Recent advances in rational structure design for nonlinear optical crystals: leveraging advantageous templates

Nonlinear optical (NLO) crystals that can expand the spectral range of laser outputs have attracted significant attention for their optoelectronic applications. The research progress from the discovery of new single crystal structures to the realization of final device applications involves many key steps and is very time consuming and challenging. Consequently, exploring efficient design strategies to shorten the research period and accelerate the rational design of novel NLO materials has become imperative to address the pressing demand for advanced materials. The recent shift in paradigm toward exploring new NLO crystals involves significant progress from extensive "trial and error" methodologies to strategic approaches. This review proposes the concept of rational structure design for nonlinear optical crystals leveraging advantageous templates. It further discusses their optical characteristics, promising applications as second-order NLO materials, and the relationship between their structure and performance, and highlights urgent issues that need to be addressed in the field of NLO crystals in the future. The review aims to provide ideas and driving impetus to encourage researchers to achieve new breakthroughs in the next generation of NLO materials.

d6versus d10, Which Is Better for Second Harmonic Generation Susceptibility? A Case Study of K2TGe3Ch8 (T = Fe, Cd; Ch = S, Se)

Two acentric chalcogenide compounds, K₂CdGe₃S₈ and K₂CdGe₃Se₈, were synthesized via conventional high-temperature solid-state reactions. The crystal structures of K₂CdGe₃S₈ and K₂CdGe₃Se₈ were accurately determined by single-crystal X-ray diffraction and crystallize in the K₂FeGe₃S₈ structure type. K₂CdGe₃S₈ is isostructural to K₂FeGe₃S₈ with superior nonlinear optical properties. For the second harmonic generation (SHG) response, K₂CdGe₃S₈ is 18× K₂FeGe₃S₈ for samples of particle size of 38-55 μm. The superior nonlinear optical properties of K₂CdGe₃S₈ over K₂FeGe₃S₈ are mainly contributed by the chemical characteristics of Cd compared with Fe, which are elucidated by nonlinear optical property measurements, electronic structure calculations, and density functional theory calculations. The [CdS₄] tetrahedra within K₂CdGe₃S₈ exhibit a higher degree of distortion and larger volume compared to the [FeS₄] tetrahedra in K₂FeGe₃S₈. This study possesses a good platform to investigate how d-block elements contribute to the SHG response. The fully occupied d¹⁰-elements are better for SHG susceptibility than d⁶-elements in this study. K₂CdGe₃S₈ is a good candidate as an infrared nonlinear optical material of high SHG response (2.1× AgGaS₂, samples of particle size of 200-250 μm), type-I phase-matching capability, high laser damage threshold (6.2× AgGaS₂), and good stability.

Two Nonlinear Optical Thiophosphates Cu5Hg0.5P2S8 and AgHg3PS6 Activated by Their Tetrahedra-Stacking Architecture

Infrared (IR) lasers are very critical in military and civil affairs, but it is also challenging and difficult to develop new infrared nonlinear optical (NLO) crystals. Herein, two new mixed-metal thiophosphates, Cu₅Hg_0.5P₂S₈ and AgHg₃PS₆ were discovered with the noncentrosymmetric (NCS) space group Pmn2₁ (No. 31) and Cc (No. 9). Cu₅Hg_0.5P₂S₈ displays a three-dimensional (3D) defective diamond-like structure stacked by _∞²[Cu_2.5Hg_0.25PS₈]^8- layers. AgHg₃PS₆ is characterized by a 3D framework consisting of distorted tetrahedrons. Moreover, the optical spectra show the band gaps of Cu₅Hg_0.5P₂S₈ and AgHg₃PS₆ are 2.12 and 1.85 eV, respectively, with a broad transparent range of 0.7-16.0 μm. In these two compounds, the dipole moments of nonlinear active units are strengthened due to the high-valence element P and the Hg-S bonds with large susceptibility. Therefore, AgHg₃PS₆ exhibits a moderate second harmonic generation (SHG) response that is half that of AgGaS₂ (AGS) at 30-45 μm, while Cu₅Hg_0.5P₂S₈ performs a phase-matching (PM) behavior with a good SHG signal of 0.8 × AGS at 150-200 μm. The origin of NLO performance and electronic structures were revealed by the calculated dipole moments of distorted tetrahedra and theoretical calculations on the basis of density functional theory.

AAg3Ga8Se14 (A = Rb, Cs): Second-Harmonic Generation Responses Realized through the Parallel Arrangement of AgSe4 and GaSe4 Tetrahedrons

Two new quaternary selenides AAg3Ga8Se14, (A = Rb, 1; Cs, 2) were synthesised via sold-state reaction in sealed silica tubes. Compounds 1 and 2 crystallised in the monoclinic space group...

Cu4MnGe2S7 and Cu2MnGeS4: two polar thiogermanates exhibiting second harmonic generation in the infrared and structures derived from hexagonal diamond

