Materials For Second Harmonic Generation Research Articles

Characterization of organic crystals for second-harmonic generation.

Second-harmonic generation (SHG) is a common technique with many applications. Common inorganic single-crystalline materials used to produce SHG light are effective using short IR/visible wavelengths but generally do not perform well at longer, technologically relevant IR wavelengths such as 1300, 1550, and 2000 nm. Efficient SHG materials possess many of the same key material properties as terahertz (THz) generators, and certain single-crystalline organic THz generation materials have been reported to perform at longer IR wavelengths. Consequently, this work focuses on characterizing three efficient organic THz generators for SHG, namely, DAST (trans-4-[4-(dimethylamino)-N-methylstilbazolium] p-tosylate), DSTMS (4-N,N-dimethylamino-4'-N'-methylstilbazolium 2,4,6-trimethylbenzenesulfonate), and the recently discovered generator PNPA ((E)-4-((4-nitrobenzylidene)amino)-N-phenylaniline). All three of these crystals outperform the beta-barium borate (BBO), an inorganic material commonly used for SHG, using IR pump wavelengths (1200-2000 nm).

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.

Designing a Strong Second-Harmonic Generation Polar Iodate by the Structure-Directing Properties of the “Tumbler-like” [Zn(IO3)(I2O5(OH))] Polyanions

Materials with second-harmonic generation (SHG) properties have attracted great attention in the field of photon technology. However, synthesis of materials with strong SHG responses and polar structure still faces enormous challenges. Herein, a new iodate K2Zn(IO3)3(I2O5(OH))(IO2(OH))(H2O) (KZIOH) was synthesized by coupling distorted ZnO6 octahedra with three diverse asymmetrical chromophores to assemble “tumbler-like” clusters, [Zn(IO3)(I2O5(OH))], which was the first noncentrosymmetric (NCS) and polar iodate containing isolated [IO3], [I2O5(OH)], and [IO2(OH)]. KZIOH exhibits the strongest SHG response (∼12 × KH2PO4 (KDP)@1064 nm) among the polyiodates with two or more types of isolate units and a short ultraviolet cutoff edge (262 nm). It also possesses a moderate birefringence (0.055@532 nm) and a wide band gap (4.0 eV), indicating KZIOH is a promising SHG material. Theoretical calculations and crystal structure analysis show that the asymmetric and top-heavy configurations of I–O groups around Zn-centered octahedral units with the aid of hydrogen bonding favor its polar structure and strong SHG response.

Theoretical Prediction‐Assisted Synthesis and Characterization of Infrared Nonlinear Optical Material NaSrBS3

AbstractNaSrBS3, the first noncentrosymmetric alkali and alkaline earth metal mixed thioborate with the isolated [BS3] as the building units, has been synthesized successfully under the guidance of theoretical prediction. Comprehensive characterizations reveal that NaSrBS3 exhibits wide bandgap (3.94 eV), large birefringence (0.185 at 1064 nm), a high laser‐induced damage threshold (9 × AGS), and a suitable phase‐matching second‐harmonic generation (SHG) response (0.3 × AGS). Structure‐properties analyses illustrate that the [BS3] units determine the bandgap and contribute the most to the birefringence and SHG response. This work is the first successful case of “prediction to synthesis” involving infrared (IR) nonlinear optical (NLO) crystals in the thioborate system and provides an avenue to SHG materials design.

Nanoencapsulation-Induced Second Harmonic Generation in Pillararene-Based Host-Guest Complex Cocrystals.

The features of molecularly preferable centrosymmetric arrangements exclude organic nonlinear optical (NLO) materials for second harmonic generation (SHG) when used in the solid and crystalline states, which greatly limits their applications in optoelectronic devices. Herein, a pillar[5]arene (BrP5) is used as the macrocyclic host to encapsulate NLO molecules, 4-[4'-methoxystyryl]-1-methylpyridinium iodide (OM), 4-[2'-(5'-(dimethylamino)thiophen-2'-yl)vinyl]-1-methylpyridinium iodide (DAST), and 4-methoxy-β-nitrostyrene (MNS), to alter the solid-state packing of these NLO molecules and manipulate their centrosymmetric arrangements. BrP5 forms 2:1 host-guest complexes with OM and DAST, while it forms a 1:1 host-guest complex with MNS. Experimental results show that the pillar[5]arene and each of these three NLO guests form a nanocapsule architecture along with an overall centrosymmetric crystal structure. However, the random orientation of OM and DAST molecules inside the 2:1 host-guest complex nanocapsules breaks the local centrosymmetric arrangement of the NLO molecules, resulting in strong SHG. On the contrary, for BrP5⊃MNS, the MNS molecules inside the pillar[5]arene cavities are unable to break the centrosymmetry. They have only one determined orientation in the one-dimensional (1D) channels of BrP5, while other MNS molecules in adjacent channels have the opposite direction. The centrosymmetry of the dipolar chains is strictly maintained with the cancellation of nonlinear polarization, resulting in the quenching of SHG. Furthermore, an ultrasound-induced host-guest crystallization method is developed for the fast preparation of these host-guest composite materials with NLO activity. This work opens a new way to construct solid-state organic NLO materials, which have potential in high-power lasers, optical switches, and imaging applications.

