NH4 Be2 BO3 F2 and γ-Be2 BO3 F: Overcoming the Layering Habit in KBe2 BO3 F2 for the Next-Generation Deep-Ultraviolet Nonlinear Optical Materials

Theoretical Understanding of Nonlinear Optical Properties in Solids: A Perspective

Nonlinear optical (NLO) crystals have become a hot topic in chemical science and material physics, due to their essential role in laser technology, optical information, optoelectronics, and precision measurements. In this Perspective, we provide an overview of recent advances in second-order nonlinear optics, with a focus on two critical topics: second harmonic generation (SHG) and the bulk photovoltaic effect (BPVE). For SHG, we discuss recent progress in deep-ultraviolet (DUV) materials, highlighting their structural characteristics and nonlinear groups that contribute to their exceptional performance. For BPVE, we concentrate on the emerging field of low-dimensional materials, emphasizing their potential in a shift current. Additionally, we discuss the development of regulation approaches for NLO materials, which is vital for their practical application. Finally, we address the outlook for the field, including the challenges that must be overcome to further advance NLO materials research.

NaRb6(B4O5(OH)4)3(BO2) Featuring Noncentrosymmetry, Chirality, and the Linear Anionic Group BO2. J Am Chem Soc 2023;145:4928-4933. [PMID: 36811389 DOI: 10.1021/jacs.2c12069]

Noncentrosymmetric (NCS) structures are of particular interest owing to their symmetry-dependent physical properties, e.g., pyroelectricity, ferroelectricity, piezoelectricity, and nonlinear optical (NLO) behavior. Among them, chiral materials exhibit polarization rotation and host topological properties. Borates often contribute to NCS and chiral structures via their triangular [BO₃] and tetrahedral [BO₄] units and their numerous superstructure motifs. However, no chiral compound with the linear [BO₂] unit has been reported to date. Herein, an NCS and chiral mixed-alkali-metal borate, NaRb₆(B₄O₅(OH)₄)₃(BO₂), with a linear BO₂^- unit in the structure was synthesized and characterized. The structure features a combination of three types of basic building units (BBUs), [BO₂], [BO₃], and [BO₄] with sp-, sp²-, and sp³-hybridization of boron atoms, respectively. It crystallizes in the trigonal space group R32 (No. 155), one of the 65 Sohncke space groups. Two enantiomers of NaRb₆(B₄O₅(OH)₄)₃(BO₂) were found, and their crystallographic relationships are discussed. These results not only expand the small family of NCS structures with the rare linear BO₂^- unit but also prompt recognition to the fact that NLO materials have generally overlooked the existence of two enantiomers in achiral Sohncke space groups.

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.

Double-Modification Oriented Design of a Deep-UV Birefringent Crystal Functionalized by [B12 O16 F4 (OH)4 ] Clusters

Polarization modulation of deep-UV light is of significance to current technologies, and to this end, the birefringent crystal has emerged as an invaluable material as it allows for effective light modulation. Herein, a double-modification strategy driven by F and OH anions that makes double effects towards the critical property enhancement of deep-UV birefringent crystals is proposed. This leads to a new hydroxyborate (NH₄ )₄ [B₁₂ O₁₆ F₄ (OH)₄ ] with giant cluster as a deep-UV birefringent crystal with large birefringence (Δn_exp. =0.12@546.1 nm). This birefringence is a record among inorganic hydroxyborates with experimentally measured birefringence. Structural analysis shows that the near-plane arrangement of [B₁₂ O₁₆ F₄ (OH)₄ ] cluster is responsible for the large optical anisotropy. Theoretical calculations indicate that its π-conjugated [BO₃ ] and [BO₂ OH] units are the main source of this large optical anisotropy.

KF·B(OH)3: a KBBF-type material with large birefringence and remarkable deep-ultraviolet transparency

A designed material with large birefringence and remarkable deep-ultraviolet transparency, KF·B(OH)₃, has been discovered. The well-aligned [F·B(OH)₃]^- layers endow a very large bandgap (7.63 eV), comparably large birefringence (0.114 @1064 nm), and the reinforced interlayer bonding of ca. 2.16 × KBe₂BO₃F₂.

