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Deep ultraviolet (DUV) nonlinear optical (NLO) crystals have important applications in lithography, microfabrication, and high-resolution photoelectric spectrometer. In recent years, significant progress has been made in both the theoretical design and experimental research of DUV NLO crystals. This review aims to offer a comprehensive perspective on exploring a new generation of DUV NLO materials. First, we summarize various computer-aided strategies for crystal structure design and emphasize their significant role in advancing the discovery of DUV NLO materials. Then, we outline several representative DUV crystals of experimental synthesis. Finally, we discuss the future prospects for exploring new generations of DUV NLO materials. We believe that employing data-driven, computer-aided methods to explore DUV NLO materials will help address the current challenges in the field of DUV NLO materials research. The close integration of calculation and experimentation will unlock new opportunities. [ABSTRACT FROM AUTHOR]

There is still a high demand for new deep ultraviolet (deep-UV) nonlinear optical (NLO) materials, but synthesizing an ideal deep-UV NLO crystal remains a significant challenge. In this work, three borophosphate crystals, LiAB2P2O9 (A = NH4, K) and LiNaB2P2O8(OH)2·H2O, have been obtained via a solid-state reaction in a closed system. LiAB2P2O9 (A = NH4, K) crystallize in the acentric P212121 space group with a three-dimensional (3D) anion framework, while LiNaB2P2O8(OH)2·H2O belongs to the C2 space group with a two-dimensional (2D) anion framework. The mechanism of the nonpolar to polar space group transition of LiAB2P2O9 (A = NH4, K) and LiNaB2P2O8(OH)2·H2O is investigated. Furthermore, the structure of borophosphates with a B/P ratio of 1 : 1 is discussed, which possess a variety of functional building blocks (FBBs) and connection modes. LiAB2P2O9 (A = NH4, K) exhibit short cutoff edges (< 200 nm), and second harmonic generation (SHG) responses of 0.4, and 0.9 × KH2PO4 (KDP), respectively. Additionally, structure–function correlation was investigated through theoretical calculations. This work offers valuable insight for the subsequent exploration of borophosphate deep-UV NLO materials. [ABSTRACT FROM AUTHOR]

Birefringence is crucial for birefringent crystals to modulate polarized light and nonlinear optical crystals to enable phase-matching. It is closely related to the arrangement of advantageous functional groups with large microcosmic polarizability anisotropy in the structure. Currently, designing crystals with large birefringence by modulating the arrangement of advantageous functional groups is still a challenge. In this work, the arrangement of the advantageous [B3O7] groups is optimized by introducing rigid tetrahedral [SO4] groups, resulting in a novel boron-rich borosulfate, Na2B6SO13, which exhibits unprecedented [B6SO13]∞ double chain and an enhanced birefringence (Δn = 0.07 at 589.3 nm) against the borate parent LiB3O5. Theoretical calculations confirm that the birefringence is mainly attributable to the uniform alignment of [B3O7] groups. Na2B6SO13 also possesses a deep ultraviolet (DUV) cutoff edge (λ < 185 nm). This work provides a feasible method to design DUV birefringent crystals with large birefringence by inducing the arrangement of advantageous functional groups. [ABSTRACT FROM AUTHOR]

Deep‐ultraviolet (UV) birefringent materials are urgently needed to facilitate light polarization in deep‐UV lithography. Maximizing anisotropy by regulating the alignment of functional modules is essential for improving the linear optical performance of birefringent materials. In this work, we proposed a strategy to design deep‐UV birefringent materials that achieve functional module ordering via weak interactions. Following this strategy, four compounds CN4H7SO3CF3, CN4H7SO3CH3, C(NH2)3SO3CH3, and C(NH2)3SO3CF3 were identified as high‐performance candidates for deep‐UV birefringent materials. The millimeter‐sized crystals of CN4H7SO3CF3, CN4H7SO3CH3, and C(NH2)3SO3CH3 were grown, and the transmittance spectra show that their cutoff edges are below 200 nm. CN4H7SO3CF3 exhibits the largest birefringence (0.149 @ 546 nm, 0.395 @ 200 nm) in the deep‐UV region among reported sulfates and sulfate derivatives. It reveals that the hydrogen bond can modulate the module ordering of the heteroleptic tetrahedra and planar π‐conjugated cations, thus greatly enhancing the birefringence. Our study not only discovers new deep‐UV birefringent materials but also provides an upgraded strategy for optimizing optical anisotropy to achieve efficient birefringence. [ABSTRACT FROM AUTHOR]

