Large Crystal Growth and New Crystal Exploration of Mid-Infrared Second-Order Nonlinear Optical Materials

K2CdGe3S8: A New Infrared Nonlinear Optical Sulfide

A quaternary metal chalcogenide, namely K2CdGe3S8 (I), is obtained through a high-temperature solid-state approach. Compound I crystallizes with the non-centrosymmetric space group P212121. It features a 2D layer structure with [CdGe3S8] layers consisting of tetrahedral GeS4 and CdS4 units, and counter K+ embedded between the layers. The compound exhibits a powder second-harmonic generation (SHG) response of ~0.1 times that of KH2PO4 (KDP) with phase-matchable character at the laser wavelength of 1064 nm. Remarkably, it has a wide band gap (3.20 eV), which corresponds to a favorable high laser-induced damage threshold of 6.7 times that of AgGaS2. In addition, the calculated birefringence (Δn) is 0.039 at the wavelength of 1064 nm, which satisfies the Δn criteria for a promising infrared NLO material.

Rational design via dual-site aliovalent substitution leads to an outstanding IR nonlinear optical material with well-balanced comprehensive properties

The acquisition of a non-centrosymmetric (NCS) structure and achieving a nice trade-off between a large energy gap (E g > 3.5 eV) and a strong second-harmonic generation (SHG) response (d eff > 1.0 × benchmark AgGaS2) are two formidable challenges in the design and development of infrared nonlinear optical (IR-NLO) candidates. In this work, a new quaternary NCS sulfide, SrCdSiS4, has been rationally designed using the centrosymmetric SrGa2S4 as the template via a dual-site aliovalent substitution strategy. SrCdSiS4 crystallizes in the orthorhombic space group Ama2 (no. 40) and features a unique two-dimensional [CdSiS4]2- layer constructed from corner- and edge-sharing [CdS4] and [SiS4] basic building units (BBUs). Remarkably, SrCdSiS4 displays superior IR-NLO comprehensive performances, and this is the first report on an alkaline-earth metal-based IR-NLO material that breaks through the incompatibility between a large E g (>3.5 eV) and a strong phase-matching d eff (>1.0 × AgGaS2). In-depth mechanism explorations strongly demonstrate that the synergistic effect of distorted tetrahedral [CdS4] and [SiS4] BBUs is the main origin of the strong SHG effect and large birefringence. This work not only provides a high-performance IR-NLO candidate, but also offers a feasible chemical design strategy for constructing NCS structures.

Material research from the viewpoint of functional motifs

As early as 2001, the need for the ‘functional motif theory’ was pointed out to assist the rational design of functional materials. The properties of materials are determined by their functional motifs and by how they are arranged in the materials. Uncovering the functional motifs and their arrangements is crucial in understanding the properties of materials and rationally designing new materials of desired properties. The functional motifs of materials are the critical microstructural units (e.g. constituent components and building blocks) that play a decisive role in generating certain material functions, and could not be replaced with other structural units without losing or significantly suppressing the relevant functions. The role of functional motifs and their arrangements in materials with representative examples was presented. These examples could be classified into six types of material microscopic structures on a length scale smaller than ∼10 nm with maximum subatomic resolution, i.e. the crystal, magnetic, aperiodic, defect, local, and electronic structures. The method of functional motif analysis could be employed in the function-oriented design of materials, as elucidated by taking infrared nonlinear optical materials as an example. Machine learning is more efficient in predicting material properties and screening materials with high efficiency than high-throughput experimentation and high-throughput calculations. In extracting the functional motifs and finding their quantitative relationships, developing sufficiently reliable databases for material structures and properties is imperative.

Li4 MgGe2 S7 : The First Alkali and Alkaline-Earth Diamond-Like Infrared Nonlinear Optical Material with Exceptional Large Band Gap

Large band gap and strong nonlinear optical (NLO) effect are two valuable but contradictory parameters, which are difficult to balance in one infrared (IR) NLO material. Herein, the first alkali and alkaline-earth metal diamond-like (DL) IR NLO material Li₄ MgGe₂ S₇ , presenting a honeycomb-like 3D framework constructed by 6-membered LiS₄ rings and GeMgS₆ zigzag chains, was rationally designed and synthesized. The introduction of rigid alkali metal and alkaline-earth metal LiS₄ and MgS₄ tetrahedra effectively broadens the band gap of DL compound to 4.12 eV (the largest one in the reported quaternary metal chalcogenides), generating a high laser damage threshold of 7 × AgGaS₂ at 1064 nm. Furthermore, Li₄ MgGe₂ S₇ displays a suitable SHG response (0.7 × AgGaS₂ ) with a type I phase-matching behavior. The results indicate that Li₄ MgGe₂ S₇ is a promising IR NLO material for the high-power laser application and it provides an insight into the design of new DL compound with outstanding IR NLO performances.

Uncovering a Functional Motif of Nonlinear Optical Materials by In Situ Electron Density and Wavefunction Studies Under Laser Irradiation

Exploring nonlinear optical (NLO) functional motifs (FM, the structural origin of NLO efficiency) is vital for the rational design of NLO materials. Normal spectrum techniques applied in studying photon exciting materials are invalid for NLO materials, in which electrons are not excited substantially but only distorted under laser. A general strategy of determining NLO FM is proposed by comparative studies of experimental electron density (ED) without and under the laser. The in situ experimental ED and wavefunction of typical NLO material LiB₃ O₅ (LBO) under dark and 360 and 1064 nm lasers are investigated. Compared with the initial state under dark, the ED of [B₃ O₅ ]^- unit at functional states under laser irradiation exhibits remarkable changes of topological atomic and bond properties, confirming the NLO FM being [B₃ O₅ ]^- . The work extracts for the first time the FM of a NLO material experimentally and highlights the crucial role of in situ ED analysis in studying NLO mechanisms.

