Strong Second Harmonic Generation Effect Research Articles

A Rare Earth Chalcogenide Nonlinear Optical Crystal KLaGeS4: Achieving Good Balance among Band Gap, Second Harmonic Generation Effect, and Birefringence.

Midinfrared nonlinear optical (NLO) rare earth chalcogenides have attracted extensive research interest in recent several decades. Employing charge-transfer engineering strategy in the early stage, rigid tetrahedral [GeS4] was introduced into rare-earth sulfides to synthesize KYGeS4, which had an enlarged band gap while maintaining a strong second harmonic generation (SHG) effect. Based on KYGeS4, La was equivalently substituted to successfully synthesize KLaGeS4 with a stronger SHG effect (dij = 1.2 × AgGaS2) and lower cost. Meanwhile, a larger band gap (Eg = 3.34 eV) was retained and realized phase matching (Δn = 0.098 @ 1064 nm). KLaGeS4 enabled an effective balance among band gap, SHG effect, and birefringence, making it a promising candidate for infrared NLO optical materials among various rare-earth sulfides.

Polysulfide Anions [Sx]2- (x=2, 3, 4, 5): Promising Functional Building Units for Infrared Nonlinear Optical Materials.

Infrared nonlinear optical (IR NLO) materials play significant roles in laser technology. The novel functional building units (FBUs) are of great importance in constructing NLO materials with strong second harmonic generation (SHG). Herein, polysulfide anion [Sx]2- (x=2, 3, 4, 5) units are investigated on NLO-related properties and structure-performance relationships. Theoretical calculations uncover that the [Sx]2- (x=2, 3, 4, 5) units are potential IR NLO FBUs with large polarizability anisotropy (δ), hyperpolarizability (β) and wide HOMO-LUMO gap. Fourteen crystals including [Sx]2- (x=2, 3, 4, 5) units are calculated and analyzed. The results show that these units can result in a wide IR transmittance range, significant SHG effects, wide band gap Eg (Na2S4: Eg=3.09eV), and large birefringence Δn [BaS3 (P21212): Δn=0.70]. More importantly, it is highlighted that the crystal materials including with [Sx]2- (x=2, 3, 4, 5) groups are good candidates for the exploration of the outstanding IR NLO materials.

(C5H6N2)2H](Sb4F13): a polyfluoroantimonite with a strong second harmonic generation effect.

It is of great difficulty to create a new antimonite with second-harmonic-generation (SHG) intensity larger than 6 times that of KDP. In this study, a polyfluoroantimonite strategy has been proposed to explore fluoroantimonites with large nonlinear optical (NLO) coefficients. Under the cooperation of chemical (highly asymmetric π-conjugated organic amine) and physical (viscous reaction medium ethylene glycol) methods, two novel polyfluoroantimonites, namely, (3PC)2(Sb4F14) and (3AP)2(Sb4F13), have been achieved. Interestingly, these two structures contain two new polyfluoroantimonite groups respectively, an isolated (Sb4F14)2- four-member polyhedral ring and an infinite [Sb4F13]∞- helical chain. More importantly, the polar (3AP)2(Sb4F13) displays a strong SHG intensity of 8.1 × KDP, a large birefringence of 0.258@546 nm and a high laser-induced damage threshold (LIDT) value of 149.7 MW cm-2. Theoretical calculations indicated that its strong SHG effect stems from the synergistic effect of the helical [Sb4F13]∞- polyfluoroantimonite chain and π-conjugated 3AP+ cation, with a contribution ratio of 48.93% and 50.77% respectively. This work provides a new approach for the design and synthesis of high-performance fluoroantimonites.

Flexible Anionic Groups-Activated Structure Dissymmetry for Strong Nonlinearity in Ln2 Ae3 MIV 3 S12 Family.

