Excellent Infrared Nonlinear Optical Crystals BaMO(IO?)? (M = V, Ta) Predicted by First Principle Calculations.
ABSTRACT: Two nonlinear optical crystals, BaVO(IO?)? and BaTaO(IO?)?, are designed by substituting Nb with V and Ta, respectively, in BaNbO(IO?)?, which is itself a recently synthesized infrared nonlinear optical (NLO) material. The designs of BaVO(IO?)? and BaTaO(IO?)? from BaNbO(IO?)? are based on the following motivation: BaVO(IO?)? should have a larger second-harmonic generation (SHG) coefficient than BaNbO(IO?)?, as V will result in a stronger second-order Jahn-Teller effect than Nb due to its smaller ion radius; at the same time, BaTaO(IO?)? should have a larger laser-damage threshold, due to the fact that Ta has a smaller electronegativity leading to a greater band-gap. Established on reliable first-principle calculations, it is demonstrated that BaVO(IO?)? has a much larger SHG coefficient than BaNbO(IO?)? (23.42 × 10-9 vs. 18.66 × 10-9 esu); and BaTaO(IO?)? has a significantly greater band-gap than BaNbO(IO?)? (4.20 vs. 3.55 eV). Meanwhile, the absorption spectra and birefringences of both BaVO(IO?)? and BaTaO(IO?)? are acceptable for practice, suggesting that these two crystals can both be expected to be excellent infrared NLO materials.
Project description:Nonlinear optical (NLO) switchable materials are important for photonic and optoelectronic technologies. One important issue for NLO photoswitching, the most studied physical switching approach, is how to improve the switching contrast of second harmonic generation (SHG) in crystals, because the known values are generally below 3 times. Thermoswitching, as another approach, has shown impressive high SHG-switching contrasts (4-? times), but the fast decay of thermally induced states demands constant heat sources to maintain specific SHG intensities. We have synthesized a photochromic and thermochromic bistable acentric compound, ?-[(MQ)ZnCl<sub>3</sub>] (MQ<sup>+</sup> = <i>N</i>-methyl-4,4'-bipyridinium), which represents the first crystalline compound with both photo- and heat-induced SHG-switching behavior and the first example of a thermoswitchable NLO crystal that can maintain its expected second-order NLO intensity without any heat source. The SHG-switching contrast can reach about 8 times after laser irradiation or 2 times after thermal annealing. The former value is the highest recorded for photoswitchable NLO crystals. This work also indicates that higher SHG-switching contrasts may be obtained through increasing electron-transfer efficiency, variation of permanent dipole moment, and self-absorption.
Project description:Mid-Infrared nonlinear optical (Mid-IR NLO) crystals with excellent performances play a particularly important role for applications in areas such as telecommunications, laser guidance, and explosives detection. However, the design and growth of high performance Mid-IR NLO crystals with large NLO efficiency and high laser-damage threshold (LDT) still face numerous fundamental challenge. In this study, two potential Mid-IR NLO materials, Rb2LiVO4 (RLVO) and Cs2LiVO4 (CLVO) with noncentrosymmetric structures (Orthorhombic, Cmc21) were synthesized by high-temperature solution method. Thermal analysis and powder X-ray diffraction demonstrate that RLVO and CLVO melt congruently. Centimeter sized crystals of CLVO have been grown by the top-seeded solution growth method. RLVO and CLVO exhibit strong second harmonic generation (SHG) effects (about 4 and 5 times that of KH2PO4, respectively) with a phase-matching behavior at 1.064??m, and a wide transparency range (0.33-6.0??m for CLVO). More importantly, RLVO and CLVO possess a high LDT value (~28?×?AgGaS2). In addition, the density functional theory (DFT) and dipole moments studies indicate that the VO4 anionic groups have a dominant contribution to the SHG effects in RLVO and CLVO. These results suggest that the title compounds are promising NLO candidate crystals applied in the Mid-IR region.
