High-Q MgF₂ whispering gallery mode resonators for refractometric sensing in aqueous environment

[CON3H6BF4]: An Efficient Metal-Free Borate with Optimized Birefringence Achieved by Structural Design Strategy

Deep-ultraviolet (UV) birefringent materials are urgently needed for advancing light polarization in deep-UV lithography. In crystal engineering, maximizing anisotropy by classical template alteration demonstrates its efficiency in achieving superior performance characteristics. Consequently, a novel planar deep-UV functional building unit (FBU), [CON3H5], is proposed, acquired by amino modulation within the urea template. This article presents a methodical investigation of novel fluoroborate [(CON3H6)BF4] (AUBF), designed by unifying a planar π-conjugated functional cation with a fully fluorinated non-π-conjugated tetrahedron in a single system. The compound exhibits well-balanced properties, owing to its parallel molecular arrangement, and has been characterized as a high-performing deep-UV birefringent material. As is known to all, it is the first semi-organic urea-based compound to reach the deep-UV region with a cutoff edge at 196 nm, and a substantial birefringence of 0.127@546 nm, similar to that of the previously reported urea-based compounds as well as commercial birefringent crystal α-BBO (0.123@546 nm). This work not only identifies a novel birefringent gene with improved optical anisotropy but also opens the door for synthesizing novel deep-UV birefringent materials via a structural regulation design strategy.

NH4PO3HCH3 and NH4PO3CH2NH3: Two Phosphate Derivatives with Deep-UV Cutoff Edges and Moderate Birefringence

To maintain moderate birefringent properties without compromising the wide optical transmittance window characteristic of phosphate materials, a crucial approach focuses on designing innovative functional units with great performance. This research has demonstrated that [PO3HCH3] and [PO3CH2NH3] units serve as effective birefringent-active components in the DUV regions. This study successfully prepared two novel crystalline materials, NH4PO3HCH3 and NH4PO3CH2NH3, which demonstrated both deep-UV transparency and moderate birefringence. These properties exceed those of conventional MgF2 crystals and the majority of known phosphate compounds in the DUV region. Computational investigations indicate that the observed birefringence is from the [PO3HCH3] and [PO3CH2NH3] building blocks, highlighting the potential of tetrahedral coordination environment engineering as a viable strategy for developing new optical functional units.

Metal Cation Engineered UV Birefringent Crystals in π-Conjugated Sulfonates with Record-High Optical Anisotropy

Exploring excellent UV birefringent crystals is very urgent for use in advanced optical technologies to manipulate light polarization of great significance. Planar π-conjugated groups are of great interest to researchers in the exploration of optical materials with pronounced birefringence (∆n). However, the systematic understanding of cation and anion groups-regulated structure-property relationships requires further investigation. Herein, metal cation engineering is applied to achieve controlled synthesis of novel compounds, enabling precise coordination modulation and structure-property correlation studies. In this work, eight new 3-pyridinesulfonate crystals with the formula A(3-C5H4NSO3)x·yH2O (A = Li, Na, Mg, Ca, Zn, Sr, and Ba; x = 1 and 2, y = 0, 4, and 6) and Na(3-C5H4NSO3)·2NaBF4·(3-C5H4NSO3H) are synthesized and characterized. Notably, these crystals exhibit strong optical anisotropy, with birefringence (∆n) values ranging from 0.106 to 0.345 (measured at 546 nm) and short UV cut-off edges below 280 nm. Specifically, Li(3-pySO3) exhibits an exceptionally large birefringence (∆n = 0.345 at 546 nm), which represents the highest value within the metal-containing 3-pyridinesulfonate system and surpasses that of most commercially available birefringent crystals. Moreover, this analysis of the eight crystals elucidates a comprehensive cation-structure-birefringence correlation framework, thereby establishing a cation-driven design principle for high-performance birefringent crystals.

Breaking Performance Barriers in KBe2BO3F2 (KBBF) Analogs by Functional Group Self-Polymerization

Enhancing the conversion efficiency of all-solid-state lasers through the rational design of crystal materials with superior linear and nonlinear optical (NLO) properties remains a formidable challenge. Herein, we present a novel approach to optimizing these properties in KBe2BO3F2 (KBBF)-analog crystals via functional group self-polymerization. This strategy led to the synthesis of two new optical crystals: noncentrosymmetric CsAs2O3Br and centrosymmetric CsAs4O6Br. By incorporating highly optically active [AsO3]3- units into the classical 2D [Be2BO3F]∞ - framework, we facilitated the self-assembly of [As2O3]∞ layers, forming a densely packed and highly ordered structure that enhances macroscopic optical activity. CsAs2O3Br exhibited an extraordinary second-harmonic generation (SHG) response, 20.5 times stronger than KH2PO4 (KDP), while CsAs4O6Br demonstrated exceptional birefringence (0.26 at 546 nm), setting new performance benchmarks among KBBF analogs. Theoretical analyses reveal that these superior properties arise from the efficient alignment and high density of self-polymerized functional units. This work represents a significant advancement in the design of high-performance UV NLO materials, particularly for fourth-harmonic generation, and paves the way for future innovations in photonic technologies, including solar-blind UV laser systems and advanced photonic devices.

Enhanced Birefringence and Excellent Thermal Stability in Two Tellurium(IV) Vanadates: KTeOVO4 and Ca2TeV2O9

Two tellurium(IV) vanadates, KTeOVO4 and Ca2TeV2O9, were synthesized by integrating Te4+ with lone-pair of electrons and V5+ exhibiting the second-order Jahn-Teller effect (SOJT) active centers. Both compounds feature unique one-dimensional chain structures formed by [TeO4] and [VOx] polyhedra, resulting in notable birefringence values of 0.162 and 0.128 at 546 nm, respectively, higher than those of commonly used materials like α-BaB2O4 (0.116@1064 nm) and LiNbO3 (0.074@1440 nm). Additionally, they demonstrate excellent thermal stability (up to 500 and 700 °C) and broad transparency ranges (0.39-10.00 μm and 0.53-10.68 μm), enabling potential applications in near- and mid-infrared optical devices. Structural and theoretical analyses reveal that the enhanced birefringence arises from the synergistic effects of Te4+ lone-pair electrons and the anisotropic polarizability of V5+ centers.

