Mixed-Metal Carbonate Fluorides as Deep-Ultraviolet Nonlinear Optical Materials

2D van der Waals Layered [C(NH2)3]2SO3S Exhibits Desirable UV Nonlinear-Optical Trade-Off

It remains a challenge to develop UV nonlinear optical (NLO) crystals that can achieve a desirable trade-off on UV absorption edge, second harmonic generation (SHG), and birefringence. Here we report a thiosulfate UV NLO crystal of a 2D van der Waals layered structure, [C(NH₂)₃]₂SO₃S. Remarkably, this thiosulfate realizes the desired trade-off, with a short absorption edge of 254 nm, a strong SHG response of approximately 2.8 times that of the benchmark KH₂PO₄, and a sufficient birefringence of 0.073 at the wavelength of 546 nm. In addition, it exhibits strong in-plane anisotropy of the SHG intensity. According to the first-principles calculations, the non-π-conjugated [SO₃S]^2- anion is the dominant SHG functional gene, while the π-conjugated [C(NH₂)₃]⁺ cation serves as the functional gene of birefringence. This is different from common UV NLO materials whose functionals of SHG and birefringence are the same. These findings indicate that combining different function genes may be an effective strategy to develop outstanding NLO materials with the desirable property trade-off.

Non-π-Conjugated Deep-Ultraviolet Nonlinear Optical Crystal K2Zn3(SO4)(HSO4)2F4

Deep-ultraviolet (deep-UV) (wavelengths of <200 nm) nonlinear optical (NLO) materials are playing an increasingly important role because of their significant technological applications in advanced scientific instruments. In recent years, the non-π-conjugated systems have received extensive attention as new emerging sources of deep-UV NLO materials. Here, a new non-π-conjugated deep-UV NLO material, K₂Zn₃(SO₄)(HSO₄)₂F₄, has been successfully obtained by the hydrothermal method. It has a layered structure formed by [Zn₃(SO₄)(HSO₄)₂F₃]_∞ layers bridged via K-O and K-F bonds. Powder second-harmonic generation shows that K₂Zn₃(SO₄)(HSO₄)₂F₄ can achieve phase matching, and the response is ∼0.3 times that of KH₂PO₄. Remarkably, the single-crystal transmittance spectrum confirms that the absorption edge of K₂Zn₃(SO₄)(HSO₄)₂F₄ is below 200 nm, and the experimentally measured birefringence is 0.0126 at λ = 546 nm. In-depth first-principles calculations illustrate well the microscopic origin of the optical properties. This work enriches the structrual diversity of non-π-conjugated deep-UV NLO materials.

A Congruent-Melting Mid-Infrared Nonlinear Optical Vanadate Exhibiting Strong Second-Harmonic Generation

Study of mid-infrared (mid-IR) nonlinear optical (NLO) materials is hindered by the competing requirements of optimized second-harmonic generation (SHG) coefficient d_ij and laser-induced damage threshold (LIDT) as well as the harsh synthetic conditions. Herein, we report facile hydrothermal synthesis of a polar NLO vanadate Cs₄ V₈ O₂₂ (CVO) featuring a quasi-rigid honeycomb-layered structure with [VO₄ ] and [VO₅ ] polyhedra aligned parallel. CVO possesses a wide IR-transparent window, high LIDT, and congruent-melting behavior. It has very strong phase-matchable SHG intensities in metal vanadate family (12.0 × KDP @ 1064 nm and 2.2 × AGS @ 2100 nm). First-principles calculations suggest that the exceptional SHG responses of CVO largely originate from virtual electronic transitions within [V₄ O₁₁ ]_∞ layer; the excellent optical transmittance of CVO arises from the special characteristics of vibrational phonons resulting from the layered structure.

UV Solar-Blind-Region Phase-Matchable Optical Nonlinearity and Anisotropy in a π-Conjugated Cation-Containing Phosphate

Wide ultraviolet (UV) transparency, strong second-harmonic generation (SHG) response, and sufficient optical birefringence for phase-matching (PM) at short SHG wavelengths are vital for practical UV nonlinear optical (NLO) materials. However, simultaneously optimizing these properties is a major challenge, particularly for metal phosphates. Herein, we report a non-traditional π-conjugated cation-based UV NLO phosphate [C(NH₂ )₃ ]₆ (PO₄ )₂ ⋅3 H₂ O (GPO) with a short UV cutoff edge. GPO is SHG active at 1064 nm (3.8 × KH₂ PO₄ @ 1064 nm) and 532 nm (0.3 × β-BaB₂ O₄ @ 532 nm) and also possesses a significant birefringence (0.078 @ 546 nm) with a band gap >6.0 eV. The PM SHG capability of GPO can extend to 250 nm, indicating GPO is a promising UV solar-blind NLO material. Calculations and crystal structure analysis show that the rare coexistence of wide UV transparency, large SHG response, and optical anisotropy is due to the introduction of π-conjugated cations [C(NH₂ )₃ ]⁺ and their favorable arrangement with [PO₄ ]^3- anions.