Structural dissymmetry and strong second-harmonic generation (SHG) responses are key conditions for nonlinear optical (NLO) crystals, and targeted combinatorial screening of suitable anionic groups has become extremely effective. Herein, optimal combination of flexible SnSn (n=5, 6) groups and highly electropositive cations (lanthanides (Ln3+ ) and alkaline earth (Ae2+ : Sr, Ca) metals) affords the successful synthesis of 12 NLO thiostannates including Ln2 Sr3 Sn3 S12 (Pmc21 ) and Ln2 Ca3 Sn3 S12 (P-62m); whereas 17 rigid GeS4 or SiS4 tetrahedra-constructed Ln2 Ae3 Ge3 S12 and Ln2 Ae3 Si3 S12 crystallize in the centrosymmetric (CS) Pnma. This unprecedented CS to noncentrosymmetric (NCS) structural transformation (Pnma to P-62m to Pmc21 ) in the Ln2 Ae3 MIV 3 S12 family indicates that chemical substitution of the tetrahedral GeS4 /SiS4 units with SnSn breaks the original symmetry to form the requisite NCS structures. Remarkably, strong polarization anisotropy and hyperpolarizability of the Sn(4+) S5 unit afford huge performance improvement from the nonphase-matching (NPM) SHG response (1.4×AgGaS2 and Δn=0.008) of La2 Ca3 Sn3 S12 to the strong phase-matching (PM) SHG effect (3.0×AgGaS2 and Δn=0.086) of La2 Sr3 Sn3 S12 . Therefore, Sn(4+) S5 is proven to be a promising "NLO-active unit." This study verifies that the coupling of flexible SnSn building blocks into structures opens a feasible path for designing targeted NCS crystals with strong nonlinearity and optical anisotropy.

Ca2 Ln(BS3 )(SiS4 ) (Ln = La, Ce, and Gd): Mixed Metal Thioborate-Thiosilicates as Well-Performed Infrared Nonlinear Optical Materials.

The first examples of thioborate-thiosilicates, namely Ca2 Ln(BS3 )(SiS4 ) (Ln = La, Ce, and Gd), are synthesized by rationally designed high-temperature solid-state reactions. They crystalize in the polar space group P63 mc and feature a novel three-dimensionalcrystal structure in which the discrete [BS3 ]3- and [SiS4 ]4- anionic groups are linked by Ca2+ and Ln3+ cations occupying the same atomic site. Remarkably, all three compounds show comprehensive properties required as promising infrared nonlinear optical materials, including phase-matchable strong second harmonic generation (SHG) responses at 2.05µm (1.1-1.2 times that of AgGaS2 ), high laser-induced damage thresholds (7-10 times that of AgGaS2 ), wide light transmission range (0.45-11µm), high thermal stabilities (>800 °C), and large calculated birefringence (0.126-0.149 @1064nm), which justify the material design strategy of combining [BS3 ]3- and [SiS4 ]4- active units. Theoretical calculations suggest that their large SHG effects originate mainly from the synergy effects of the LnS6 , BS3 , and SiS4 groups. This work not only broadens the scope of research on metal chalcogenides but also provides a new synthetic route for mixed anionic thioborates.

Large Difference in Nonlinear Optical Activity of Rare Earth Ion Substitution of Bi3+ in A3TeBO9 (A = Bi, La, Pr, Nd, Sm-Dy).

A series of oxides with high rare earth contents, RE3TeBO9 (RE = La, Pr, Nd, Sm-Dy), were synthesized, and they all crystallize in the noncentrosymmetric space group P63 resembling that of a previously reported Bi3TeBO9 (J. Am. Chem. Soc. 2016, 138, 14190-14193), which showed a very strong second harmonic generation (SHG) signal believed to come from all three structural units [BO3], [TeO6], and [BiO6]. Surprisingly, compared with the isostructural compound Bi3TeBO9, both theoretical calculations and powder SHG tests show that RE3TeBO9 has an abnormally small SHG effect (0.2 × KDP). Based on the group theory analysis, the possible reasons are attributed to the misalignments and lack of distortions of the [BO3], [TeO6], and [REO8] groups in the unit cell. The 21 symmetric operation (a subset operation of the 63-screw axis) of the [TeO6], [BO3], and [REO8] groups leads to complete cancellation of their contributions to the xy directions, while the absence of lone pair electrons in RE3+ and Te6+ results in small distortion in the c axis, hence small contributions to the SHG coefficients dijk involving the z axis. Diffuse reflectance spectra of RE3TeBO9 (RE = La, Pr, Nd, Sm-Dy) show typical absorption of different RE3+, and the similarity of their infrared spectra is due to the same crystal structure and same structural units of the [TeO6] and [BO3] groups. The strong fluorescence intensities of Eu3TeBO9 and Tb3TeBO9 show the characteristic emissions of Eu3+ and Tb3+, which may serve as luminescence materials with proper doping.

