NLO) crystals continues to pose significant challenges due to the difficulty of assembling polar NLO-active modules in an optimal additive fashion. We report herein the first NLO-active mercuric nitrates A2 Hg(NO3 )4 (A=(KHNO), Rb (RHNO)), for which assembly is induced by ionic polarization of the d10 cations. The two new crystalline compounds are isostructural, featuring interesting pseudo-diamond-like structures with parallel [Hg(NO3 )4 ] modules, and leading to strong powder second-harmonic generation (SHG) responses of 9.2 (KHNO) and 8.8 (RHNO) times that of KH2 PO4 . In combination with the simple solution preparation of centimeter-scale crystals, sufficient birefringence, and short ultraviolet (UV) cutoff edges, these attributes make KHNO and RHNO promising candidates for UV NLO materials. Theoretical calculations and single-crystal structure analysis reveal that the newly-developed highly condensed and distorted [Hg(NO3 )4 ] module, with an Hg2+ cation that is quadruply bidentate nitrate-ligated, is crucial for the significant SHG responses. This work highlights the potential importance of modules with multiple bidentate ligands for the development of high-performing next-generation NLO materials.

Cd2 Nb2 Te4 O15 : A Novel Pseudo-Aurivillius-Type Tellurite with Unprecedented Nonlinear Optical Properties and Excellent Stability

Oxides are emerging candidates for mid-infrared (mid-IR) nonlinear optical (NLO) materials. However, their intrinsically weak second harmonic generation (SHG) effects hinder their further development. A major design challenge is to increase the nonlinear coefficient while maintaining the broad mid-IR transmission and high laser-induced damage threshold (LIDT) of the oxides. In this study, it is reported on a polar NLO tellurite, Cd2 Nb2 Te4 O15 (CNTO), featuring a pseudo-Aurivillius-type perovskite layered structure composed of three types of NLO active groups, including CdO6 octahedra, NbO6 octahedra, and TeO4 seesaws. The uniform orientation of the distorted units induces a giant SHG response that is ≈31 times larger than that of KH2 PO4 , the largest value among all reported metal tellurites. Additionally, CNTO exhibits a large band gap (3.75 eV), a wide optical transparency window (0.33-14.5 µm), superior birefringence (0.12@ 546 nm), high LIDT (23 × AgGaS2 ), and strong acid and alkali resistance, indicating its potential as a promising mid-IR NLO material.

NaCl·CH4N2O·H2O: An Organic-Inorganic Hybrid Ultraviolet Nonlinear Optical Crystal with Optimized Comprehensive Properties

Ultraviolet (UV) nonlinear optical (NLO) crystals are a vital component in laser systems to modulate frequency. Organic groups have the advantages of easy synthesis, structural diversity, and large hyperpolarizations (β), which can be combined with inorganic groups to form unique organic-inorganic hybrid NLO crystal materials. Herein, a urea-containing crystal NaCl·CH4N2O·H2O is screened out and large-size crystals have been grown by a simple aqueous solution method. Compared with most reported organic-inorganic hybrid NLO materials, it has optimized comprehensive properties including a large SHG effect of 1.53 × KDP, a moderate birefringence (0.084@1064 nm), and a short UV cutoff edge of 209 nm. First-principles studies reveal that the dominant contributor to the linear and nonlinear optical properties is the urea molecule.

Insights into molecular cluster materials with adamantane-like core structures by considering dimer interactions

A class of adamantane-like molecular materials attracts attention because they exhibit an extreme non-linear optical response and emit a broad white-light spectrum after illumination with a continuous-wave infrared laser source. According to recent studies, not only the nature of the cluster molecules, but also the macroscopic structure of the materials determines their non-linear optical properties. Here we present a systematic study of cluster dimers of the compounds AdR₄ and [(RT)₄ S₆ ] (T = Si, Ge, Sn) with R = methyl, phenyl or 1-naphthyl to gain fundamental knowledge about the interactions in the materials. For all compounds, a similar type of dimer structures with a staggered arrangement of substituents was determined as the energetically most favorable configuration. The binding energy between the dimers, determined by including London dispersion interactions, increases with the size of the core and the substituents. The cluster interactions can be classified as substituent-substituent-dominated (small cores, large substituents) or core-core-dominated (large cores, small substituents). Among various possible dimer conformers, those with small core-core distances are energetically preferred. Trimer and tetramer clusters display similar trends regarding the minimal core-core distances and binding energies. The much lower energy barrier determined for the rotation of substituents as compared to the rotation of the cluster dimers past each other indicates that the rotation of substituents more easily leads to different conformers in the material. Thus, understanding the interaction of the cluster dimers allows an initial assessment of the interactions in the materials.