Intermolecular interactions enhanced second harmony generation of iridium complex for bio-labeling

Second harmonic generation (SHG) microscopy avoids the inherent drawbacks of traditional fluorescent probes and brings a major breakthrough in live cell imaging. However, not all ordered non-centrosymmetric matter has the ability to generate SHG signals due to the different modes of intermolecular interaction. Here, we designed a non-centrosymmetric metal iridium complex (Ir-SHG) and obtained single crystals with two different intermolecular interaction modes in red (Ir-R) and yellow (Ir-Y), in which Ir-Y was shown to acquire a significant SHG effect. Interestingly, the SHG imaging of Ir-SHG exhibits high imaging signal-to-ratio (SNR) and brightness, as well as can be used in combination with a large number of fluorophores without signal cross-talk. We believe this work brings new insights into the development of SHG materials for multimodal nonlinear optical imaging.

Analysis of various properties of l-alaninium p-toluenesulfonate single crystal for nonlinear optical applications

L-alaninium p-toluenesulfonate single crystal is a promising organic nonlinear optical material for second harmonic generation (SHG) and optical limiting applications. In this work, the slow evaporation solution growth technique was employed to grow a good quality single crystal at ambient conditions. The obtained crystal’s structure, unit cell dimensions and phase identification were done by Powder X-ray diffraction (PXRD) and found that it crystallizes in orthorhombic structure with space group P212121 and exhibits non-centrosymmetric nature. Its crystalline purity was characterized by High-Resolution X-ray diffraction (HRXRD) and found highly crystalline with minor grain boundaries. The Hirschfield surface analysis was carried out to identify major interactions. The photopyroelectric (PPE) technique was employed to determine the specific heat capacity (Cp) and thermal conductivity (k) of the grown sample. A low value of inhomogeneity in the birefringence of the titled crystal suggests its better optical quality. Photo-conductivity studies inferred that the titled compound features negative photoconductivity and the currents produced were in nanoscale. Further, the Piezoelectric coefficient (d33) was also measured. The mechanical strength was studied by giving shock waves to the crystals in terms of shock damage threshold (SDT) and by giving static indentations to the crystals in Vickers’s microhardness test. The third-order nonlinear optical properties were studied by Z-scan analysis.

Synthesis, structural characterization, and Hirshfeld surface analysis of novel NLO active 1-methyl-1,3,5,7-tetraazaadamantan-1-ium 3-carboxy-5-nitrobenzoate hydrate

Single crystals of 1-methyl-1,3,5,7-tetraazaadamantan-1-ium 3-carboxy-5-nitrobenzoate hydrate (I) were grown from an aqueous solution containing hexamethylenetetramine (HMT) and 5-nitroisophthalic acid (5NIPA) by slow evaporation solution growth technique after the mild hydrothermal process. The crystal structure is elucidated by single-crystal X-ray diffraction analysis. It crystallizes in the monoclinic system with acentric space group Cc. The cation and anion present in the adduct are bridged by a water molecule, which is interconnected with CH···O and OH···O hydrogen bonds. The relative second harmonic generation (SHG) efficiency measurements characterized by the Kurtz powder technique reveal that (I) is an active SHG material. The grown crystals were further characterized by FT-IR, UV-DRS, and photoluminescence spectral analysis. Theoretical calculations were performed using density functional theory to derive the dipole moment, polarizability, and first-order molecular hyperpolarizability (β). The intermolecular hydrogen bonding interactions are analysed by Hirshfeld surface analysis and fingerprint plots. Theoretically estimated high β rationalized by the experiment. The molecular electrostatic potential (MEP) maps and Mulliken atomic charge were calculated using density functional theory (DFT) calculations.