Explorations of Second-Order Nonlinear Optical Materials in the Monovalent Trithiocyanurate System

Two π-conjugated trithiocyanurates A(H2C3N3S3) (A = NH4, Cs) were successfully synthesized via a mild aqueous solution method or hydrothermal technique, and the ammonium bicarbonate and cesium hydroxide were used as deprotonation agents. NH4(H2C3N3S3) crystallizes in a symmetric space group P1̅ with flat [H2C3N3S3]∞ chain separated by NH4+, whereas Cs(H2C3N3S3) belongs to an asymmetric space group Cc [Z = 4, a = 5.405(6) Å, b = 21.263(3) Å, c = 14.996(2) Å, β = 92.408(10)°] with wavy [H2C3N3S3]∞ chain separated by Cs+. More importantly, Cs(H2C3N3S3) displays a strong second-harmonic generation effect (4.6 × KH2PO4), a large birefringence (Δn = 0.196@1064 nm), and a thermal stability up to 271.3 °C. In addition, a theoretical study further reveals that the π-conjugated (H2C3N3S3)− group has made a primary contribution toward enhancing the linear and nonlinear optical properties in the above two compounds.

Perfectly Encoding π‐Conjugated Anions in the RE5(C3N3O3)(OH)12 (RE=Y, Yb, Lu) Family with Strong Second Harmonic Generation Response and Balanced Birefringence

AbstractNonlinear optical (NLO) crystal, which simultaneously exhibits strong second‐harmonic‐generation (SHG) response and desired optical anisotropy, is a core optical material accessible to the modern optoelectronics. Accompanied by strong SHG effect in a NLO crystal, a contradictory problem of overlarge birefringence is ignored, leading to low frequency doubling efficiency and poor beam quality. Herein, a series of rare earth cyanurates RE5(C3N3O3)(OH)12 (RE=Y, Yb, Lu) were successfully characterized by 3D electron diffraction technique. Based on a “three birds with one stone” strategy, they enable the simultaneous fulfillment of strong SHG responses (2.5–4.2× KH2PO4), short UV cutoff (ca. 220 nm) and applicable birefringence (ca. 0.15 at 800 nm) by the introduction of rare earth coordination control of π‐conjugated (C3N3O3)3− anions. These findings provide high‐performance short‐wavelength NLO materials and highlight the exploration of cyanurates as a new research area.

Perfectly Encoding π-Conjugated Anions in the RE5 (C3 N3 O3 )(OH)12 (RE=Y, Yb, Lu) Family with Strong Second Harmonic Generation Response and Balanced Birefringence.

Nonlinear optical (NLO) crystal, which simultaneously exhibits strong second-harmonic-generation (SHG) response and desired optical anisotropy, is a core optical material accessible to the modern optoelectronics. Accompanied by strong SHG effect in a NLO crystal, a contradictory problem of overlarge birefringence is ignored, leading to low frequency doubling efficiency and poor beam quality. Herein, a series of rare earth cyanurates RE5 (C3 N3 O3 )(OH)12 (RE=Y, Yb, Lu) were successfully characterized by 3D electron diffraction technique. Based on a "three birds with one stone" strategy, they enable the simultaneous fulfillment of strong SHG responses (2.5-4.2× KH2 PO4 ), short UV cutoff (ca. 220 nm) and applicable birefringence (ca. 0.15 at 800 nm) by the introduction of rare earth coordination control of π-conjugated (C3 N3 O3 )3- anions. These findings provide high-performance short-wavelength NLO materials and highlight the exploration of cyanurates as a new research area.

Mid-Infrared Nonlinear Optical Halides with Diamond-like Structures: A Theoretical and Experimental Study

As an important family of potential mid-IR nonlinear optical (NLO) crystals, halides exhibit superiority in large optical band gaps and wide IR transparency. However, their second-harmonic generation (SHG) effects are generally small, limiting their frequency conversion efficiency in NLO processes. Diamond-like structures, due to the intrinsic polarization-parallel alignment of tetrahedral frameworks, enable sufficiently large SHG effects while maintaining balanced NLO performance in the mid-IR spectral region. Accordingly, in this study, we focused on the NLO performance mining and assembly of tetrahedral polar motifs in halides with unique diamond-like structures. We screened almost all diamond-like structures in inorganic halide systems and investigated in detail how their balanced NLO performances evolve with ideal and defective diamond-like structures. We predicted that the three existing diamond-like iodides, CuGaI4, Ag2CdI4, and Ag2ZnI4, can satisfy the balanced requirement of strong SHG effects (>3.9 pm/V) and large optical band gaps (>3 eV) by analyzing the microstructural NLO origin. Preliminary experiments also verified our predictions with Ag2CdI4 (1.4 × AgGaS2 and 3.3 eV) and Ag2HgI4 (2.2 × AgGaS2 and 2.4 eV). More importantly, a map with the diamond-like halide family tree was illuminated, unlocking dozens of diamond-like structures with balanced NLO properties, from ternary AgCdI3, Ag2Cd3I8, Ag4CdI6, and AgInI4 to quaternary AgCuCdI4, Ag8HgCd3I16, Ag3HgGaI8, AgCd2InI8, and Ag2CdI2Br2. As long as following this map, one may design and discover potentially promising mid-IR NLO halide crystals with diamond-like structures.