Alignment of Λ-Shaped Basic Building Units to Construct One New KMoO3(IO3) Polar Polymorph

Nonlinear optical (NLO) crystals assume an irreplaceable role in the development of laser science and technologies, yet the reasonable design of a high-performance NLO crystal remains difficult owing to the unpredictability of inorganic structures. In this research, we report the fourth polymorph of KMoO₃(IO₃), i.e., δ-KMoO₃(IO₃), to understand the effect of different packing patterns of basic building units on structures and properties. Among four polymorphs of KMoO₃(IO₃), the different stacking patterns of Λ-shaped cis-MoO₄(IO₃)₂ units result in α- and β-KMoO₃(IO₃) featuring nonpolar layered structures, whereas γ- and δ-KMoO₃(IO₃) display polar frameworks. Theoretical calculations and structure analysis reveal that IO₃ units can be regarded as the main source of its polarization in δ-KMoO₃(IO₃). Further property measurements show that δ-KMoO₃(IO₃) exhibits a large second-harmonic generation response (6.6 × KDP), a wide band gap (3.34 eV), and a broad mid-infrared transparency region (∼10 μm), which demonstrates that adjusting the arrangement of the Λ-shaped basic building units is an effective approach for rationally designing NLO crystals.

Intense d-p Hybridization in Nb3 O15 Tripolymer Induced the Largest Second Harmonic Generation Response and Birefringence in Germanates

We herein report two asymmetric germanate crystals, KNbGe₃ O₉ and K₃ Nb₃ Ge₂ O₁₃ , with different structures and optical properties derived from divergent polymerized forms of GeO₄ and NbO₆ groups. Remarkably, K₃ Nb₃ Ge₂ O₁₃ achieved a rare combination of the strongest second harmonic generation (SHG) response of 17.5×KDP @ 1064 nm and the largest birefringence of 0.196 @ 546 nm in germanates. It features unique [Nb₃ O₁₂ ]_∞ tubular chains constructed by circular Nb₃ O₁₅ tripolymers. Theoretical calculations reveal that the d-p interactions in the Nb₃ O₁₅ group are responsible for outstanding optical properties. This work emphasizes the significance of the polymerizable functional units in obtaining high-performance nonlinear optical (NLO) crystals.

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.

Oxychalcogenides as Promising Ultraviolet Nonlinear Optical Candidates: Experimental and Theoretical Studies of AEGeOS2 (AE = Sr and Ba)

Oxychalcogenides have gained widespread attention as promising infrared nonlinear optical (IR-NLO) candidates. However, high-performance oxychalcogenides have rarely been reported in the ultraviolet (UV) region owing to the low energy gaps (E_g < 4.0 eV). Herein, two non-centrosymmetric (NCS) oxychalcogenides with one-dimensional (1D) chain structures and wide E_g (>4.3 eV), namely, AEGeOS₂ (AE = Sr and Ba), have been discovered by combined experiments and theory calculations as a new source of UV-NLO materials. Significantly, they exhibit excellent comprehensive performance comparable to the commercial UV-NLO material KH₂PO₄ (KDP), including large phase-matching ranges (>380 nm), sufficient second harmonic generation intensities (0.7-1.1 × KDP), high laser-induced damage thresholds (1.2 × KDP), wide transparent regions (0.26-12.2 μm), and good thermal stability (up to 1100 K). Moreover, systematic structure-activity relationship analysis illustrates that the 1D homochiral helical [GeOS₂]^2- chains composed of heteroanionic [GeS₂O₂] units make major contribution to the desirable UV-NLO performance. This work makes the two compounds shine out as new energy in the UV-NLO field and offers a new perspective for the exploration of structure-driven functional oxychalcogenides.

