Birefringent Materials Research Articles

Bound states in the continuum in asymmetric one-dimensional photonic crystal systems guided by anisotropy.

Bound states in the continuum (BICs) have been widely observed in many symmetric geometries in the optical system during the last decade. Here, we consider a scenario in which the structure is designed asymmetrically with anisotropic birefringent material embedded in one-dimensional photonic crystals. This kind of new shape opens the possibility of obtaining symmetry-protected BICs (SP-BICs) and Friedrich-Wintgen BICs (FW-BICs) form in tunable anisotropy axis tilt. Interestingly, these BICs can be observed as high-Q resonances by variation of the system's parameters, such as the incident angle, which means the structure without being injected at Brewster's angle can also achieve BICs. Our findings might achieve active regulation and are easy to manufacture.

Two Carboxylate-Cyanurates with Strong Optical Anisotropy and Large Band Gaps.

The first metal carboxylate-cyanurates, namely, K(H3C3N3O3)(HCO2) (I) and Ba2(H2C3N3O3)(CH3CO2)3(H2O) (II), which contain π-conjugated carboxylate and cyanurate groups, have been synthesized by hydrothermal methods. They crystallize in centrosymmetric space groups of P1̅ and P21/n, respectively. Compound I exhibits a novel three-dimensional (3D) structure based on a [K(H3C3N3O3)]+ cationic framework with 12-membered ring (12-MR) channels, and the (HCO2)- anions are located within the 12-MR channels. The [K(H3C3N3O3)]+ cationic framework is composed of K+ ions interconnected by H3C3N3O3 ligands. Compound II features a 3D network formed by [Ba2(CH3CO2)3]+ cationic double chains bridged by (H2C3N3O3)- anions. The [Ba2(CH3CO2)3]+ cationic double chain is composed of (CH3CO2)- anions and Ba2+ ions. Optical property measurements show that both compounds exhibit short ultraviolet cutoff edges (I, 208 nm; II, 218 nm) and wide band gaps (I, 5.43 eV; II, 5.20 eV). Importantly, K(H3C3N3O3)(HCO2) (I) features a large birefringence of 0.285@532 nm due to the parallel alignment of π-conjugated H3C3N3O3 and (HCO2)- groups, indicating that K(H3C3N3O3)(HCO2) (I) is a promising short-wave ultraviolet birefringent material. Detailed theoretical calculations elucidate that their excellent optical properties originate from the synergetic effect of both types of π-conjugated groups.

Characteristics of surface plasmonic modes in cylindrical chiral-graphene-dielectric waveguide structure

A cylindrical chiral-graphene-dielectric waveguide structure is proposed. Correspondingly, characteristics of surface plasmonic mode are theoretically investigated, including dispersion relation, intensity, phase and polarization distribution. The proposed waveguide can only support the vortex modes with the hybrid polarization distribution, which originates from the spin-momentum locking of evanescent electromagnetic waves. The circular birefringence of chiral materials releases the degeneracy between same-order vortex modes. In addition, the number of modes can be controlled by changing the radius of the dielectric nanowire and the Fermi level of graphene. The effective index and corresponding propagation length of the mode are sensitive to the chiral parameter. We believe the proposed waveguide can find some potential applications in multiplex communication, chiral sensing and the fabrication of tunable nano-photonic devices.

Dramatically improved optical anisotropy by realizing stereochemically active lone pairs in a sulfate system, K2SO4·HIO3

A novel birefringent material, K2SO4·HIO3, was developed by introducing a polyhedron of lone pair cations. This material shows a 70-fold increase in birefringence compared to the corresponding monosulfate, K2SO4.

Linear π-conjugated units of the D∞h point group as superior ultraviolet birefringent units.

