Recent progress on new infrared nonlinear optical materials with application prospect

K2HgGe3Se8: A Quaternary Hg-Based Selenide with Nonlinear Optical Properties

A new quaternary Hg-based selenide K2HgGe3Se8 was successfully synthesized and characterized as an engaging infrared (IR) nonlinear optical (NLO) material, which crystallizes in the P21 space group of the noncentrosymmetric monoclinic crystal system. The crystal structure of K2HgGe3Se8 is characterized by the stacking of countless [ H g G e 3 S e 8 ] 2 - ∞ 2 layers with K+ cations filling the interlayers. The UV-vis-NIR diffuse reflectance spectrum shows that K2HgGe3Se8 possesses an indirect band gap of 2.13 eV. The second harmonic generation (SHG) response of K2HgGe3Se8 is approximately 1.1 times that of AgGaS2 (AGS) within the particle size of 20-50 μm along with a nonphase-matching (NPM) behavior at 2090 nm, as indicated by the SHG test. K2HgGe3Se8 also has a large powder laser-induced damage threshold (∼ 2.3 × AGS). As revealed by theoretical calculations, the optical properties of K2HgGe3Se8 are predominantly determined by [HgSe4] and [GeSe4] tetrahedra, and K2HgGe3Se8 possesses a calculated optical band gap of 1.44 eV together with a maximum SHG coefficient d22 of -7.70 pm V-1.

Sr2HgGe2OS6: A Hg-Based Oxychalcogenide Infrared Nonlinear Optical Material Exhibiting Favorable Balance between a Large Band Gap and Strong Second Harmonic Generation Response

Currently, oxychalcogenides with mixed-anion groups that integrate the property advantages of oxides (wide optical band gap) and chalcogenides [strong second harmonic generation (SHG) response] through chemical substitution engineering have attracted widespread interest and are considered to be important candidates for infrared (IR) nonlinear optical (NLO) materials. Herein, the first Hg-based oxychalcogenide Sr2HgGe2OS6 with mixed anion [GeOS3] units has been successfully synthesized through a spontaneous crystallization method, which exhibits a favorable balance between the strong SHG response (0.7 × AgGaS2) and large optical band gap (2.9 eV). In addition, Sr2HgGe2OS6 shows high laser-induced damage threshold (LIDT, 2.1 × AgGaS2) as well as phase-matching (PM) performance. Theoretical calculations indicate that the Sr2HgGe2OS6 encompasses large birefringence of 0.128@2090 nm (3.3 × AgGaS2) and its SHG density mainly comes from [HgS4] tetrahedra and [GeOS3] units. This work not only demonstrates that Sr2HgGe2OS6 is a promising IR NLO material but also provides new ideas for the exploration of Hg-based oxychalcogenide IR NLO materials.

Theoretically designed M@diaza[2.2.2]cryptand complexes: the role of non-covalent interactions in promoting NLO properties of organic electrides

Organic excess electron compounds with significant nonlinear optical (NLO) properties are widely employed in optoelectronic applications. Herein, single-alkali metals with diaza[2.2.2] cryptand (M@crypt,M=Li, Na, and K) are investigated for optoelectronic and NLO properties by using the density functional theory. Thermodynamic and kinetic stabilities of present complexes are computed through interaction energy (Eint) and ab-initio molecular dynamic (AIMD) simulations. M@crypt complexes carry excess electrons and mimic molecular electrides. Quantum theory of atoms in molecules (QTAIM) analysis and reduced density gradient (RDG) spectra demonstrate the roles of the weak van der Waals (vdW) interactions between metal and complexant. The remarkable hyperpolarizability (βo) value up to 1.41 × 106 au may be credited to the presence of loosely bound excess electrons. The hyper Rayleigh scattering hyperpolarizability (βHRS) is recorded up to 1.31 × 106 au for the K@crypt. Furthermore, frequency-dependent first-order and second-order hyperpolarizability is more prominent at the applied frequency of ω = 0.042823 au. The electron localizing function (ELF) and localized orbital locator (LOL) analysis further disclose the nature of interaction between alkali metal and complexant. The TD-DFT method is adopted to get excited state parameters and absorbance properties. An electron density difference map (EDDM) is exploited to evaluate the orbital contributions in excited states. Hence, the studied electride may become a promising candidate for NLO materials. We anticipate that the present work will provide insight into further development of molecular electride for optoelectronic applications.