Mixed-Anion-Oriented Design of LnMGa3S6O (Ln = La, Pr, and Nd; M = Ca and Sr) Nonlinear Optical Oxysulfides with Targeted Property Balance

Nonlinear optical (NLO) crystals are of importance on extending infrared (IR) laser wavelengths. Considering their performance drawbacks in commercial IR NLO crystals, a recent challenge in exploring new excellent IR NLO crystals is how to break the inherent conflict between a wide bandgap (E_g ≥ 3.0 eV) and large NLO effect (d_ij ≥ 0.5 × AgGaS₂) and simultaneously enlarge the birefringence (Δn) for a requisite phase-matching (PM) behavior. For that reason, rational combination of mixed-anion functional groups into a crystal structure affords the successful design and synthesis of six LnMGa₃S₆O (Ln = La, Pr, and Nd; M = Ca and Sr) NLO oxysulfides. Among them, LaMGa₃S₆O satisfy the property-balance demand (E_g: 3.21-3.27 eV and d_ij: 0.9-1.0 × AgGaS₂) as promising PM NLO crystals through gathering their property advantages between LaMGa₃O₇ and LaMGa₃S₇ by mixed-anion-oriented performance engineering. A study on the structure-property relationship indicates that heteroleptic (Ln/M)S₇O and GaS₃O anionic groups are proven as promising NLO-active units and offer a great synergistic effect on the NLO origin. This work as a visualized model not only provides a first clear cognition on varying properties from oxide to sulfide to oxysulfide but also highlights the feasibility of mixed-anion-oriented design of new NLO candidates with balanced performances.

Pb5(GeO4)(Ge2O7) and Pb3.32Ca1.68(GeO4)(Ge2O7): Two Nonlinear Optical Germanates Induced by Diverse PbOx Polyhedra

Oxide nonlinear optical (NLO) crystals have drawn wide interest for their comprehensive physical performances including wide infrared (IR) transparency ranges, large band gaps, and good stability in open air. Here, two isostructural germanate oxides, Pb₅(GeO₄)(Ge₂O₇) (1) and Pb_3.32Ca_1.68(GeO₄)(Ge₂O₇) (2), adopting the noncentrosymmetric (NCS) space group P6̅, were composed via a conventional solid-state reaction. The latter was designed by the partial cation substitution strategy based on parent 1. The whole structures of 1 and 2 are composed of isolated distorted GeO₄ tetrahedra, Ge₂O₇ dimers, and diverse M (M = Pb, Ca, or Pb/Ca)-centered polyhedra. They exhibit second-harmonic generation (SHG) responses around 3.3 and 1.4 times that of KH₂PO₄ (KDP) under 1.064 μm laser radiation, respectively. Theoretical calculation results reveal that the Pb²⁺ cations with stereo-active long pair (SCALP) electrons of 1 favor the large SHG response, while Pb-based polyhedra showing inert SCALP electrons make predominant contributions to the moderate SHG effect of 2.

Bi3TeBO9: A Promising Mid-Infrared Nonlinear Optical Crystal with a Large Laser Damage Threshold

Mid-infrared (mid-IR) nonlinear optical (NLO) crystals are key materials in the field of laser technology. Nevertheless, the applications of these significant optical materials are always limited by their low laser damage threshold (LDT). Here, an oxide mid-IR NLO crystal, Bi₃TeBO₉, with a large LDT was discovered. The centimeter-sized Bi₃TeBO₉ single crystal was successfully grown by the top-seeded solution growth (TSSG) method. The Bi₃TeBO₉ features a high LDT of 450 MW/cm² (∼15 × AgGaS₂) and the widest transparent range (0.35-7.10 μm) among known borate crystals. The second-harmonic generation (SHG) effect is about 1.6 times that of KDP. First-principles calculations and structural analysis show that the optical characteristics of Bi₃TeBO₉ are mainly affected by distorted [BiO₆] groups. Furthermore, its thermal characteristics including specific heat, thermal diffusivity, thermal conductivity, and thermal expansion were also measured.

Cd4InO(BO3)3: A New Nonlinear Optical Crystal Exhibiting Strong Second Harmonic Generation Effect and Moderate Birefringence

A new noncentrosymmetric cadmium indium borate, Cd4InO(BO3)3 (CIBO) has been successfully developed via a standard solid-state reaction. Its crystal structure was confirmed by the single crystal X-ray diffraction, which shows that CIBO belongs to the non-centrosymmetric and polar space group Cm. Its structure contains the distorted InO6 and CdOn (n = 6, 8) polyhedra, which link together by sharing an edge or corner to build a three dimensions framework with BO3 triangles accommodated in tunnels. Benefiting from the approximately parallel configuration of BO3 triangles, CIBO exhibited a strong second harmonic generation (SHG) effect (3 × KDP), and moderate birefringence of 0.077@1064 nm. Further optical and thermal characterizations suggest that CIBO possesses a wide transparent window and good thermal stability. Theoretical calculation reveals that the macroscopic SHG coefficients of CIBO results from the synergistic effect of the parallel arrangement of BO3 groups and d10 Cd2+ cation.