At present, birefringent materials face a limited selection of large structural anisotropic functional modules (FMs). In this paper, we present a series of linear units which belong to the D∞h point group represented by (BO2)- proposed as novel birefringent active FMs. By analyzing the molecular orbital of the (BO2)- unit, it is found that there are relatively fewer non-bonding orbitals in (BO2)- than in (BO3)3- and the delocalized π bonds in (BO2)- appear in shallow energy levels, which are easily excited. Through first-principles modeling and simulation, it is found that the delocalized π bonds in (BO2)- can still show obvious transition processes, which produce a significant gain to the birefringence. Besides, a series of compounds containing linear anionic frameworks which also belong to the D∞h point group show excellent optical anisotropy in the same way. Therefore, the linear anionic basic units which belong to the D∞h point group have the great potential to become new birefringent FMs.

RbCl·(H2SeO3)2: A Salt-Inclusion Selenite Featuring Short UV Cut-Off Edge and Large Birefringence

Birefringent materials are key components to control the light polarization in laser science and technology as well as optical communication. However, the performance of current commercial birefringent materials has been limited by the magnitude of birefringence, optical transparency range, or the attainability of large-scale single crystals. To explore new birefringent materials, we strategically incorporated a lone pair cation (Se4+) with large optical anisotropy, an alkali metal, and halogen ions (Rb+ and Cl-) with superior UV transparent capacity; thus a new compound, RbCl·(H2SeO3)2, was successfully discovered with the aid of the facile hydrothermal method. Interestingly, Rb-Cl chains locate in the [H2SeO3]∞ skeleton, which makes RbCl·(H2SeO3)2 a salt-inclusion selenite. Millimeter-sized single crystals (up to 4 × 2 × 1 mm3) were obtained, and the transmittance spectrum revealed that its UV cut-off edge can be as low as 230 nm. In addition, the calculated birefringence of RbCl·(H2SeO3)2 is 0.14 at 589 nm that is similar to the birefringent value of famous α-BaB2O4. Wide UV transparency, large birefringence, and feasible crystal growth make RbCl·(H2SeO3)2 a new member of birefringent materials for UV light applications.

Evaluation of Fossil Amber Birefringence and Inclusions Using Terahertz Time-Domain Spectroscopy.

Using a cross-polarization transmission geometry, stress maps for the normalized birefringence and intrinsic stress direction of polymeric materials may be obtained using terahertz nondestructive evaluation. The analysis method utilizes a deconvolution method to determine the arrival times and amplitude of the cross-polarized terahertz pulses through a birefringent material. Using amber (a naturally occurring polymer) as a material of interest, stress maps show that inclusion-free Lebanese amber samples behave as classic uniaxial birefringent (photoelastic) materials whose principal stress directions, as inferred in the terahertz spectral range, agree well with visible photoelasticity measurements. Since amber samples, depending upon their source, may be either transparent or opaque to visible light, comparing birefringence measurements in the visible and terahertz spectral ranges cross-validates the stress measurements, thereby establishing a strong and unique stress analysis methodology for visibly opaque samples. While the material of interest for this paper is amber, the method is generally applicable for any terahertz-transparent polymer. The cross-polarization experimental configuration enables stress levels within the amber matrix to be visualized while also outlining highly localized regions of stress surrounding inclusions. Birefringence stress maps clearly show localized increases in stress magnitude and directional changes surrounding inclusions.

Broadband Wollaston prism with a large output beam separation based on mercurous halides.

The paper proposes a Wollaston-type crystal polarizer suitable for broadband operation within the visible spectral band up to the far infrared band based on unique optical materials, mercurous halides (Hg2X2). This paper introduces the general characteristics and optical properties of these birefringent tetragonal optical materials, as well as the general description of a Wollaston prism and the process of its parameter optimization. In general, the Wollaston polarizer is constructed from two combined wedge-shaped prisms. The key parameters that affect the properties of the Wollaston polarizer are then the cut angle of these two prisms and the refractive index of the exploited optical cement (immersion) that bonds the prisms together. The optimal prism cut angles and immersion refractive index are investigated to maximize the Wollaston parameters, such as the transmittance of the polarized radiation and the separation angle of the output orthogonally polarized beams. This process is significantly dependent on the characteristics of all selected mercurous halides (Hg2Cl2, Hg2Br2, Hg2I2). The optimal values of the prism cut angle for each material are selected based on the outlined results. In addition, the Wollaston prism behaviour regarding real radiation propagation is modelled in detail via the Zemax optical studio. The presented models aim to aid in the real design and fabrication of a broadband Wollaston polarizer based on mercurous halides.

