Nonlinear Optical Compound Research Articles

Unusual Thermo-Enhanced Second Harmonic Generation in Organic Configurationally-Locked Polyene Crystals.

To modulate nonlinear optical (NLO) effects of crystalline material holds great application potential in the photoelectronic and optical fields. Organic configurationally-locked polyene represents an exciting NLO family with large second harmonic generation (SHG) effects, whereas it is a huge blank to switch and modulate their NLO property through external stimuli. For the first time, here present unusual thermo-enhanced SHG activities are presented in a polyene-based NLO compound, 2-{3-[2-(4-pyrrolidinphenyl)vinyl]-5,5-dimethylcyclohex-2-enylidene}malononitrile (1), giving a record-high magnitude of SHG enhancement up to ≈170% during its isomorphic phase transition. Theoretical analysis discloses this behavior stems from the reduced degree of torsion in the π-conjugated structures in 1, as verified by dihedral angles between its pyrrolidine and phenyl planes. As the first study on thermo-enhanced SHG properties of organic crystals, this work affords a new avenue of modulating physical properties to fabricate high-performance photoelectronic and optical devices.

ACuGa6S10 (A = Rb, Cs): Design and Synthesis of Two New Cavity-Chalcopyrite Chalcogenides Based on "Iterative Substitution" Strategy.

Two new non-centrosymmetric chalcogenides, ACuGa6S10 (A = Rb, Cs) have been successfully synthesized by an "iterative substitution" strategy based on chalcopyrite CuFeS2 structural template. Benefiting from the substitution of Fe3+ cations by Ga3+ cations, ACuGa6S10 (A = Rb, Cs) exhibit wide suitable band gap of 2.48 and 2.40 eV, respectively, which is about five times higher than their structure template CuFeS2, and the large second harmonic generation response (1.5 and 1.8 × AgGaS2). Combining theoretical calculation and structural analysis confirm that the [GaS4] tetrahedra make the main contribution on their good liner and nonlinear optical (NLO) performances. The "iterative substitution" strategy expands the design idea of materials and can lead to the discovery of a large number of IR NLO compounds.

A Congruent-Melt Infrared Nonlinear Optical Crystal Na4SrAs2S8.

A quaternary metal thioarsenate infrared (IR) nonlinear optical (NLO) compound Na4SrAs2S8 was successfully prepared by a high-temperature reaction method from stoichiometric agents. It crystallizes in the tetragonal P4n2 with the unit cell parameters a = 10.0393(3) Å, c = 6.9638(3) Å, and Z = 2, with the basic structural groups of isolated AsS4 tetrahedra. Compared with benchmark IR NLO crystal AgGaS2 (AGS), the title compound exhibits balanced optical properties of large band gap (3.05 eV vs. 2.60 eV) and considerable second harmonic generation response (0.95 × AGS). Moreover, the combined powder X-ray diffraction and differential scanning calorimetry analyses demonstrate that Na4SrAs2S8 is a congruent-melting compound with a low melting point of 668 °C, which is benefical for bulk single crystal growth. Experimental and theoretical calculation results indicate that Na4SrAs2S8 is a practically usable IR NLO crystal, also motivating the exploration of thioarsenates as high-performance IR NLO candidates.

Synthesis, structure, growth, characterization and quantum chemical studies on 2-amino-5-bromopyridinium trichloroacetate: A novel phasematchable organic NLO single crystal

