Giant birefringent optics in multilayer polymer mirrors

Unusual Triple-State Switching of Thermally Induced Birefringence in a Two-Dimensional Perovskite Ferroelectric

Birefringent crystals hold a significant position in optical and optoelectronic fields due to their capability to control polarized light. Despite various chemical strategies devoted to designing birefringent crystals, it remains a challenge to switch and manipulate birefringence under physical stimuli. Here we present an unusual triple-state switching of birefringence in a 2D perovskite ferroelectric, (N-methylcyclohexylammonium)2PbCl4 (1), which exhibits two reversible phase transitions at 361 and 373 K. The in-plane birefringence of 1 (Δnbc) shows three distinctive states inside the bc plane, namely, the low-, high-, and zero-Δnbc states. Strikingly, a huge augmentation of Δnbc is solidly confirmed up to ∼400% between its low and high states, far beyond other birefringent materials. The origin of this triple-state switching of birefringence involves the variation of the ferroelastic strain and domain in the vicinity of the phase transition. As an entirely new mode of switching birefringence, this work facilitates the further development of new intelligent nonlinear optics.

Breaking Boundaries: Giant Ultraviolet Birefringence in Dimension-Reduced Zn-Based Crystals

Birefringent crystals have essential applications in optical communication areas. Low-dimensional structures with inherited structural anisotropy are potential systems for investigating birefringent materials with large birefringence. In this work, the zero-dimensional (0D) [(p-C5H5NO)2ZnCl2] (1) and [p-C5H6NO]2[ZnCl4] (2) were obtained by introducing the π-conjugated p-C5H5NO (4HP) into the three-dimensional (3D) ZnCl2. Remarkably, 1 exhibits a giant birefringence of 0.482@546 nm, which is the largest among Zn-based ultraviolet (UV) compounds and 160 times that of ZnCl2. According to structural and theoretical calculation analyses, the large optical polarizability, high spatial density, ideal distribution of the [(4HP)2ZnCl2]0 cluster, and the low dimension of 1 result in the dramatically increased birefringence compared to ZnCl2. This work will provide a valid route for accelerating the design and synthesis of compounds with excellent birefringence in low-dimensional systems.

Organic Crystals and Optical Functions in Biology: Knowns and Unknowns

Organic crystals are widely used by animals to manipulate light for producing structural colors and for improving vision. To date only seven crystal types are known to be used, and among them β-guanine crystals are by far the most widespread. The fact that almost all these crystals have unusually high refractive indices (RIs) is consistent with their light manipulation function. Here, the physical, structural, and optical principles of how light interacts with the polarizable free-electron-rich environment of these quasiaromatic molecules are addressed. How the organization of these molecules into crystalline arrays introduces optical anisotropy and finally how organisms control crystal morphology and superstructural organization to optimize functions in light reflection and scattering are also discussed. Many open questions remain in this fascinating field, some of which arise out of this in-depth analysis of the interaction of light with crystal arrays. More types of organic crystals will probably be discovered, as well as other organisms that use these crystals to manipulate light. The insights gained from biological systems can also be harnessed for improving synthetic light-manipulating materials.

Proximal High-Index Metamaterials based on a Superlattice of Gold Nanohexagons Targeting the Near-Infrared Band

Plasmonic nanoparticles can be assembled into a superlattice, to form optical metamaterials, particularly targeting precise control of optical properties such as refractive index (RI). The superlattices exhibit enhanced near-field, given the sufficiently narrow gap between nanoparticles supporting multiple plasmonic resonance modes only realized in proximal environments. Herein, the planar superlattice of plasmonic Au nanohexagons (AuNHs) with precisely controlled geometries such as size, shape, and edge-gaps is reported. The proximal AuNHs superlattice realized over a large area with selective edge-to-edge assembly exhibited the highest-ever-recorded RI values in the near-infrared (NIR) band, surpassing the upper limit of the RI of the natural intrinsic materials (up to 10.04 at λ = 1.5 µm). The exceptionally enhanced RI is derived from intensified in-plane surface plasmon coupling across the superlattices. Precise control of the edge-gap of neighboring AuNHs systematically tuned the RI as confirmed by numerical analysis based on the plasmonic percolation model. Furthermore, a 1D photonic crystal, composed of alternating layers of AuNHs superlattices and low-index polymers, is constructed to enhance the selectivity of the reflectivity operating in the NIR band. It is expected that the proximal AuNHs superlattices can be used as new optical metamaterials that can be extended to the NIR range.

