Rational Design of Novel Polar Nonlinear Optical Materials in Alkali Metal Rare Earth Iodates.

Four new potassium rare earth iodates, namely, acentric K2Lu(IO3)5 and KM(IO3)4(HIO3)0.33 (M = Ce/Pr) and centric KLa(IO3)4, were successfully grown by mild hydrothermal reactions. Three of them exhibit polar structures; K2Lu(IO3)5, KCe(IO3)4(HIO3)0.33, and KPr(IO3)4(HIO3)0.33 show second-harmonic generation (SHG) responses of 3.0, 1.0, and 0.8 × KDP, respectively. These three iodates are phase-matchable for second-harmonic generation. The influence of changes in the radius and coordination mode of rare earth ions on the crystal structure and SHG response has been discussed in detail. Our findings suggest that in the alkali metal rare earth iodate, modulating the arrangement of iodate groups by changing the coordination geometry of rare earth ions is an effective strategy for designing polar NLO materials.

Polar-Layer-Driven Reconstruction of a Noncentrosymmetric Vandate Mid-Infrared Nonlinear-Optical Crystal.

Noncentrosymmetry (NCS) is essential for a second-harmonic nonlinear-optical (NLO) crystal; however, the tailored synthesis of NCS materials has historically posed significant challenges because the adjacent dipole moments of constituted NLO-active units in the structure tend to align in opposite directions, resulting in the NLO effect being canceled as in a centrosymmetric (CS) crystal. In this work, we propose a polar-layer-driven strategy, wherein a polarization-layered framework is constructed to constrain the dipole moment to align in the same direction, thereby facilitating the formation of the NCS structure. Taking the layered structure CS K2ZnV2O7 as a prototype compound, a novel vandate K4ZnV5O15Br (KZVB) was rationally synthesized via a multiple sites-oriented cosubstition method. KZVB containing two types of NLO-active units of V5+ d0 and Zn2+ d10 cations can be utilized as a mid-infrared NLO crystal with a remarkable NLO response comparable to that of nonoxide AgGaS2. This work not only broadens the effective strategy for designing novel NCS compounds but also provides a progressive development of NLO materials.

Polymorphism of Niobium Phosphate Bronze Na4Nb8P4O32 Deduced by the Packings of Structural Units: A Centrosymmetric Phase.

Herein, a new centrosymmetric phase Na4Nb8P4O32 (referred to as CS-Na4Nb8P4O32) was obtained by a molten salt method, which is a polymorph of niobium phosphate bronze Na4Nb8P4O32. CS-Na4Nb8P4O32 displays high structural similarity to the noncentrosymmetric Na4Nb8P4O32 phase (referred to as NCS-Na4Nb8P4O32): Distorted NbO6 octahedra are corner-coordinated to form ReO3-type layers, which are further joined together by isolated PO4 tetrahedra. However, two polymorphous phases adopt different packings of structural units, resulting in distinct symmetries. NbO3 layers and PO4 tetrahedra are reversely arranged along the crystallographic a direction in CS-Na4Nb8P4O32, thereby producing a centrosymmetric structure. The reverse packing cancels out all contributions of dipole moments originating from the distorted NbO6 octahedra; NCS-Na4Nb8P4O32 exhibits the C2-rotation distribution of NbO3 layers and PO4 tetrahedra, thus generating a noncentrosymmetric and polar structure. The C2-rotation packing of structural units brings a constructive addition of the dipole moments, further obtaining large calculated independent second harmonic generation susceptibilities. The study of structural evolution deduced by the packings of structural units in polymorphous Na4Nb8P4O32 might provide valuable insights into polymorphism and structural regulation.

Antithermal-Quenching All-Inorganic Perovskite Crystals with Green Ultralong Persistent Luminescence for Advanced Anticounterfeiting Applications.

Long persistent luminescence (PersL) materials have revolutionized many fields of optoelectronics and photonics due to their applications in anticounterfeiting, information encryption, and in vivo bioimaging. Here, we reported a novel PersL crystal prepared by the heterovalent doping of Sb3+ into perovskite tetragonal phase RbCdCl3, comparing with the pristine non-perovskite orthorhombic phase analogue without PersL property. Surprisingly, under the UV light irradiation, the title crystals concurrently exhibit green ultralong PersL (>2400 s), high photoluminescence quantum yield (49.1%), and antithermal quenching in the range from 148 to 328 K. It was revealed by experimental results and theoretical analyses that green ultralong PersL and antithermal quenching of perovskite-phase RbCdCl3/Sb3+ crystals originate from the electron transition between the 5s2 level of the dopant Sb3+ and the electronic defect-induced trap states. Enlightened by the excellent optical properties, the tetragonal perovskite-phase RbCdCl3/Sb3+ PersL materials show promising application prospects in anticounterfeiting and encryption of information.