Breaking Through the Trade-Off Between Wide Band Gap and Large SHG Coefficient in Mercury-Based Chalcogenides for IR Nonlinear Optical Application

It is substantially challenging for non-centrosymmetric (NCS) Hg-based chalcogenides for infrared nonlinear optical (IR-NLO) applications to realize wide band gap (Eg > 3.0 eV) and sufficient phase-matching (PM) second-harmonic-generation intensity (deff > 1.0 × benchmark AgGaS2 ) simultaneously due to the inherent incompatibility. To address this issue, this work presents a diagonal synergetic substitution strategy for creating two new NCS quaternary Hg-based chalcogenides, AEHgGeS4 (AE = Sr and Ba), based on the centrosymmetric (CS) AEIn2 S4 . The derived AEHgGeS4 displays excellent NLO properties such as a wide Eg (≈3.04-3.07 eV), large PM deff (≈2.2-3.0 × AgGaS2 ), ultra-high laser-induced damage threshold (≈14.8-15 × AgGaS2 ), and suitable Δn (≈0.19-0.24@2050 nm), making them highly promising candidates for IR-NLO applications. Importantly, such excellent second-order NLO properties are primarily attributed to the synergistic combination of tetrahedral [HgS4 ] and [GeS4 ] functional primitives, as supported by detailed theoretical calculations. This study reports the first two NCS Hg-based materials with well-balanced comprehensive properties (i.e., Eg > 3.0 eV and deff > 1.0 × benchmark AgGaS2 ) and puts forward a new design avenue for the construction of more efficient IR-NLO candidates.

Giant Mid-Infrared Second-Harmonic Generation Response in a Densely-Stacked Van Der Waals Transition-Metal Oxychloride

Second-harmonic generation (SHG) is a fundamental optical property of nonlinear optical (NLO) crystals. Thus far, it has proved difficult to engineer large SHG responses, particularly in the mid-infrared region, owing to the difficulty in simultaneously controlling the arrangement and density of functional NLO-active units. Herein, a new assembly strategy employing functional modules only, and aimed at maximizing the density and optimizing the spatial arrangement of highly efficient functional modules, has been applied to the preparation of NLO crystals, affording the van der Waals crystal MoO2 Cl2 . This exhibits the strongest powder SHG response (2.1×KTiOPO4 (KTP) @ 2100 nm) for a transition-metal oxyhalide, a wide optical transparency window, and a sufficient birefringence. MoO2 Cl2 is the first SHG-active transition-metal oxyhalide effective in the infrared region. Theoretical studies and crystal structure analysis suggest that the densely packed, optimally-aligned [MoO4 Cl2 ] modules within the two-dimensional van der Waals layers are responsible for the giant SHG response.

Three-in-One Strategy Constructing the First High-Performance Nonlinear Optical Sulfide Crystallizing with the P43 Space Group

The balance between large nonlinear optical (NLO) effect and wide bandgap is the key scientific issue for the exploration of infrared NLO materials. Targeting this issue, two new pentanary chalcogenides KGaGe1.37 Sn0.63 S6 (1) and KGaGe1.37 Sn0.63 Se6 (2) are obtained by the three-in-one strategy, viz. three types of fourfold-coordinated metal elements co-occupying the same site. They crystallize in the tetragonal P43 (1) and monoclinic Cc (2) space group. Their structures can be evolved from benchmark AgGaS2 (AGS) by suitable substitution. Remarkably, 1 is the first NLO sulfide crystallizing with the P43 space group, representing a new structure-type NLO material. The structural relationship between 1 and 2 and the evolution from 1, 2 to AGS are also analyzed. Both 1 and 2 show balanced NLO properties. Specifically, 1 exhibits phase-matchable SHG response of 0.6 × AGS, a wide bandgap of 3.50 eV, and a high laser damage threshold of 6.24 × AGS. Theoretical calculation results suggest that the Ga/Ge/Sn element ratios of the co-occupied sites of 1 and 2 are the most appropriate for stabilizing the structures. The strategy adopted here will provide some inspiration for exploring new high-performance NLO materials.