Sb4O5I2: Enhancing Birefringence through Optimization of Sb/I Ratio for Alignment of Stereochemically Active Lone Pairs

Birefringent crystals are the key components of functional optics, contributing significantly to scientific and technological advancements. To enhance birefringence, the presence of stereochemically active lone pairs offers a unique opportunity. In fact, strengthening the stereochemical activity and aligning uniformly lone pairs face tough challenges. Herein, an anisotropic layered crystal, Sb4O5I2, is discovered to exhibit enhanced birefringence. The influence of crystal symmetry on the birefringence of Sb4O5X2 (X = Cl, Br, or I) is found to be minor. Instead, the asymmetric nature of ABUCBs (i.e., cis-X3[SbO3]6- and cis-X3[SbO4]8-) plays a crucial role in enhancing the optical anisotropy. And the orientation of these ABUCBs is equally important. We demonstrate that by adjusting the Sb/I ratio from 5:1 to 2:1, all of the intralayer Sb atoms in Sb5O7I-P63 are forced onto the surface position. This structural adjustment leads to strengthened ionic bonding interactions, enhanced activity of the lone pairs, and uniform alignments of the ABUCBs in Sb4O5I2. Consequently, this results in a 6-fold increase in birefringence.

PbTeB4O9: a lead tellurium borate with unprecedented fundamental building block [B4O10] and large birefringence

Herein, the first lead tellurium borate, PbTeB4O9, with an unprecedented fundamental building block [B4O10] was successfully synthesized. The near-parallel alignment of [B4O10] groups and [TeO3] polyhedra resulted in a high birefringence (0.099@1064 nm). The structure-property relationship was discussed by using the first-principles calculations.

A Two-Dimensional Hybrid Perovskite With Heat Switching Birefringence

Birefringent crystals that can switch light polarization have important applications in optoelectronics. In the last decades, birefringence is mostly optimized by chemical strategies. Recently, switching birefringence by physical means has attracted much attention. Here, this work reports the observation of heat switching birefringence in a 2D layered hybrid halide perovskite (C2 N3 H4 )2 PbCl4 ((C2 N3 H4 )+ =1,2,4-triazolium). This heat switching birefringence leads to a significant change in the interference color for the crystal plate under the illumination of orthogonal polarized light. Structure analyses reveal a heat dependent structure transition in (C2 N3 H4 )2 PbCl4 , whose birefringence is switched by the change in the distortion degree of PbCl6 octahedron. This discovery may be beneficial to the further development of stimuli-responsive polarization optical devices.

Visible to mid-infrared giant in-plane optical anisotropy in ternary van der Waals crystals

Birefringence is at the heart of photonic applications. Layered van der Waals materials inherently support considerable out-of-plane birefringence. However, funnelling light into their small nanoscale area parallel to its out-of-plane optical axis remains challenging. Thus far, the lack of large in-plane birefringence has been a major roadblock hindering their applications. Here, we introduce the presence of broadband, low-loss, giant birefringence in a biaxial van der Waals materials Ta2NiS5, spanning an ultrawide-band from visible to mid-infrared wavelengths of 0.3-16 μm. The in-plane birefringence Δn ≈ 2 and 0.5 in the visible and mid-infrared ranges is one of the highest among van der Waals materials known to date. Meanwhile, the real-space propagating waveguide modes in Ta2NiS5 show strong in-plane anisotropy with a long propagation length (>20 μm) in the mid-infrared range. Our work may promote next-generation broadband and ultracompact integrated photonics based on van der Waals materials.

Stereochemically Active Lone-Pair Containing Metal Substitution in Polar Axis toward a Giant Phase-Matchable Optical Nonlinear Silicate Crystal Li3(OH)PbSiO4

Atoms in special lattice sites can play a crucial role in realizing materials properties, which is long pursued but difficult to control. Herein, by adopting a stereochemically active lone-pair-containing metal substitution strategy, a nonlinear-optical (NLO) silicate crystal Li3(OH)PbSiO4 was successfully synthesized, featuring [PbSiO4]∞ layers with the perfect orientation of the stereochemically active lone-pair Pb(II) cation in the polar-axis lattice. Li3(OH)PbSiO4 overcomes the long-standing problem of silicates, that is, poor nonlinear properties because it exhibits both the largest birefringence of 0.082 and the largest phase-matchable second-harmonic-generation (SHG) efficiency of 21 × KDP among the known silicates. The successful polar-axis lattice substitution could offer a new direction for realizing the rational control of materials structures and properties.

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.