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.

Sulfamide: A Promising Deep-Ultraviolet Nonlinear Optical Crystal Assembled from Polar Covalent [SO2 (NH2 )2 ] Tetrahedra

For the first time, the polar covalent tetrahedron [SO₂ (NH₂ )₂ ] is revealed as a new deep-ultraviolet (DUV) nonlinear optical (NLO)-active unit according to theoretical calculations. Furthermore, sulfamide consisting of polar [SO₂ (NH₂ )₂ ] units was confirmed as an excellent candidate as a DUV NLO crystal. Sulfamide provides the optimal balance between composition, structure, and properties, in addition to a very short absorption of 160 nm. It achieves multiple optical performance records for non-π-conjugated DUV NLO materials, including the strongest second harmonic generation (SHG) efficiency (about 4 times that of KDP), the largest birefringence (obv.: 0.07@589.3 nm) and the shortest SHG wavelength predicted as 188 nm.

Modified KBBF-like Material for Energy Storage Applications: ZnNiBO3(OH) with Enhanced Cycle Life

Not only are new and novel materials sought for electrode material development, but safe and nontoxic materials are also highly being intensively investigated. Herein, we prepare ZnNiBO₃(OH) (ZNBH), a modified and Be-free KBe₂BO₃F₂ (KBBF) family member as an effective electrode material. The novel ZNBH resembles the KBBF structure but with reinforced structure and bonding, in addition to well-incorporated conductive metals benefiting supercapacitor applications. The enhanced electronic properties of ZNBH are further studied by means of density functional theory calculations. The as-prepared ZNBH electrode material exhibits a specific capacity of 746 C g^-1 at a current density of 1 A g^-1. A hybrid supercapacitor (HSC) device is fabricated and successfully illuminated multiple color LEDs. Interestingly, even after being subjected to long charge-discharge for 10 000 cycles, the ZNBH//AC HSC device retains 97.2% of its maximum capacity, indicating the practicality of ZNBH as an electrode material.

Na4B8O9F10: A Deep-Ultraviolet Transparent Nonlinear Optical Fluorooxoborate with Unexpected Short Phase-Matching Wavelength Induced by Optimized Chromatic Dispersion

Exploring significant ultraviolet/deep-ultraviolet nonlinear optical (NLO) materials is hindered by rigorous and contradictory requirements, especially, possessing a moderate optical birefringence to meet phase-matching (PM). Except for suitable birefringence, small chromatic dispersion is also crucial to blue-shift the PM wavelength. Here, the introduction of fluorinated tetrahedral boron-centred chromophore strategy was proposed to optimize the chromatic dispersion. Herein, [BF4]- unit with large HOMO-LUMO band gap was introduced to Na-B-O-F system and Na4B8O9F10 was designed and synthesized successfully for the first time. Na4B8O9F10 with optimized chromatic dispersion can achieve a short second harmonic generation PM wavelength of 240 nm with a relatively small birefringence (cal. 0.036@1064 nm). Notably, Na4B8O9F10 is the first acentric crystal with [BF4]- unit among reported metal-fluorooxoborate system, involving isolated [BF4]- and novel [B7O10F6]5- fundamental building blocks.

Designing A New Infrared Nonlinear Optical Material, β-BaGa2 Se4 Inspired by the Phase Transition of the BaB2 O4 (BBO) Crystal

Infrared nonlinear optical (IR NLO) crystals play a significant role in the development of IR laser technology. But rationally designing a new IR NLO crystal remains a huge challenge because of the unpredictability of inorganic structures. Herein, inspired by phase transformation of the famous BaB₂ O₄ (BBO) crystal, a temperature-induced centrosymmetric (CS) to noncentrosymmetric (NCS) transformation approach is employed in CS BaGa₂ Se₄ to uncover a new NCS ternary chalcogenide, β-BaGa₂ Se₄ which can exhibit the well-balanced NLO properties, including moderate phase-matching SHG response (≈0.6×AgGaS₂ ), high LDT (10×AgGaS₂ ), and large birefringence (Δn=0.18 in the visible region). Therefore, β-BaGa₂ Se₄ will be a promising IR NLO crystal. The analysis for the structure-property relationship shows that the excellent NLO properties of β-BaGa₂ Se₄ mainly originate from edge-sharing GaSe₄ tetrahedral chains, which are different from B₃ O₆ groups in β-BBO and respect a new direction for the design of IR NLO crystals. So, we believe that the current research provides not only a new IR NLO crystal but also some insights for the design of new IR NLO crystals.