O‐C5H4NHOH]2[I7O18(OH)]⋅3 H2O: An Organic–Inorganic Hybrid SHG Material Featuring an [I7O18(OH)] Branched Polyiodate Chain

AbstractAn organic–inorganic hybrid polyiodate, namely, [o‐C5H4NHOH]2[I7O18(OH)]⋅3 H2O (I), featuring a novel branched polyiodate chain has been obtained by evaporation method. [o‐C5H4NHOH]2[I7O18(OH)]⋅3 H2O (I) crystalizes in the polar space group Ia and features an [I7O18(OH)] branched polyiodate chain in which [I3O9]3− trimers are grafted on the same side of the one‐dimensional (1D) chain based on [I4O11(OH)]3− tetramers. The asymmetric organic amine groups are beneficial to the polymerization of iodate groups and inducing the formation of the non‐centrosymmetric (NCS) structure. Compound I exhibits a rather large Second‐Harmonic‐ Generation (SHG) signal of 8.5×KH2PO4 (KDP) upon 1064 nm laser radiation, a moderate band gap of 3.90 eV and a high laser‐induced‐damage‐threshold (LIDT) of 182.34 MW cm−2, hence it is a promising new SHG material. The relationship between the structures of the organic amine groups and the overall structures has been also analyzed.

O‐C5H4NHOH]2[I7O18(OH)]⋅3 H2O: An Organic–Inorganic Hybrid SHG Material Featuring an [I7O18(OH)] Branched Polyiodate Chain

An organic-inorganic hybrid polyiodate, namely, [ o -C 5 H 4 NHOH] 2 [I 7 O 18 (OH)]·3H 2 O ( I )， featuring a novel branched polyiodate chain has been obtained by evaporation method. [ o -C 5 H 4 NHOH] 2 [I 7 O 18 (OH)]·3H 2 O ( I ) crystalizes in the polar space group Ia and features an [I 7 O 18 (OH)] 2- ∞ branched polyiodate chain in which [I 3 O 9 ] 3- trimers are grafted on the same side of the one-dimensional (1D) chain based on [I 4 O 11 (OH)] 3- tetramers. The asymmetric organic amine groups are beneficial to the polymerization of iodate groups and inducing the formation of the non-centrosymmetric (NCS) structure. Compound I exhibits a rather large Second-Harmonic- Generation (SHG) signal of 8.5 × KH 2 PO 4 (KDP) upon 1064 nm laser radiation, a moderate band gap of 3.90 eV and a high laser-induced-damage-threshold (LIDT) of 182.34 MW cm -2 , hence it is a promising new SHG material. The relationship between the structures of the organic amine groups and the overall structures has been also analyzed.

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.

Noncentrosymmetric Tetrel Pnictides RuSi4P4 and IrSi3P3: Nonlinear Optical Materials with Outstanding Laser Damage Threshold

AbstractNoncentrosymmetric (NCS) tetrel pnictides have recently generated interest as nonlinear optical (NLO) materials due to their second harmonic generation (SHG) activity and large laser damage threshold (LDT). Herein nonmetal‐rich silicon phosphides RuSi4P4 and IrSi3P3 are synthesized and characterized. Their crystal structures are reinvestigated using single crystal X‐ray diffraction and 29Si and 31P magic angle spinning NMR. In agreement with previous report RuSi4P4 crystallizes in NCS space group P1, while IrSi3P3 is found to crystallize in NCS space group Cm, in contrast with the previously reported space group C2. A combination of DFT calculations and diffuse reflectance measurements reveals RuSi4P4 and IrSi3P3 to be wide bandgap (Eg) semiconductors, Eg = 1.9 and 1.8 eV, respectively. RuSi4P4 and IrSi3P3 outperform the current state‐of‐the‐art infrared SHG material, AgGaS2, both in SHG activity and laser inducer damage threshold. Due to the combination of high thermal stabilities (up to 1373 K), wide bandgaps (≈2 eV), NCS crystal structures, strong SHG responses, and large LDT values, RuSi4P4 and IrSi3P3 are promising candidates for longer wavelength NLO materials.