A novel borate Ca3Al2B8O18, with two types of topological six-membered rings (6-MRs) in its graphene-like layer, was successfully fabricated by introducing [AlO4] tetrahedra. The 6-MRs [B5AlO17] in Ca3Al2B8O18 can be identified as unique ones, and Ca3Al2B8O18 exhibits a new fundamental building block [B8O19] greatly enriching the structural diversity of B–O configurations. [ABSTRACT FROM AUTHOR]

Finding birefringent material with giant optical anisotropy is urgent for photonic applications. However, the application of existing birefringent materials is limited by restricted transparency wavelength or optical anisotropy. The process of trial and error is time‐consuming and inefficient in terms of resources, especially when compared to the vast design spaces for materials. The immense chemical space necessitates a high‐throughput approach for giant optical anisotropy materials design. Here, a full‐applied wavelength map of birefringent materials is constructed, from conventional visible expanding to ultraviolet and deep‐ultraviolet region by high‐throughput screening methods. And highlighted 579 materials birefringence exceeds the one of available commercial birefringent materials in corresponding regions. Significantly, 54 materials possess giant birefringence above 0.5 @1064 nm. Via developing a high‐throughput screening technique for microscopic structures with given coordination number, the study further characterizes a comprehensive map of functional modules featuring birefringent activeness. According to the macroscopic and microscopic dual‐functional maps, and a novel crystal Li2(HC3N3S3)∙5H2O with birefringence difference 0.532 @546 nm is designed and synthesized successfully. This study identifies desired materials with high performance in unheeded chemical spaces. [ABSTRACT FROM AUTHOR]

As an exceptional optical gene, the [PO4] tetrahedron is indispensable in the field of optical crystals due to its gain in the band gap of solid materials. However, the high symmetry of [PO4] tetrahedron hinders the achievement of large optical anisotropy in the lattice for phosphate crystal forms. In this work, the heteroleptic tetrahedra strategy, which involves replacing some of the oxygen atoms on regular oxy‐tetrahedra, is proven to be a feasible approach for preserving the wide transmission of the original [PO4] tetrahedra and improving the polarizability anisotropy. Based on this, eight methylphosphates are designed and synthesized for the phosphate system by substituting a [CH3] group for the O atom on the [PO4] tetrahedra. Theoretically, as compared to [PO4] units, [CH3PO3] and [CH3PO3H] units can improve the polarizability anisotropy, particularly [CH3PO3H] units, which have the potential to be birefringence‐active. The strong birefringence (exp. 0.108@546.1 nm) and short deep‐UV cutoff edge (195 nm) of nonmetallic methylphosphates [C(NH2)3][CH3PO3H] in the series suggest that it may be a possible short‐wave UV birefringent crystal. In addition to providing new birefringence‐active units for optical material design, this study validates the viability of modifying tetrahedra to improve inherent short board of nearly rigid [PO4] tetrahedra in phosphate family. [ABSTRACT FROM AUTHOR]

Birefringent materials play indispensable roles in modulating the polarization of light and are vital in laser science and technology. Units with π‐conjugation are regarded as potential functional modules for large optical anisotropy, such as planar [BO3]3−, [B3O6]3−, and [B3O3(OH)3]. However, the discovery of deep‐ultraviolet (DUV, λ ≤200 nm) birefringent materials still faces a serious challenge due to the limited band gap. Replacing hydroxyl anions [OH]− with fluorine anions F− in borates can cause the blueshift of the ultraviolet (UV) cutoff edge, however, the effect of this replacement on π‐conjugation units has not been systematically studied. Herein, based on template materials metaboric acid α‐B3O3(OH)3 with planar [B3O3(OH)3] units, we proposed an OH/F substitution strategy designing potential DUV birefringent materials. The designed B3O3(OH)2F, B3O3(OH)F2, and B3O3F3 exhibit large birefringence from 0.123 to 0.146 at 546 nm with DUV transparency based on first‐principles calculations, exceeding the performance of traditional α‐BaB2O4. Further electronic structure analysis reveals the band edge and bonding difference between [OH]− and F−, indicating the corresponding designed novel [BO2F], [B3O3(OH)2F], [B3O3(OH)F2] and [B3O3F3] units can act as potential DUV birefringent‐active functional modules with large structure anisotropy. This work offers the chemical difference of F− and [OH]− anions, and design‐strategy of new DUV birefringent materials. [ABSTRACT FROM AUTHOR]