Colossal Optical Anisotropy from Atomic-Scale Modulations

Materials with large birefringence (Δn, where n is the refractive index) are sought after for polarization control (e.g., in wave plates, polarizing beam splitters, etc.), nonlinear optics, micromanipulation, and as a platform for unconventional light-matter coupling, such as hyperbolic phonon polaritons. Layered 2D materials can feature some of the largest optical anisotropy; however, their use in most optical systems is limited because their optical axis is out of the plane of the layers and the layers are weakly attached. This work demonstrates that a bulk crystal with subtle periodic modulations in its structure-Sr9/8 TiS3 -is transparent and positive-uniaxial, with extraordinary index ne = 4.5 and ordinary index no = 2.4 in the mid- to far-infrared. The excess Sr, compared to stoichiometric SrTiS3 , results in the formation of TiS6 trigonal-prismatic units that break the chains of face-sharing TiS6 octahedra in SrTiS3 into periodic blocks of five TiS6 octahedral units. The additional electrons introduced by the excess Sr form highly oriented electron clouds, which selectively boost the extraordinary index ne and result in record birefringence (Δn > 2.1 with low loss). The connection between subtle structural modulations and large changes in refractive index suggests new categories of anisotropic materials and also tunable optical materials with large refractive-index modulation.

Heterologous Isomorphic Substitution Induces Optical Property Enhancement for Deep-UV Crystals: a Case in Rb[B3O3F2(OH)2]

Optical anisotropy is pivotal for optical crystals, and it can be characterized by the maximum algebraic difference in refractive indices. Improving the optical anisotropy, especially for deep-ultraviolet (UV) crystals, is still a challenge and of interest. Herein, a new hydroxyfluorooxoborate, Rb[B3O3F2(OH)2], was obtained by the heterologous isomorphic substitution strategy. Dual enhancement for the band gap and birefringence compared with the parent A[B3O3F2(OH)2] (A = [Ph4P]/[Ph3MeP]) compounds was achieved in Rb[B3O3F2(OH)2]. This considerable enhancement originates from the removal of organic components and the retention of a birefringence-active anionic framework. This enhancement pushes the application region from UV to deep-UV. This discovery not only expands the structural chemistry of borates but also demonstrates the viability of heterologous isomorphic substitution to design deep-UV crystals with enhanced optical property.

Designing a Dimension Reduced Hybrid Perovskite with Robust Large Birefringence by Expanding Cationic π-Delocation

It is in great demand to discover new materials with large birefringence for the miniaturization of optical communication devices. In this work, a new one-dimensional hybrid halide perovskite, (C6 N10 H8 )Pb2 Br6 , is obtained successfully through structural design of dimension reduction from the notable three-dimensional halide perovskite CsPbBr3 . Remarkably, (C6 N10 H8 )Pb2 Br6 exhibits a significantly enhanced birefringence of ∆n = 0.42@550 nm, which is the largest among halide perovskites so far. Furthermore, its birefringence performance is robust in a wide temperature range of 300-440 K. Theoretical calculations reveal that this outstanding birefringence results from the synergistic effect of [PbBr6 ]4- octahedra and [C6 N10 H8 ]2+ cations with expanding π-delocation. According to further structural analyses, the structural dimension reduction cooperating with the increase of [PbBr6 ]4- octahedral distortion leads to the enhanced birefringence. This work uncovers the great promise of hybrid halide perovskites as robust birefringent crystals in future optical communication and would shed useful insights on the design and synthesis of new birefringent crystals.

Development and analysis of a nano-triangular wave-shaped polarizer

As society becomes smarter, advanced optical sensing and imaging technologies utilizing visible and near-infrared regions have become increasingly prevalent. Wire-grid polarizers, which are available for broadband electromagnetic waves, are effective in improving the signal-to-noise ratio of such optical systems and enabling more advanced object detection and analysis. However, to be implemented in everyday products, low-cost manufacturing methods must be developed while maintaining high-performance optical functions. To meet these requirements, we conducted an analysis of the geometry of wire-grid polarizers, and designed and developed a wire-grid polarizer with a nano-triangular wave-shaped structure that can be fabricated using general-purpose manufacturing equipment. Once the mould is prepared, this polarizer can be fabricated via nanoimprinting and metal deposition with a normal angle or electroless plating processes. The polarizer fabricated through electroless Ni plating achieves a transmittance of 40%, which is approximately 1.4 times higher than that achieved in a previous study using electroless Ni plating on a rectangular structure with the same period. In addition, the polarizer fabricated through normal angle Al deposition operates over a wide range of wavelengths from visible light to near-infrared, and achieves a polarization extinction ratio of 24 dB at a wavelength of 550 nm and a high transmittance of 81%. High-performance polarizers can be obtained through normal-angle deposition using general-purpose equipment in contrast to the oblique-angle deposition method employed in the manufacture of conventional rectangular structure-based wire-grid polarizers, thereby contributing to cost reduction and improved manufacturability.

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.