A General Principle for DUV NLO Materials: π-Conjugated Confinement Enlarges Band Gap*

Current nonlinear optical materials face a conventional limitation in the trade-off between the band gap and birefringence, especially in the deep UV spectral region. To circumvent this dilemma, we propose a general principle, π-conjugated confinement, to partially decouple the interunit π-conjugated interactions by the separation of non-π-conjugated units. The goal is to further enlarge the band gap to a value larger than that of the singular π-conjugated counterpart and to maintain a suitable density of π-conjugated units to gain a large optical anisotropy. We reveal that π-conjugated confinement is a shared structural feature for all DUV NLO materials known to date, and thus, it provides a novel and essential design criterion for future design synthesis. Guided by this principle, the carbonophosphates are predicted to be a new promising DUV candidate system. Sr₃ Y[PO₄ ][CO₃ ]₃ (1) and Na₃ X[PO₄ ][CO₃ ] (X=Ba, Sr, Ca, Mg, 2-5) exhibit not only greatly enhanced birefringence that is 3-24 times larger than that of singular phosphates but also enhanced band gaps that are 0.2-1.7 eV wider than those of singular carbonates.

Strong SHG Responses in a Beryllium-Free Deep-UV-Transparent Hydroxyborate via Covalent Bond Modification

Deep-ultraviolet (deep-UV) nonlinear optical (NLO) crystals are key materials in creating tunable deep-UV lasers for frequency conversion technology. However, practical application of the sole usable crystal, KBe₂ BO₃ F₂ , has been hindered by the high toxicity of beryllium and its layering tendency in crystal growth. Herein, we report a beryllium-free deep-UV NLO material NaSr₃ (OH)(B₉ O₁₆ )[B(OH)₄ ] (NSBOH), synthesized by a covalent bond modification strategy under hydrothermal conditions. Moisture-stable NSBOH exhibits strong second-harmonic generation (SHG) at 1064 nm (3.3 × KH₂ PO₄ ) and 532 nm (0.55 × β-BaB₂ O₄ ), both amongst the largest powder SHG responses for a deep-UV borate, with good phase-matchability and a short wavelength cutoff edge (below 190 nm). NSBOH possesses a 3D covalent anionic [B₉ O₁₉ ]_∞ honeycomb-like framework with no layering. The Sr²⁺ and Na⁺ ions, residing in the cavities of the anionic framework, act as templates for the assembly and favorable alignment of NLO-active groups, resulting in an optimal balance between strong SHG activities and wide UV transparency. These merits indicate NSBOH is a very attractive candidate for deep-UV NLO applications.

Novel van der Waals Deep-UV Nonlinear Optical Materials

Van der Waals (vdW) deep-UV (DUV) nonlinear optical (NLO) crystal is an important material system recently developed. Herein, we review its concept and original intention, and then summarized the discovery process of related materials, including the role of A-site cations and the resulting two-/one-dimensional vdW DUV NLO systems. Finally, we evaluate the practical DUV NLO performance and prospected the opportunities and challenges.

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.

