Large Second Harmonic Generation Effect Research Articles

AllHgMIVS4 (All = Sr, Ba, MIV = Si, Ge): A Series of Materials with Large Second Harmonic Generation Response and Wide Band Gaps

AbstractWide band gaps and large second harmonic generation (SHG) response are two crucial yet contradictory parameters for nonlinear optical (NLO) crystals. Herein, through exploring the influence of anion groups on structural crystallization dimension and space group, a series of new NLO Hg‐based chalcogenides AIIHgMIVS4 (AII = Sr, Ba, MIV = Si, Ge) with wide band gaps and large SHG responses have been successfully designed and synthesized. The structures of these compounds consist of 2D [HgMS4]2− (M = Si and Ge) layers composed of distorted (HgS4 and MS4) tetrahedra arranged alternately, and the layers are separated by eight‐coordinated A2+ (A = Sr and Ba) cations. Their band gaps and SHG responses meet the basic requirements (SHG > 1 × AgGaS2; Eg > 2.33 eV). Especially, SrHgSiS4 and SrHgGeS4 exhibit both large SHG effects (2 and 2.4 × AgGaS2) and wide band gaps (3.06 and 2.97 eV), respectively, indicating they are promising IR NLO crystals. This work opens a new avenue for new NLO material design to achieve the balance between strong SHG response and large band gaps.

α- and β-(C4H5N2O)(IO3)·HIO3: Two SHG Materials Based on Organic-Inorganic Hybrid Iodates.

Organic-inorganic hybrid nonlinear optical (NLO) materials are highly anticipated because of the integration of both merits of the organic and inorganic moieties. Herein, the 2-pyrimidinone cation (C4H5N2O)+ has been incorporated into the iodate system to form two polymorphic organic-inorganic hybrid iodates, namely, α- and β-(C4H5N2O)(IO3)·HIO3. They crystallize in different polar space groups (Ia and Pca21), and their structures feature one-dimensional (1D) chain structures composed of (C4H5N2O)+ cations, IO3- anions, and HIO3 molecules interconnected via hydrogen bonds. α- and β-(C4H5N2O) (IO3)·HIO3 exhibit strong and moderate second-harmonic-generation (SHG) responses of 6.4 and 0.9 × KH2PO4 (KDP), respectively, the same band gaps of 3.65 eV, and high powder laser-induced damage threshold (LIDT) values [51 and 57 × AgGaS2 (AGS)]. The results of theoretical calculations revealed that the large SHG effect of α-(C4H5N2O)(IO3)·HIO3 originated from the IO3 and HIO3 groups. This work indicates that (C4H5N2O)+ is a potential group for designing new NLO materials with brilliant optical performances.

Design of the Ionic Organic Nonlinear Optical Material NH4 [LiC3 H(CH3 )O4 ] with Ultrawide Band Gap and Moderate Birefringence.

Ionic organic crystals containing organic planar π-conjugated units has become one of the hot spots as nonlinear optical (NLO) materials. However, although this type of ionic organic NLO crystals commonly have remarkable second harmonic generation (SHG) responses, they also suffer from overlarge birefringences and relatively small band gaps that be hardly beyond 6.2 eV. Herein, a flexible π-conjugated [C3 H(CH3 )O4 ]2- unit was theoretically revealed, showing great potential for designing NLO crystals with balanced optical properties. Accordingly, through the reasonable NLO-favourable layered design, a new ionic organic material, NH4 [LiC3 H(CH3 )O4 ], was successfully obtained. As expected, it achieves not only a large SHG effect (4×KDP), but also a suitable birefringence (0.06@546 nm) and an ultrawide band gap (>6.5 eV). This study provides a new flexible π-conjugated NLO-active unit, contributing to design more ionic organic NLO materials with excellent balanced optical properties.

Design of the Ionic Organic Nonlinear Optical Material NH4[LiC3H(CH3)O4] with Ultrawide Band Gap and Moderate Birefringence

AbstractIonic organic crystals containing organic planar π‐conjugated units has become one of the hot spots as nonlinear optical (NLO) materials. However, although this type of ionic organic NLO crystals commonly have remarkable second harmonic generation (SHG) responses, they also suffer from overlarge birefringences and relatively small band gaps that be hardly beyond 6.2 eV. Herein, a flexible π‐conjugated [C3H(CH3)O4]2− unit was theoretically revealed, showing great potential for designing NLO crystals with balanced optical properties. Accordingly, through the reasonable NLO‐favourable layered design, a new ionic organic material, NH4[LiC3H(CH3)O4], was successfully obtained. As expected, it achieves not only a large SHG effect (4×KDP), but also a suitable birefringence (0.06@546 nm) and an ultrawide band gap (>6.5 eV). This study provides a new flexible π‐conjugated NLO‐active unit, contributing to design more ionic organic NLO materials with excellent balanced optical properties.

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.

Synergistic Effect of π-Conjugated [C(NH2)3] Cation and Sb(III) Lone Pair Stereoactivity on Structural Transformation and Second Harmonic Generation.