The new, quaternary diamond-like semiconductor (DLS) Cu₄MnGe₂S₇ was prepared at high-temperature from a stoichiometric reaction of the elements under vacuum. Single crystal X-ray diffraction data were used to solve and refine the structure in the polar space group Cc. Cu₄MnGe₂S₇ features [Ge₂S₇]^6- units and adopts the Cu₅Si₂S₇ structure type that can be considered a derivative of the hexagonal diamond structure. The DLS Cu₂MnGeS₄ with the wurtz-stannite structure was similarly prepared at a lower temperature. The achievement of relatively phase-pure samples, confirmed by X-ray powder diffraction data, was nontrival as differential thermal analysis shows an incongruent melting behaviour for both compounds at relatively high temperature. The dark red Cu₂MnGeS₄ and Cu₄MnGe₂S₇ compounds exhibit direct optical bandgaps of 2.21 and 1.98 eV, respectively. The infrared (IR) spectra indicate potentially wide windows of optical transparency up to 25 μm for both materials. Using the Kurtz-Perry powder method, the second-order nonlinear optical susceptibility, χ⁽²⁾, values for Cu₂MnGeS₄ and Cu₄MnGe₂S₇ were estimated to be 16.9 ± 2.0 pm V^-1 and 2.33 ± 0.86 pm V^-1, respectively, by comparing with an optical-quality standard reference material, AgGaSe₂ (AGSe). Cu₂MnGeS₄ was found to be phase matchable at λ = 3100 nm, whereas Cu₄MnGe₂S₇ was determined to be non-phase matchable at λ = 1600 nm. The weak SHG response of Cu₄MnGe₂S₇ precluded phase-matching studies at longer wavelengths. The laser-induced damage threshold (LIDT) for Cu₂MnGeS₄ was estimated to be ∼0.1 GW cm^-2 at λ = 1064 nm (pulse width: τ = 30 ps), while the LIDT for Cu₄MnGe₂S₇ could not be ascertained due to its weak response. The significant variance in NLO properties can be reasoned using the results from electronic structure calculations.

Design and synthesis of Ba3SiSe5 with suitable birefringence modulated via MIV atoms in the Ba-MIV-Q (MIV = Si, Ge; Q = S, Se) system

A new ternary Ba-based selenide, Ba₃SiSe₅, was synthesized by a high-temperature solid-state method. It crystallizes in the centrosymmetric space group Pnma (no. 62) of the orthorhombic system. The structure of the title compound consists of unique Se(4)Ba layers and discrete SiSe₄ tetrahedra. The structure and computational properties of Ba₃SiSe₅ are systematically studied together with those of the Ba-M^IV-Q (M^IV = Si, Ge; Q = S, Se) system, and show an interesting difference in dimensions formed by one of the crystallographic Ba atoms and M^IVQ₄ tetrahedra, as well as optical property transformations modulated by M^IV atoms. First principles methods were employed to obtain a better understanding of the relationship between structures and properties. Ba₃SiSe₅ maintains a moderate birefringence of 0.044@1064 nm and the real space atom cutting method indicates that the SiSe₄ tetrahedra make the major contribution to its birefringence.

A3Mn2Sb3S8 (A = K and Rb): a new type of multifunctional infrared nonlinear optical material based on unique three-dimensional open frameworks

A new type of multifunctional IR-NLO material, A₃Mn₂Sb₃S₈ (A = K and Rb), with unique 3D open frameworks has been developed using a facile surfactant–thermal method.

Ultralow thermal conductivity in the quaternary semiconducting chalcogenide Cs4[Ho26Cd7Se48] with an unprecedented closed cavity architecture

A quaternary semiconducting chalcogenide, Cs₄[Ho₂₆Cd₇Se₄₈], with an unprecedented closed cavity architecture exhibits ultralow thermal conductivity.

A new type of novel salt-inclusion chalcogenide with ultralow thermal conductivity

The design and development of novel chalcogenides with ultralow thermal conductivity is extremely important but very challenging for promoting the efficiencies of thermoelectric (TE) materials. Herein, a new type of salt-inclusion chalcogenide (SIC), [Rb6Cl][RE23Mn7Se44] (RE = Ho-Yb), was discovered via a modified flux method. They possessed [RESe6] and [MSe6] (M = RE/Mn) octahedra as basic building units, which interlinked to form a three-dimensional quasi-NaCl-type [RE23Mn7Se44]5- host framework, where the [Rb6Cl]5+ guest ions resided. Interestingly, these isomorphic compounds showed ultralow thermal conductivities (0.28-0.37 W m-1 K-1) at 673 K, which are reported for the first time in SICs. This work not only enriches SIC chemistry but also broadens the application of SICs in the TE field.

Coordinated regulation on critical physiochemical performances activated from mixed tetrahedral anionic ligands in new series of Sr6A4M4S16 (A = Ag, Cu; M = Ge, Sn) nonlinear optical materials

Coordinated regulation on physiochemical performances activated from mixed anionic ligands in a new series of IR NLO materials was systematically investigated.

ABaSbQ3 (A = Li, Na; Q = S, Se): diverse arrangement modes of isolated SbQ3 ligands regulating the magnitudes of birefringences

The interrelation of arrangement modes of isolated SbQ₃ ligands on structures and birefringences has been systematically studied in series of chalcogenides.

Na6Zn3MIII2Q9 (MIII = Ga, In; Q = S, Se): four new supertetrahedron-layered chalcogenides with unprecedented vertex-sharing T3-clusters and desirable photoluminescence performances

Four new supertetrahedron-layered chalcogenides as the first example of exhibiting vertex-sharing T₃-clusters in the known quaternary chalcogenides were discovered.

NaBaMIIIQ3 (MIII = Al, Ga; Q = S, Se): first quaternary chalcogenides with isolated edge-sharing (MIII2Q6)6− dimers

New series of quaternary chalcogenides with firstly discovered isolated edge-sharing (MIII2Q₆)⁶⁻ dimers showing obvious optical anisotropy were reported.

Syntheses and characterization of three new sulfides with large band gaps: acentric Ba4Ga4SnS12, centric Ba12Sn4S23 and Ba7Sn3S13

Three new sulfides with large band gaps, NCS Ba₄Ga₄SnS₁₂, CS Ba₁₂Sn₄S₂₃ and Ba₇Sn₃S₁₃ were synthesized for the first time. Ba₄Ga₄SnS₁₂ exhibits a desired balance between the band gap (2.90 eV) and the SHG intensity (34 × KDP).