Deep-Ultraviolet Nonlinear-Optical van-der-Waals Beryllium Borates*

Van-der-Waals (vdW) deep-ultraviolet (DUV) nonlinear-optical (NLO) materials hold great potential to extend DUV NLO applications to two dimensions, but they are rare in nature. In this study, we propose a design principle to realize vdW DUV NLO materials via structural evolution from the non-vdW (BO₃ )-(BeO₃ F) layers in KBe₂ BO₃ F₂ (KBBF) to the vdW (BO₃ )-(BeO₄ H) layers in berborite Be₂ BO₅ H₃ (BBH) and the vdW (BO₄ )-(BeO₄ ) layers in beryllium metaborate BeB₂ O₄ (BEBO). Based on first-principles calculations, the fundamental NLO properties of BBH and BEBO demonstrate that a balanced DUV NLO performance can be achieved in these two systems. Importantly, BBH, a layered material existing in nature, can achieve an available DUV phase-matched output with strong second harmonic generation (SHG) for 177.3/193.7 nm DUV lasers, which is almost identical to that of KBBF. Remarkably, BEBO shows an excellent DUV SHG capacity and an even shorter phase-matching wavelength than KBBF. Therefore, the newly discovered vdW BBH and BEBO, once verified by experiments, could provide an ideal platform to study DUV NLO effects in three dimensions and two dimensions.

Discovery of First Magnesium Fluorooxoborate with Stable Fluorine Terminated Framework for Deep-UV Nonlinear Optical Application

The generated light can be tuned to cover almost the entire spectral range from deep-ultraviolet to terahertz wavelengths by utilizing the nonlinear optical crystals with a simple frequency doubling process. Among them, the discovery of novel candidates for the production of deep-ultraviolet light is by extension a great challenge toward realizing the vast potential. Actually, the availability for this process mainly depends on whether the critical performance can be well coexisted in one practical crystal. Herein, the first magnesium fluorooxoborate MgB₅ O₇ F₃ was synthesized as a new competitive candidate for deep-ultraviolet nonlinear optical application. It has a sufficiently large nonlinearity and a deep-ultraviolet phase matching wavelength, indicating that it holds great potential for the production of coherent light below 200 nm. The critical performance enhancement of MgB₅ O₇ F₃ when compared with its isomorphic phases was demonstrated and discussed. More importantly, we proposed that fluorooxoborate system with the general formula of MB₅ O₇ F₃ (M=divalent metal) possesses stable fluorine terminated framework, which makes them tend to retain their crystallized space groups unchanged.

Prediction of Novel van der Waals Boron Oxides with Superior Deep-Ultraviolet Nonlinear Optical Performance

Deep-ultraviolet nonlinear optical (DUV NLO) materials are attracting increasing attention because of their structural diversity and complexity. Using the two-dimensional (2D) crystal structure prediction method combined with the first-principles calculations, here we propose layered 18-membered-ring (18MR) boron oxide B₂ O₃ polymorphs as high-performance NLO materials. 18MR-B₂ O₃ with the AA and AB stackings are potential DUV NLO materials. The superior performing 18MR-B₂ O₃ ^AB has an unprecedentedly high second harmonic generation coefficient of 1.63 pm V^-1 , the largest among the DUV NLO materials, three times larger than that of the advanced DUV NLO material KBe₂ BO₃ F₂ and comparable to that of β-BaB₂ O₄ . Its unusually large birefringence of 0.196 at 400 nm guarantees the phase-matching wavelength λ_PM to reach this material's extreme absorption edge of ≈154 nm.

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.

Anionic Aliovalent Substitution from Structure Models of ZnS: Novel Defect Diamond-like Halopnictide Infrared Nonlinear Optical Materials with Wide Band Gaps and Large SHG Effects

To design pnictide nonlinear optical materials with wide band gap and large second-harmonic generation, the heavy halogen I was introduced into pnictides through anionic aliovalent substitution with diamond-like ZnS as templates. Thus, four excellent halopnictide-based infrared nonlinear optical crystals, M^II ₃ PnI₃ (M^II =Zn, Cd; Pn=P, As), were obtained. They all exhibited defect diamond-like structures with highly parallel-oriented [M^II PnI₃ ] mixed-anionic tetrahedral groups, leading to excellent physical properties including wide band gaps (2.38-2.85 eV), large second harmonic generation responses (2.7-5.1×AgGaS₂ ), high laser-induced damage thresholds (5.5-10.7×AgGaS₂ ), and good IR transparency. In particular, Cd₃ PI₃ and Cd₃ AsI₃ achieved phase-matching (Δn=0.035 and 0.031) that their template β-ZnS could not do. Anionic aliovalent substitution provides a feasible strategy to design novel promising halopnictide IR NLO materials.