Synthesis, Structure, and Magnetic Properties of Cyanurates RE5(C3N3O3)(OH)12 (RE = Gd-Lu): Cryogenic Magnetocaloric Candidate Gd5(C3N3O3)(OH)12.

Rare-earth (RE)-based frustrated magnets are fertile playgrounds for discovering exotic quantum phenomena and exploring adiabatic demagnetization refrigeration applications. Here, we report the synthesis, structure, and magnetic properties of a family of rare-earth cyanurates RE5(C3N3O3)(OH)12 (RE = Gd-Lu) with an acentric space group P6̅2m. Magnetic susceptibility χ(T) and isothermal magnetization M(H) measurements manifest that RE5(C3N3O3)(OH)12 (RE = Gd, Dy-Yb) compounds exhibit no magnetic ordering down to 2 K, while Tb5(C3N3O3)(OH)12 shows long-range magnetic ordering around 3.6 K. Among them, magnetically frustrated spin-7/2 Gd5(C3N3O3)(OH)12 shows long-range magnetic ordering around 1.25 K and a large magnetocaloric effect with a maximum magnetic entropy change ΔSm of up to 58.1 J kg-1 K-1 at ΔH = 7 T at liquid-helium temperature regimes.

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 and Luminescence Properties of Mn-doped MgAl2O4 Red-Emitting Phosphors with Varying Sintering Temperature.

Oxide matrix red-emitting phosphors are deemed as excellent color converters for white light emitting diodes (WLEDs) and laser diodes (LDs). Manganese-doped MgAl2O4 powder was synthesized by a solid-state reaction method at different sintering temperatures. Microstructure shows that grain size is mainly in the range of 0.2-5 µm, and grain agglomeration occurs with increased sintering temperature. XPS analysis indicates that the doped Mn ion exhibits a valence state of + 4 within the MgAl2O4 matrix. The diffraction peak of the phosphors is shifted by the sintering temperature, which affects lattice constant. Upon excitation by 300 nm ultraviolet light, the samples emit asymmetric broadband red light within the range of 620-720 nm, attributed to Mn4+ ion's transition from 2Eg to 4A2g states. With the increasing temperature, the main emission peak shifts from 677 nm to 650 nm, ascribed to the change in energy level (2Eg) resulting from the reduction of Al2O3 phase. Crystal field theory confirmed that Mn4+ ions are within a strong crystal field environment created by MgAl2O4 matrix. By affecting particle size and crystallinity, the sintering temperature influences the fluorescence lifetime of the Mn4+ ion. Notably, these red-emitting phosphors exhibits remarkable thermal stability as their emission intensity remains approximately at 58% of initial intensity even at elevated temperature (435 K). Consequently, Mn4+: MgAl2O4 red-emitting phosphors with high thermal stability render them promising candidates for WLED applications.

Ba1.09Pb0.91Be2(BO3)2F2: The First Pb-Containing Beryllium Borate Fluoride with Trigonal Prismatic PbO6 and 2D [Be3B3O6F3]∞ Layers.

Ba1.09Pb0.91Be2(BO3)2F2 (BPBBF), a previously unreported lead-containing beryllium borate fluoride, has been successfully grown through a high-temperature flux method. Its structure is solved by single-crystal X-ray diffraction (SC-XRD), and it is optically characterized via infrared, Raman, UV-vis-IR transmission, and polarizing spectra as well. SC-XRD data suggests that it can be indexed by a trigonal unit cell (space group P3m1) with lattice parameters a = 4.7478(6) Å, c = 8.3856(12) Å, Z = 1, and V = 163.70(5) Å. This material could be considered as a derivative of the Sr2Be2B2O7 (SBBO) structural motif. It consists of 2D [Be3B3O6F3]∞ layers in the crystallographic ab plane, with divalent Ba2+ or Pb2+ cations serving as spacers among the adjacent layers. Ba and Pb were found to adopt a disordered arrangement in the trigonal prismatic coordination within the BPBBF structural lattice, which is evidenced by both structural refinements against SC-XRD data and energy dispersive spectroscopy. The UV absorption edge (279.1 nm) and birefringence (Δn = 0.054@ 546.1 nm) of BPBBF are confirmed by UV-vis-IR transmission and polarizing spectra, respectively. The discovery of this previous unreported SBBO-type material, BPBBF, along with other reported analogues such as BaMBe2(BO3)2F2 (M = Ca, Mg, and Cd), provide a prodigious example for tuning the bandgap, birefringence, and short UV absorption edge via simple chemical substitution.