Organic crystals have become excellent candidates towards nonlinear optical (NLO) applications as they possess superior second and third order optical nonlinearities and high electro-optic coefficients when compared with their inorganic counterparts. In this direction, 2-Amino-5-bromopyridinium trichloroacetate (2A5BPT), a new organic second-order nonlinear optical compound is synthesized and optical grade single crystals are formed utilizing a slow evaporation solution growth approach at room temperature while using methanol as the solvent. Single crystal X-ray diffraction study shows that the title compound is assigned to monoclinic crystal system having non-centrosymmetric space group Cc. The crystal is transparent ( ̴ 85%) throughout the whole visible spectrum, according to the Ultraviolet-Visble (UV–Vis) spectra, with a cutoff wavelength of 367 nm and an optical energy band gap Eg of 3.32 eV. The crystal possesses good thermal stability. The compound has a 1.6 times greater powder second harmonic generation (SHG) efficiency than potassium dihydrogen phosphate (KDP). The phase matchability of the 2A5BPT is studied through particle size dependence of SHG. The intermolecular connection demonstrated by numerous inter contacts has been shown by a molecular Hirshfeld surface analysis of the crystal structure, and the related 2-dimensional finger print analysis further evaluates the individual contribution from each atom. In order to examine the first order hyperpolarizability, molecular electrostatic potential, and Highest occupied molecular orbital-Lowest unoccupied molecular orbital (HOMO-LUMO) energies, the quantum chemical calculations for the title molecule (2A5BPT) were carried out by density functional theory (DFT) utilizing the B3LYP/6–311++G(d,p) basis set. According to these results, the 2A5BPT crystal could be suitable for optoelectronic applications.

Theoretical study of static and frequency-dependent nonlinear optical properties of 1-phenyl-2,5-di(thiophen-2-yl)-1H-pyrrole based D-π-A chromophores through π-linkers modifications: A gateway towards giant nonlinear optical compounds

Designing materials with proficient nonlinear optical (NLO) amplitudes is the frontier area of research. To address the briskly mounted demand for potentially useful NLO materials in optoelectronic and electrical applications, static and frequency-dependent NLO response properties of 1-phenyl-2,5-di(thiophen-2-yl)-1H-pyrrole (PDTP) based D-π-A chromophores were computed at 1907.21 nm, 532 nm, and 1064 nm laser wavelength for exploring SHG β (−2ω, ω, ω) and EOPE β (−ω, ω, 0). The PDTP-based organic dye (MC28) is selected to serve as a reference, and the quantum chemical models of its ten derivatives (SLN1-SLN10) are constructed by incorporating a variety of previously reported π-linkers into the reference. DFT and TD-DFT calculations reveal that π-linker modification tunes the NLO characteristics in SLN1-SLN10. The photophysical, electronic aspects, molecular electrostatic potential (MEP), frontier molecular orbital (FMO), density of state (DOS), transition density matrix (TDM), natural population analysis (NPA), and frequency-dependent NLO response properties of investigated dyes were evaluated. Giant NLO responses due to a much lower energy gap than the reference dye was found in SLN1-SLN10 confirmed by FMO analysis. UV–Vis analysis determined that the maximum absorption wavelength of the studied dyes was in the visible spectrum (383–413 nm). The proposed dye’s active NLO sensitivity and charge separation state was shown to have resulted from the migration of electrons from D to A via π-linkers using natural population analysis (NPA) analysis. In addition, molecular electrostatic potential (ESP), electron localization function (ELF), and localized orbital locator (LOL) topological assessments are carried out to evaluate the transfer of charge that takes place within SLN1-SLN10 as well as high NLO features. Maximum values of linear polarizability (αo) and first-hyperpolarizability (βo) were calculated to be 5.64 × 102 a.u. and 1.89 × 104 a.u., respectively in SLN3. At 1907.21 nm, SLN3 with terthiophene linker showed a remarkable SHG β(−2ω,ω,ω) and EOPE β (−ω,ω,0) response of 2.60 × 104 a.u. and 2.09 × 104 a.u. respectively. The theoretical structure provided conclusive evidence that the dyes under investigation possess excellent static and NLO potential at different laser wavelengths and should be considered for use in future NLO applications.