(NH4 )Bi2 (IO3 )2 F5 : An Unusual Ammonium-Containing Metal Iodate Fluoride Showing Strong Second Harmonic Generation Response and Thermochromic Behavior

An ammonium-containing metal iodate fluoride compound, (NH₄ )Bi₂ (IO₃ )₂ F₅ , featuring a two-dimensional double-layered framework constructed by [BiO₂ F₅ ]^6- and [BiO₄ F₄ ]^9- polyhedra, as well as [IO₃ ]^- groups, was successfully synthesized. The well-ordered alignment of these SHG-active units leads to an extraordinary strong SHG response of 9.2 times that of KDP. Moreover, this compound possesses a large birefringence (Δn=0.0690 at 589.3 nm), a wide energy band gap (E_g =3.88 eV), and a high laser damage threshold (LDT; 40.2×AgGaS₂ ). In particular, thermochromic behavior was observed for the first time in this type of compound. Such multifunctional crystals will expand the application of nonlinear optical materials.

LiMII (IO3 )3 (MII =Zn and Cd): Two Promising Nonlinear Optical Crystals Derived from a Tunable Structure Model of α-LiIO3

Excellent nonlinear optical materials simultaneously meet the requirements of large SHG response, phase-matching capability, wide transparency windows, considerable energy band-gap, good thermal stability and structure stability. Herein, two new promising nonlinear optical (NLO) crystals LiM^II (IO₃ )₃ (M^II =Zn and Cd) are rationally designed by the aliovalent substitution strategy from the commercialized α-LiIO₃ with the perfect parallel alignment of IO₃ groups. Compared with parent α-LiIO₃ and related A^I ₂ M^IV (IO₃ )₆ , the title compounds exhibit more stable covalent 3D structure, and overcome the racemic twinning problem of A^I ₂ M^IV (IO₃ )₆ . More importantly, both compounds inherit NLO-favorable structure merits of α-LiIO₃ and show larger SHG response (≈14× and ≈12×KDP), shorter absorption edge (294 and 297 nm) with wider energy band-gap (4.21 and 4.18 eV), good thermal stability (460 and 430 °C), phase-matching behaviors, wider optical transparency window and good structure stability, achieving an excellent balance of NLO properties.

Ba10In6Zn7S26- nZnS: An Inorganic Composite System with Interface Phase-Matching Tuned for High-Performance Infrared Nonlinear Optical Materials

Mid- and far-infrared nonlinear optical (MFIR NLO) materials are important in modern laser technologies. However, it is very challenging to develop materials that can achieve a subtle balance between the key requirements, such as large NLO response, high laser-induced damage threshold (LIDT), wide IR transparency, and phase-matching. In this work, a new wide IR transparency (0.38-15.3 μm) NLO crystal Ba₁₀In₆Zn₇S₂₆ (SS26) is synthesized. Further, its composite system Ba₁₀In₆Zn₇S₂₆- nZnS is synthesized by eutectic reaction. In particular, Ba₁₀In₆Zn₇S₂₆-14ZnS (SS40) shows excellent balanced NLO performance that includes a large band gap of 3.05 eV, high LIDT (13.3 × AgGaS₂), large second harmonic generation (SHG) response (2.1 × AgGaS₂ at 2050 nm, 5.2 × KDP at 1064 nm), and wide optical transmission window (0.37-15.4 μm). Importantly, the phase-matching condition is realized for SS40 by interfaces formed between the crystal face (112) of matrix SS26 and the crystal face (111) of reinforcement cubic ZnS by topological chemical reaction, and the NLO performance can be tuned by different concentrations of ZnS. First-principles simulations are employed to study NLO properties of SS26 and the interfaces. This work demonstrates that SS40 is a promising MFIR NLO material, and tuning components of the composite material system is a useful way to develop new MFIR NLO materials with excellent comprehensive performance.