A BaGa4Se7 crystal based pulsed mid-infrared light source with a narrow linewidth in 4-12 µm

We present a BaGa4Se7 (BGSe) crystal based coherent pulsed light source for high resolution mid-infrared (MIR) spectroscopy in the 4-12 µm region. The all-solid-state system consists of an injection seeded optical parametric generator (OPG) and an optical parametric amplifier (OPA) using two KTiOPO4 crystals. The idler output of OPG-OPA and the fundamental output (1064 nm) of a wavelength stabilized Nd:YAG laser are employed for difference frequency generation of MIR pulses in the BGSe crystal. Pulsed MIR radiation in the 4-12 µm range is obtained with typical pulse energies higher than 100 µJ and pulse durations of ∼5 ns. By measuring H2O absorption lines in the 8 µm region with this MIR light source and a cavity ring-down spectrometer, the linewidth of the MIR source is inferred as 120 ± 10 MHz, which is very close to the Fourier-transform limited linewidth of 5 ns laser pulses.

Structure and Optical Properties of LixAg1-xGaSe2 and LixAg1-xInSe2

Complete substitution of Li atoms for Ag atoms in AgGaSe₂ and AgInSe₂ was achieved, resulting in the solid solutions Li_xAg_1-xGaSe₂ and Li_xAg_1-xInSe₂. The detailed crystal structures were determined by single-crystal X-ray diffraction and solid-state ⁷Li nuclear magnetic resonance spectroscopy, which confirm that Li atoms occupy unique sites and disorder only with Ag atoms. The tetragonal CuFeS₂-type structure (space group I4̅2d) was retained within the entirety of the Ga-containing solid solution Li_xAg_1-xGaSe₂, which is noteworthy because the end-member LiGaSe₂ normally adopts the orthorhombic β-NaFeO₂-type structure (space group Pna2₁). These structures are closely related, being superstructures of the cubic sphalerite and hexagonal wurtzite prototypes adopted by diamond-like semiconductors. For the In-containing solid solution Li_xAg_1-xInSe₂, the structure transforms from the tetragonal to orthorhombic forms as the Li content increases past x = 0.50. The optical band gaps increase gradually with higher Li content, from 1.8 to 3.4 eV in Li_xAg_1-xGaSe₂ and from 1.2 to 2.5 eV in Li_xAg_1-xInSe₂, enabling control to desired values, while the second harmonic generation responses become stronger or are similar to those of benchmark infrared nonlinear optical materials such as AgGaS₂. All members of these solid solutions remain congruently melting at accessible temperatures between 800 and 900 °C. Electronic structure calculations support the linear trends seen in the optical band gaps and confirm the mostly ionic character present in Li-Se bonds, in contrast to the more covalent character in Ga-Se or In-Se bonds.

Bridging oxygen atoms in trigonal prism units driven strong second-harmonic-generation efficiency in Sr3Ge2O4Te3

Herein, a novel IR NLO oxytelluride Sr₃Ge₂O₄Te₃ was successfully designed and synthesized through a "partial O-to-Te substitution" strategy. Compared with the parent oxide, Sr₃Ge₂O₄Te₃ not only successfully achieves a phase-matchability transition (from NPM to PM), but also greatly improves the linear and NLO performances, including a wide band gap (2.26 eV), strong SHG response (1.3 × AgGaSe₂) and large optical anisotropy (Δn = 0.152@2090 nm). The analyses of the structure-property relationship and SHG-density indicate that the bridging oxygen in the [O₃Ge-O-GeTe₃] prism unit plays the most important role in the multiplication SHG effect. This work provides some insights into the design and exploration of novel IR NLO materials.

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.