Atomic Substitution to Tune ScO6 Distortion in Ba2MSc2(BO3)4 (M = Na, K, Ba) to Acquire a Large Birefringence.

Replacing alkali metals (K, Na atoms) by an alkaline-earth metal (Ba atom), α-Ba3Sc2(BO3)4 (high-temperature phase) is successfully obtained by a molten salt method, taking Ba2K1.6Na0.4Sc2(BO3)4 as the parent template. Although both of them exhibit similar layered structures composed of ScO6 and BO3 units, α-Ba3Sc2(BO3)4 shows largely distorted ScO6 octahedra (Δd = 0.56) forced by the uniform tension of a larger space effect from BaO12 polyhedrons, rather than regular ScO6 octahedra like in Ba2K1.6Na0.4Sc2(BO3)4. Experimental measurements and calculated analyses elucidate that distorted ScO6 octahedra in α-Ba3Sc2(BO3)4, displaying a second-order Jahn-Teller (SOJT) effect, enlarge the experimental birefringence up to 0.14@550 nm, while Ba2K1.6Na0.4Sc2(BO3)4 with regular ScO6 octahedra only shows Δn = 0.11 under the same condition. In addition, other optical and thermal properties of the two title compounds were characterized. The experimental results indicate that Ba2K1.6Na0.4Sc2(BO3)4 and α-Ba3Sc2(BO3)4 are promising birefringent materials.

Antimony Doping Enabled Photoluminescence Quantum Yield Enhancement in 0D Inorganic Bismuth Halide Crystals.

Owing to the exterior self-trapped excitons (STEs) with adjustable fluorescence beams, low-dimensional ns2-metal halides have recently received considerable attention in solid-state light-emitting applications. However, the photoluminescence (PL) mechanism in metal halides remains a major challenge in achieving high efficiency and controllable PL properties because the excited-state energy of ns2 conformational ions varies inhomogeneously with their coordination environments. Here, a novel zero-dimensional (0D) lead-free bismuth-based Rb3BiCl6·0.5H2O crystal was reported as a pristine crystal to modulate the optical properties. By doping Sb3+ ions with 5s2 electrons into Rb3BiCl6·0.5H2O crystals, bright orange emission at room temperature was obtained with a photoluminescence quantum yield of 39.7%. Optical characterizations and theoretical studies show that the Sb3+ doping can suppress the strong exciton-phonon coupling, optimize the electronic energy band structure, improve the thermal activation energy, soften the structural lattice of the host crystals, deepen the STE states, and ultimately lead to strong photoluminescence. This work manifests a fruitful manipulation in ripening bismuth-based halides with high-efficiency PL properties, and the PL enhancement mechanisms will guide future research in the exploration of emerging luminescent materials.

Efficient Synthesis and In Vitro Hypoglycemic Activity of Rare Apigenin Glycosylation Derivatives.

Apigenin is a natural flavonoid with significant biological activity, but poor solubility in water and low bioavailability limits its use in the food and pharmaceutical industries. In this paper, apigenin-7-O-ß-(6″-O)-d-glucoside (AG) and apigenin-7-O-ß-(6″-O-succinyl)-d-glucoside (SAG), rare apigenin glycosyl and succinyl derivatives formed by the organic solvent-tolerant bacteria Bacillus licheniformis WNJ02 were used in a 10.0% DMSO (v/v) system. The water solubility of SAG was 174 times that of apigenin, which solved the application problem. In the biotransformation reaction, the conversion rate of apigenin (1.0 g/L) was 100% at 24 h, and the yield of SAG was 94.2%. Molecular docking showed that the hypoglycemic activity of apigenin, apigenin-7-glucosides (AG), and SAG was mediated by binding with amino acids of α-glucosidase. The molecular docking results were verified by an in vitro anti-α-glucosidase assay and glucose consumption assay of active compounds. SAG had significant anti-α-glucosidase activity, with an IC50 of 0.485 mM and enhanced glucose consumption in HepG2 cells, which make it an excellent α-glucosidase inhibitor.

Perfectly Encoding π-Conjugated Anions in the RE5 (C3 N3 O3 )(OH)12 (RE=Y, Yb, Lu) Family with Strong Second Harmonic Generation Response and Balanced Birefringence.