Electrically tunable dual-layer twisted nematic liquid crystal THz phase shifters with intermediate composite polymer thin film

We demonstrate electrically tunable twisted nematic (TN) aligned liquid crystal (LC) terahertz phase shifters with a novel structure, in which a single LC composite polymer thin film is inserted in the middle of the LC cell. For designing such types of phase shifters, we have applied the Mauguin formalism used in LC-based switches for applications in the visible. Experimental results are in good agreement with those predicted by the theoretical model. Using this design, the phase shift has been increased by 14.3° at 1.2 THz in comparison to the conventional design (98.1°). The threshold voltage has been lowered from 0.81 VRMS to 0.48 VRMS while the driving voltage for quarter-wave (π/2) phase shift operation has been reduced from 8.4 VRMS to 6.3 VRMS at 1.2 THz. The dynamic response of devices also enhances significantly. Such design has also successfully been adopted to demonstrate a 2π phase shifter with similar improvement. Full-wave operation is achieved at a bias of 25 VRMS at 1.2 THz using a highly birefringent LC material. Order parameter calculations show that the polymer film improves significantly LC molecular alignment in the thick LC cell for THz applications. We have also compared the figure of merits (FOMs) of the present devices with previously reported THz π/2 and 2π phase shifters.

Li0.5Na0.5AlB2O4F2: Fluoroaluminoborate with Aligned 1∞[BO2] Chain Induced by Unprecedented [AlO3F3]6− Species Features Enhanced Birefringence

AbstractBirefringent materials have been used extensively owing to their indispensable role in creating and modulating polarized light, while birefringent materials for deep ultraviolet (DUV, λ < 200 nm) applications are still urgently needed. Herein, a strategy to induce the formation of birefringence‐active 1∞[BO2] chains driven by AlOmFn units is proposed for designing DUV birefringent materials, and a new fluoroaluminoborate, Li0.5Na0.5AlB2O4F2, with aligned 1∞[BO2] chains and an unprecedented [AlO3F3]6− motif is synthesized. The birefringence of Li0.5Na0.5AlB2O4F2 achieves significant enhancement compared with maternal structure LiAlB2O5 due to the optimization of functional units and arrangement via co‐substitution and fluorination tactics. The design strategy provides a propitious and feasible way to construct novel structures with excellent optical properties.

Tilting nondispersive bands in an empty microcavity

Recently, microcavities with anisotropic materials were shown to be able to create bands with non-zero local Berry curvature. The anisotropic refractive index of the cavity layer is believed to be critical in opening an energy gap at the tilted Dirac points. In this work, we show that the anticrossing between a cavity mode and a Bragg mode can also be realized within an empty microcavity without any birefringent materials in the cavity layer. Nondispersive bands are observed within the energy gap due to the particular refractive index distribution of the sample. The intrinsic TE-TM splitting and XY splitting of DBR mirrors induce the squeezing of the cavity modes in momentum space, so that the nondispersive bands are tilted and spin-dependent. Our results pave the way to investigate interesting physical phenomena of photonic modes close to or in the nondispersive bands without anisotropic cavity layers.

Time-Resolved Four-Channel Jones Matrix Measurement of Birefringent Materials Using an Ultrafast Laser.

A method for ultrafast time-resolved four-channel Jones matrix measurement of birefringent materials using an ultrafast laser is investigated. This facilitated the acquisition of a four-channel angular multiplexing hologram in a single shot. The Jones matrix information of a birefringent sample was retrieved from the spatial spectrum of a hologram. The feasibility of this approach was established by measuring the Jones matrix of starch granules in microfluidic chips and the complex amplitude distribution and phase delay distribution of liquid crystal cell at different voltages. Moreover, when the picosecond laser was switched to a femtosecond laser, ultrafast measurements were possible provided that the time interval between two detection pulses was larger than the pulse width.