The search for nonlinear optical (NLO) crystals with excellent comprehensive properties is a formidable challenge. In this work, two guanidine antimony fluorides, C(NH2)3Sb2F7 and C(NH2)3SbF4, were obtained by conjunction of [C(NH2)3] groups with π-conjugated configuration and stereochemically active Sb3+ cations. Due to the different coordination modes of Sb-F bonds and H-F hydrogen bonds, the crystal structure of C(NH2)3Sb2F7 is centrosymmetric (CS), while C(NH2)3SbF4 is noncentrosymmetric (NCS). Optical measurements show that the UV cutoff wavelengths of the title compounds were both less than 240 nm. Thermal studies indicate that these crystals are stable up to 250 °C. In addition, the second harmonic generation (SHG) response of C(NH2)3SbF4 is 2 times that of KH2PO4 (KDP) with the phase-matchable capacity. Theoretical calculations reveal that the large SHG effects of C(NH2)3SbF4 were attributed to the synergy between the planar [C(NH2)3] units and the distorted [SbF4] groups. These results demonstrate that the guanidine antimony fluorides will have potential value as UV NLO materials.

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.

Why Some Noncentrosymmetric Borates Do Not Make Good Nonlinear Optical Materials: A Case Study with K3B5O8(OH)2.

Synthesis of deep-ultraviolet (DUV) transparent materials, especially those with noncentrosymmetric structures, remains a great challenge in the solid-state chemistry community. A new DUV transparent borate, K3B5O8(OH)2, was discovered with a short absorption edge below 200 nm. The title compound crystallizes in a noncentrosymmetric, polar space group, Fdd2 (point group mm2), with the following cell parameters: a = 13.736(9) Å, b = 19.317(12) Å, c = 7.606(5) Å. The structure of K3B5O8(OH)2 features a 3D framework composed of [B5O8(OH)2]3- basic building units that are linked by contacts with K+ cations and O-H···O hydrogen bonds. Second harmonic generation (SHG) measurements were performed, and an SHG efficiency of 0.5 × SiO2 was observed. Symmetry dictates that the χ14 component of the macroscopic NLO susceptibility is equal to zero in mm2, which prevents the maximal component of the microscopic NLO susceptibility for [B5O8(OH)2]3- units (χ14) from contributing to the macroscopic NLO susceptibility of the crystal and therefore limits the SHG efficiency. In contrast, large SHG effects can be observed from compounds containing [B5O10] units that crystallize in point groups with nonzero χ14, such as 222. These findings provide insight into understanding the relationship between crystal structure and SHG efficiency in [B5O10]-based compounds and discovering other borate-based DUV materials.

Noncentrosymmetric Organic Solid and Its Zinc Coordination Polymer with Diamonded Network Prepared from an Ionothermal Reaction: Syntheses, Crystal Structures, and Second-Order Nonlinear Optics Properties

An organic ligand 4-(1,2,4-triazol-1-yl) benzoic acid (Htzbc) with a nonlinear optical (NLO) chromophore has been synthesized. Reaction of Zn(NO3)2 with Htzbc ligand in 1-n-butyl-3-methylimidazolium tetrafluoroborate ionic liquid afforded compound Zn(tzbc)2 (1). The Htzbc organic solid and 1 crystallize in the acentric Pn and Cc space groups, respectively. The tetrahedral Zn2+ center in 1 coordinates to two triazole nitrogen atoms and two carboxylate oxygen atoms from four tzbc– ligands, leading to a noncentrosymmetric 5-fold interpenetrating diamondoid network. Second-harmonic generation (SHG) measurements revealed the Htzbc solid and 1 have SHG activities. The Htzbc compound has a large SHG effect, which is approximately 3 times that of KDP and is type-I phase matchable, while 1 also belongs to phase-matchable class with a SHG response of about 1.2 times that of KDP. The SHG active of the organic solid can be derived from the inherently dipolar Htzbc molecule as well as the optimal relative orientation of the molecular NLO-phores in the crystal packing. The odd number-fold interpenetrated diamondoid network with dipolar NLO chromophoric units based on asymmetrical bridging ligands and metal centers results in the SHG activity of 1. In addition, elemental analysis, IR spectra, powder X-ray diffraction patterns (PXRD), and thermogravimetric analysis of both compounds are described.

Ab initio studies on the mechanism for linear and nonlinear optical effects in YAl3(BO3)4

First-principles studies of the linear and nonlinear optical properties for YAl3(BO3)4 (YAB) are presented. Based upon the electronic band structure, the optical refractive indices, birefringence, and second harmonic generation (SHG) coefficients of YAB are calculated, which are in good agreement with experimental values. In addition, the SHG-weighted electron density analysis and the real-space atom-cutting method are adopted to elucidate the origin of the linear and nonlinear optical effects in YAB. The results show that the anionic (BO3) groups have dominant contributions to the birefringence. The contribution of the Al cations to the optical effects is negligibly small. However, the Y cations bond to the neighbor O anions and form the deformed (YO6) octahedra, which results in the large SHG effects in YAB.