M(NH2 SO3 )2 (M=Sr, Ba): Two Deep-Ultraviolet Transparent Sulfamates Exhibiting Strong Second Harmonic Generation Responses and Moderate Birefringence

Over the last few decades, the development of each new nonlinear optical (NLO)-active functional unit has led to the discoveries of a series of excellent NLO materials. In the present work, based on first-principles studies, we identified a novel deep-UV (DUV) NLO-active functional unit, a non-π-conjugated group viz. (NH₂ SO₃ )^- . By combining alkaline-earth metals with (NH₂ SO₃ )^- group, two DUV transparent NLO sulfamates, M(NH₂ SO₃ )₂ (M=Sr, Ba) with superior optical properties including strong SHG responses (1.2 and 2.7 × KH₂ PO₄ (KDP)), short UV cut-off edge (<190 nm) and moderate birefringence (0.056@589.3 nm for Sr(NH₂ SO₃ )₂ ) were successfully synthesized. Our work has provided not only two promising DUV transparent NLO crystals, but also an innovative non-π-conjugated unit for developing more DUV transparent NLO materials.

Deep-ultraviolet transparent alkali metal–rare earth metal sulfate NaY(SO4)2·H2O as a nonlinear optical crystal: synthesis and characterization

The first SHG-active alkali metal–rare earth metal sulfate NaY(SO₄)₂·H₂O, which manifests a short ultraviolet cutoff edge below 200 nm and exhibits a phase-matching SHG response of 0.6 × KDP, has been obtained using a mild hydrothermal method.

In situ hydrothermal synthesis of polar second-order nonlinear optical selenate Na5(SeO4)(HSeO4)3(H2O)2

An alkali–metal selenate nonlinear optical (NLO) crystal, Na₅(SeO₄)(HSeO₄)₃(H₂O)₂, which possesses good second-order nonlinear optical properties, has been obtained by mild in situ hydrothermal synthesis.

Ba3 (BO3 )(CO3 )F: The First Borate Carbonate Fluoride Synthesized by the High-Temperature Solution Method

In contrast to the well-investigated halogen-containing borates and carbonates, very few halogen-containing borate carbonate compounds have been reported. Specifically, no example of borate carbonate fluoride has been synthesized successfully until now. Herein, the planar π-conjugated units [BO₃ ]^3- and [CO₃ ]^2- and the F^- ions are introduced simultaneously into one crystal structure resulting in the first borate carbonate fluoride, Ba₃ (BO₃ )(CO₃ )F, by the high-temperature solution method in the atmosphere. Its structure features a hexagonal channel formed by the [BO₃ ]^3- and [CO₃ ]^2- units with the [F₃ Ba₈ ]¹³⁺ trimers filled in the channel. Various characterizations including single crystal- and powder-XRD, EDX, IR, UV-vis-NIR, and TG-DSC, together with the first principles calculation have been carried out to verify the structure and fully understand the structure-property relationships.

NaZnCO3(OH): A High-Performance Carbonate Ultraviolet Nonlinear Optical Crystal Derived from KBe2BO3F2

Currently, to obtain new ultraviolet (UV) nonlinear optical (NLO) materials with large second-harmonic generation (SHG) response and short SHG phase-matching cutoff wavelength (λ_PM) is still very challenging. Herein, through evolving the KBe₂BO₃F₂ (KBBF) structure to a carbonate system, a novel hydroxycarbonate UV NLO crystal, NaZnCO₃(OH), was successfully synthesized. It not only possessed the typical structural characteristics of KBBF but also enhanced the bonding force between functional layers. Remarkably, in this crystal, the rare coexistence of a very large SHG response of 5.2 × KH₂PO₄ (KDP, @1064 nm) and short type-I SHG λ_PM of 201 nm had been achieved, indicating that it was a promising UV NLO crystal. This work showed the great significance of KBBF structure evolution and provided new opportunity to design UV NLO materials with good performance.