Nonlinear optical (NLO) crystal, which simultaneously exhibits strong second-harmonic-generation (SHG) response and desired optical anisotropy, is a core optical material accessible to the modern optoelectronics. Accompanied by strong SHG effect in a NLO crystal, a contradictory problem of overlarge birefringence is ignored, leading to low frequency doubling efficiency and poor beam quality. Herein, a series of rare earth cyanurates RE5 (C3 N3 O3 )(OH)12 (RE=Y, Yb, Lu) were successfully characterized by 3D electron diffraction technique. Based on a "three birds with one stone" strategy, they enable the simultaneous fulfillment of strong SHG responses (2.5-4.2× KH2 PO4 ), short UV cutoff (ca. 220 nm) and applicable birefringence (ca. 0.15 at 800 nm) by the introduction of rare earth coordination control of π-conjugated (C3 N3 O3 )3- anions. These findings provide high-performance short-wavelength NLO materials and highlight the exploration of cyanurates as a new research area.

Low loss modified Bezier bend waveguide.

We proposed and experimentally demonstrated a low loss modified Bezier bend for silicon and silicon nitride photonic integrated circuits. Both simulation and experimental results confirm that the modified Bezier bend can effectively reduce the bend loss for silicon and silicon nitride platform. At a bend radius of 1 µm, the reduction of bend loss from 0.367 dB/90° of circular bend and 0.35 dB/90° of traditional Bezier bend to 0.117 dB/90° of modified Bezier bend for silicon platform was experimentally demonstrated. For a 12-µm radius silicon nitride bend, the bend loss reduction from 0.65 dB/90° of circular bend and 0.575 dB/90° of traditional Bezier bend to 0.32 dB/90° was achieved. The proposed modified Bezier bend design can also be applied to other material systems, such as InP, LN, GaAs, etc., to effectively reduce the bend waveguide loss.

Mode selection in InGaAs/InGaAsP quantum well photonic crystal lasers based on coupled double-heterostructure cavities.

Photonic crystal lasers with a high-Q factor and small mode volume are ideal light sources for on-chip nano-photonic integration. Due to the submicron size of their active region, it is usually difficult to achieve high output power and single-mode lasing at the same time. In this work, we demonstrate well-selected single-mode lasing in a line-defect photonic crystal cavity by coupling it to the high-Q modes of a short double-heterostructure photonic crystal cavity. One of the FP-like modes of the line-defect cavity can be selected to lase by thermo-optically tuning the high-Q mode of the short cavity into resonance. Six FP-like modes are successively tuned into lasing with side mode suppression ratios all exceeding 15 dB. Furthermore, we show a continuous wavelength tunability of about 10 nm from all the selected modes. The coupled cavity system provides a remarkable platform to explore the rich laser physics through the spatial modulation of vacuum electromagnetic field at submicron scale.

PT symmetric single-mode line-defect photonic crystal lasers with asymmetric loss design.

The exploration of parity-time (PT) symmetry in micro-/nano-cavity lasers has recently gained immense research interest. The PT symmetric phase transition to single-mode lasing has been achieved by arranging the spatial distribution of optical gain and loss in single or coupled cavity systems. In terms of photonic crystal (PhC) lasers, a non-uniform pumping scheme is usually employed to enter the PT symmetry-breaking phase in a longitudinal PT symmetric system. Instead, we use a uniform pumping scheme to enable the PT symmetric transition to the desired single lasing mode in line-defect PhC cavities based on a simple design with asymmetric optical loss. The flexible control of gain-loss contrast is realized by removing a few rows of air holes in PhCs. We obtain single-mode lasing with a side mode suppression ratio (SMSR) of around 30 dB without affecting the threshold pump power and linewidth. The output power of the desired mode is six times higher than that in multimode lasing. This simple approach enables single-mode PhC lasers without sacrificing the output power, threshold pump power, and linewidth of a multimode cavity design.

Synthesis, Structure Determination, and Characterizations of a Polar Salt-Inclusion Scandium Germanate, Rb10Li3Sc4Ge12O36F.

A noncentrosymmetric salt-inclusion germanate, Rb10Li3Sc4Ge12O36F, was grown through spontaneous crystallization from a LiF-RbF flux. It crystallizes in the polar space group P31c with cell parameters of a = 10.7587(3) Å, c = 21.6691(10) Å, and Z = 2. Its structure features a complex 3D framework composed of helical [Ge4O12] chains from condensed [GeO4] tetrahedra running along the c axis, which are interconnected by the [ScO6] octahedra. Voids of the 3D net are filled with Rb+ ions, Li+ ions, and isolated trigonal-bipyramidal [Rb3Li2F] superalkali clusters. The title compound has a large band gap of 5.6 eV, a moderate powder second-harmonic-generation response of 0.9KDP, and an extremely small birefringence of 0.001, as was further unraveled by theoretical calculations.