Dual Lone Pair Strategy for High Birefringence in Antimony(III)-Based Tellurite Sulfates with Short-Wave UV Transparency

We present two novel antimony(III)-based tellurite sulfate crystals, Sb2(TeO3)(SO4)2-P1̅ (I) and Sb2(TeO3)(SO4)2-P21/c (II), synthesized using a dual lone pair strategy that incorporates Sb3+ and Te4+ ions into a sulfate framework. This approach significantly enhances the birefringence of these compounds, with values of 0.11 and 0.10 at 546 nm, respectively, surpassing those of many previously reported mixed alkali metal/alkaline earth metal sulfates and tellurites. Both compounds also exhibit excellent UV transparency with cutoff edges at 292 and 294 nm, making them promising candidates for short-wave UV optical applications. Structural and theoretical analyses reveal that the enhanced birefringence arises from the synergistic effects of the stereochemically active lone pairs of Sb3+ and Te4+, which contribute to the optical anisotropy. This work highlights the potential of dual SCALP elements in improving the optical properties of sulfate-based crystals and provides new insights into designing high-performance optical materials with tailored properties for advanced UV technologies.

Sulfate Derivatives with Heteroleptic Tetrahedra: New Deep-Ultraviolet Birefringent Materials in which Weak Interactions Modulate Functional Module Ordering

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.

The New Paradigm of Ligand Substitution-Driven Enhancement of Anisotropy from SO4 Units in Short-Wavelength Region

For non-π-conjugated [SO4] units, it is challenging to generate sufficient birefringence, owing to the high symmetry of the regular tetrahedron. Unlike the traditional trial-and-error approach, we propose a new paradigm for birefringence engineering to tune the optical properties based on [SO4] units. Through the strategy of ligand substitution, we can predict its effect on the band gap and anisotropy. Theoretical evaluations reveal generalized results that the anisotropic electron distribution of new functional groups induced by the suitable ligand substitution contributes to the band gap and birefringence. To further validate the correctness of the paradigm, we experimentally synthesized and characterized nine novel compounds with selected functional modules. By the new paradigm of ligand substitution, they can reach up to 4-6 times the birefringence of the corresponding sulfate and maintain the wide bandgap. Through rational design, (CN4H7)SO3NH2 exhibits about 35 times the birefringence of Li2SO4, which is a significant order of magnitude improvement and verifies the superiority of our proposed paradigm. This work provides a new paradigm for the modification to the non-π-conjugated group and will guide and accelerate the exploration of novel birefringent crystals in the short-wavelength region.

[C(NH2)3]2S2O6: A SBBO-Like Dithionate Crystal with Large Optical Anisotropy

Investigating ultraviolet (UV) birefringent crystals is a focal point of research in recent years. The development of superior birefringent materials faces substantial challenges, primarily due to the need to pinpoint optimal fundamental building blocks and to perfect their geometric arrangement within the crystal lattice. By selecting a planar π-conjugated [C(NH2)3] group and a staggered [S2O6] group, we have successfully synthesized an organic-inorganic hybrid crystal, [C(NH2)3]2S2O6. Similar to the famous inorganic crystal Sr2Be2B2O7 (SBBO), [C(NH2)3]2S2O6 exhibits a two-dimensional double-layered crystal structure connected by N-H···O hydrogen bonding. Due to the ideal spatial arrangements of the planar π-conjugated [C(NH2)3] group, this crystal displays a large birefringence (0.150 at 546 nm) among sulfate derivatives in the short-wave UV region. Meanwhile, [C(NH2)3]2S2O6 has a large band gap of 5.23 eV. This work indicates that the [S2O6] unit, acting as a direct structural motif, is beneficial for the discovery of UV birefringent crystals.

Exploring short-wavelength birefringent crystals via triggering cooperative arrangement between different π-conjugated groups

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.

C9H7NBrX (X = Cl, Br, NO3): Three Excellent Birefringent Crystals with Distinct Optical Anisotropy Regulated by Anions

A large optical anisotropy is the most important parameter of birefringent crystals. Integrating π-conjugated groups with large polarizable anisotropy into target compounds is a common strategy for constructing brilliant birefringent crystals. However, the key problem is to enhance the density of the birefringence-active units and further arrange them parallelly. In this study, three novel birefringent crystals, C9H7NBrX (X = Cl, Br, NO3), are successfully synthesized by introducing a new birefringence-active [C9H7NBr]+ unit. Interestingly, these compounds feature similar layered structures but exhibit different optical anisotropies at 550 nm (0.277 for C9H7NBrCl, 0.328 for C9H7NBrBr, and 0.401 for C9H7NBrNO3) owing to the different anions in them. Particularly, the small trigonal planar NO3 anions perfectly fill the interstices of the π-conjugated [C9H7NBr]+ groups with large optical anisotropy, with the resulting compound C9H7NBrNO3 showing superior optical properties compared to the others. The above findings provide strategies for designing new optical materials with large birefringence by matching birefringence-active groups of different sizes. Additionally, a new theory for predicting and comparing the polarizability anisotropy of compounds is proposed, which would guide in exploring large birefringent crystals.