Synthesis, growth and mechanical properties of mid-infrared nonlinear optical crystal CdGeAs2

CdGeAs2 crystal is a promising nonlinear optical material for second harmonic generation (SHG) and optical parameter oscillation (OPO) applications in mid-infrared range. In this work, CdGeAs2 polycrystalline was synthesized using temperature oscillation method and CdGeAs2 single crystals with diameters of 30 mm were grown by modified vertical Bridgeman method. The synthesized polycrystalline was characterized by X-ray diffraction (XRD). The composition and transmission of wafers cut from a single crystal were characterized by energy dispersive spectrometer (EDS) and IR Prestige-21 spectrophotometer. The mechanical properties such as Young's modulus and hardness of (001), (100), (102) and (112) faces of CdGeAs2 were measured by nanoindentation technique. The measured Young's moduli established a relationship with the bonding direction, i.e., the closer the indentation direction was to the bonding direction, the stronger the stiffness will be.

Second Harmonic Generation Susceptibilities from Symmetry Adapted Wannier Functions.

Elucidating the orbital level origin of second harmonic generation (SHG) in materials and identifying the local contributions is a long-standing challenge. We report a first principles approach for the SHG where the contributions from individual orbitals or atoms can be evaluated via symmetry adapted Wannier functions without semiempirical parameters. We apply this method to the common SHG materials KBe_{2}BO_{3}F_{2}, KCaCO_{3}F, and β-BaB_{2}O_{4}, and show that the orbitals on noncentrosymmetric sublattices are responsible for SHG effect and the energies of these orbitals control the magnitude.

Quantitative Polarization-Resolved Second-Harmonic-Generation Microscopy of Glycine Microneedles.

Second-harmonic generation (SHG) is a nonlinear optical process that can provide disease diagnosis through characterization of biological building blocks such as amino acids, peptides, and proteins. The second-order nonlinear susceptibility tensor χ(2) of a material characterizes its tendency to cause SHG. Here, a method for finding the χ(2) elements from polarization-resolved SHG microscopy in transmission mode is presented. The quantitative framework and analytical approach that corrects for micrometer-scale morphology and birefringence enable the determination and comparison of the SHG susceptibility tensors of β- and γ-phase glycine microneedles. The maximum nonlinear susceptibility coefficients are d33 = 15 pm V-1 for the β and d33 = 5.9 pm V-1 for the γ phase. The results demonstrate glycine as a useful biocompatible nonlinear material. This combination of the analytical model and polarization-resolved SHG transmission microscopy is broadly applicable for quantitative SHG material characterization and diagnostic imaging.

Synthesis, growth and characterization of N, N-dimethyl-4-[2-(2-quinolyl) vinyl] aniline (DADMQ): An SHG material for NLO applications

The organic nonlinear optical (NLO) material N, N-dimethyl-4-[2-(2-quinolyl) vinyl] aniline (DADMQ) has been synthesized and high-quality single crystal with the dimension of 8 × 1 × 0.5 mm3 was grown from methanol solvent by slow cooling method for the first time. Single crystal and powder XRD studies show that DADMQ crystal belongs to the orthorhombic space group Pna21. All the functional groups and structure of the synthetic material can be determined from FT-IR and 1H NMR spectral analyses. The thermal properties can be studied through TG/DTG and DSC analyses, and its melting point is confirmed at 184.9 ℃. Some critical optical characteristics like the optical transmittance, optical absorption coefficient (α), energy band gap (Eg), cut-off wavelength (λmax) and extinction coefficient (κ) can be carried out through UV–vis-NIR spectral analysis. The dielectric features of the title crystal have also been measured. Furthermore, there is a blue emission appeared at 493 nm through PL spectral analysis. The second harmonic generation (SHG) efficiency of the title crystal has been measured, which indicates that DADMQ has 3.64 times greater than KDP crystal.