Impact of π-Linkers Modifications on Tuning Nonlinear Optical Amplitudes of Pyridoquinazolinone-Based Aromatic Dyes: A Rational Entry to Novel D-π-A NLO Materials

Developing efficient nonlinear optical (NLO) materials is a cutting-edge field of study. The desirability of promising NLO materials for optoelectronic applications drove us to investigate the pyridoquinazolinone-based D-π-A architectured chromophores for predicting proficient NLO compounds. Pyridoquinazolinone-based dye (WL-O) is chosen as a reference and its nine derivatives (ASH1-ASH9) are quantum chemically modeled by inserting variously reported π-linkers into the D-π-A framework. Simulations using DFT and TD-DFT provide insight into how π-linker alteration affects the NLO properties. The potential use of investigated dyes for NLO response applications is confirmed by evaluating photophysical aspects, frontier molecular orbital (FMO), natural population analysis (NPA), molecular electrostatic potential (MEP), the density of state (DOS), transition density matrix (TDM) analysis, and static, frequency-dependent NLO response properties. FMO analysis shows that all dyes’ energy gaps are lower when compared to the reference, resulting in a strong NLO response. The highest absorption wavelength of the studied dyes was found to be in the visible spectrum, as determined by UV-Vis analysis. NPA-based findings demonstrated the movement of the electrons from D to A through π-linkers resulting in the formation of a charge separation state and active NLO sensitivity of the proposed dyes. Moreover, MEP, LOL, and ELF topological investigations are performed to appraise the charge transfer inside ASH1-ASH9 and high NLO characteristics. The robust NLO response revealed the eye-catching linear polarizability(α0 ), first-hyperpolarizability (β), particularly ASH3, with the highest α0 and β0 computed to be 520.44 a.u. and 26166.79 a.u., respectively. Additionally, frequency-dependent hyperpolarizability SHG β(−2ω,ω,ω) and EOPE β(−ω,ω,0) calculations for commonly used lasers were performed at 1907.21, 532, and 1064 nm. ASH3 with terthiophene linker exhibits a striking EOPE β(−ω,ω,0) and SHG β(−2ω,ω,ω) response of 2.94 × 104 a.u. and 3.80 × 104 a.u. respectively at 1907.21 nm. This theoretical framework confirmed that the investigated dyes have fine NLO potential and are recommended for future NLO applications.

Theoretical Perspective toward Designing of 5-Methylbenzo [1,2-b:3,4-b':6,5-b″] trithiophene-Based Nonlinear Optical Compounds with Extended Acceptors.

A series of benzotrithiophene-based compounds (DCTM1-DCTM6) having D1-π1-D2-π2-A configuration were designed using a reference molecule (DCTMR) via incorporating pyrrole rings (n = 1-5) as the π-spacer (π2). Quantum chemical calculations were performed to determine the impact of the pyrrole ring on the nonlinear optical (NLO) behavior of the above-mentioned chromophores. The optoelectronic properties of the compounds were determined at the MW1PW91/6-311G(d,p) functional. Among all of the derivatives, DCTM5 exhibited the least highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) band gap (Eg) 0.968 eV with a high softness of 0.562 eV-1, and hence possessed the highest polarizability. Interestingly, transition density matrix (TDM) findings demonstrated that DCTM5 with an effective diagonal charge transmission proportion at the acceptor group supports the frontier molecular orbital (FMO) results. Additionally, the exciton binding energy values for DCTM1-DCTM6 were found to be less than that for DCTMR and thus, the effective charge transfer was examined in the derivatives. All of the derivatives exhibited effective NLO outcomes with the highest magnitude of linear polarizability ⟨α⟩, and first (βtot) and second (γtot) hyperpolarizabilities relative to the parent compound. Nevertheless, the highest βtot and γtot were obtained for DTCM1 and DTCM6, 7.0440 × 10-27 and 22.260 × 10-34 esu, respectively. Hence, through this structural tailoring with a pyrrole spacer, effective NLO materials can be obtained for optoelectronic applications.