Comprehensive Density Functional Theory Studies of Vibrational Spectra of Carbonates

Within the framework of the density functional theory (DFT) and the hybrid functional B3LYP by means of the CRYSTAL17 program code, the wavenumbers and intensities of normal oscillations of MgCO3, CaCO3, ZnCO3, CdCO3 in the structure of calcite; CaMg(CO3)2, CdMg(CO3)2, CaMn(CO3)2, CaZn(CO3)2 in the structure of dolomite; BaMg(CO3)2 in the structure of the norsethite type; and CaCO3, SrCO3, BaCO3, and PbCO3 in the structure of aragonite were calculated. Infrared absorption and Raman spectra were compared with the known experimental data of synthetic and natural crystals. For lattice and intramolecular modes, linear dependences on the radius and mass of the metal cation are established. The obtained dependences have predictive power and can be used to study solid carbonate solutions. For trigonal and orthorhombic carbonates, the linear dependence of wavenumbers on the cation radius RM (or M-O distance) is established for the infrared in-plane bending mode: 786.2-65.88·RM and Raman in-plane stretching mode: 768.5-53.24·RM, with a correlation coefficient of 0.87.

Explorations of Second-Order Nonlinear Optical Materials in the Alkaline Earth Barbiturate System: Noncentrosymmetric Ca(H3C4N2O3)2·H2O and Centrosymmetric Sr(H5C8N4O5)2·4H2O

Two new alkaline earth barbiturates, Ca(H₃C₄N₂O₃)₂·H₂O and Sr(H₅C₈N₄O₅)₂·4H₂O, were synthesized via the mild hydrothermal technique. For Ca(H₃C₄N₂O₃)₂·H₂O, the stacking (H₃C₄N₂O₃)^- anions along the c axis are interconnected by CaO₇ polydedra forming a three-dimensional structure. Also, for Sr(H₅C₈N₄O₅)₂·4H₂O, it has two-dimensional layers composed by SrO₇ polyhedra and (H₅C₈N₄O₅)^- anions. The (H₅C₈N₄O₅)^- anions can be seen as two (H₃C₄N₂O₃)^- anions connected each other via the N-C bond. Powder second-harmonic generation (SHG) measurements revealed that Ca(H₃C₄N₂O₃)₂·H₂O is a phase-matchable material with a moderate SHG response (ca. 1.15× that of KDP). Furthermore, the birefringence values of the two crystals were measured as 0.49 and 0.475 at 546.1 nm, respectively. The theoretical calculations showed that the SHG response and large birefringence were primarily caused by the (H₃C₄N₂O₃)^- and (H₅C₈N₄O₅)^- groups.

Cs3B3O3F6 with a Deep-Ultraviolet Cutoff Edge and a Suitable Birefringence as the Potential Zero-Order Waveplate Material

The zero-order waveplates, the essential materials in altering the polarization state of optical waves, are significant in polarimetry and the laser industry. Restricted by birefringence and absorption edge, few materials can have moderately small birefringence (Δn < 0.01) and a deep-ultraviolet (DUV; λ < 200 nm) transparency range. We used the B₃O₃F₆ unit for designing the zero-order waveplate material, which is composed of three BO₂F₂ tetrahedra interconnected by corner sharing. Here we obtained a new Cs₃B₃O₃F₆ compound with the isolated B₃O₃F₆ unit by a high-temperature solution method, which has the DUV cutoff edge and suitable birefringence (0.0069 at 532 nm) based on the experimental and computational results. This demonstrates that the isolated B₃O₃F₆ unit is a potential group, which favors the search for suitable compounds for DUV zero-order waveplate materials.

(NH4)3[B(OH)3]2(COOH)3: a graphite-like UV nonlinear optical material with a large birefringence via structural optimization

A novel noncentrosymmetric (NCS) metal-free formic-borate, (NH4)3[B(OH)3]2(COOH)3, has been discovered, which exhibits an infrequent graphite-like structure. The alliance of two types of π-conjugated planar anions BO33- and COOH- produced an optimized layered structure to maximize the anisotropic polarizability, resulting in an extremely large birefringence (0.156@546 nm), larger than that of the classical commercial UV birefringent material α-BBO (0.122@546 nm). This strategy that structural optimization could enhance birefringence will guide the discovery of large birefringence materials, especially in the UV region.