Metal oxyhalides: an emerging family of nonlinear optical materials

Second-order nonlinear optical (NLO) materials have drawn enormous academic and technological attention attributable to their indispensable role in laser frequency conversion and other greatly facilitated applications. The exploration of new NLO materials with high performances thus has long been an intriguing research field for chemists and material scientists. However, an ideal NLO material should simultaneously satisfy quite a few fundamental yet rigorous criteria including a noncentrosymmetric structure, large NLO coefficients, desired transparent range, large birefringence, high laser damage threshold, and availability of a large-size single crystal. Therefore, the identification of promising compound systems, targeted design, and experience-based syntheses are crucial to discover novel NLO materials working in the spectral region of interest. As an important family of mixed-anion compounds, versatile metal oxyhalides containing metal-centered oxyhalide functional units ([MO m X n ] (X = F, Cl, Br, and I)) are becoming a marvelous branch for interesting NLO materials. Especially, when the central metals are d0/d10 transition metals or heavy post-transition metals, a number of novel NLO materials with superior functionalities are expected. Our thorough review on the recent achievements of metal oxyhalides for NLO materials are divided into the fast-growing NLO metal oxyhalides with single type halogen anions and the newly identified NLO metal oxyhalides with mixed halogen anions. Here we mainly focus on the design strategy, structural chemistry, NLO-related properties, and structure-property correlation of the metal oxyhalides with relatively large NLO responses. We hope this review can provide an insight on the rational design and future development of emerging metal oxyhalides for NLO and other applications.

Ba2HgTe5: a Hg-based telluride with giant birefringence induced by linear [HgTe2] units

Ba2HgTe5, the first Hg-based telluride birefringent material, was successfully synthesized. The analysis of the response electron distribution anisotropy illustrates that the large birefringence of Ba2HgTe5 originates from the linear [HgTe2] unit.

Ba6In2Ge2Te15: A THz Birefringent Material with an Intriguing Quasi-[Te5]4- Chain Possessing Giant Optical Anisotropy and Ultrawide Transmission Range

Exploring mid- and far-infrared (MFIR) and even terahertz (THz) birefringent crystal with a large optical anisotropy remains a great challenge. Herein, a novel THz birefringent material Ba6In2Ge2Te15, which contains the...

KCs2Pb2(HCOO)2Cl5: A Lead Formate with Strong Second-Harmonic-Generation Response Obtained by an Anionic Substitution

Nonlinear optical (NLO) materials are playing an increasingly vital role in laser science and technology. Herein, we report a new lead formate, KCs₂Pb₂(HCOO)₂Cl₅, successfully synthesized by using mild mix-solvothermal methods. By the anionic substitution of a [CH₃COO] group with a [HCOO] group, the dipole moment of the distorted [PbCl₄O₂] polyhedron is increased. In contrast, polarization from the anionic group and the volume of the unit cell are reduced. Hence, KCs₂Pb₂(HCOO)₂Cl₅ possesses a strong second-harmonic-generation (SHG) response of 4.2 times that of KH₂PO₄ (KDP), a transparent window covering 0.43-1.87 μm, a large band gap of 3.52 eV, and moderate birefringence (0.11@1064 nm). Systematic theoretical calculations illustrate the origin of the linear and nonlinear optical properties of KCs₂Pb₂(HCOO)₂Cl₅. This research provides a viable strategy for the design and synthesis of new NLO crystals.

Synthesis-enabled exploration of chiral and polar multivalent quaternary sulfides

An innovative method of synthesis is reported for the large and diverse (RE)6(TM) x (Tt)2S14 (RE = rare-earth, TM = transition metals, Tt = Si, Ge, and Sn) family of compounds (∼1000 members, ∼325 contain Si), crystallizing in the noncentrosymmetric, chiral, and polar P63 space group. Traditional synthesis of such phases involves the annealing of elements or binary sulfides at elevated temperatures. The atomic mixing of refractory components technique, presented here, allows the synthesis of known members and vastly expands the family to nearly the entire transition metal block, including 3d, 4d, and 5d TMs with oxidation states ranging from 1+ to 4+. Arc-melting of the RE, TM, and tetrel elements of choice forms an atomically-mixed precursor, which readily reacts with sulfur providing bulk powders and large single crystals of the target quaternary sulfides. Detailed in situ and ex situ experiments show the mechanism of formation, which involves multiphase binary sulfide intermediates. Crystal structures and metal oxidation states were corroborated by a combination of single crystal X-ray diffraction, elemental analysis, EPR, NMR, and SQUID magnetometry. The potential of La6(TM) x (Tt)2S14 compounds for non-linear optical applications was also demonstrated.