Project description:Combining high-throughput screening and machine learning models is a rapidly developed direction for the exploration of novel optoelectronic functional materials. Here, we employ random forests regression (RFR) model to investigate the second harmonic generation (SHG) coefficients of nonlinear optical crystals with distinct diamond-like (DL) structures. 61 DL structures in Inorganic Crystallographic Structure Database (ICSD) are selected, and four distinctive descriptors, including band gap, electronegativity, group volume and bond flexibility, are used to model and predict second-order nonlinearity. It is demonstrated that the RFR model has reached the first-principles calculation accuracy, and gives validated predictions for a variety of representative DL crystals. Additionally, this model shows promising applications to explore new crystal materials of quaternary DL system with superior mid-IR NLO performances. Two new potential NLO crystals, Li2CuPS4 with ultrawide bandgap and Cu2CdSnTe4 with giant SHG response, are identified by this model.
Project description:Mid-far infrared (IR) non-linear optical (NLO) materials are of great importance in military and civil fields. However, commercial IR-NLO crystals (such as AgGaS<sub>2</sub>, AgGaSe<sub>2</sub> and ZnGeP<sub>2</sub>) do not currently satisfy the requirements of large second-harmonic generation (SHG) and high laser induced damage thresholds (LIDTs), which seriously limits their practical applications. Herein, we have developed a new series of salt-inclusion chalcogenides, [A<sub>3</sub>X][Ga<sub>3</sub>PS<sub>8</sub>] (A = K, Rb; X = Cl, Br), which are constructed from alternate stacking of adamantane-like [Ga<sub>3</sub>PS<sub>10</sub>]<sup>6-</sup> cluster layers and cationic [A<sub>3</sub>X]<sup>2+</sup> salt layers. Importantly, they display both large SHG responses of several-fold and high LIDTs for dozens of times that of commercial AgGaS<sub>2</sub>, which exhibit the highest LIDTs among the reported IR-NLO materials with a larger SHG conversion efficiency than that of AgGaS<sub>2</sub>. These properties together with wide transparent region, type I phase-matching features and congruent-melting behaviors indicate they are promising IR-NLO materials.
Project description:First principles methods are used to explicitly calculate the nonlinear susceptibility (?<sup>(2)</sup>(2?, ?, ?)) representing the second harmonic generation (SHG) of two dimensional semiconducting materials, namely transition metal dichalcogenides (TMDs) and Boron Nitride (BN). It is found that alloying TMDs improves their second harmonic response, with MoTeS alloys exhibiting the highest of all hexagonal alloys at low photon energies. Moreover, careful examination of the relationship between the concentration of Se in Mo<sub>x</sub>Se<sub>y</sub>S<sub>z</sub> alloys shows that the SHG intensity can be tuned by modifying the stoichiometry. In addition, materials with curvature can have large second harmonic susceptibility. Of all the calculated monolayer structures, the hypothetical TMD Haeckelites NbSSe and Nb<sub>0.5</sub>Ta<sub>0.5</sub>S<sub>2</sub> exhibit the highest ?<sup>(2)</sup>, while one of the porous 3D structures constructed from 2D hBN exhibits a larger ?<sup>(2)</sup> than known large band gap 3-D materials.
Project description:We demonstrate the formation of uniform and oriented metal-organic frameworks using a combination of anion effects and surface chemistry. Subtle but significant morphological changes result from the nature of the coordinative counteranion of the following metal salts: NiX2 with X = Br-, Cl-, NO3-, and OAc-. Crystals could be obtained in solution or by template surface growth. The latter results in truncated crystals that resemble a half structure of the solution-grown ones. The oriented surface-bound metal-organic frameworks (sMOFs) are obtained via a one-step solvothermal approach rather than in a layer-by-layer approach. The MOFs are grown on Si/SiOx substrates modified with an organic monolayer or on glass substrates covered with a transparent conductive oxide (TCO). Regardless of the different morphologies, the crystallographic packing is nearly identical and is not affected by the type of anion or by solution versus the surface chemistry. A propeller-type arrangement of the nonchiral ligands around the metal center affords a chiral structure with two geometrically different helical channels in a 2:1 ratio with the same handedness. To demonstrate the accessibility and porosity of the macroscopically oriented channels, a chromophore (resorufin sodium salt) was successfully embedded into the channels of the crystals by diffusion from solution, resulting in fluorescent crystals. These "colored" crystals displayed polarized emission (red) with a high polarization ratio because of the alignment of these dyes imposed by the crystallographic structure. A second-harmonic generation (SHG) study revealed Kleinman symmetry-forbidden nonlinear optical properties. These surface-bound and oriented SHG-active MOFs have the potential for use as single nonlinear optical (NLO) devices.