Optimization of Functional Building Blocks Generates a Substantial Improvement in Birefringence from Sn2OSO4 to Sn3O2(OH)(HSO4)

In this work, two tin(II)-based sulfates, Sn2OSO4 and Sn3O2(OH)(HSO4), were synthesized via the mild hydrothermal method. Both compounds employ the Sn2+ cation with stereochemically active lone pair (SCALP) electrons and non-π-conjugated tetrahedral anionic groups SO4 as the functional structural blocks. Interestingly, the experimental birefringence of Sn3O2(OH)(HSO4) is 0.169@546 nm, approximately 42 times larger than that of Sn2OSO4, which is 0.004@546 nm. Detailed structural analysis and theoretical calculations suggest that this significant birefringence difference arises from the optimization of functional building blocks in coordination environments and spatial arrangements. Furthermore, both compounds exhibit ultraviolet absorption edges at 308 and 307 nm, respectively. This indicates that Sn3O2(OH)(HSO4) has the potential to be a candidate for an ultraviolet (UV) birefringent crystal. This study offers inspiration for further exploration of tin(II)-based compounds with excellent comprehensive properties.

Tunable Optical Anisotropy in Rare-Earth Borates with Flexible [BO3] Clusters

Borates have garnered a lot of attention in the realm of solid-state chemistry due to their remarkable characteristics, in which the synthesis of borates with isolated [BO3] by adding rare-earth elements is one of the main areas of structural design study. Five new mixed-metal Y-based rare-earth borates, Ba2ZnY2(BO3)4, KNa2Y(BO3)2, Li2CsY4(BO3)5, LiRb2Y(BO3)2, and RbCaY(BO3)2, have been discovered using the high-temperature solution approach. Isolated [BO3] clusters arranged in various configurations comprise their entire anionic framework, allowing for optical anisotropy tuning between 0.024 and 0.081 under 1064 nm. In this study, we characterize the relative placements of their [BO3] groups and examine how their structure affects their characteristics. The origin of their considerable optical anisotropy has been proven theoretically. This study unequivocally demonstrates that even a slight alteration to borates' anionic structure can result in a significant improvement in performance.

Sb2O2SeO3 and Sb2O(SeO3)2: Two-Layered Antimony(III) Selenites with Enhanced Birefringence

Two antimony selenites, Sb2O2SeO3 and Sb2O(SeO3)2, were synthesized by simultaneously incorporating stereochemically active lone pair electrons containing SeO32- and Sb3+. These compounds are structured with [SbOx] polyhedra and [SeO3] units within a two-dimensional framework. Both of them exhibit cutoffs at 300 and 330 nm within the ultraviolet (UV) range and demonstrate significant birefringence, with indices of 0.069 and 0.126 at 546 nm, respectively. These properties highlight their potential as UV birefringent materials. Structural analyses and theoretical calculations reveal that their exceptional birefringence results from the synergistic interactions between SeO32- anions and Sb3+ cations.

(C8H7N2O2)2[Bi2Br8]·2H2O and (C8H7N2O2)6[Bi2Cl10]Cl2·2H2O: Exploring Birefringent Crystals in Hybrid Halide Systems

In this study, new hybrid birefringent crystals of (C8H7N2O2)2[Bi2Br8]·2H2O and (C8H7N2O2)6[Bi2Cl10]Cl2·2H2O were successfully synthesized by introducing a new birefringent group [C8H7N2O2]+ by a simple aqueous solution evaporation method. They crystallize in the P21/n space group, and their structure consists mainly of the π-conjugated group [C8H7N2O2]+ and the octahedron centered on Bi3+. By first-principles calculations, the birefringence response comes from the [C8H7N2O2]+ group with a planar π-conjugated structure. Meanwhile, the synthesis, structure, first-principles calculations, and optical properties are reported in this paper.

Na3K6(CO3)3(NO3)2X·6H2O (X = NO3, Cl, Br): Exploring High-Performance UV Birefringent Crystals Induced by Coplanar π-Conjugated CO3 and NO3 Groups

Planar π-conjugated groups, like CO3, NO3, and BO3 triangles, are ideal functional units for designing birefringent materials due to their large optical anisotropy and wide band gap. The key point for designing birefringent crystals is to select appropriate functional building blocks (FBBs) and the proper arrangement mode. It is well known that the substitution strategy has proven to be a promising and accessible approach. In this work, alkali metals were chosen to regulate and control two different π-conjugated groups, CO3 and NO3, to build new compounds with large birefringence. Subsequently, three new compounds, Na3K6(CO3)3(NO3)2X·6H2O (X = NO3, Cl, Br), were successfully synthesized using the hydrothermal method. The aliovalent substitution between the [NO3]- anionic group and halogen anions [Cl]-/[Br]- has been achieved in these compounds. Na3K6(CO3)3(NO3)2X·6H2O feature the well-coplanar CO3 and NO3 groups in their crystal structure. This coplanar arrangement mode may effectively enhance the anisotropic polarizability of Na3K6(CO3)3(NO3)2X·6H2O. And their experimental birefringence can reach 0.094-0.131 at 546 nm. Diffuse reflectance spectra demonstrate that these compounds exhibit short ultraviolet (UV) absorption edges of ∼235 nm. Meanwhile, Na3K6(CO3)3(NO3)2X·6H2O also have an easily grown capacity under facile conditions. This work not only reports three new potential UV birefringent crystals but also provides a strategy to make the π-conjugated MO3 group coplanar.