Rational design via dual-site aliovalent substitution leads to an outstanding IR nonlinear optical material with well-balanced comprehensive properties.

The acquisition of a non-centrosymmetric (NCS) structure and achieving a nice trade-off between a large energy gap (E g > 3.5 eV) and a strong second-harmonic generation (SHG) response (d eff > 1.0 × benchmark AgGaS2) are two formidable challenges in the design and development of infrared nonlinear optical (IR-NLO) candidates. In this work, a new quaternary NCS sulfide, SrCdSiS4, has been rationally designed using the centrosymmetric SrGa2S4 as the template via a dual-site aliovalent substitution strategy. SrCdSiS4 crystallizes in the orthorhombic space group Ama2 (no. 40) and features a unique two-dimensional [CdSiS4]2- layer constructed from corner- and edge-sharing [CdS4] and [SiS4] basic building units (BBUs). Remarkably, SrCdSiS4 displays superior IR-NLO comprehensive performances, and this is the first report on an alkaline-earth metal-based IR-NLO material that breaks through the incompatibility between a large E g (>3.5 eV) and a strong phase-matching d eff (>1.0 × AgGaS2). In-depth mechanism explorations strongly demonstrate that the synergistic effect of distorted tetrahedral [CdS4] and [SiS4] BBUs is the main origin of the strong SHG effect and large birefringence. This work not only provides a high-performance IR-NLO candidate, but also offers a feasible chemical design strategy for constructing NCS structures.

A Lead Mixed Halide with Three Different Coordinated Anions and Strong Second-Harmonic Generation Response.

Nonlinear optical (NLO) crystals are widely applied in information technology, micro-manufacturing and medical treatment. Herein, a new lead mixed halide with strong second-harmonic generation (SHG) response, Cs3 Pb2 (CH3 COO)2 Br3 I2 , has been designed and rationally synthesized. Cs3 Pb2 (CH3 COO)2 Br3 I2 represents the rare NLO crystal featuring that three different anions (I- , Br- and O2- ) simultaneously coordinate the Pb(II) atom to form a severely distorted [PbBr2 I2 O2 ] polyhedron with a large polarizability. Remarkably, Cs3 Pb2 (CH3 COO)2 Br3 I2 not only exhibits a very strong phase-matching SHG response of 9×KH2 PO4 (KDP), but also possesses a large birefringence (0.27@1064 nm) and high laser damage threshold (LDT). The strong SHG effect of Cs3 Pb2 (CH3 COO)2 Br3 I2 mainly originates from the oriented arrangement of [Pb2 Br3 I2 ] chains. This study points out an effective strategy to develop new NLO crystals with strong SHG response.

GaSeCl5O: A Molecular Compound with Very Strong SHG Effect.

GaSeCl5O is a new inorganic molecular compound prepared from SeO2, SeCl4, and GaCl3 at 50 °C in quantitative yield. The structure of the title compound is described by GaCl3(OSeCl2) molecules with a tetrahedrally coordinated Ga atom and a pseudo-tetrahedrally coordinated Se atom (including lone pair of Se(IV)) that are bridged by oxygen. GaSeCl5O crystallizes in the polar chiral space group P61, which is rarely observed for molecular structures. The compound is characterized by X-ray structure analysis based on single crystals and powder samples, thermogravimetry, infrared and Raman spectroscopy as well as by second harmonic generation (SHG) measurements. The experimental data are complemented by density functional theory calculations. GaSeCl5O shows one of the strongest SHG signals known in the visible part of the electromagnetic spectrum (480-700 nm) with an SHG intensity 10 times higher than potassium dihydrogen phosphate (KDP). This is in accordance with the phase matchability and a strong dipole moment (|μ| = 8.3 D for a molecule in the crystal lattice). Such a strong SHG effect is also remarkable since GaSeCl5O-unlike most of the materials with strong SHG intensity-is an inorganic molecular compound.