Li2PbB2O5: A Pyroborate with Large Birefringence Induced by the Synergistic Effect of Stereochemical Active Lone Pair Cations and π-Conjugated [B2O5] Groups.

Through the inclusion of two kinds of birefringence reactive groups, that is, Pb2+ cations with stereochemically active lone pairs and π-conjugated [B2O5] groups, a new pyroborate Li2PbB2O5 used to be acquired by the usage of the high-temperature solution method. The three-dimensional structural framework of Li2PbB2O5 is made up of highly distorted [PbO5] polyhedra and parallelly arranged π-conjugated [B2O5] groups via the connection of four- and five-coordinated Li+ cations. The birefringence of Li2PbB2O5 is 0.091 at 1064 nm, which is relatively large and revealed by the first-principles calculations. The contribution from the synergistic effect of the two kinds of birefringence reactive groups was substantiated via the analysis of the real-space atom cutting method. Also, the diffuse reflectance spectrum suggests that Li2PbB2O5 features a band gap of 4.0 eV, which corresponds to an ultraviolet (UV) cutoff edge. We hope that this work will foster further improvements in the design of UV birefringent crystal materials in the borate system.

ZrF2(IO3)2 and RbGaF3(IO3): Two Promising Birefringent Crystals Featuring 1D Metal-Fluoride Cationic Chains and Wide Bandgaps

Herein, two novel metal iodate-fluorides, ZrF2(IO3)2 and RbGaF3(IO3), have been prepared via hydrothermal reactions. In the structure of ZrF2(IO3)2, neighboring Zr atoms are connected via a pair of F atoms into the one-dimensional (1D) [ZrF2]2+ cationic chain, which further attaches two iodate groups forming 1D ZrF2(IO3)2, whereas the 1D cationic chain in RbGaF3(IO3) is [GaF]2+, formatted by the bridging of Ga3+ cations and axial F- anions, which further connect with one iodate group and two terminal F- anions into 1D [GaF3(IO3)]- anionic chains. According to the experimental characterization and theoretical calculations, both of them exhibit wide bandgaps (4.24 and 4.61 eV), short absorption edges (242 and 216 nm), and large birefringence (Δncal = 0.329 @1064 nm and Δncal = 0.174 @1064 nm for ZrF2(IO3)2 and RbGaF3(IO3), respectively), indicating that they can be candidates as UV birefringent materials.

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.

A new birefringent material, HfF2(IO3)2, with a large birefringence and improved overall performances achieved by the integration of functional groups

Birefringent materials have been widely used in optical devices. An excellent birefringent material, HfF2(IO3)2, was rational designed and synthesized by integrating iodate and fluoride functional groups. HfF2(IO3)2 inherits the huge optical anisotropy of iodates and the wide energy gap of fluorides and possesses outstanding properties including a large birefringence of 0.333 and a wide energy gap of 4.11 eV (a short ultraviolet absorption cutoff edge of 253 nm), superior to the commercial birefringent crystals such as YVO4 (Δn, 0.204 at 532 nm; Eg, 3.10 eV) and TiO2 (Δn, 0.256 at 1530 nm; Eg, 3.20 eV). HfF2(IO3)2 also has a high thermal stability and a wide transparent spectral range, and it is non-hygroscopic. Our investigations showed that the birefringences are linearly proportional to the parallelisms and the spatial densities of the functional lone-pair groups. This work demonstrates that combining functional groups is an effective strategy for designing promising inorganic functional materials.

Angularly resolved polarization microscopy for birefringent materials with Fourier ptychography.