Structure Tuning, Strong Second Harmonic Generation Response, and High Optical Stability of the Polar Semiconductors Na1-xKxAsQ2

The mixed cation compounds Na_1-xK_xAsSe₂ (x = 0.8, 0.65, 0.5) and Na_0.1K_0.9AsS₂ crystallize in the polar noncentrosymmetric space group Cc. The AAsQ₂ (A = alkali metals, Q = S, Se) family features one-dimensional (1D) ¹/_∞[AQ₂^-] chains comprising corner-sharing pyramidal AQ₃ units in which the packing of these chains is dependent on the alkali metals. The parallel ¹/_∞[AQ₂^-] chains interact via short As···Se contacts, which increase in length when the fraction of K atoms is increased. The increase in the As···Se interchain distance increases the band gap from 1.75 eV in γ-NaAsSe₂ to 2.01 eV in Na_0.35K_0.65AsSe₂, 2.07 eV in Na_0.2K_0.8AsSe₂, and 2.18 eV in Na_0.1K_0.9AsS₂. The Na_1-xK_xAsSe₂ (x = 0.8, 0.65) compounds melt congruently at approximately 316 °C. Wavelength-dependent second harmonic generation (SHG) measurements on powder samples of Na_1-xK_xAsSe₂ (x = 0.8, 0.65, 0.5) and Na_0.1K_0.9AsS₂ suggest that Na_0.2K_0.8AsSe₂ and Na_0.1K_0.9AsS₂ have the highest SHG response and exhibit significantly higher laser-induced damage thresholds (LIDTs). Theoretical SHG calculations on Na_0.5K_0.5AsSe₂ confirm its SHG response with the highest value of d₃₃ = 22.5 pm/V (χ₃₃₃⁽²⁾ = 45.0 pm/V). The effective nonlinearity for a randomly oriented powder is calculated to be d_eff = 18.9 pm/V (χ_eff⁽²⁾ = 37.8 pm/V), which is consistent with the experimentally obtained value of d_eff = 16.5 pm/V (χ_eff⁽²⁾ = 33.0 pm/V). Three-photon absorption is the dominant mechanism for the optical breakdown of the compounds under intense excitation at 1580 nm, with Na_0.2K_0.8AsSe₂ exhibiting the highest stability.

Synthesis and Characterizations of Two Tellurides β-BaGa2Te4 and Ba5Ga2Ge3Te12 with Flexible Chain Structure

Demands for IR birefringent materials are increasing due to the rapid developments of IR laser applications. Herein, two new chain tellurides β-BaGa₂Te₄ and Ba₅Ga₂Ge₃Te₁₂ have been discovered. β-BaGa₂Te₄ crystallizes in the orthorhombic space group Imma (no. 74) with unit cell constants of a = 23.813(3) Å, b = 11.9673(19) Å, and c = 6.7215(9) Å, while Ba₅Ga₂Ge₃Te₁₂ crystallizes in the monoclinic space group P2₁/c (no. 14) with unit cell constants of a = 13.6540(3) Å, b = 9.6705(2) Å, and c = 23.1134(7) Å. The structure of β-BaGa₂Te₄ can be considered to be the antiparallel arrangement of one-dimensional (1D) [GaTe₂] chains formed by edge-sharing GaTe₄ tetrahedra, which are separated by Ba²⁺ cations. In the crystal structure of Ba₅Ga₂Ge₃Te₁₂, two kinds of 1D chains, namely chain 1 _∞¹[(GaGe)₃Te₈] and chain 2 _∞¹[(GaGe)₂Te₄], are stacked alternately and put together by the coulomb force with Ba²⁺ cations. In addition, First-principles calculations indicate that β-BaGa₂Te₄ has a large birefringence, ∼0.325 at 2050 nm, derived from the superposition of the polarizabilities of GaTe₄ tetrahedra, implying that it has potential as an IR birefringent material. This work may provide some guidance for exploring new IR birefringent crystals.

Investigation of Mid-Infrared Broadband Second-Harmonic Generation in Non-Oxide Nonlinear Optic Crystals