HgTeO2F(OH): A Nonlinear Optical Oxyfluoride Constructed of Active [TeO2F(OH)]2- Pyramids and V-Shaped [HgO2]2- Groups

Oxyhalides possessing the merits of oxides and halides have widely received attention for their comprehensive physical performances, especially as potential nonlinear optical (NLO) crystals. Here, based on conventional strategy for obtaining acentric compounds, a Te⁴⁺ lone-pair cation was introduced into oxyhalides, and one oxyfluoride, HgTeO₂F(OH), was obtained via a hydrothermal reaction. Crystallized in the polar space group Pca2₁, the layered structure of HgTeO₂F(OH) is composed of V-shaped [HgO₂]^2- groups and [TeO₂F(OH)]^2- pyramids, in which the [TeO₂F(OH)]^2- pyramid first served as the NLO functional motif. Its powder sample exhibits a phase-matchable SHG response of 1.1 × KH₂PO₄ at 1064 nm, and its birefringence (0.09@1064 nm) is sufficient for phase-matchable behavior, which manifests its comprehensive capacity as a promising NLO candidate. Theoretical calculations about electronic structure and optical properties are also carried out, revealing that the Te⁴⁺ lone-pair cation makes the predominant contribution to the SHG effect and synergizes with the [HgO₂]^2- groups.

Unprecedented mid-infrared nonlinear optical materials achieved by crystal structure engineering, a case study of (KX)P2S6 (X = Sb, Bi, Ba)

Three superior type-I phase-matching middle infrared nonlinear optical materials (Ea > 4.0 eV, dij > 2× AGS, LDT > 8× AGS) were achieved via crystal structure engineering.

Band Gap Modulation and Properties of Quaternary Tellurates Li2GeTeO6

Tellurate crystals are attractive for developing new () materials in the mid-infrared region due to their wide transmission window. In this work, we report a quaternary tellurate oxide crystal, Li2GeTeO6,...

Investigations on the Synthesis, Crystal Structure, Linear- and Nonlinear-Optical Properties of the Zinc Germanate Rb2ZnGe2O6

A quaternary nonlinear-optical (NLO) zinc germanate, Rb₂ZnGe₂O₆ (RZGO), was prepared from its stoichiometric melts, which belongs to the noncentrosymmetric space group C222₁ (No. 20) and features a 3D framework formed by GeO₄ tetrahedra, ZnO₄ tetrahedra, and Rb⁺ cations filling into the void space. RZGO displays a good phase-matchable second-harmonic-generation (SHG) intensity similar to that of KH₂PO₄. The UV cutoff edge for RZGO was found to be approximately 0.37 μm with a broad optical band gap of 3.22 eV, indicating a large laser-induced damage threshold. Further characterizations suggested that this compound displays an excellent thermal stability (1014 °C) and melts congruently. Besides, theoretical analysis was also implemented to investigate the electronic structures and effective NLO coefficients as well as the origin of the observed SHG response for RZGO. The present study can enrich the diversity of acentric germanate structures and pave the way for the synthesis of new NLO compounds.

A new broad-band infrared window material CdPbOCl2 with excellent comprehensive properties

Herein, a new IR window material CdPbOCl₂ is rationally designed and fabricated by a heavy-metal oxide and halide combined strategy. The millimeter-scale CdPbOCl₂ single crystal exhibits a wide IR transparent region (1.4-18.0 μm) and excellent comprehensive properties. The results provide an insight into the exploration of broad-band IR window materials.

Ca3(TeO3)2(MO4) (M = Mo, W): Mid-Infrared Nonlinear Optical Tellurates with Ultrawide Transparency Ranges and Superhigh Laser-Induced mage ThreDasholds