Borates have emerged as a significant resource for exploring birefringent materials in the UV frequency region. The π‐conjugated planar anions, such as the [BO3], [B2O5], and [B3O6] units, exhibiting strong polarizability anisotropy, are traditionally good structural models. Both fluorination and hydroxylation have been proven effective strategies to modulate the optical properties of the planar anions. Controlling the ratio between F and OH is the key to realizing a balanced performance between birefringence and bandgap. Based on a fluorination mechanism proposed recently, a novel hydroxyfluorooxoborate (C3N2H5)B3O3F2(OH)2 with an optimal F/OH ratio of 1/1 is synthesized. During the synthesis, another strategy known as "cation activation" is also employed, which further increases birefringence by introducing active cation units. With its superior performance, this compound exhibits a short UV cutoff edge at 214 nm and a substantial birefringence of 0.205 at 546 nm, surpassing the commercial birefringent crystal α‐BBO (0.122@532 nm). The synergy between F/OH‐ratio optimization and cation activation offers a practical methodology for developing UV birefringent materials. [ABSTRACT FROM AUTHOR]

Since the study of short‐wave ultraviolet nonlinear optical crystals based on the quasi‐phase‐matching principle will effectively expand the development field of nonlinear optical materials, it is urgent to achieve their directional design. Here, it is proposed and implemented on account of the hexagonal wurtzite structure. Thus, the LiRbSeO4 crystal is obtained with [SeO4] groups as the main tetrahedra and [LiO4] as the separation. It exhibits a wide transparency window (0.22–5.57 µm) and an excellent nonlinear optical effect (≈1.7 × KH2PO4), which originates from the synergy of distorted tetrahedral groups and their uniform arrangement. More importantly, the LiRbSeO4 crystal is confirmed to be a ferroelectric with a typical polarization‐electric field hysteresis loop (a coercive field of 27.71 kV cm−1, a remanent polarization of 8.60 µC cm−2 at 373 K). Its ferroelectricity is further identified by the domain engineering studies, involving piezoelectricity and the chirality. Besides, ferroelectricity is attributed to the ordered arrangement of tetrahedra with the polarization reversal involving atomic displacements. Therefore, the LiRbSeO4 crystal is a promising short‐wave ultraviolet nonlinear optical candidate based on the quasi‐phase‐matching principle. This work also provides a new horizon for the design of nonlinear optical crystals. [ABSTRACT FROM AUTHOR]

We herein reported our strategy to assemble planar π-conjugated [B(OH)3] and [C2O4]2−/[HC2O4]−/[C4O4]2− functional groups to explore short-wavelength birefringent crystals. Three compounds, K2C2O4·B(OH)3, Cs4(HC2O4)2(C2O4)·[B(OH)3]2 and K(C4O4)0.5·B(OH)3, which all exhibit a layered structure favorable for generating large optical anisotropy, were synthesized and characterized. The strategy of assembling π-conjugated [B(OH)3] and C–O functional modules shows the potential to explore promising UV birefringent materials. [ABSTRACT FROM AUTHOR]

Two borates, K3Al2B11O21 (KABO) and K3Al2MoB9O21 (KAMBO), with edge-sharing [BO4] (ES-[BO4]) tetrahedra and different B–O configurations in one structure, were rationally designed and prepared at atmospheric pressure by introducing covalent tetrahedra. KABO is the first borate identified to have ES-[BO4] tetrahedra and two different dimensional B–O groups, namely, isolated [B6O12] groups with ES-[BO4] and one-dimensional infinite chains. In addition, KAMBO is a rare borate that features ES-[BO4] tetrahedra and two distinct isolated B–O groups ([B6O12] and [B3O6]). The discovery of these two compounds not only enriches the structures of borates, but also suggests a novel method for creating borates with ES-[BO4] tetrahedral structures and flexible B–O configurations at atmospheric pressure. [ABSTRACT FROM AUTHOR]