Emerging Materials and Strategies for Passive Daytime Radiative Cooling

In recent decades, the growing demands for energy saving and accompanying heat mitigation concerns, together with the vital goal for carbon neutrality, have drawn human attention to the zero-energy-consumption cooling technique. Recent breakthroughs in passive daytime radiative cooling (PDRC) might be a potent approach to combat the energy crisis and environmental challenges by directly dissipating ambient heat from the Earth to the cold outer space instead of only moving the heat across the Earth's surface. Despite significant progress in cooling mechanisms, materials design, and application exploration, PDRC faces potential functionalization, durability, and commercialization challenges. Herein, emerging materials and rational strategies for PDRC devices are reviewed. First, the fundamental physics and thermodynamic concepts of PDRC are examined, followed by a discussion on several categories of PDRC devices developed to date according to their implementation mechanism and material properties. Emerging strategies for performance enhancement and specific functions of PDRC are discussed in detail. Potential applications and possible directions for designing next-generation high-efficiency PDRC are also discussed. It is hoped that this review will contribute to exciting advances in PDRC and aid its potential applications in various fields.

LiSrSbS3: parallel configurations of lone pair electrons inducing a large birefringence

Enhancement of birefringence is significant since the birefringent materials can create and control polarized light and be used extensively in various advanced optical systems. By optimizing the arrangement of [SbS₃] units with stereo-chemical active lone pair electrons, a new quaternary thioantimonate LiSrSbS₃ with a large birefringence has been successfully synthesized by a high temperature solid-state reaction method. LiSrSbS₃ crystallizes in the monoclinic space group of P2₁/c. In the structure, the isolated infinite [LiS₄] chains and zigzag [SrS₆] chains are alternately connected with each other to compose a three-dimensional (3D) framework, and the isolated pyramid [SbS₃] units are located between them. To analyze the source of large birefringence, the electronic structure and optical properties of LiSrSbS₃ were further investigated by the first-principles method, and the results show that the optimized arrangement [SbS₃] trigonal pyramid induces a large birefringence.

KC9H5O6(H2O): A Promising UV Nonlinear-Optical Material with Large Birefringence Based on a π-Conjugated (C9H5O6)- Group

In order to meet the growing needs for the laser technology and optics industries, the goal is to find suitable fundamental building blocks with large nonlinear-optical (NLO) coefficients and birefringence for an excellent-performance NLO or birefringent system. Via preliminary investigations and calculations, it has been found that the planar π-conjugated group (C₉H₅O₆)^- possesses large polarizability anisotropy (δ) and hyperpolarizability (β_max), comparable to well-known groups such as (B₃O₆)^3-, (C₃N₃O₃)^3-, etc. Herein, we report a new alkali-metal 3,5-dicarboxybenzoate, KC₉H₅O₆(H₂O) (KH₂BTC), which crystallized in the acentric space group Pna2₁. Second-harmonic-generation (SHG) measurements of KH₂BTC under 1064 nm laser radiation show that the SHG response of KH₂BTC is 1.2 times that of KDP with type I phase-matching behavior. Birefringence measurements show that KH₂BTC owns a large birefringence of about 0.372 at 550 nm. The band gap of KH₂BTC obtained by ultraviolet (UV) diffuse-reflectance spectroscopy is 3.91 eV, indicating that KH₂BTC has potential applications as UV NLO or birefringent materials. Theoretical calculation further confirmed that the impressive optical properties of KH₂BTC are derived from the large polarizability anisotropy of the (C₉H₅O₆)^- anions.

Ba2Zn2B6O13: coplanar [B2O5] in unnoted U-shaped [B6O13] groups achieving large birefringence

Currently, π-conjugated [B₂O₅] moieties are rarely studied for designing deep-UV birefringent crystals. Here, we report a new deep-UV birefringent crystal Ba₂Zn₂B₆O₁₃ with a deep-UV cut-off edge of 190 nm and large birefringence (Δn = 0.085@ 532 nm), indicating that it can be used as the birefringent material in the DUV area. The first-principles calculation analyses suggest that its large birefringence mainly originates from the coplanar [B₂O₅] dimers in the unnoted U-shaped [B₆O₁₃].

Fabrication of Bragg Mirrors by Multilayer Inkjet Printing

Bragg mirrors are widely applied in optical and photonic devices due to their capability of light management. However, the fabrication of Bragg mirrors is mainly accomplished by physical and chemical vapor deposition processes, which are costly and do not allow for lateral patterning. Here, the fabrication of Bragg mirrors by fully inkjet printing is reported. The photonic bandgap of Bragg mirrors is tailored by adjusting the number of bilayers in the stack and the layer thickness via simply varying printing parameters. An ultrahigh reflectance of 99% is achieved with the devices consisting of ten bilayers only, and the central wavelength of Bragg mirrors is tuned from visible into near-infrared wavelength range. Inkjet printing allows for fabricating Bragg mirrors on various substrates (e.g., glass and foils), in different sizes and variable lateral patterns. The printed Bragg mirrors not only exhibit a high reflection at designed wavelengths but also show an outstanding homogeneity in color over a large area. The approach thus enables additive manufacturing for various applications ranging from microscale photonic elements to enhanced functionality and aesthetics in large-area displays and solar technologies.