Na4Sn4(C2O4)3F6 and NaSnC2O4F·H2O: two tin(II) fluoride oxalates display large birefringence

Birefringent materials play an important role in laser techniques as an essential part of optical devices. Therefore, the exploration of high-performance birefringent materials has been a central focus of researchers. Herein, two tin(II) fluoride oxalates Na4Sn4(C2O4)3F6 and NaSnC2O4F·H2O were gained by the combination of birefringence-active groups of Sn2+ with stereochemically active lone pairs and planar π-conjugated [C2O4]2- groups. These groups assemble into low-dimensional structures of 0D [C2O4F4]6- clusters and 1D [SnC2O4F]∞- chains in Na4Sn4(C2O4)3F6, and double [Sn2(C2O4)2F2]∞2- chains in NaSnC2O4F·H2O, which gives rise to the large birefringence of 0.160@546 nm and 0.189@546 nm, respectively. Detailed structure-property analysis and theoretical calculations indicate that strong optical anisotropy can be induced by the rational arrangement of the Sn2+-polyhedra and [C2O4]2- groups.

Two Short-Wave UV Antimony(III) Sulfates Exhibiting Large Birefringence

In the present work, we have successfully obtained two new UV antimony-based sulfates, NH4Sb(SO4)2 and Ca2Sb2O(SO4)4, by a conventional hydrothermal method. Interestingly, both compounds share similar structural building blocks, such as SbO4 seesaws and SO4 tetrahedra, yet they endow discrepant birefringence values measured at 546 nm with values of 0.150 and 0.114, respectively, owing to the different distortions of the SbO4 groups with SCALP electrons. Moreover, both compounds display large band gaps (4.32 and 4.43 eV, respectively), so they can be used as short-wavelength UV birefringent materials. Moreover, NH4Sb(SO4)2 is a noncentrosymmetric compound, showing a frequency doubling effect of 0.2 × KDP. Detailed structural analyses and calculations confirm the source of superior optical performance and the reasons for the different birefringence of the two compounds. This work provides ideas for the following discovery of antimony-based optical materials with excellent properties.

AX·(H2SeO3)n (A = K, Cs; X = Cl, Br; n = 1, 2): A Series of Ionic Cocrystals as Promising UV Birefringent Materials with Large Birefringence and Wide Band Gap

The design and syntheses of new birefringent crystals will be of great importance in commercial applications and materials science. A series of ultraviolet (UV) birefringent crystals, AX·(H2SeO3)n (A = K, Cs; X = Cl, Br; n = 1, 2), with large sizes up to 23 × 6 × 3 mm3, was successfully synthesized by simple aqueous solution method. These four compounds belong to three different space groups. Isomorphic KCl·(H2SeO3)2 and CsCl·(H2SeO3)2 crystallize in the P 1 ¯ space group, while CsBr·(H2SeO3)2 and CsCl·H2SeO3 crystallize in the P21/m and P21/c space groups, respectively. They exhibit cocrystal structures composed of [2(H2SeO3)]∞ and [AX]∞ frameworks, ingeniously inheriting the large optical anisotropy of selenite and the wide band gap of alkali metal halide. And it proves that these compounds indeed possess large birefringence (0.1-0.17 at 532 nm) and short UV cutoff edges (227-239 nm), achieving a balance of optical properties. This research affords a simple and viable strategy for the design and syntheses of new UV birefringent crystals. Besides, it is also found that the n value and ionic size (A and X ions) have important influences on the crystal structures and optical properties of AX·(H2SeO3)n. And this will promote further understanding of the alkali metal halide selenite family.

(C(NH2)3)2(I2O5F)(IO3)(H2O) and C(NH2)3IO2F2: Two Guanidine Fluorooxoiodates with Wide Band Gap and Large Birefringence

Enhancing anisotropy through an effective synergistic arrangement of anionic and cationic groups is crucial for improving the birefringence optical properties of materials. In this work, by transforming I-O into I-F through the fluorination strategy, two metal-free guanidine fluorooxoiodates (C(NH2)3)2(I2O5F)(IO3)(H2O) and C(NH2)3IO2F2 and one guanidine iodate C(NH2)3IO3 were successfully synthesized using the hydrothermal method. An unprecedented dimer [I2O5F] formed by [IO3F] and [IO3] in (C(NH2)3)2(I2O5F)(IO3)(H2O) was found, which greatly enriches the structural diversity of fluorooxoiodates. All three compounds feature a relatively large birefringence (Δn = 0.068, 0.110 and 0.075 at 546 nm) and a short ultraviolet cutoff edge. The theoretical calculation was carried out to understand the electronic structures and linear optical properties.

Two van der Waals Layered Antimony(III) Phosphites as UV Optical Materials

Herein, two new Sb3+-based phosphites, Sb2O2(HPO3) (I) and Sb2O(HPO3)2 (II), were successfully obtained by ingeniously combining Sb3+-based polyhedra containing stereochemically active lone pair (SCALP) and HPO3 polar groups. Both reported compounds exhibit unique 2D van der Waals layered structures, [Sb4O4(HPO3)2]∞ and [Sb2O(HPO3)2]∞, respectively, which favors compounds with large optical anisotropy. Interestingly, the different curvatures of the two layers resulted in the two title compounds showing significantly different birefringences (0.079@546 and 0.046@546 nm, respectively). Both compounds endow wide optical band gaps (4.32 and 4.54 eV, respectively), which indicates their potential as promising ultraviolet (UV) birefringent crystals. The synthesis of the two title compounds enriched Sb3+-based phosphites in the UV region and provided guidance for the subsequent synthesis of superior optical materials.

KRb2(NO3)2Cl: a new birefringent crystal exhibiting a perovskite-related framework and a short cutoff edge

The combination of π-conjugated groups [NO3] and Cl-centered polyhedra generates a new birefringent crystal with a perovskite-related framework, KRb2(NO3)2Cl, which is the first alkali metal nitrate chloride synthesized by a mild hydrothermal method. It crystallizes in the orthorhombic space group pbam (no. 55). In addition, KRb2(NO3)2Cl crystals with dimensions up to 7 × 1.5 × 1 mm3 were grown. Notably, KRb2(NO3)2Cl has a short UV cut-off edge (below 228 nm) and a significantly enhanced birefringence (Δn = 0.084 at 1064 nm). Theoretical calculations indicate that the birefringence enhancement mainly derives from π-conjugated [NO3] plane triangles.