Project description:A novel nonlinear optical (NLO) carbonatoperoxovanadate, Cs3VO(O2)2CO3, with an exceptionally high thermostability was successfully synthesized by introducing highly polarizable Cs+ cations and inorganic polydentate carbonate anions into asymmetric peroxovanadates. The structure of Cs3VO(O2)2CO3 is composed of distorted [VO(O2)2CO3]3- units and charge balancing Cs+ cations. The title compound exhibits the largest NLO intensity ever found in the current carbonate NLO materials, i.e., 23.0 times that of KH2PO4 (KDP). The remarkably strong second-harmonic generation (SHG) response originates from the synergistic effect of the exceedingly polarizable Cs+ cations, distortive polyhedra of the V5+ cation, delocalized ? orbitals in CO3 groups, and distorted localized ? orbitals in O2 groups. First-principles calculations indicated that introducing the polarizable cations into peroxovanadates not only induces the enhancement of the SHG response but also improves the thermal stability of the framework.
Project description:Second-order nonlinear optics (NLO) is the foundation of frequency conversion for the generation of coherent light at frequencies where lasers have no emissions or operate poorly. The prerequisite for NLO materials is noncentrosymmetric symmetry that can generate an effectively non-counterbalanced spontaneous electronic polarization. Here, we propose that this material restriction can be broadened by controlling the electron distribution with a local internal electrostatic field (IEF), and we demonstrate artificially created and manipulated second harmonic generation (SHG) in a centrosymmetric optical material, a superimposed Co2+- and Mo6+-doped BiVO4 thin film with 2/m point group symmetry, where a homojunction producing tunable effective polarization is formed. The SHG was characterized and tuned by IEF. This work breaks the structural symmetry constraint on NLO materials. Besides, the phase-matching-like condition was realized for the further improvement of the efficient frequency conversion. Because polarization is also a prerequisite for many other functions besides SHG, we believe that this work should provide some inspiration for the further development of optoelectronic, photonic, and electronic materials.
Project description:The first trivalent rare-earth iodate fluoride nonlinear optical (NLO) crystal, Y(IO3)2F (YIF), was successfully designed and synthesized, featuring polarization-favorable helical chains constructed from trans-YO6F2 polyhedra and IO3 groups. It exhibited a suitable balance of a wide transparency range of 0.26-10.0 ?m, high laser damage threshold (LDT) of 39.6 × AgGaS2, and moderate second harmonic generation (SHG) effect of 2 × KDP. A series of doped RE:YIF (RE = Pr, Nd, Dy, Ho, Er, Tm, and Yb) crystals were easily synthesized benefiting from the spring-shaped helix structure, which possess wide absorption and emission peaks as well as long lifetime, especially in the visible and near-infrared regions. Particularly, the remarkable fluorescence properties of Nd and Yb doped YIF crystals are comparable to and even better than those of traditional self-frequency doubling (SFD) crystals such as YAB, YCOB, and GdCOB. Thus, these RE-doped YIF crystals are promising laser SFD crystals. This work also indicated that constructing helical chains should be an effective strategy for the design of inorganic polar materials.
Project description:During our research into novel nonlinear optical materials using 1,10-phenanthroline as an appending ligand on lanthanide iodates, crystals of an infinite layered Dy(III) iodate compound, Dy(IO(3))(3)(H(2)O)·H(2)O, were obtained under hydro-thermal conditions. The Dy(III) cation has a dicapped trigonal prismatic coordination environment consisting of one water O atom and seven other O atoms from seven iodate anions. These iodate anions bridge the Dy(III) cations into a two-dimensional structure. Through O-H?O hydrogen bonds, all of these layers stack along , giving a supra-molecular channel, with the solvent water mol-ecules filling the voids.