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.

Assembly of Guanine Crystals as a Low-Polarizing Broadband Multilayer Reflector in a Spider, Phoroncidia rubroargentea

The diminishing of the polarization effect is important in the applications of dielectric multilayer reflectors in many optical systems, such as low-loss broadband waveguides, optical fibers, and LEDs. Low-polarizing broadband reflections were identified from birefringent-guanine-crystal-based multilayer reflectors in the skins of some fish. Previous models for these intriguing natural optical phenomena suggested the combined action of two populations of guanine crystals with an orthogonal low-refractive-index optic axis. Here we report a novel realization of polarization-insensitive broadband reflectivity in a spider, Phoroncidia rubroargentea, based solely on the type of guanine crystals with the low-refractive-index optic axis normal to the crystal plates. We examined the three-dimensional structure of the guanine assembly in the spider and performed finite-difference time-domain (FDTD) optical modeling of the guanine-based multilayer reflector. Comparative modeling studies reveal that the biological selection of the guanine crystal type and specific spatial arrangement work synergistically to optimize the polarization-insensitive broadband reflection. This study demonstrates the importance of both crystallographic characteristics and 3D arrangement of guanine crystals in understanding relevant natural optical effects and also provides new insights into similar broadband, low-polarizing reflections in biological optical systems. Learning from relevant biofunctional assembly of guanine crystals could promote the bioinspired design of nonpolarizing dielectric multilayer reflectors.

Promising Deep-Ultraviolet Birefringent Materials via Rational Design and Assembly of Planar π-Conjugated [B(OH)3 ] and [B3 O3 (OH)3 ] Functional Species

Birefringent materials play a significant role in modulating polarized light in optical communication and the laser industry. However, the discovery of deep ultraviolet (DUV, λ<200 nm) birefringent materials still faces a serious challenge. Herein, we propose hydroxylated π-conjugated [B(OH)₃ ] and [B₃ O₃ (OH)₃ ] units for designing DUV birefringent materials. Innovatively, four new hydroxyborates have been synthesized under mild synthesis conditions. They present four novel pseudo layers that benefit from the large degree of freedom assembly modes of [B(OH)₃ ] and [B₃ O₃ (OH)₃ ] genes and large birefringence (0.057-0.123@532 nm). Moreover, the Cs₃ [B(OH)₃ ]₂ Cl₃ crystal features a short DUV cutoff edge (180 nm), which further indicates that the reported compounds are potential DUV birefringent crystals. Free and flexible assembly modes of π-conjugated [B(OH)₃ ] and [B₃ O₃ (OH)₃ ] groups endow them a particular advantage as significant genes for exploring promising DUV birefringent materials.

Linear unit BN2: a novel birefringence-enhanced fundamental module with sp orbital hybridization

Inorganic planar π-conjugated groups are advantageous to generate large birefringence in optical functional materials, and many excellent materials contain CO3, BO3 or B3O6, such as CaCO3, α/β-BaB2O4 (α/β-BBO), and KBe2BO3F2 (KBBF). In view of their microscopic structures, the common characteristics are the planar structures, which are regarded as birefringence-enhanced fundamental modules (FMs). Nowadays, exploring novel birefringence-enhanced FMs is becoming a burning issue. Herein, we investigated the birefringence-enhanced FMs in B-N systems and found that the BN2 linear unit could produce great birefringence. Through the investigation based on the Inorganic Crystal Structure Database, some compounds with the BN2 linear group were screened out with the formulas A3BN2 (A = Li, Na), A3BN3 (A = Mg, Ca), and Ba3(BN2)2. Particularly, Ca3(BN2)N exhibits a great birefringence of about 0.411 at 1064 nm, which is 3.5, 2.5 and 2.0 times those of the most commercially used birefringent crystals α-BaB2O4 (Δn = 0.116 at 1064 nm), CaCO3 (Δn = 0.164 at 1064 nm) and YVO4 (Δn = 0.208 at 1064 nm), respectively. To find the origins of the optical properties of compounds with the BN2 linear group, the first-principles, REDA and polarizability anisotropy analysis methods were used. Owing to the structural arrangement and the polarization anisotropy of the BN2 linear group, it can influence the birefringence significantly. This work will provide a general way for exploring birefringence-enhanced FMs in B-N compounds.