Intense interests in mid-infrared (MIR) nonlinear optical (NLO) crystals have erupted in recent years due to the development of optoelectronic applications ranging from remote monitoring to molecular spectroscopy. Here, two polar crystals Ca₃(TeO₃)₂(MO₄) (M = Mo, W) were grown from TeO₂-MO₃ flux by high-temperature solution methods. Ca₃(TeO₃)₂(MoO₄) and Ca₃(TeO₃)₂(WO₄) are isostructural, which feature novel structures consisting of asymmetric MO₄ tetrahedra and TeO₃ trigonal pyramids. Optical characterizations show that both crystals display ultrawide transparency ranges (279 nm to 5.78 μm and 290 nm to 5.62 μm), especially high optical transmittance over 80% in the important atmospheric transparent window of 3-5 μm, and superhigh laser damage thresholds (1.63 GW/cm² and 1.50 GW/cm²), 54.3 and 50 times larger than that of state-of-the-art MIR NLO AgGaS₂, respectively. Notably, they exhibit the widest band gaps and the loftiest laser-induced threshold damages among the reported tellurates so far. Moreover, Ca₃(TeO₃)₂(MO₄) exhibit type I phase matching at two working wavelengths owing to their large birefringence and strong second-harmonic generation responses from the distorted anions, as further elucidated by the first-principles calculations. The above characteristics indicate that Ca₃(TeO₃)₂(MO₄) crystals are high-performance MIR NLO materials, especially applying in high-power MIR laser operations.

Li2 MTeO6 (M=Ti, Sn): Mid-Infrared Nonlinear Optical Crystal with Strong Second Harmonic Generation Response and Wide Transparency Range

Oxide crystals have been widely used in nonlinear optics (NLO) in the ultraviolet-visible and near-infrared regions. Most traditional oxide crystals are restricted to the mid-infrared region due to their narrow transmission window. Hence, attempting to extend infrared cutoff wavelength of oxides has attracted much attention. Herein, we report two new tellurates Li₂ TiTeO₆ (LTT) and Li₂ SnTeO₆ (LST) with broad transparent regions of 0.38-6.72 and 0.38-6.86 μm, respectively, as excellent candidates for mid-infrared NLO applications. Both LTT and LST crystallize in the orthorhombic space group Pnn2. The LTT crystal exhibits intense powder second-order generation efficiency (26×KDP) under the fundamental wavelength of 1064 nm. First-principles calculations and dipole moments were used to illustrate the results of the powder second-harmonic generations based on the crystal structures. Our results provide a novel oxide NLO crystal with a strong SHG and wide transparency range. They also pave a way for the design of new oxide mid-IR NLO crystals.

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

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

Breaking through the "3.0 eV wall" of energy band gap in mid-infrared nonlinear optical rare earth chalcogenides by charge-transfer engineering

Increasing the energy band gap under the premise to maintain a large nonlinear optical (NLO) response is a challenging issue for the exploration and molecular design of mid-infrared nonlinear optical crystals. Utilizing a charge-transfer engineering method, we designed and synthesized a rare earth chalcogenide, KYGeS₄. With an NLO effect as large as that in AgGaS₂, KYGeS₄ breaks through the limitation of energy band gap, i.e., the "3.0 eV wall", in NLO rare earth chalcogenides, and thus exhibits an excellent comprehensive NLO performance. First-principles electronic structure analysis demonstrates that the large band gap in KYGeS₄ is ascribed to the decreased covalency of Y-S bonds by transferring charge from [YS₇] to [GeS₄] polyhedra. The charge-transfer engineering strategy would have significant implications for the exploration of good-performance NLO crystals.

Mid-infrared pulsed nanosecond difference frequency generation of oxide LGN crystal up to 5.7 µm

We demonstrate the tunable difference frequency generation (DFG) of an oxide La₃Ga_5.5Nb_0.5O₁₄ (LGN) crystal pumped by near-infrared lasers with nanosecond pulses for the first time to our knowledge. The type I and II phase-matching conditions of DFG were calculated in the mid-infrared region. With the processed LGN crystals, tunable lasers in the wavelength range from 4.4 to 5.7 µm and 4.56 to 5.6 µm were achieved under type II and I phase-matching conditions, respectively, with the maximum output energy of 13.1 µJ, which agreed well with the theoretical calculation. This work provides the kind of promising mid-infrared nonlinear crystals for the pumping of nanosecond pulsed lasers as well as a tunable mid-infrared laser source at a wavelength over 5 µm in further photonic applications.

Bi32Cd3P10O76: a new congruently melting nonlinear optical crystal with a large SHG response and a wide transparent region

Large size single crystal Bi₃₂Cd₃P₁₀O₇₆ with a large SHG response (4 × KDP) and a wide transmission range covering 0.36–4.9 μm.