First-principles study on AgBi(CrO4)2:An effective infrared nonlinear optical compound

The investigation of nonlinear optical (NLO) materials is critically important in developing laser technologies. The AgBi(CrO4)2 which crystallizes in the tetragonal lattice with a space group of I 4¯ is a potential candidate for the infrared (IR) region due to the large birefringence (Δn = 0.18), strong NLO response than the KDP crystal and the high electro-optical coefficients are justified their good performance NLO applications. The investigation of the elastic constant of AgBi(CrO4)2 indicates the ductile nature and the large piezoelectric coefficients make this crystal has great attention as bifunctional material.

Influence of azo-based donor modifications on nonlinear optical amplitude of D-π-A based organic chromophores: A DFT/TD-DFT exploration

Organic heterocyclic fused ring compounds possess remarkable nonlinear optical (NLO) amplitude, are potentially employed in optical computing and nano-photonics. Herein, ITTBCR1 was quantum chemically modified into a series of novel D-π-A configured NLO compounds (ITTBCD2-ITTBCD6) by substituting various efficient donor moieties. To investigate the electronic, photophysical, and NLO properties, ITTBCR1 and ITTBCD2-ITTBCD6 were subjected to density functional theory (DFT) and time-dependent DFT calculations at M06/6–311 G(d,p) level of theory. Frontier molecular orbital analysis (FMOs) illustrated that the newly designed chromophores exhibit smaller energy gaps (1.625–2.013 eV) compared to ITTBCR1 (2.384 eV). The global reactivity parameters estimated through Egap revealed higher softness (σ = 0.49–0.61 Eh) values, implying enhanced reactivity of ITTBCD2-ITTBCD6 compounds. An efficient push-pull mechanism disclosed positive charges on donors and π-spacers, while the acceptor moieties exhibited a negative charge. The presence of charge separation states and the substantial involvement of molecular orbitals in the charge transfer were explored by natural bond orbitals (NBOs), transition density matrix and density of states analyses. The UV-Vis study of the designed compounds displayed larger bathochromic shifts with wider absorption spectra. Also, a significantly large NLO behavior was found in all the novel entities (ITTBCD2-ITTBCD6), thus potentially employable as NLO materials. Inclusively, compound ITTBCD5 demonstrated highest linear polarizability (〈α〉 = 2.250 ×103a.u.), and first hyperpolarizability (βtot = 70.155 ×104a.u.), therefore proposed as the most proficient NLO material. Comparative study for NLO properties at various functional showed that at M06/6–311 G (d,p) significant NLO results could be obtained. The current investigation revealed that by controlling the type of donor motifs, numerous NLO compounds can be designed, which are equally productive in hi-tech NLO applications.

Synthesis, crystal growth, thermal, density functional theory, linear and nonlinear optical properties of 3-Aminopyridinium-2‑chloro-4-nitrobenzoate single crystal

In the present work, a 3rd-order nonlinear optical compound 3-Aminopyridinium-2‑chloro-4-nitrobenzoate (3AP2C4N) was successfully synthesised by following the slow-cooling solution growth technology 3AP2C4N single crystals are grown. The crystalline structure, established by single crystal x-ray diffraction investigation, revealed that 3AP2C4N belong to the triclinic system with the centro-symmetric space group of P-1. Powder x-ray diffraction measurement results support the formation of crystalline phase. The Fourier transform infrared analysis revealed the existence of different functional groups establishing the formation of 3AP2C4N molecule. Adopting the UV–Vis-NIR spectrometer, the absorption coefficient was measured and the calculated bandgap for 3AP2C4N was 3.25 eV. The luminescence properties of the title compound, which was excited at 320 nm, were studied using photoluminescence spectroscopy. TG-DTA measurements were used to investigate the thermal behaviour of 3AP2C4N. From the Z-scan approach it is evident that 3AP2C4N crystals are much suitable for optical limiting applications. DFT simulations at the B3LYP / 6–311++G (d, p) level enabled to evaluate the molecule's HOMO-LUMO, molecular electrostatic potential and contour map. The Hirshfeld surface investigations showed the presence of intermolecular interactions in the molecule.