Second harmonic generation responses of KH2PO4: importance of K and breaking down of Kleinman symmetry

The second harmonic generation (SHG) responses of the paraelectric and ferroelectric phases of KH₂PO₄ (KDP) were calculated by first-principles density functional theory (DFT) calculations, and the individual atom contributions to the SHG responses were analyzed by the atom response theory (ART). We show that the occurrence of static polarization does not enhance the SHG responses of the ferroelectric KDP, and that the Kleinman symmetry is reasonably well obeyed for the paraelectric phase, but not for the ferroelectric phase despite that the latter has a larger bandgap. This is caused most likely by the fact that the ferroelectric phase has lower-symmetry local structures than does the paraelectric phase. The contribution to the SHG response of an individual K⁺ ion is comparable to that of an individual O^2- ion. The contributions of the O^2- and K⁺ ions arise overwhelmingly from the polarizable parts of the electronic structure, namely, from the valence bands of the O-2p nonbonding states and from the conduction bands of the K-3d states.

Helix-constructed polar rare-earth iodate fluoride as a laser nonlinear optical multifunctional material

The first trivalent rare-earth iodate fluoride nonlinear optical (NLO) crystal, Y(IO₃)₂F (YIF), was successfully designed and synthesized, featuring polarization-favorable helical chains constructed from trans-YO₆F₂ polyhedra and IO₃ groups.

The first trivalent rare-earth iodate fluoride nonlinear optical (NLO) crystal, Y(IO₃)₂F (YIF), was successfully designed and synthesized, featuring polarization-favorable helical chains constructed from trans-YO₆F₂ polyhedra and IO₃ groups. It exhibited a suitable balance of a wide transparency range of 0.26–10.0 μm, high laser damage threshold (LDT) of 39.6 × AgGaS₂, and moderate second harmonic generation (SHG) effect of 2 × KDP. A series of doped RE:YIF (RE = Pr, Nd, Dy, Ho, Er, Tm, and Yb) crystals were easily synthesized benefiting from the spring-shaped helix structure, which possess wide absorption and emission peaks as well as long lifetime, especially in the visible and near-infrared regions. Particularly, the remarkable fluorescence properties of Nd and Yb doped YIF crystals are comparable to and even better than those of traditional self-frequency doubling (SFD) crystals such as YAB, YCOB, and GdCOB. Thus, these RE-doped YIF crystals are promising laser SFD crystals. This work also indicated that constructing helical chains should be an effective strategy for the design of inorganic polar materials.

Novel ultraviolet (UV) nonlinear optical (NLO) materials discovered by chemical substitution-oriented design

Exploring novel functional materials via chemical substitution-oriented design is an emerging strategy. The method can be expanded to the discovery of high performance ultraviolet (UV) nonlinear optical (NLO) solid state materials by a careful tuning of the substituted atoms. This minireview presents a brief introduction to chemical substitution-oriented design including single-site substitution, dual-site substitution, and multisite substitution. Several state-of-the-art UV NLO materials such as K₃VO(O₂)₂CO₃-type, KBe₂BO₃F₂ (KBBF)-type, Ca₅(PO₄)₃(OH)-type, and KTiOPO₄ (KTP)-type phases successfully discovered by the chemical substitution method are discussed.

A brief introduction to chemical substitution-oriented design to discover high performance ultraviolet (UV) nonlinear optical (NLO) solid state materials is presented.

Pb6O5(NO3)2: A Nonlinear Optical Oxynitrate Structurally Based on Lead Oxide Framework

A high second harmonic generation response is demonstrated by a Pb₆O₅(NO₃)₂ lead oxynitrate whose identity was verified upon reinvestigation of the PbO-Pb(NO₃)₂ system. Its crystal structure exhibits a unique cationic [Pb₆O₅]²⁺ framework hosting orientationally ordered NO₃^- triangles in the channels. Easy preparation and high thermal stability (until ∼500 °C in air) suggest it to be a new promising NLO material.

Optimal arrangement of π-conjugated anionic groups in hydro-isocyanurates leads to large optical anisotropy and second-harmonic generation effect

Optimal arrangements of π-conjugated anions in alkaline earth metal hydro-isocyanurates with rich structures result in large birefringence and a giant nonlinear optical effect.

Inorganic planar π-conjugated groups in nonlinear optical crystals: review and outlook

Nonlinear optical crystals with various inorganic planar π-conjugated groups are reviewed based on their bandgaps.

Second-order nonlinear optical materials with a benzene-like conjugated π system

This feature article highlights the strategies used to design UV/DUV NLO materials based on benzene-like π-conjugated units.