The mid-infrared (mid-IR) continuum generation based on broadband second harmonic generation (SHG) (or difference frequency generation) is of great interest in a wide range of applications such as free space communications, environmental monitoring, thermal imaging, high-sensitivity metrology, gas sensing, and molecular fingerprint spectroscopy. The second-order nonlinear optic (NLO) crystals have been spotlighted as a material platform for converting the wavelengths of existing lasers into the mid-IR spectral region or for realizing tunable lasers. In particular, the spectral coverage could be extended to ~19 µm with non-oxide NLO crystals. In this paper, we theoretically and numerically investigated the broadband SHG properties of non-oxide mid-IR crystals in three categories: chalcopyrite semiconductors, defect chalcopyrite, and orthorhombic ternary chalcogenides. The technique is based on group velocity matching between interacting waves in addition to birefringent phase matching. We will describe broadband SHG characteristics in terms of beam propagation directions, spectral positions of resonance, effective nonlinearities, spatial walk-offs between interacting beams, and spectral bandwidths. The results will show that the spectral bandwidths of the fundamental wave allowed for broadband SHG to reach several hundreds of nm. The corresponding SH spectral range spans from 1758.58 to 4737.18 nm in the non-oxide crystals considered in this study. Such broadband SHG using short pulse trains can potentially be applied to frequency up-conversion imaging in the mid-IR region, in information transmission, and in nonlinear optical signal processing.

Investigation of the Anisotropic Thermal Expansion Mechanism of AgxGaxGe1-xSe2 Crystals

Quaternary nonlinear optical single crystals Ag_xGa_xGe_1-xSe₂ (x = 0.250, 0.167) were grown by the Bridgman method in a four-zone furnace. The thermal expansion behavior of Ag_xGa_xGe_1-xSe₂ (x = 0.25, 0.167) was studied by the method of single-crystal X-ray diffraction from 150 to 295 K and powder X-ray diffraction in the range of 298-773 K. Both results show the crystals have positive linear thermal expansion coefficients in different directions and a positive volume thermal expansion coefficient, and it is observed that they satisfy the relationship of α_a > α_c > α_b and α_V ≈ α_a + α_b + α_c for the orthorhombic structure. It is found that the Ag_xGa_xGe_1-xSe₂ (x = 0.25, 0.167) unit cells varying with temperature were mainly dominated by variations in framework geometry (AgSe₄ tetrahedron), and the thermal motion of Ag atoms in the AgSe₄ tetrahedron. As it was revealed, according to the powder X-ray diffraction, it is found that the isotropic thermal atomic displacement parameter of the Ag atoms is much larger than those of the Se and Ga(Ge) atoms in the AgSe₄ tetrahedron. Furthermore, anisotropic atomic displacement parameters (ADPs) of Ag atoms are extracted from the single-crystal diffraction; the ADPs along the a axis, b axis, and c axis have a significant difference, which means the thermal vibration of Ag atoms is anisotropic. It is of great significance for improving crystal growth technology and understanding the thermal properties of this kind of crystals.

Broadband, few-cycle mid-infrared continuum based on the intra-pulse difference frequency generation with BGSe crystals

We demonstrate for the first time the generation of octave-spanning mid-infrared using a BGSe nonlinear crystal. A Cr:ZnS laser system delivering 28-fs pulses at a central wavelength of 2.4 µm is used as the pump source, which drives the intra-pulse difference frequency generation inside the BGSe crystal. As a result, a coherent broadband mid-infrared continuum spanning from 6 to 18 µm has been obtained. It shows that the BGSe crystal is a promising material for broadband, few-cycle mid-infrared generation via frequency down conversion with femtosecond pump sources.

Functional Chalcogenide Na2HgSn2Se6 and K2MnGe2Se6 Exhibiting Flexible Chain Structure and Intriguing Birefringence Tunability

The two functional chalcogenides K₂MnGe₂Se₆ and Na₂HgSn₂Se₆, featuring a straight-chain structure, have been successfully prepared and fully characterized. K₂MnGe₂Se₆ shows paramagnetic behavior. The birefringence of Na₂HgSn₂Se₆ is as large as 0.3107 and derives from the superposition of the polarizabilities of its fundamental building blocks, on the basis of first-principles calculations. Moreover, the flexible framework of the A₂M^IIM^IV₂Se₆ family enables a variety of heterogeneous substitutions and thus offers possible birefringence tunability, which may inspire the design and exploratory synthesis of IR birefringent materials.

3.5 W long-wave infrared ZnGeP2 optical parametric oscillator at 9.8 µm

We demonstrated a high-power long-wave infrared optical parametric oscillator at 9.8 µm based on a type-I phase-matching ${{\rm ZnGeP}_2}$ZnGeP₂ crystal. By using a ${Q}$Q-switched 2091 nm Ho:YAG laser with pulse repetition frequency of 10 kHz as the pump source, the maximum average output power of 3.51 W at 9.8 µm was achieved with incident pump power of 90 W, corresponding to a slope efficiency of 4.81% and conversion efficiency at maximum pump power of 3.9%. The pulse width of 19.6 ns and linewidth of 142 nm were obtained at maximum output level. In addition, the beam quality factor ${M^2}$M² was measured to be ${\sim}{2.2}$∼2.2.