The induced polarization enhanced birefringence in AlPS4 family: A first-principles investigation

As an important infrared nonlinear optical compound, metal-sulfur compounds have been widely used in various advanced optical systems. In this paper, the electronic structures, optical properties, and polarization under different external electronic field of AlPS4 (I222, P222,P4¯2c) compounds have been investigated using the first-principles method. The obtained results show that all these compounds have relatively large birefringence which are 0.256, 0.096, and 0.079@1064 nm, respectively. The relatively large birefringence has relation with the induced polarization under external electronic field rather than distorted polyhedron and spontaneous dipole moment. Both AlS4 and PS4 tetragonal give main contribution to the birefringence and second harmonic generation response.

Unbiased Screening of Novel Infrared Nonlinear Optical Materials with High Thermal Conductivity: Long-neglected Nitrides and Popular Chalcogenides.

Traditional infrared (IR) nonlinear optical (NLO) materials such as AgGaS2 are crucial to key devices for solid-state lasers, however, low laser damage thresholds intrinsically hinder their practical application. Here, a robust strategy is proposed for unbiased high-throughput screening of more than 140 000 materials to explore novel IR NLO materials with high thermal conductivity and wide band gap which are crucial to intrinsic laser damage threshold. Via our strategy, 106 compounds with desired band gaps, NLO coefficients and thermal conductivity are screened out, including 8 nitrides, 68 chalcogenides, in which Sr2 SnS4 is synthesized to verify the reliability of our process. Remarkably, thermal conductivity of nitrides is much higher than that of chalcogenides, e.g., 5×AgGaS2 (5.13 W/m K) for ZrZnN2 , indicating that nitrides could be a long-neglected system for IR NLO materials. This strategy provides a powerful tool for searching NLO compounds with high thermal conductivity.

Unbiased Screening of Novel Infrared Nonlinear Optical Materials with High Thermal Conductivity: Long‐neglected Nitrides and Popular Chalcogenides

AbstractTraditional infrared (IR) nonlinear optical (NLO) materials such as AgGaS2 are crucial to key devices for solid‐state lasers, however, low laser damage thresholds intrinsically hinder their practical application. Here, a robust strategy is proposed for unbiased high‐throughput screening of more than 140 000 materials to explore novel IR NLO materials with high thermal conductivity and wide band gap which are crucial to intrinsic laser damage threshold. Via our strategy, 106 compounds with desired band gaps, NLO coefficients and thermal conductivity are screened out, including 8 nitrides, 68 chalcogenides, in which Sr2SnS4 is synthesized to verify the reliability of our process. Remarkably, thermal conductivity of nitrides is much higher than that of chalcogenides, e.g., 5×AgGaS2 (5.13 W/m K) for ZrZnN2, indicating that nitrides could be a long‐neglected system for IR NLO materials. This strategy provides a powerful tool for searching NLO compounds with high thermal conductivity.

SbI3·3S8: A Novel Promising Inorganic Adducts Crystal for Second Harmonic Generation.

In the past twenty years, the basic investigation of innovative Non-Linear Optical (NLO) crystals has received significant attention, which has built the crucial heritage for the use of NLO materials. Fundamental research is essential given the scarcity of materials for NLO compounds, especially in the deep ultraviolet (DUV) and middle- and far-infrared (MFIR) regions. In the present work, we synthesized high-quality MFIR SbI3·3S8 NLO crystals having a length in the range of 1-5 mm through rapid facile liquid phase ultrasonic reaction followed by the assistance of instantaneous natural evaporation phenomenon of the solvent at room temperature. X-ray diffraction (XRD) results ratify the hexagonal R3m structure of SbI3·3S8 crystal, and energy-dispersive X-ray spectroscopy (EDX) demonstrates that the elemental composition of SbI3·3S8 crystal is similar to that of its theoretical composition. The direct and indirect forbidden energy gaps of SbI3·3S8 were measured from the optical transmittance spectra and they were shown to be 2.893 eV and 1.986 eV, respectively. The green sparkling signal has been observed from the crystal during the second harmonic generation (SHG) experiment. Therefore, as inorganic adducts are often explored as NLO crystals, this work on the MFIR SbI3·3S8 NLO crystal can bring about additional investigations on this hot topic in the near future.