Lead Tellurite Crystals BaPbTe2O6 and PbVTeO5F with Large Nonlinear-/Linear-Optical Responses due to Active Lone Pairs and Distorted Octahedra

The exploration of nonlinear-/linear-optical crystal materials with high performance is an extremely difficult research project. Herein, the two new lead tellurite crystals BaPbTe₂O₆ and PbVTeO₅F were successfully obtained through a facile hydrothermal synthesis strategy. BaPbTe₂O₆ lies in the noncentrosymmetric (NCS) and chiral orthorhombic space group P2₁2₁2₁, featuring a unique _∞¹[PbTe₂O₆] chain consisting of the PbO₄ and TeO₃ building units, while PbVTeO₅F belonging to the centrosymmetric (CS) orthorhombic space group Pbca manifests a 2D layer made up of _∞¹[PbO₄F₂] chains and novel [V₂Te₂O₁₀F₂] clusters. Further, a systematic analysis of lead tellurites finds that the coordination geometries of the Pb atom exert a considerable influence on the connection modes of Pb-O and Te-O building units. BaPbTe₂O₆ shows a great second-harmonic-generation (SHG) effect of ∼5× the benchmark KH₂PO₄ (KDP) and a large optical birefringence of 0.086 at 590 ± 3 nm. PbVTeO₅F demonstrates a remarkably larger birefringence of 0.142 at 590 ± 3 nm, benefiting from the introduction of the VO₅F octahedral unit. Theoretical studies reveal that the large SHG and birefringence in BaPbTe₂O₆ can be attributed to TeO₃ and PbO₄ polyhedra with active lone pairs, while the remarkably enlarged birefringence in PbVTeO₅F is attributable to the highly distorted octahedral VO₅F. The functional orientations of active building units may offer a practical insight into the design of the desired optical functional materials.

Hexameric poly-fluoroberyllophosphate Na4Be2PO4F5 with moderate birefringence and deep-ultraviolet transmission as a potential zero-order-waveplate crystal

Na4Be2PO4F5 features novel [Be4P2O8F10] hexameric isolated groups, caused due to a cut-off effect from F atoms, and it is an optimal choice to act as a deep-UV zero-order waveplate material.

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....

Enhancement of band gap and birefringence induced via π-conjugated chromophore with “tail effect”

The enhancement of band gap and birefringence of template compounds can be attributed to this “tail effect” caused by the extended electronic distribution from [CO3]2−/[BO3]3−/[C3N3O3]3− to [HCO3]−/[HBO3]2−/[HC3N3O3]2−.

KRE(CO3)2 (RE = Eu, Gd, Tb): new mixed metal carbonates with strong photoluminescence and large birefringence

Three new potassium rare earth carbonates KRE(CO3)2 (RE = Eu, Gd, Tb) with strong photoluminescence and large birefringence were synthesized successfully.

Polymerizations with Elemental Sulfur: From Petroleum Refining to Polymeric Materials

The production of elemental sulfur from petroleum refining has created a technological opportunity to increase the valorization of elemental sulfur by the synthesis of high-performance sulfur-based plastics with improved optical, electrochemical, and mechanical properties aimed at applications in thermal imaging, energy storage, self-healable materials, and separation science. In this Perspective, we discuss efforts in the past decade that have revived this area of organosulfur and polymer chemistry to afford a new class of high-sulfur-content polymers prepared from the polymerization of liquid sulfur with unsaturated monomers, termed inverse vulcanization.

Sn2 PO4 I: An Excellent Birefringent Material with Giant Optical Anisotropy in Non π-Conjugated Phosphate

Exploring non π-conjugated phosphate birefringent crystal with a large birefringence has been a great challenge. Herein, based on the unique two-dimensional layered structure in KBe₂ BO₃ F₂ (KBBF), two new compounds, Sn₂ PO₄ I and Sn₂ BO₃ I, were designed and synthesized successfully, maintaining the layer structural feature and enhancing the optical anisotropy of crystals. In particular, the birefringence of Sn₂ PO₄ I is larger than or equal to 0.664 @546 nm, which is largest among the reported borates and phosphates, even surpassing commercial birefringent crystals YVO₄ and TiO₂ . This work indicates that a breakthrough in birefringence of inorganic compound was achieved. Also, it provides a guiding idea for exploring large birefringence materials in the future.

Anisotropic Iridescence and Polarization Patterns in a Direct Ink Written Chiral Photonic Polymer

The iridescence of structural color and its polarization characteristics originate from the nanoscale organization of materials. A major challenge in materials science is generating the bright, lustrous hues seen in nature through nanoscale engineering, while simultaneously controlling interaction of the material with different light polarizations. In this work, a suitable chiral nematic liquid crystal elastomer ink is synthesized for direct ink writing, which self-assembles into a chiral photonic structure. Tuning the writing direction and speed leads to the programmed formation of a slanted photonic axis, which exhibits atypical iridescence and polarization selectivity. After crosslinking, a freely programmable, chiroptical photonic polymer material is obtained. The strongly perspective-dependent appearance of the material can function as specialized anticounterfeit markers, as optical elements in decorative iridescent coatings, or, as demonstrated here, in optically based signaling features.