Hg3P2S8: A New Promising Infrared Nonlinear Optical Material with a Large Second-Harmonic Generation and a High Laser-Induced Damage Threshold

The development of new infrared (IR) nonlinear optical (NLO) materials with strong second-harmonic generation (SHG) responses and high laser-induced damage thresholds (LIDTs) is urgent but challenging. Herein, the defective diamond-like chalcogenide Hg3P2S8 (HPS) was rationally designed and fabricated by a flexible and rigid tetrahedral motif combined strategy. HPS exhibits a phase-matching SHG response of similar to 3.6 x AGS (the largest one in the reported ternary sulfides), high LIDT (similar to 3 x AGS), wide IR transparent region (0.43-16.3 mu m), moderate birefringence (0.043 at 1064 nm), and good physicochemical stability and crystal growth habits. Theoretical analyses confirm that the large SHG effect originates from the synergistic effects between distorted HgS4, PS4, and vacancy-induced nonbonding electrons. The results indicate that HPS is a promising NLO candidate for high-power, high-efficiency laser output in the mid-IR region, which provides an insight into the exploration of new IR NLO materials.

Two Tellurium(IV)-Based Sulfates Exhibiting Strong Second Harmonic Generation and Moderate Birefringence as Promising Ultraviolet Nonlinear Optical Materials.

Two tellurium(IV)-based sulfate nonlinear optical (NLO) materials, Te2O3(SO4) and Te(OH)3(SO4)·H3O, were successfully synthesized via the mild hydrothermal method. Te2O3SO4 has a two-dimensional (2D) structure consisting of [Te6O12]∞ layers as well as [SO4] groups. Te(OH)3(SO4)·H3O features a simple 0D structure made up of an isolated [TeO3] pyramid and a [SO4] tetrahedra. Both of them are phase-matching materials and show remarkable powder second harmonic generation (SHG) efficiencies about 6 and 3 times that of KH2PO4 (KDP), respectively, for Te2O3SO4 and Te(OH)3(SO4)·H3O. Especially for Te(OH)3(SO4)·H3O, in addition to a large SHG response, it possesses a short UV cutoff edge (∼233 nm) as well as moderate birefringence (0.052@546.1 nm). Furthermore, theoretical calculations confirmed that their strong SHG effects are due to the synergistic effect of the [TeO3] pyramid and [SO4] tetrahedra.

High-Performance Second-Harmonic-Generation (SHG) Materials: New Developments and New Strategies.

ConspectusSecond-harmonic-generation (SHG) causes the frequency doubling of light, which is very useful for generating high-energy lasers with specific wavelengths. Noncentrosymmetry (NCS) is the first requirement for an SHG process because the SHG coefficient is zero (χ2 = 0) in all centrosymmetric structures. At this stage, developing novel NCS crystals is a crucial scientific topic. Assembling polar units in an addictive fashion can facilely form NCS crystals with outstanding SHG performance. In this way, our group has obtained many different NCS crystals with extremely large SHG intensities (>5 × KDP or 1 × KTP). In this Account, we first provide a brief review of the development of SHG materials and concisely highlight the features of the excellent SHG materials. Then, we present four facile and rational molecular design strategies: (1) Traditional BO33--based crystals feature short absorption edges but usually suffer from relatively weak SHG performance (<5 × KDP). The combination of two types of pure π-conjugated anions (BO33- and NO3-) in a parallel fashion in the same compound has afforded a metal borate nitrate with a strong SHG effect. (2) To overcome the problems of the weak SHG effect and small birefringence in the less anisotropic QO4-based compounds, highly polarizable cations such as Hg2+ and Bi3+ are introduced into these systems, which greatly enhances both SHG effects and birefringence. (3) Iodate anions can be condensed into polynuclear iodate anions with a higher density of I5+ per unit cell, hence polyiodate anions can serve as excellent SHG-active groups. We developed a novel synthesis method for hydrothermal reactions under a phosphoric acid medium and obtained a series of metal polyiodates with strong SHG effects. In addition, as the number of iodate groups increases, the structural configuration of the polyiodate anion changes from linear to bent. (4) We introduce the concept of aliovalent substitution which features site-to-site atomic displacement at the structural level. Such aliovalent substitution led to new materials that have the same chemical stoichiometries or structural features as their parent compounds. Thus, aliovalent substitution can provide more experimental opportunities and afford new high-performance SHG materials. The introduction of a fluoride anion and the replacement of metal cations in the MO6 octahedron can result in new metal iodates with balanced properties including a large SHG effect, a wide band gap, and a high laser-induced damage threshold (LIDT) value. Finally, we briefly discuss several problems associated with the studies of SHG materials and give some prospects for SHG materials in the future.