Enhanced Second-Harmonic Generation of van der Waals CuInP2S6 via Pressure-Regulated Cationic Displacement

Design and exploration of high-performance nonlinear optical (NLO) materials have long been sought with the goal of tunable local structures and NLO properties for advanced laser technology. Thus far, the design strategies for second-order NLO materials have been mainly focused on anionic groups that have made great progress in the development of new NLO compounds. However, few studies have focused on the effects of cationic components and their contributions to NLO properties have long been underappreciated and remain unclear. Here, by introducing pressure to continuously tune the Cu displacement, we demonstrate the significant role of the cationic configuration in NLO properties, and a remarkable enhancement by one order of magnitude in second-harmonic generation (SHG) has been achieved at 3.4 GPa in thiophosphate CuInP2S6 (CIPS). In situ high-pressure structural characterization indicates that Cu cations move from the van der Waals edge to the cages of S6 octahedra during compression and subsequently form highly distorted [CuS6] octahedra at 3.2 GPa. We quantitively demonstrate the contributions of cationic displacement on the octahedral distortion and interband dipole moments, which dominate optical nonlinearity and determine the enhanced SHG. This work deepens the fundamental understanding of the relationship between cationic displacement and SHG properties, thus providing new paths to NLO material design by optimizing the cationic configurations.

Third-order nonlinear optical studies of Bis(4-methylbenzylammonium) tetrachloridocuprate metal-organic crystal with optical limiting behavior: Experimental and theoretical investigations

The metal-organic nonlinear optical (NLO) compound Bis(4-methylbenzylammonium) tetrachloridocuprate (4MBA) was synthesized and grown by the slow evaporation method. X-ray crystal analysis revealed the crystal structure and conformation. Density functional theory (DFT) at B3PW91/LanL2DZ basis set level was applied and computed the optimized geometry, electronic structure, vibrational spectra, frontier molecular orbitals (FMO), molecular electrostatic potential (MEP), and natural bonding orbitals (NBO). UV–Vis-NIR spectroscopic measurements revealed the optical transparency range from 310 to 1100 nm and excited-state properties. The thermal behavior and stability of the compound were analyzed using TGA/DTA, and the suitability of the optical material is weighed. The dielectric parameters and AC conductivity were investigated as a function of frequency and temperature. Second-harmonic generation (SHG) efficiency was measured using the Kurtz and Perry powder approach and found 4MBA to be 0.8 times the KDP. The nonlinear refractive index (n2), nonlinear optical absorption (β) and third-order nonlinear optical susceptibility (χ(3)) measured values are 1.548 × 10−9 cm2 W−1, 0.0538 × 10−4 cm W−1, and 6.049 × 10−8 esu respectively, and shows that the 4MBA molecule is a third-order nonlinear optical material. The optical limiting threshold value is determined as 7.632 × 103 W cm2 for 532 nm continuous-wave laser, representing that the 4MBA is a strong candidate for the optical limiting application. The static and dynamic nonlinear optical parameters were computed with three different functions including B3PW91, M06 and PBE0 using LanL2DZ basis set. The outcomes revealed that the 4MBA molecule under study might be a promising NLO material for frequency generators and optical limiters applications.

Design of Infrared Nonlinear Optical Compounds with Diamond-like Structures and Balanced Optical Performance.