Polarization light microscopy is a very popular approach for structural imaging in optics. So far these methods mainly probe the sample at a fixed angle of illumination. They are consequently only sensitive to the polarization properties along the microscope optical axis. This paper presents a novel method to resolve angularly the polarization properties of birefringent materials, by retrieving quantitatively the spatial variation of their index ellipsoids. Since this method is based on Fourier ptychography microscopy the latter properties are retrieved with a spatial super-resolution factor. An adequate formalism for the Fourier ptychography forward model is introduced to cope with angularly resolved polarization properties. The inverse problem is solved using an unsupervised deep neural network approach that is proven efficient thanks to its performing regularization properties together with its automatic differentiation. Simulated results are reported showing the feasibility of the methods.

Birefringence in the Polarized Raman Scattering of Biaxial van der Waals α‐MoO3

AbstractThe effect of birefringence in anisotropic materials has been a long‐term issue for polarized Raman scattering. In this work, the polarization‐dependent Raman scattering in anisotropically birefringent materials is modeled with birefringence considered in the fundamental polarization selection rule. The birefringence‐induced polarization transformation is treated as a tensor in the calculation of Raman scattering intensity. The validity of this theory is further demonstrated in experiments by taking angle‐resolved polarized Raman measurements on the basal and cross planes of α‐MoO3—a typical biaxial van der Waals crystal with strong in‐plane and out‐of‐plane anisotropy. The anomalous angular dependency of polarized Raman scattering intensity can be well reproduced by the modified theoretical model with fitted real‐valued Raman tensors and birefringence‐related parameters. It can be concluded that this work can provide a valuable reference and guidance for quantitative analyses of anisotropic materials by using polarization‐resolved Raman scattering spectroscopy.

Orthogonal separation of arbitrary vector beams from non-polarized light waves based on a patterned liquid-crystal photo-alignment.

An effective method for orthogonally separating arbitrary vector polarized beams from non-polarized incident light waves is proposed in this Letter. A tunable patterned spatial distribution of liquid-crystal (LC) molecules can be effectively constructed based on both the initial photo-alignment and the electrically controlled birefringence of nematic LC materials. The LC photo-alignment over a smooth surface without any common nano-grooves leads to a highly efficient light-wave transformation by inducing a desired initial arrangement of LC directors and then acquiring extraordinary light waves with the needed, or even arbitrary, spatial polarization. The vector polarized beams can be highly converged according to a microhole-patterned electrode and a gradient refractive index distribution of the LC layer, which is driven and adjusted by an applied signal voltage. Due to the intrinsic polarization sensitivity of nematic LC materials, the formed gradient refractive index appearance only corresponds to extraordinary light waves. The proposed approach provides a way to achieve the orthogonal separation of arbitrary vector beams from non-polarized light waves. Moreover, it can be further utilized to generate and obtain arbitrary vector beams, as well as to perform adaptive light-beam convergence or even the focusing of arbitrary vector beams, which is expected to advance the development of vector beam generation and manipulation, thereby stimulating potential applications.

Improved light transmission in nanocrystalline aluminum nitride (AlN)—Enabling a lightweight, thermal shock resistant transparent ceramic

Optically transparent ceramics were traditionally only viable using optically isotropic materials and limited birefringent materials with small Δn such as alumina (Δn≈0.008). Thus heavily birefringent materials such as AlN (Δn≈0.045), polycrystalline ceramics are usually not considered for optical applications due to their high scattering loss, despite its exceptionally appealing thermal conductivity, mechanical stiffness/toughness and relatively low density—attributes that are ideal for light weight optical windows for harsh environments. To further improve the transparency over coarse grain AlN ceramics that have been reported, subwavelength grain size is desired. Here we present significant improvements in visible, NIR and mid-IR transparency in AlN ceramics by reducing the average grain size to 230 nm. A newly developed, non-carbon thermal reduction for producing clean powders at low temperatures is key to the success. The nanocrystalline AlN ceramics reach full density at a densification temperature as low as 1500 °C and showed an in-line transparency of over 55% at 2.5 μm and over 70% in the mid-IR which is the highest transparency in AlN ceramics reported. The successful demonstration of transparent nanocrystalline AlN ceramics makes it the ideal choice for optical windows in quickly fluctuating temperature environments where weight is a consideration such as air and space vehicles.