Development of dual-ended depth-of-interaction detectors using laser-induced crystals for small animal PET systems

Sensitivity and spatial resolution of positron emission tomography (PET) scanners can be improved by using thicker scintillation crystals with depth-of-interaction (DOI) encoding. Subsurface laser engraving (SSLE) can be used to segment crystals of a scintillation detector in order to fabricate a DOI detector. We previously applied SSLE to crystal bars of 3 × 3 × 20 mm³and 1.5 × 1.5 × 20 mm³and developed two dual-ended detectors with DOI segments of 3 mm and 1.5 mm, respectively. To further improve the DOI resolution, our SSLE detector design can be used with smaller pitch crystal bars, making them excellent detector candidates for small animal PET scanners with submillimetre resolution. In the present study, three small crystal bars of 1 × 1 × 20 mm³, 2 × 1 × 20 mm³, and 2 × 1 × 40 mm³were laser engraved to 12, 20 and 40 segments, respectively, by applying SSLE in their height directions. The segmented crystal bars were characterised in three prototype detector arrangements. First, the 1 × 1 × 20 mm³crystal bars were characterised in an 8 × 8 crystal array designed for DOI encoding along crystal height in a conventional small animal PET design. Second, a 4 × 8 crystal array of 2 × 1 × 20 mm³crystal bars was characterised for using the DOI information for crystal interaction positioning along the axial axis of a small animal PET scanner. Finally, the third part of the study was performed on a single 2 × 1 × 40 mm³crystal bar with 40 segments to investigate the feasibility of DOI estimation in longer crystals for application in a system with extended axial length. We evaluated the capability of segment identification and energy resolution of theses detectors. The 3D position maps of the detectors were obtained using the Anger-type calculation and the crystal identification performance was evaluated for each detector. Clear segment separation was obtained for the crystal arrays with 12 (segment pitch of 1.67 mm) and 20 (segment pitch of 1 mm) segments. Mean energy resolutions of 8.8% ± 0.4% and 9.6% ± 0.8% at 511 keV were obtained for the segments in the central regions of the 8 × 8 array with 12 segments and the 4 × 8 array with 20 segments, respectively. Clear segment identification was found to be difficult for the detector with 40 segments, especially for the segments at the middle of the crystal. Energy and interaction positioning characterisation results suggest that both prototype detectors with 12 and 20 segments are well suited for small animal PET scanners with high spatial resolution.

Li3La2(BO3)3 and Li1.75Na1.25La2(BO3)3: A Great Enhancement in Birefringence Induced by Optimal Arrangement of π-Conjugated [BO3] Units

In virtue of the essential role in controlling polarized light, outstanding ultraviolet (UV) and deep-ultraviolet (DUV) birefringent crystals are imperative in many advanced optical instruments. To design the UV and DUV crystals with large birefringence, we paid more attention to combining the excellent gene, π-conjugated [BO₃] unit and metal cations beneficial to blue-shift the cutoff edge; finally, two rare-earth borates Li₃La₂(BO₃)₃ and Li_1.75Na_1.25La₂(BO₃)₃ have been synthesized using the high-temperature solution method. Compared with Na₃La₂(BO₃)₃, Li₃La₂(BO₃)₃ and Li_1.75Na_1.25La₂(BO₃)₃ are isostructural, and the isolated [BO₃] units are arranged nearly parallel to each other in the structure, which is conducive to generating a larger birefringence. The structural comparison between the two crystals and Na₃La₂(BO₃)₃ indicates that the various coordination environments of alkali metal cations play an important role in the evolution of the crystal structure from Li₃La₂(BO₃)₃ and Li_1.75Na_1.25La₂(BO₃)₃ to Na₃La₂(BO₃)₃. This work can contribute to a better understanding of the enhancement in birefringence from Na₃La₂(BO₃)₃ (0.023 @ 1064 nm) to Li₃La₂(BO₃)₃ (0.078 @ 1064 nm) with the perspective of structure-property relationships. Meanwhile, the two title crystals possess the DUV cutoff edge (<190 nm), suggesting that they can be applied as the DUV birefringent crystals.