Mixed Alkali Metal and Alkaline Earth Metal Scandium Borate Birefringence Material with Layered Structure and Short Ultraviolet Cutoff Edge

Birefringent crystals are essential in the domains of linear and nonlinear optics that need light wave polarization control. Rare earth borate has become a popular study material for ultraviolet (UV) birefringence crystals due to its short cutoff edge in the UV area. RbBaScB₆O₁₂, a two-dimensional layered structure compound with the B₃O₆ group, was effectively synthesized through spontaneous crystallization. The UV cutoff edge of RbBaScB₆O₁₂ is shorter than 200 nm, and the experimental birefringence is 0.139 @ 550 nm. Theoretical research indicates that the large birefringence originates from the synergistic impact of the B₃O₆ group and the ScO₆ octahedron. RbBaScB₆O₁₂ is an outstanding candidate material for birefringence crystals in the UV and even deep UV regions due to its short UV cutoff edge and significant birefringence.

Cs2Mg(H2C3N3S3)4·8H2O: An Excellent Birefringent Material with Giant Optical Anisotropy in π-Conjugated Trithiocyanurate

The design of giant birefringence was performed by adjusting cations to make parallel and compact alignments of π-conjugated (H_xC₃N₃S₃)^x-3, where x = 1 and 2) groups with large polarizability anisotropy. Finally, the first mixed alkali/alkali-earth-metal trithiocyanurates, A₂B(H₂C₃N₃S₃)₄·nH₂O (A = K, Rb, Cs; B = Mg, Sr; n = 5-8, 12), were designed and synthesized successfully. Importantly, Cs₂Mg(H₂C₃N₃S₃)₄·8H₂O (III) and K₂Sr(H₂C₃N₃S₃)₄·5H₂O (IV) possess large birefringences of 0.580 and 0.194 at 800 nm, respectively, of III has the largest birefringence among all practical birefringent crystals, cyanurates, and hydroisocyanurates.

Optical monitoring of detergent pollutants in greywater

Large amount of wastewater is produced by washing machines and dishwashers, which are used in a daily basis. This domestic wastewater generated in households or office buildings (also called greywater) is drained directly to the drainpipes without differentiation from that with fecal contamination from toilets. Detergents are arguably the pollutants most frequently found in greywater from home appliances. Their concentrations vary in the successive stages in a wash cycle, which could be taken into account in a rational design of home appliances wastewater management. Analytical chemistry procedures are commonly used to determine the pollutant content in wastewater. They require collecting samples and their transport to properly equipped laboratories, which hampers real time wastewater management. In this paper, optofluidic devices based on planar Fabry-Perot microresonators operating in transmission mode in the visible and near infrared spectral ranges have been studied to determine the concentration of five brands of soap dissolved in water. It is found that the spectral positions of the optical resonances redshift when the soap concentration increases in the corresponding solutions. Experimental calibration curves of the optofluidic device were used to determine the soap concentration of wastewater from the successive stages of a washing machine wash cycle either loaded with garments or unloaded. Interestingly, the analysis of the optical sensor indicated that the greywater from the last water discharge of the wash cycle could be reused for gardening or agriculture. The integration of this kind of microfluidic devices into the home appliances design could lead to reduce our hydric environmental impact.

Distance calibration via Newton's rings in yttrium lithium fluoride whispering gallery mode resonators

In this work, we analyze the first whispering gallery mode resonator (WGMR) made from monocrystalline yttrium lithium fluoride (YLF). The disc-shaped resonator is fabricated using single-point diamond turning and exhibits a high intrinsic quality factor (Q) of 8×10⁸. Moreover, we employ a novel, to the best of our knowledge, method based on microscopic imaging of Newton's rings through the back of a trapezoidal prism. This method can be used to evanescently couple light into a WGMR and monitor the separation between the cavity and the coupling prism. Accurately calibrating the distance between a coupling prism and a WGMR is desirable as it can be used to improve experimental control and conditions, i.e., accurate coupler gap calibration can aid in tuning into desired coupling regimes and can be used to avoid potential damage caused by collisions between the coupling prism and the WGMR. Here, we use two different trapezoidal prisms together with the high-Q YLF WGMR to demonstrate and discuss this method.

Large optical anisotropy-oriented construction of a carbonate-nitrate chloride compound as a potential ultraviolet birefringent material

The design of new birefringent materials is very significant owing to their indispensable role in modulating the polarization of light and is vital in laser technology. Herein, by applying a large optical anisotropy-oriented construction induced by a synergy effect of multiple anionic groups, a promising carbonate-nitrate chloride, Na3Rb6(CO3)3(NO3)2Cl·(H2O)6, has been designed and synthesized successfully by the solvent evaporation method and single crystals of centimeter size were obtained by the recrystallization method in aqueous solution. It crystallizes in the hexagonal P63/mcm space group, the RbO9Cl polyhedra and the NaO7 polyhedra construct a three-dimensional (3D) framework by sharing O or Cl atoms and trigonal plane units (CO3 and NO3). The transmittance spectrum based on a 1 mm thick single-crystal plate shows that its short UV cut-off edge is about 231 nm. And the refractive index differences (0.14 @ 546 nm) measured by using a polarizing microscope on the (101) crystal plane, proves that Na3Rb6(CO3)3(NO3)2Cl·(H2O)6 has a large birefringence, which has potential application in the solar blind ultraviolet region. The theoretical calculations reveal that the π-conjugated CO3 and NO3 groups are the main cause of the birefringence. It demonstrates that combining π-conjugated CO3 and NO3 groups in one structure is an extremely effective strategy to explore new UV birefringent crystals.

