Qun Jing

Cs3Zn6B9O21: a chemically benign member of the KBBF family exhibiting the largest second harmonic generation response.

Nonlinear optical (NLO) crystals are essential materials for generation of coherent UV light in solid state lasers. KBBF is the only material that can achieve coherent light below 200 nm by direct second harmonic generation (SHG). However, its strong layer habits and the high toxicity of the beryllium oxide powders required for synthesis limit its application. By substituting Be with Zn and connecting adjacent [Zn2BO3O2]∞ layers by B3O6 groups, a new UV nonlinear optical material, Cs3Zn6B9O21, was synthesized. It overcomes the processing limitations of KBBF and exhibits the largest SHG response in the KBBF family.

Pb2Ba3(BO3)3Cl: A Material with Large SHG Enhancement Activated by Pb-Chelated BO3 Groups

Pb(II) has long been associated with lone pair activity and is often substituted in alkali earth metal borates to create new nonlinear optical (NLO) materials with enhanced second harmonic generation (SHG) capabilities. However, large enhancement in isomorphic Pb-free analogues is rare. Here we report a new NLO material Pb2Ba3(BO3)3Cl with a phase-matching SHG response approximately 3.2× that of KDP and 6× higher than its isomorphic compound Ba5(BO3)3Cl. We show that the enhanced SHG response originates from a unique edge-sharing connection between lead-oxygen polyhedra and boron-oxygen groups, making the dielectric susceptibility more easily affected by the external electric field of an incident photon. This understanding provides a route to identify systems that would benefit from SHG-active cation substitution in isomorphic structures that exhibit weak or null SHG responses.

Simulated pressure-induced blue-shift of phase-matching region and nonlinear optical mechanism for K3B6O10X (X = Cl, Br)

Birefringence plays a great role in phase matching of the nonlinear optical (NLO) crystals. Small birefringence restricts various crystals from achieving deep-ultraviolet laser output although they exhibit short UV cutoff edges and high second-harmonic generation (SHG) intensities. An access to achieve deeper coherent light output through external pressure on NLO crystal, K3B6O10Cl is proposed and demonstrated through computer experiment based on the first principles theory. The “hot spot” in structure that determine the SHG effects and birefringence were highlighted. The shortest achievable phase-matching wavelengths are predicted based on calculated refractive indices. It is found that the quasi-planar (B6O10)2− group is the dominant contributing unit to optical anisotropy. The pressure-induced increase of polarizability anisotropy of (B6O10)2− group can notably enlarge birefringence which extends the shortest achievable wavelength of K3B6O10Cl frequency conversion. The results show that pressure engineering may be a promising scheme to overcome the drawback of small birefringence of some NLO crystals.

p–(p,π*) interaction mechanism revealing and accordingly designed new member in deep-ultraviolet NLO borates LinMn−1B2n−1O4n−2 (M = Cs/Rb, n = 3, 4, 6)

Exploration on the compounds in the complex alkali metal borate system had resulted in the discovery of a class of deep-ultraviolet second-order nonlinear optical (NLO) materials, including Li3Cs2B5O10 (L3CBO), Li4Cs3B7O14 (L4CBO), and Li6Rb5B11O22 (L6RBO), which can be reduced to a general formula of LinMn−1B2n−1O4n−2 (M = Cs/Rb, n = 3, 4, 6). All of the three crystals exhibit a short UV cutoff edge (below 190 nm) and share a class of topologically similar BO groups interconnected by LiOn (n = 4, 5), Rb/CsOn (n = 8, 9, 10). The mechanism of NLO properties of this class of crystals was studied using band-resolved and SHG-density methods based on the first-principles theory. The results reveal that the “charge-transfer excitation” from the non-bonding 2p occupied states of O atoms to the π* and 2p unoccupied states of the BO3 substructure in BO groups is the key mechanism of NLO properties of this material family. Through systematic analyses on the relationship between crystal structure and NLO effects, a new crystal, Li4Rb3B7O14, was designed and subsequently synthesized through solid state reaction, which is isomorphic with Li4Cs3B7O14 and exhibits a short UV cutoff edge (below 190 nm) and a SHG response of 2/3 × KDP.