Nonlinear Optical Oxythiophosphate Approaching the Good Balance with Wide Ultraviolet Transparency, Strong Second Harmonic Effect, and Large Birefringence

AbstractUltraviolet (UV) transparency, second harmonic effect and optical birefringence are three vital but mutually restrictive factors in the application of UV nonlinear optical (NLO) materials. It is difficult for traditional phosphates to achieve a good balance among these factors. In this communication, we propose that the structural evolution of the NLO motif from traditional phosphate to oxythiophosphate would enhance the birefringence and second harmonic generation (SHG) effect while maintaining wide UV transparency, which is confirmed by first‐principles calculations and preliminary experimental measurements. Following this strategy, we predict that, compared with the well‐known NLO phosphate KH2PO4, the oxythiophosphate KH2PO3S exhibits better balance for the UV NLO performance, including wide UV transparency (UV cutoff down to 203 nm), strong SHG effect (ca. 0.9 pm V−1), and large birefringence (ca. 0.1 at 1 μm) with short phase‐matching SHG output wavelength (≈214 nm).

Layered oxide B2S2O9 with a deep-ultraviolet band gap and a strong and robust second-harmonic generation

Two-dimensional (2D) layered semiconductors with both ultrawide bandgap and strong second harmonic generation (SHG) are essential for expanding the nonlinear optical (NLO) applications to deep-ultraviolet (DUV) region in nanoscale. Unfortunately, these materials are rare in nature and have not been discovered until now. In this Letter, we predict the B2S2O9 (BSO), an existing layered oxide, can exhibit both DUV bandgap and strong SHG effects, comparable to the best known DUV NLO bulks. The strong SHG intensities in BSO, originated from the ordered arrangement of polar SO4 and BO4 tetrahedra forming planar structure, are linearly tunable by the layer thickness. Surprisingly, the spontaneous rotations of rigid tetrahedra under strains can induce the (nearly) zero Poisson's ratios in BSO, which simultaneously result in the robust SHG effects against large strains, fundamentally differing from other known 2D NLO semiconductors. The discovery of BSO may provide an unprecedented opportunity to explore DUV NLO physics and applications in 2D limit.

Realizing Deep-Ultraviolet Second Harmonic Generation by First-Principles-Guided Materials Exploration in Hydroxyborates.

Aimed to realize deep-ultraviolet (DUV, λ < 200 nm) second harmonic generation (SHG), herein we systematically investigate the scheme for screening DUV nonlinear optical (NLO) material "genes" from traditional borates to hydroxyborates. From our first-principles studies, it is demonstrated that, if hydroxyborate motifs can be aligned by reasonable layered crystallization types, the resulting crystal will exhibit potential DUV NLO capability. Guided by the theoretical analysis and prediction, two targeted hydroxyborates AEB8O15H4 (AE = Sr, Ca) screened from the inorganic crystal structure database are obtained in the experiments. Preliminary measurements including powder absorption spectra and SHG effects are in good agreement with our simulation results, well confirming our prediction that AEB8O15H4 (AE = Sr, Ca) would be promising DUV NLO crystals with large band gaps, strong SHG effects, and sufficient birefringence. This work is the first demonstration that hydroxyborates can realize DUV SHG, which may bring new opportunities for finding DUV NLO materials in the hydroxyborate system.

Strong Second Harmonic Generation in a Tungsten Bronze Oxide by Enhancing Local Structural Distortion.

To discover the nonlinear optical (NLO) materials with strong second harmonic generation (SHG), the design of NLO-active molecular units with large polarization is considered as a common strategy. Herein, we propose that the local structural distortion induced with vacancies, apart from the NLO-active units, can be employed to improve the NLO effect in solids as well. Accordingly, a new tungsten bronze (TB) oxide, Pb2(Pb0.15Li0.7□0.15)Nb5O15 (□ representing vacancies), is successfully designed and prepared, which exhibits a strong SHG response of 39 times that of KH2PO4. The detailed analysis reveals that the local structural distortions enhanced by the vacancies in PLN strengthen the local dipole moments of neighboring NbO6 octahedra, and thus significantly prompt the SHG effect. Moreover, a series of new TB compounds with large NLO effects are discovered by this molecular design strategy, which are perspectives for new NLO materials synthesis.