Realization of Enlarged Birefringence from BaCdBe2(BO3)2F2 to NaMgBe2(BO3)2F via the Cation Size Effect as a Potential Deep-Ultraviolet Birefringent Material.

Birefringence, as one of the most important factors for birefringent materials, governs their performances in applications. In this study, two previously unreported beryllium borates, BaCdBe2(BO3)2F2 (BDBBF) and NaMgBe2(BO3)2F (NMBBF), have been rationally designed by modulating interstitial cations. When smaller sizes of the cations are used, the crystal structure of NMBBF exhibits closer-packed 2D [Be6B6O12F3]∞ double layers rather than the 2D [Be3B3O6F3]∞ single layers in the crystal structure of BDBBF. The ultraviolet (UV) transmittance spectrum indicates that the short UV absorption edges of BDBBF and NMBBF are below 200 nm. The results from both theoretical calculations (theo.) and experimental characterizations (exp.) reveal enlarged birefringence from BDBBF (0.067 at 589 nm from theo. and 0.059 at 546.1 nm from exp.) to NMBBF (0.078 at 589 nm from theo. and 0.081 at 546.1 nm from exp.). Because of its excellent structure-based optical properties, NMBBF has the potential to be a deep-UV birefringent material. Our structural comparison and discussion provide a scope to aid in the design of potential deep-UV birefringent materials with large birefringence.

Design and analysis of an InGaAs/InGaAsP quantum well microlaser with longitudinal periodical strain distribution for single-mode lasing.

Single-mode lasing for small size semiconductor laser is significantly important in the on-chip optical signal processing, data storage, and dense optical integrated systems. This paper presents new, to the best of our knowledge, single-mode quantum well microlasers by distributing periodical strain along the longitudinal laser cavity. The quantum transmission line modeling (Q-TLM) method is employed to establish the model for strained microlasers. The dynamic output of quantum well microlasers with longitudinal periodical strain (LPS) distribution is analyzed in the time and frequency domains, and it is found that the introduction of LPS significantly improves the single-mode output of quantum well microlasers by increasing the side mode suppression ratio (SMSR) from 8.44 to 28.29 dB. The study results confirm that well-controlled periodical strain along the longitudinal laser cavity provides an alternative routine for realizing single-mode lasing by strain engineering.

Ca3(TeO3)2(MO4) (M = Mo, W): Mid-Infrared Nonlinear Optical Tellurates with Ultrawide Transparency Ranges and Superhigh Laser-Induced Damage Thresholds.

Intense interests in mid-infrared (MIR) nonlinear optical (NLO) crystals have erupted in recent years due to the development of optoelectronic applications ranging from remote monitoring to molecular spectroscopy. Here, two polar crystals Ca3(TeO3)2(MO4) (M = Mo, W) were grown from TeO2-MO3 flux by high-temperature solution methods. Ca3(TeO3)2(MoO4) and Ca3(TeO3)2(WO4) are isostructural, which feature novel structures consisting of asymmetric MO4 tetrahedra and TeO3 trigonal pyramids. Optical characterizations show that both crystals display ultrawide transparency ranges (279 nm to 5.78 µm and 290 nm to 5.62 µm), especially high optical transmittance over 80% in the important atmospheric transparent window of 3-5 µm, and superhigh laser damage thresholds (1.63 GW/cm2 and 1.50 GW/cm2), 54.3 and 50 times larger than that of state-of-the-art MIR NLO AgGaS2, respectively. Notably, they exhibit the widest band gaps and the loftiest laser-induced threshold damages among the reported tellurates so far. Moreover, Ca3(TeO3)2(MO4) exhibit type I phase matching at two working wavelengths owing to their large birefringence and strong second-harmonic generation responses from the distorted anions, as further elucidated by the first-principles calculations. The above characteristics indicate that Ca3(TeO3)2(MO4) crystals are high-performance MIR NLO materials, especially applying in high-power MIR laser operations.

Widely tunable long-wave infrared difference frequency generation with a BaGa4Se7 crystal.

We report an experimental study of long-wave infrared difference frequency generation based on BaGa4Se7 crystal. The sources of two input wavelengths were the fundamental output of a Nd:YAG laser and its second-harmonic pumped ∼1.2µmKTiOPO4 optical parametric oscillator. A wide tuning range of 7.9-17.5 µm (>1.14 octave) was achieved, which reached the upper limit of the BaGa4Se7 transparency region. The spectra and pulse widths, input-output relationship, beam profile, wavelength tolerance, and angular acceptance of the phase-matching were characterized in detail. This presented coherent source can potentially be applied in multiple gas analyses and spectral imaging.