Janus XM-GaS (M=Si, Ge, Sn; X=N, P) monolayers: Multifunctional properties for photocatalysis, piezoelectricity and second harmonic generation

Two dimensional (2D) Janus monolayers attract increasing attention since successful synthesis of MoSSe monolayer. In this work, we study the stability and multifunctional applications of Janus monolayers MX-GaS (M = Si, Ge; Sn X = N, P) using first-principles calculations. 2D MX-GaS monolayers are energetically, mechanically and dynamically stable. Among MX-GaS, single-layer SiP-GaS and GeP-GaS possess suitable band edges for photocatalytic water splitting and strong visible-light absorbance. The in-plane and out-of-plane second harmonic generation (SHG) coefficients of single-layer MX-GaS are comparable or beyond that of typical 2D SHG materials GaS, GaSe and MoS2. Single-layer SiN-GaS, GeN-GaS, SnN-GaS and SnP-GaS display in-plane piezoelectric coefficients, while single-layer SiN-GaS exhibits considerable out-of-plane piezoelectricity. This work will stimulate discoveries of novel properties and applications among 2D Janus Ⅴ-Ⅳ-Ⅲ-Ⅵ (Ⅴ = N, P; Ⅳ = Si, Ge, Sn; Ⅲ = Ga, In; Ⅵ = S, Se, Te) materials.

Nanostructured Borate Halides for Optical Second Harmonic Generation at Surfaces

Optical second harmonic generation (SHG) at surfaces and interfaces is an exciting new technique. In order to achieve high surface specificity and sensitivity of the SHG technique, smaller particles of SHG materials are highly desired. Borate halides are regarded as a great source of SHG materials because of their outstanding nonlinear optical properties. Phase‐pure nanostructured samples of three borate halides of the type A2B5O9X were synthesized at low temperature in task‐specific ionic liquids (IL). Optimized reactions of acetates A(OAc)2 (A = Sr, Ba) and boric acid B(OH)3 with the mixed IL [P66614]X (X = Cl, Br) and LiNTf2 led to nano‐rods of Sr2B5O9Cl, nano‐needles of Sr2B5O9Br, and nano‐sheets of Ba2B5O9Cl. The reaction route presented here might trigger the synthesis of other borate compounds. Efficient second‐order nonlinear conversion by the nanostructured borate halides was confirmed using confocal SHG microscopy.

Resource-Efficient Low-Temperature Synthesis of Microcrystalline Pb2 B5 O9 X (X = Cl, Br) for Surfaces Studies by Optical Second Harmonic Generation.

Optically nonlinear Pb2 B5 O9 X (X = Cl, Br) borate halides are an important group of materials for second harmonic generation (SHG). Additionally, they also possess excellent photocatalytic activity and stability in the process of dechlorination of chlorophenols, which are typical persistent organic pollutants. It would be of great interest to conduct in situ (photo-) catalysis investigations during the whole photocatalytic process by SHG when considering them as photocatalytic materials. In order to get superior photocatalytic efficiency and maximum surface information, small particles are highly desired. Here, a low-cost and fast synthesis route that allows growing microcrystalline optically nonlinear Pb2 B5 O9 X borate halides at large quantities is introduced. When applying the ionothermal growth process at temperatures between 130 and 170 °C, microcrystallites with an average size of about 1 µm precipitate with an orthorhombic hilgardite-like borate halide structure. Thorough examinations using powder X-ray diffraction and scanning electron microscopy, the Pb2 B5 O9 X microcrystals are indicated to be chemically pure and single-phased. Besides, the Pb2 B5 O9 X borate halides' SHG efficiencies are confirmed using confocal SHG microscopy. The low-temperature synthesis route thus makes these borate halides a highly desirable material for surface studies such as monitoring chemical reactions with picosecond time resolution and in situ (photo-) catalysis investigations.

Upper limit on shift current generation in extended systems

The response functions of a material characterize its behavior under external stimuli, such as electromagnetic radiation. Such responses may grow linearly with the amplitude of the incident radiation, as is the case of absorption, or may be nonlinear. The latter category includes a diverse set of phenomena such as second harmonic generation (SHG), shift current, sum frequency generation, and excited state absorption, among others. Despite decades of research into nonlinear response theory, and the occasional discovery of materials with large nonlinear responses, there has been no systematic investigation into the maximum amount of nonlinear optical response attainable in solid-state materials. In this work, we present an upper bound on the second-order response functions of materials, which controls the SHG and shift current responses. We show that this bound depends on the band gap, band width, and geometrical properties of the material in question. We find that Kuzyk's bound for the maximum SHG of isolated molecules can be exceeded by conjugation or condensation of molecules to form molecular solids, and that strongly coupled systems generally have larger responses than weakly coupled or isolated ones. As a proof of principle, we perform first-principles calculations of the response tensors of a wide variety of materials, finding that the materials in our database do not yet saturate the upper bound. This suggests that new large SHG and shift current materials will likely be discovered by future materials research guided by the factors mentioned in this work.