Sr4B10O18(OH)2·2H2O: a new UV nonlinear optical material with a [B10O23]16− building block

A new ultraviolet (UV) nonlinear optical (NLO) crystal Sr4B10O18(OH)2·2H2O has been synthesized using the hydrothermal method. It crystallizes in the triclinic space group P1 (no. 1) with lattice constants a = 6.4991(6) A, b = 6.5059(6) A, c = 11.3175(11) A, α = 88.458(6)°, β = 86.019(5)°, γ = 61.351(5)°, Z = 1. The fundamental building block of the structure is a [B10O23]16− unit which consists of two stereo-isomeric configurations of [B5O12]9− groups. The structure exhibits a ∞[B10O18]6− three dimensional network with 9-membered tunnels along the b-axis where the Sr cation, OH anion and H2O molecule are located. Powder second harmonic generation (SHG) measurements indicate that Sr4B10O18(OH)2·2H2O is type-I phase-matchable and displays a SHG response of about 2 × KH2PO4. The UV-Vis-NIR diffuse reflectance spectrum shows that its cutoff edge is lower than 190 nm. Density functional theory (DFT) calculations on electronic structures and SHG coefficients confirm the experimental results.

Anomalous second-harmonic generation response in SrBPO5 and BaBPO5

In order to study the mechanism of the second harmonic generation (SHG) effect in borophosphates, two large ultraviolet nonlinear optical crystals, SrBPO5 and BaBPO5, have been successfully obtained using a top-seeded solution growth method. Optical and thermal properties have been measured, and SrBPO5 and BaBPO5 are shown to display large experimental gaps (6.44 eV and 6.14 eV, respectively) and moderate SHG coefficients (0.57 and 0.43 times that of KH2PO4). More importantly, based on a further understanding of the relationship between nonlinear optical properties and crystal structure within density functional theory (DFT), we have demonstrated that BO4 groups and metal cations play an essential role in the SHG effect of SrBPO5 and BaBPO5, but not PO4 groups as anticipated. Indeed, PO4 groups play a counteractive role in BaBPO5 due to the indirect influence of metal cations.

Theoretical perspective of the lone pair activity influence on band gap and SHG response of lead borates

Metal lone pairs play an important role in determining the SHG enhancement and bandgap red shift. In this work, the electronic structures and optical properties of a class of lead borates Pb2B5O9Cl, BaPb[B5O9(OH)]·H2O, and Ba2Pb(B3O6)2 have been investigated to uncover the influence of the lead atom on the band gap and SHG response. It is found that BaPb[B5O9(OH)]·H2O and Ba2Pb(B3O6)2 have little bandgap redshift due to their weak distortion and stereochemical activity. The intensity of the lead lone pair stereochemical activity plays an important role in determining the reduction of the band-gap.

A new 12L-hexagonal perovskite Cs4Mg3CaF12: structural transition derived from the partial substitution of Mg2+ with Ca2+

A new 12L-hexagonal perovskite Cs4Mg3CaF12 was synthesized using an open high-temperature solution method. It crystallizes in the trigonal space group Rm (no. 166) with lattice constants a = 6.2196(9) A, c = 29.812(9) A, Z = 3. Cs4Mg3CaF12 is changed from the cubic phase CsMgF3 by replacing 25% Mg2+ with Ca2+. The changes derived from the substitution and the structural comparison between Cs4Mg3CaF12 and other perovskites are discussed in this paper. Thermal analysis, infrared spectroscopy, and electronic structure calculations were performed on the reported material.

M3V2B10O23 (M = Ca, Sr): two new vanadoborates with [V2B10O23]∞ double layers

Two M3V2B10O23 (M = Ca, Sr) vanadoborates were synthesized by using a high-temperature solid-state technique for the first time. The single-crystal X-ray diffraction results showed that they both crystallized in the triclinic space group P, with unit cell parameters a = 6.525(3) A, b = 11.462(4) A, c = 12.714(5) A, α = 114.126(10)°, β = 103.656(10)°, γ = 90.594(10)° for Ca3V2B10O23 and a = 6.639(2) A, b = 11.571(4) A, c = 12.998(4) A, α = 114.333(3)°, β = 103.334(3)°, γ = 90.432(3)° for Sr3V2B10O23. Their main structural frameworks were observed to be novel “infinite” 2D [V2B10O23]∞ double layers composed of the [B5O9]∞ infinite layers and VO4 tetrahedra. Functional groups present in the M3V2B10O23 (M = Ca, Sr) samples were identified in their infrared spectra. Their thermal properties, UV-Vis-NIR diffuse-reflectance spectra and elements analyses are also presented.