Infrared (IR) nonlinear optical (NLO) crystals are the major materials to widen the output range of solid-state lasers to mid-infrared regions, but they are still inadequate for application due to the difficulties in balancing the large band gaps and strong NLO response. The diamond-like structure is a potential structural template to explore IR NLO materials. Herein, a computational workflow is proposed for exploring compounds with diamond-like structures, a series of LiMgGaSe3 structures were predicted successfully through this workflow, and LiMgGaSe3-I-III exhibited good optical performances in a large band gap (2.75-2.92 eV), strong SHG response (1.2-1.3 × AGS), and suitable birefringence (0.0470-0.0783 at 1064 nm). The in-depth mechanism explorations strongly demonstrate that the synergistic effect of alkaline earth metal tetrahedral [MgSe4] and [GaSe4] units is the main origin of large SHG response. The foregoing results suggest that our workflow can accelerate the discovery of new mid-IR NLO materials with diamond-like structures.

Symmetric vs. asymmetric: Which one is the better molecular configuration for achieving robust NLO response?

The current study reports a comparative investigation for the nonlinear optical (NLO) response for some symmetric and asymmetric molecular configurations. The designed compounds are featuring a similar 2,7-bis(phenyl ethynyl)cyclopentaaceanthrylene core connected to different terminal donor and acceptor moieties such as dimethylamine, methoxy, nitro and ethene-tricarbonitrile groups based on three topologies, “D-π-D” (1–2), “A-π-A” (3–4) and “D-π-A” (5–8). A comparative analysis of these symmetrically and asymmetrically substituted chromophores is done to investigate the effect of different terminal groups on their NLO response, optical and charge transfer properties. The asymmetric compounds display larger first hyperpolarizability amplitudes (β//) than those of the symmetric compounds where a β// amplitude as large as 1851 × 10−30 esu is calculated for asymmetric compound 6. Interestingly, the average second hyperpolarizabilities (γ) for symmetric compound 4 have been calculated to be 16, 096 × 10−36 esu and for asymmetric counterpart compound 6 it is found to be 23, 162 × 10−36 esu using M06/6-311G* methodology. Our findings were also inline with experimental observations of Mathews et al. (Mathews, S.J., et al., 2007. Optics Communications, 280, 206–212), where they also experimentally determined the larger third-order susiptibility for asymmetric phthalocyanine (Pc3) than their symmetric counterparts. Additionally, the NLO polarizabilities of all the compounds have also been compared with a typical donor-acceptor molecule of p-NA to specify the comparison of designed compounds. TD-DFT studies have shown that the higher β and γ values were found for asymmetric compounds owing to their lower transition energies and larger trasition dipole moments. The depiction of molecular orbitals and electrostatic potentials have shown more efficient intramolecular charge transfer (ICT) for asymmetric compounds leading to the robust NLO response. Thus, a systematic comparison of the NLO polarizabilities and other electronic properties of our designed symmetric and asymmetric compounds shows that symmetry based designing of NLO compounds are very crucial to get robust NLO response properties.

First-principles study of the influence of lone pair electrons and molecular groups on the birefringence of phosphate and fluorophosphate

As a significant nonlinear optical compound, phosphate has been wildly used in all kinds of advanced optical systems. In this work, the electronic structures, and optical properties of a series of XPO3F and X3(PO4)2 (X = Sn, Pb, Sr, Ba) compounds were investigated using the first-principles method in both molecular and crystal levels. The results show that introducing F atoms can lead to enhanced HOMO-LUMO gap and enlarged anisotropy of polarizability. The anionic groups give main contribution to the total birefringence of alkaline earth phosphate and fluorophosphate. As for the post-transition metal phosphate and fluorophosphate, both anionic groups and cation-centric polyhedra play a critical role in determining birefringence. The X-p (X = Sn, and Pb) states nearby the Fermi level are beneficial to get enhanced birefringence, but they do not play a vital role in determining total birefringence.

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.