SbHPO3F: 2D van der Waals Layered Phosphite Exhibiting Large Birefringence

A novel antimony(III)-based phosphite, SbHPO₃F, featuring a unique two-dimensional (2D) van der Waals layered structure, has been successfully designed and synthesized via the simultaneous employment of optically active moieties including SbO₃F seesaw and tetrahedral HPO₃ groups. Its optimized layered arrangement formed by the alternating connection of 4-membered rings (4-MRs) and 8-MRs endows the title compound with desirable optical properties including a large birefringence and short ultraviolet (UV) cutoff edge, implying that it is a potential UV birefringent material.

Sharp Enhancement of Birefringence in Antimony Oxalates Achieved by the Cation-Anion Synergetic Interaction Strategy

Birefringent materials with large birefringence play an important role in in laser science and technology owing to their ability to modulate polarized light. However, the lack of systematic and effective synthesis strategies severely hinders the development of novel superior birefringent materials. Herein, the cation-anion synergetic interaction strategy was proposed to successfully synthesize two excellent UV birefringent materials, RbSb(C₂O₄)F₂·H₂O and [C(NH₂)₃]Sb(C₂O₄)F₂·H₂O. Both compounds feature unprecedented [Sb(C₂O₄)F₂]_∞^- anionic chains composed of planar π-conjugated [C₂O₄]^2- units and a distorted SbO₄F₂ complex with stereochemically active lone pairs, which induce a large optical anisotropy. Remarkably, further enhancement of birefringence in [C(NH₂)₃]Sb(C₂O₄)F₂·H₂O was achieved via cation-anion synergetic interactions between the [C(NH₂)₃]⁺ cationic groups and [Sb(C₂O₄)F₂]_∞^- anionic chains. It exhibited a giant birefringence of 0.323@546 nm, twice larger than that of its analogue RbSb(C₂O₄)F₂·H₂O (0.162@546 nm). A detailed structural analysis and theoretical calculations revealed that the cation-anion synergetic interaction strategy is an effective strategy for the efficient exploration of superior birefringent materials, which will guide the further exploration of new structure-driven functional materials.

Layered Perovskite-like Nitrate Cs2Pb(NO3)2Br2 as a Multifunctional Optical Material

A novel alkali metal lead halide nitrate, Cs₂Pb(NO₃)₂Br₂, has been successfully synthesized via a hydrothermal method. Interestingly, the title compound features a distinctive Ruddlesden-Popper perovskite-like layered structure, which induces the outstanding multifunctional optical properties, including a large birefringence (0.147@546 nm) and broad light-orange emission. Detailed structural analysis and theoretical calculations revealed that the large birefringence originates from the p-π interaction between the Pb²⁺ cations and NO₃ groups and that the excellent luminescence properties derive from the distortion of PbO₄Br₄ polyhedra. This work not only enriches the variant structure types of layered perovskites but also guides the further exploration of all-inorganic multifunctional optical materials.

From Pb(H2C3N3O3)(OH) to Pb(H2C3N3O3)F: Homovalent Anion Substitution-Induced Band Gap Enlargement and Birefringence Enhancement

Birefringent materials capable of modulating the polarization of light have attracted intensive studies because of their wide utilization in optical communication and the laser industry. Herein, two new lead(II)-based cyanurates, namely, Pb(H₂C₃N₃O₃)X (X = OH, F), were synthesized by hydrothermal methods, and the first halogen-containing metal cyanurate Pb(H₂C₃N₃O₃)F was successfully obtained by the rational substitution of a homovalent anion. Pb(H₂C₃N₃O₃)X (X = OH, F) belong to space group P1̅, and their structures display a neutral [Pb(H₂C₃N₃O₃)X] (X = OH, F) layer. The Pb²⁺ ions in Pb(H₂C₃N₃O₃)(OH) are interconnected by hydroxyl groups and oxygen atoms of cyanurate anions into a 1D [PbO(OH)]^- chain, whereas the Pb²⁺ ions in Pb(H₂C₃N₃O₃)F are interconnected by F^- anions and oxygen atoms of cyanurate anions into a 2D [PbOF]^- layer. The π-π interactions between adjacent hydroisocyanurate rings and the hydrogen bonds between neighboring neutral layers provide additional stability to the structures. Luminescent studies show that Pb(H₂C₃N₃O₃)(OH) and Pb(H₂C₃N₃O₃)F emit yellow-green and blue light, respectively. Theoretical calculations unveiled their birefringences of 0.079 and 0.203@1064 nm and their band gaps of 3.96 and 4.96 eV, respectively, for OH^- and F^- containing materials. Obviously, the substitution of OH^- by F^- with the largest electronegativity can simultaneously improve both the birefringence and band gap.

[C(NH2)3]BiCl2SO4: an excellent birefringent material obtained by multifunctional group synergy

A multifunctional group synergistic strategy was proposed to successfully develop an excellent organic–inorganic hybrid guanidine sulfate birefringent material [C(NH2)3]BiCl2SO4.

Performance of optical materials with derivative planar π-conjugated groups: Recent advances and future prospects

Planar π-conjugated groups which possess not only large hyperpolarizability but also optical anisotropy are proven to be a good functional motif for optical materials with outstanding nonlinear optics and/or birefringence....

Large optical anisotropy differentiation induced by the anion-directed regulation of structures

The modulation of optical anisotropy for two novel UV birefringent materials [C(NH2)3]2Sb3F3(HPO3)4 and [C(NH2)3]SbFPO4·H2O has been successfully achieved via anion-directing regulation structures.