Ba8SrPb24O24Cl18: the first alkali-earth metal lead(ii) oxyhalide with an intriguing multimember-ring layer

The first alkali-earth metal lead(ii) oxyhalide Ba₈SrPb₂₄O₂₄Cl₁₈ characterized by fascinating multimember-ring layers has been discovered. Theoretical and experimental investigations illustrate that Ba₈SrPb₂₄O₂₄Cl₁₈ exhibits a moderate band gap of 3.09 eV, incongruent melting behavior and birefringence of 0.014@1064 nm. This discovery may offer new ideas for regulating the optical properties of oxyhalides and broadening their structural diversity.

Alkali-earth metal lead(ii) oxyhalide Ba27Pb8O8Cl54 exhibiting interesting [Pb4Ba4O4]8+ species

A novel oxyhalide Ba₂₇Pb₈O₈Cl₅₄ exhibits interesting [Pb₄Ba₄O₄]⁸⁺ species.

BaCuSiTe3: A Noncentrosymmetric Semiconductor with CuTe4 Tetrahedra and Ethane-like Si2Te6 Units

BaCuSiTe₃ was prepared from the elements in a solid-state reaction at 973 K, followed by slow cooling to room temperature. This telluride adopts a new, hitherto unknown structure type, crystallizing in the noncentrosymmetric space group Pc with a = 7.5824(1) Å, b = 8.8440(1) Å, c = 13.1289(2) Å, β = 122.022(1)°, and V = 746.45(2) ų (Z = 4). The structure consists of a complex network of two-dimensionally connected CuTe₄ tetrahedra and ethane-like Si₂Te₆ units with a Si-Si bond. This semiconducting material has an optical band gap of 1.65 eV and a low thermal conductivity of 0.50 W m^-1 K^-1 at 300 K. Calculations of its optical properties revealed a moderate birefringence of 0.23 and a second-order harmonic generation response of d_eff = 3.4 pm V^-1 in the static limit.

Ba2Si3P6: 1D Nonlinear Optical Material with Thermal Barrier Chains

A novel barium silicon phosphide was synthesized and characterized. Ba₂Si₃P₆ crystallizes in the noncentrosymmetric space group Pna2₁ (No. 33) and exhibits a unique bonding connectivity in the Si-P polyanion not found in other compounds. The crystal structure is composed of SiP₄ tetrahedra connected into one-dimensional double-tetrahedra chains through corner sharing, edge sharing, and covalent P-P bonds. Chains are surrounded by Ba cations to achieve an electron balance. The novel compound exhibits semiconducting properties with a calculated bandgap of 1.6 eV and experimental optical bandgap of 1.88 eV. The complex pseudo-one-dimensional structure manifests itself in the transport and optical properties of Ba₂Si₃P₆, demonstrating ultralow thermal conductivity (0.56 W m^-1 K^-1 at 300 K), promising second harmonic generation signal (0.9 × AgGaS₂), as well as high laser damage threshold (1.6 × AgGaS₂, 48.5 MW/cm²) when compared to the benchmark material AgGaS₂. Differential scanning calorimetry reveals that Ba₂Si₃P₆ melts congruently at 1373 K, suggesting that large single crystal growth may be possible.

SnGa2GeSe6, a benign addition to the AMIII2MIVQ6 family: synthesis, crystal structure and nonlinear optical performance

A new noncentrosymmetric compound SnGa₂GeSe₆ exhibits a crisscross network of two types of infinite chains and intriguing nonlinear optical performance.

From CuFeS2 to Ba6Cu2FeGe4S16: rational band gap engineering achieves large second-harmonic-generation together with high laser damage threshold

From CuFeS₂, the introduction of Ge leads to an increase in band gap. The ordered arrangement of NLO active units [GeS₄] results in a strong SHG response. Finally, Ba₆Cu₂FeGe₄S₁₆ exhibits good NLO performance (SHG, 1.5 × AgGaSe₂; LDT, 2 × AgGaSe₂).