Growth of a large-aperture mid-infrared nonlinear optical La3Nb0.5Ga5.5O14 crystal for optical parametric chirped-pulse amplification

A high optical quality 60 mm-diameter LGN crystal with wide transparency was grown by the Czochralski method. The origin of the wide transparency as for a traditional oxide crystal was investigated from the viewpoint of crystal symmetry.

Optimized growth and anisotropic properties of Li2ZrTeO6 nonlinear optical crystals

Centimeter-sized and high-quality Li2ZrTeO6 crystals were grown by a modified top-seed solution growth method. The excellent thermal properties indicate that Li2ZrTeO6 is an excellent candidate suitable for high-power nonlinear optical applications.

Tuning the Langasite, La3SbZn3Si2O14, towards white light emission: synthesis, structure, SHG and photoluminescence studies

A series of single-phase color-tunable Langasite, La3SbZn3Si2O14:RE3+ (RE = Eu, Tb, and Tm) phosphors have been prepared and characterized employing X-ray diffraction (XRD), Raman, and photoluminescence (PL) studies. The substituted compounds, La3SbZn3Si2O14:Eu3+, La3SbZn3Si2O14:Tb3+ and La3SbZn3Si2O14:Tm3+ exhibit the characteristic emissions of Eu3+ (red), Tb3+ (green) and Tm3+ (blue), respectively. By carefully adjusting the concentration of Eu3+ ions in the La3SbZn3Si2O14:1% Tm3+, 2% Tb3+, x% Eu3+ system, we achieved a white emission with a reasonable quantum yield. The compounds are found to be SHG (second-harmonic generation) active with the highest values of 3.6 × KDP (potassium dihydrogen phosphate). In addition, the compounds exhibit reasonable dielectric constants with low loss. The present study demonstrates the color tunability of the Langasite family of compounds and establishes it as an adaptable structure.

Finding Optimal Mid-Infrared Nonlinear Optical Materials in Germanates by First-Principles High-Throughput Screening and Experimental Verification

Owing to wide infrared (IR) transparency ranges, high laser damage thresholds, and being easy to grow in open air, germanates are emerging as promising mid-infrared (mid-IR) nonlinear optical (NLO) materials. However, the germanates as NLO materials have not been investigated comprehensively and the crystals with large second harmonic generation (SHG) response have not been identified. Herein, we used the first-principles high-throughput screening pipeline for NLO materials to search for excellent NLO crystals from germanates collected in the inorganic crystal structure database. After two steps of screening, three crystals are picked out from 128 structures based on their predicted energy gaps, birefringences, and SHG coefficients. Subsequently, the three germanates are synthesized and measured. The results show that Pb₃Ga₂Ge₄O₁₄ and Ba₂TiGe₂O₈ exhibit a wide energy gap (>3.1 eV) and a strong phase-matchable SHG intensity that are comparable to the benchmark AgGaS₂ (0.8 and 1.2 × AgGaS₂, respectively). In addition, the statistical analyses of different categories classified according to their cations show that the d⁰-transition metal and lone pair cations are more conducive to achieving a larger SHG response and birefringence compared to other cations in germanates. It gives a guideline for exploring new mid-IR NLO materials.

“Old dog, new tricks”: the lone pair effect inducing divergent optical responses in lead cyanurates containing π-bonds

The stereochemically active lone pair effect of Pb²⁺ and induced narrower bandgaps were elaborated in Pb₃(HC₃N₃O₃)₂(OH)₂ for the first time.

Rb4 Li2 TiOGe4 O12 : A Titanyl Nonlinear Optical Material with the Widest Transparency Range

Practical mid-infrared (MIR) coherent light beams generated by frequency conversion in nonlinear optical (NLO) crystals are indispensable in time-resolved infrared vibrational spectroscopy, remote light detection and ranging, and free-space communications. Herein, a new titanyl germanate Rb₄ Li₂ TiOGe₄ O₁₂ (RLTG) MIR NLO crystal was obtained by heavier element substitution. It features a complicated structure network composed of compressed TiO₅ square pyramids and distorted GeO₄ tetrahedra, separated by Rb⁺ and Li⁺ cations. More importantly, RLTG exhibits concurrently short ultraviolet (0.28 μm) and long IR (5.58 μm) transmittance cutoffs, fully covering the atmospheric transparent window of 3-5 μm. Related to the short UV cutoff, it shows a higher laser-induced damage threshold in comparison to commercial MIR NLO crystals, about twice that of KTiOPO₄ (KTP) and 50 times that of AgGaS₂ (AGS).

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

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