Two metal-free cyanurate crystals with a large optical birefringence resulting from the combination of π-conjugated units

Benefiting from the planar π-conjugated (H_xC₃N₃O₃)^x-3 (x = 0-3) groups, cyanurate crystals have recently become a research hotspot in birefringent materials. Herein, by combining the (H_xC₃N₃O₃)^x-3 (x = 0-3) group with the (CN₃H₆)⁺ cationic group, two metal-free cyanurates, GU(H₂C₃N₃O₃) (I) and GU₃(H₂C₃N₃O₃)₃(H₃C₃N₃O₃) (II), were obtained by the hydrothermal method. These compounds have wide band gaps (∼5 eV) and a large birefringence (∼0.40@400 nm), demonstrating their potential to be ultraviolet birefringent crystals. Moreover, first-principles calculations indicate that their large birefringence values originated from the synergistic effect of the (CN₃H₆)⁺ cations and (H_xC₃N₃O₃)^x-3 (x = 0-3) groups. These findings provide a new design strategy for exploring low-cost UV birefringent crystals with a large birefringence.

Co-crystal AX·(H3C3N3O3) (A = Na, Rb, Cs; X = Br, I): a series of strongly anisotropic alkali halide cyanurates with a planar structural motif and large birefringence

Birefringent crystals with strong anisotropy are important components in modern optical devices. The newly discovered planar π-conjugated cyanurate group (H_xC₃N₃O₃)^x-3 (x = 0-3) has been demonstrated as an effective functional motif for improving birefringence in the ultraviolet region. Here, single co-crystals of alkali halide cyanurates, RbBr·(H₃C₃N₃O₃) (I), RbI·(H₃C₃N₃O₃) (II), and CsBr·(H₃C₃N₃O₃) (III) were synthesized by the ethanol solution method, and NaBr·(H₃C₃N₃O₃) (IV) was obtained via the solvent-drop grinding method. These four co-crystals feature a planar (H₃C₃N₃O₃) arrangement and exhibit wide band gaps (> 4.90 eV), tunable birefringence (Δn_exp∼ 0.124-0.256), and high thermal stability (156 °C-349 °C). In addition, first principles calculations were also carried out to evaluate the relationship between molecule density, spatial arrangement and optical birefringence, and suggested a great tailoring effect of the alkali metal and halogen species on regulating the optical anisotropy of co-crystal cyanurates.

Hydroxyfluorooxoborate Na[B3 O3 F2 (OH)2 ]⋅[B(OH)3 ]: Optimizing the Optical Anisotropy with Heteroanionic Units for Deep Ultraviolet Birefringent Crystals

Maximizing the optical anisotropy in birefringent materials has emerged as an efficient route for modulating the polarization-dependent light propagation. Currently, the generation of deep-ultraviolet (deep-UV) polarized light below 200 nm is essential but challenging due to the interdisciplinary significance and insufficiency of high-performing birefringent crystals. Herein, by introducing multiple heteroanionic units, the first sodium difluorodihydroxytriborate-boric acid Na[B₃ O₃ F₂ (OH)₂ ]⋅[B(OH)₃ ] has been characterized as a novel deep-UV birefringent crystal. Two rare heteroanionic units, [B₃ O₃ F₂ (OH)₂ ] and [B(OH)₃ ], optimally align to induce large optical anisotropy and also the dangling bonds are eliminated with hydrogens, which results in an extremely large birefringence and band gap. The well-ordered OH/F anions in [B₃ O₃ F₂ (OH)₂ ] and [B(OH)₃ ] were identified and confirmed by various approaches, and also the origin of large birefringence was theoretically discussed. These results confirm the feasibility of utilizing hydrogen involved heteroanionic units to design crystals with large birefringence, and also expand the alternative system of deep-UV birefringent crystals with new hydroxyfluorooxoborates.

AZn4(OH)4(C3N3O3)2 (A = Mg, Zn): Two Zn-Based Cyanurate Crystals with Various Cation Coordination and Large Birefringence

Cyanurate crystals have recently become a research hot spot in birefringent materials owing to the large structural and optical anisotropy of the planar π-conjugated (H_xC₃N₃O₃)^x-3 (x = 0-3) groups. In this study, two new Zn-based cyanurate crystals, Zn₅(OH)₄(C₃N₃O₃)₂ and MgZn₄(OH)₄(C₃N₃O₃)₂, were synthesized by the hydrothermal method. In Zn₅(OH)₄(C₃N₃O₃)₂, the d¹⁰ Zn²⁺ cations have three different coordinating environments, which has never been found in cyanurates. These compounds have wide band gaps (∼5 eV) and large birefringence (∼0.32 at 400 nm), indicating their potential as ultraviolet birefringence crystals.

Dielectric perturbations: anomalous resonance frequency shifts in optical resonators

Small perturbations in the dielectric environment around resonant dielectric structures usually lead to a frequency shift of the resonator modes directly proportional to the polarizability of the perturbation. Here, we report experimental observations of strong frequency shifts that can oppose and even exceed the contribution of the perturbations' polarizability. We show in particular how the mode frequencies of a lithium niobate whispering-gallery-mode resonator are shifted by planar substrates-of refractive indices ranging from 1.50 to 4.22-contacting the resonator rim. Both blue- and redshifts are observed, as well as an increase in mode linewidth, when substrates are moved into the evanescent field of the whispering gallery mode. We compare the experimental results to a theoretical model by Foreman et al. [J. Opt. Soc. Am. B33, 2177 (2016)JOBPDE0740-322410.1364/JOSAB.33.002177] and provide an additional intuitive explanation based on the Goos-Hänchen shift for the optical domain, with applications to dielectric structures ranging from meta-surfaces to photonic crystal cavities.

Zn(H2C3N3O3)2·3H2O: the first single-d10 transition metal based ultraviolet hydroisocyanurate crystal with large birefringence

The first single-d¹⁰ transition metal hydroisocyanurate crystal was designed and synthesized successfully with large band gaps and optical anisotropy.