Birefringence and Dichroism in Quasi-1D Transition Metal Trichalcogenides: Direct Experimental Investigation

Birefringence and dichroism are very important properties in optical anisotropy. Understanding the intrinsic birefringence and dichroism of a material can provide great help to utilize its optical anisotropy. But the direct experimental investigation of birefringence in nanoscale materials is rarely reported. As typical anisotropic transition metals trichalcogenides (TMTCs) materials with quasi-1D structure, TiS₃ and ZrS₃ have attracted extensive attention due to their special crystal structure and optical anisotropy characteristics. Here, the optical anisotropy properties such as birefringence and dichroism of two kinds of quasi-1D TMTCs, TiS₃ and ZrS₃ , are theoretically and experimentally studied. In experimental results, the anisotropic refraction and anisotropic reflection of TiS₃ and ZrS₃ are studied by polarization-resolved optical microscopy and azimuth-dependent reflectance difference microscopy, respectively. In addition, the birefringence and dichroism of ZrS₃ nanoribbon in experiment are directly measured by spectrometric ellipsometry measurements, and a reasonable result is obtained. This work provides the basic optical anisotropy information of TiS₃ and ZrS₃ . It lays a foundation for the further study of the optical anisotropy of these two materials and provides a feasible method for the study of birefringence and dichroism of other nanomaterials in the future.

Nanoindentation Hardness and Practical Scratch Resistance in Mechanically Tunable Anti-Reflection Coatings

This work presents fundamental understanding of the correlation between nanoindentation hardness and practical scratch resistance for mechanically tunable anti-reflective (AR) hardcoatings. These coatings exhibit a unique design freedom, allowing quasi-continuous variation in the thickness of a central hardcoat layer in the multilayer design, with minimal impact on anti-reflective optical performance. This allows detailed study of anti-reflection coating durability based on variations in hardness vs. depth profiles, without the durability results being confounded by variations in optics. Finite element modeling is shown to be a useful tool for the design and analysis of hardness vs. depth profiles in these multilayer films. Using samples fabricated by reactive sputtering, nanoindentation hardness depth profiles were correlated with practical scratch resistance using three different scratch and abrasion test methods, simulating real world scratch events. Scratch depths from these experiments are shown to correlate to scratches observed in the field from consumer electronics devices with chemically strengthened glass covers. For high practical scratch resistance, coating designs with hardness >15 GPa maintained over depths of 200–800 nm were found to be particularly excellent, which is a substantially greater depth of high hardness than can be achieved using previously common AR coating designs.

Giant optical anisotropy in transition metal dichalcogenides for next-generation photonics

Large optical anisotropy observed in a broad spectral range is of paramount importance for efficient light manipulation in countless devices. Although a giant anisotropy has been recently observed in the mid-infrared wavelength range, for visible and near-infrared spectral intervals, the problem remains acute with the highest reported birefringence values of 0.8 in BaTiS3 and h-BN crystals. This issue inspired an intensive search for giant optical anisotropy among natural and artificial materials. Here, we demonstrate that layered transition metal dichalcogenides (TMDCs) provide an answer to this quest owing to their fundamental differences between intralayer strong covalent bonding and weak interlayer van der Waals interaction. To do this, we made correlative far- and near-field characterizations validated by first-principle calculations that reveal a huge birefringence of 1.5 in the infrared and 3 in the visible light for MoS2. Our findings demonstrate that this remarkable anisotropy allows for tackling the diffraction limit enabling an avenue for on-chip next-generation photonics.

All-polymeric high-Q optofluidic Fabry-Perot resonator

A full polymeric optofluidic Fabry-Perot (FP) resonator with a high quality factor (Q) is proposed and tested. The device is based on multilayer optical polymeric films that act as high reflectivity interference mirrors. The all-polymeric laminated structure avoids any deposition or etching process, simplifying the fabrication procedure while retaining a high quality optical surface. The measured quality factor of the FP resonator is 3.03×10⁴, and the finesse is 91 around 700 nm. The refractometric sensing properties have been measured, and a sensitivity of 314 nm/RIU with a limit of detection of 2.55×10^-5RIU could be achieved. The device exhibits a very high figure of merit of 1.36×10⁴RIU^-1 that is comparable with the performance attainable with sensors based on photonic crystal and whispering gallery mode resonators.

AB11O16(OH)2 (A = K and Cs): interpenetrating 2D layers with large birefringence

By a cation substitution strategy, two new hydroxyborates AB₁₁O₁₆(OH)₂ (A = K and Cs) were synthesized, and KB₁₁O₁₆(OH)₂ exhibits a short DUV cutoff edge (195 nm) and a large birefringence.

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.

An antimony(iii) borate with large birefringence exhibiting unwonted [B5O11] fundamental building blocks and dimeric [Sb2O6] clusters

By combining π-conjugated [B₃O₆] groups and Sb³⁺ cations with SCALP, we successfully obtained the second antimony(iii) borate SbB₅O₉ with a large birefringence (0.151 @ 1064 nm), which exhibits unwonted [B₅O₁₁] FBBs and dimeric [Sb₂O₆] clusters.