Na4SnS4 and Na4SnSe4 exhibiting multifunctional physicochemical performances as potential infrared nonlinear optical crystals and sodium ion conductors

Na₄SnS₄ and Na₄SnSe₄ exhibiting excellent physicochemical performances as potential IR NLO crystals and sodium ion conductors were systematically studied.

A review of the AI2BIICIVDVI4 family as infrared nonlinear optical materials: the effect of each site on the structure and optical properties

The AI2B^IIC^IVDVI4 family as promising infrared NLO materials is summarized. The influence of each site substitutions on the structures and properties is systematically analyzed.

A2SrMIVS4 (A = Li, Na; MIV = Ge, Sn) concurrently exhibiting wide bandgaps and good nonlinear optical responses as new potential infrared nonlinear optical materials

A series of A₂SrM^IVS₄ compounds concurrently exhibiting wide bandgaps and good NLO responses were proven as promising IR NLO materials.

Exploration of new nonlinear optical (NLO) materials is of importance for infrared (IR) applications. However, it is an extremely tough challenge to design and synthesize excellent IR NLO materials with optimal performances (e.g., concurrently a large NLO response and wide bandgap). Herein, four new mixed alkali/alkaline earth metal sulfides, A₂SrM^IVS₄ (A = Li, Na; M^IV = Ge, Sn), were successfully synthesized by a motif-optimization approach using the classical AgGaS₂ as a template. Note that all of them concurrently exhibit wide bandgaps (3.1–3.8 eV) and good NLO responses (0.5–0.8 × AgGaS₂) with phase-matching behavior, which satisfy the balance conditions (E_g ≥ 3.0 eV and d_ij ≥ 0.5 × benchmark AgGaS₂) of optical performances and hence are outstanding IR NLO materials. Remarkably, both of Na₂SrM^IVS₄ have the same structure without the structural transformation (Ge to Sn) in the reported related analogues and an interesting cation-dependent structural change is also found in Na₂M^IISnS₄ (M^II: Sr, R3c vs. Ba, I4[combining macron]2d). These results verify that the above design strategy of motif-optimization provides a feasible guide for the discovery of new IR NLO candidates and the A–AE–M–S (A = alkali metal; AE = alkaline-earth metal; M = Ga, In, Ge, Sn) system was identified as the preferred system for IR NLO materials.

Investigation on two new antimony-based quaternary chalcogenides: Na6CdSb4S10 and Na3CdSbSe4

Two new Sb-based chalcogenides exhibiting interesting structural features and optical properties were successfully synthesized for the first time.

Structural modulation induced by MIIIA metals in Ba3MQ4X (M = Al, Ga, In; Q = S, Se; X = Cl, Br): an experimental and computational analysis

The optical properties of Ba₃MQ₄X (M = Al, Ga, In; Q = S, Se; X = Cl, Br) have been studied and their structures have been compared.

Structural insights into T2-cluster-containing chalcogenides with vertex-, edge- and face-sharing connection modes of NaQ6 ligands: Na3ZnMIIIQ4 (MIII = In, Ga; Q = S, Se)

New series of T₂-cluster-containing chalcogenides exhibiting the novel connection mode of NaS₆ ligands and torsional adjacent T₂-clusters were reported.

Thioborates: potential nonlinear optical materials with rich structural chemistry

This review summarizes thioborates and their structural motifs ranging from zero-dimension to three-dimension. The most commonly observed building units of these examples are planar-triangle BS₃ and tetrahedron BS₄. Thioborates possess advantages with respect to their structural diversity, optical nonlinearity, laser-induced damage threshold and transparency range, and represent potentially a rich supply of new nonlinear optical materials.

Na6Si3S8O: the first example of a sulfide silicate exhibiting unusual tri-polymerized [Si3S8O]6−units without S–O bonds

A new sulfide silicate – Na₆Si₃S₈O – featuring novel tri-polymerized [Si₃S₈O]⁶⁻units without common S–O bonds has been verified as a promising birefringent material.

The first quaternary diamond-like semiconductor with 10-membered LiS4rings exhibiting excellent nonlinear optical performances

New DLS–Li₄HgGe₂S₇with novel 10-MR LiS₄rings and excellent performances(concurrently large SHG coefficient and impressive LDT) is reported.

Synthesis, crystal structure and second-order nonlinear optical property of a novel pentanary selenide (K3I)[InB12(InSe4)3]

A novel pentanary selenide (K₃I)[InB₁₂(InSe₄)₃] (P6₃22) features InSe₄ tetrahedron consolidated B₁₂ icosahedron and 1-D chain constructed by InSe₆ octahedron and B₁₂Se₁₂ cluster. It is an indirect semiconductor with the energy gap of 1.15 eV and second harmonic generation-active.

Na9Sb(Ge2Q6)2(Q = S, Se): two new antimony(iii) quaternary chalcogenides with ethane-like [Ge2Q6]6−ligands

The changing character from different orientations of M–M bonds (M = Si, Ge) in quaternary chalcogenides was systematically studied.

An investigation of new infrared nonlinear optical material: BaCdSnSe4, and three new related centrosymmetric compounds: Ba2SnSe4, Mg2GeSe4, and Ba2Ge2S6

The crystal-structures and properties of new four compounds were systematically reported and non-centrosymmetric BaCdSnSe₄showed a good SHG response.