From BaCl2 to Ba(NO3)Cl: significantly enhanced birefringence derived from π-conjugated [NO3]

The chlorine barium nitrate Ba(NO3)Cl was synthesized for the first time to the best of our knowledge and found to exhibit a strong birefringence of 0.178 @ 1064 nm, about 19 times that of BaCl2.

Barium fluoroiodate crystals with a large band gap and birefringence

BaI₂O₅F₂ and BaIO₂F₃ have large birefringence of 0.174 and 0.133 at 1064 nm, respectively, which is owing to the rare [IO₃F]²⁻ units with high anisotropic polarizability in BaI₂O₅F₂ and the orderly arranged [IO₂F₂]⁻ units in BaIO₂F₃.

K2Pb(H2C3N3O3)4(H2O)4: a potential UV nonlinear optical material with large birefringence

K₂Pb(H₂C₃N₃O₃)₄(H₂O)₄ (I) features a 2D [K₂PbO₈(H₂O)₄]^12- anionic layer and reveals a moderate SHG signal of approximately 2.6 × KDP.

The synthesis, characterization, and theoretical analysis of (NH4)3PbCl5

(NH₄)₃PbCl₅ features a 3D structure with a short UV cutoff edge of 256 nm and simulated birefringence of about 0.050 at 1064 nm.

CsHgNO3Cl2: A New Nitrate UV Birefringent Material Exhibiting an Optimized Layered Structure

Developing effective strategies to design novel, excellent birefringent materials has become an emerging challenge. Herein, we report a new UV nitrate birefringent material, CsHgNO₃Cl₂, which is an analogue of the known UV carbonate nonlinear optical crystal KCaCO₃F. In CsHgNO₃Cl₂, a perfect layered structure was produced owing to the alliance of a planar π-conjugated anion NO₃^- and highly electropolarized Hg²⁺ cation. The optimized layered structure that is beneficial in enlarging the optical anisotropy induced a large birefringence (Δn = 0.145@546 nm) for CsHgNO₃Cl₂, even superior to that of the well-known commercial birefringent material α-BBO in the UV region. The strategy that the optimization of planar functional group arrangement is able to boost birefringence is expected to guide the development of high-performance birefringent materials in the future, especially in the field of UV.

A(H3C3N3O3)(NO3) (A = K, Rb): Alkali-Metal Nitrate Isocyanurates with Strong Optical Anisotropy

The first alkali-metal nitrate isocyanurates, A(H₃C₃N₃O₃)(NO₃) (A = K, Rb), were synthesized by the tactic of introducing (NO₃)^- into isocyanurate with a mild hydrothermal technique. They crystallized into the same monoclinic centrosymmetric (CS) space group P2₁/c, which featured a 2D [(H₃C₃N₃O₃)(NO₃)]_∞ layered structure separated by K⁺ and Rb⁺ cations, respectively. Both compounds exhibited short ultraviolet cutoff edges (λ_cutoff = 228 and 229 nm) and large birefringences (Δn = 0.253 and 0.224 at 546.1 nm). More importantly, in comparison with most of the isocyanurates and nitrates, they have better thermal stability with decomposition temperatures up to 319.8 and 324.4 °C. In addition, our theoretical calculations reveal that the π-conjugated groups play significant roles in improving the optical anisotropy. Remarkably, introducing a π-conjugated inorganic acid radical (NO₃)^- into isocyanurate is an extremely meaningful strategy to explore new UV birefringent crystals.

RE(H2C3N3O3)2·(OH)·xH2O (RE = La, Y and Gd): potential UV birefringent materials with strong optical anisotropy originating from the (H2C3N3O3)- group

A new series of dihydro-isocyanurates, namely, RE(H₂C₃N₃O₃)₂·(OH)·xH₂O (RE = La, Y and Gd) was readily obtained by a mild hydrothermal technique using lithium hydroxide as a deprotonation agent. La(H₂C₃N₃O₃)₂·OH·2H₂O and Re(H₂C₃N₃O₃)₂·OH·5H₂O (Re = Y, Gd) crystallized in the centrosymmetric (CS) space groups P2₁/c and P1[combining macron], respectively. Their intricate 3D frameworks were built by cationic polyhedrons (LaO₈N/ReO₈) and (H₂C₃N₃O₃)^- groups. The experiments and theoretical studies demonstrated that these compounds possessed short ultraviolet cut-off edges (≤240 nm) and large birefringences (Δn≥ 0.257@546.1 nm). In addition, the theoretical studies further revealed that the (H₂C₃N₃O₃)^2- group plays a pivotal role in strengthening the optical anisotropy for all compounds.

Three-Dimensional Microtubular Devices for Lab-on-a-Chip Sensing Applications

The rapid advance of micro-/nanofabrication technologies opens up new opportunities for miniaturized sensing devices based on novel three-dimensional (3D) architectures. Notably, microtubular geometry exhibits natural advantages for sensing applications due to its unique properties including the hollow sensing channel, high surface-volume ratio, well-controlled shape parameters and compatibility to on-chip integration. Here the state-of-the-art sensing techniques based on microtubular devices are reviewed. The developed microtubular sensors cover microcapillaries, rolled-up nanomembranes, chemically synthesized tubular arrays, and photoresist-based tubular structures via 3D printing. Various types of microtubular sensors working in optical, electrical, and magnetic principles exhibit an extremely broad scope of sensing targets including liquids, biomolecules, micrometer-sized/nanosized objects, and gases. Moreover, they have also been applied for the detection of mechanical, acoustic, and magnetic fields as well as fluorescence signals in labeling-based analyses. At last, a comprehensive outlook of future research on microtubular sensors is discussed on pushing the detection limit, extending the functionality, and taking a step forward to a compact and integrable core module in a lab-on-a-chip analytical system for understanding fundamental biological events or performing accurate point-of-care diagnostics.