Birefringence Research Articles

6.7–8.1 µm tunable single-frequency difference frequency generation in ZGP for high-resolution spectroscopy

Difference frequency generation (DFG) based tunable single-frequency mid-infrared (MIR) light sources are desirable for high-resolution spectroscopy, sensing, and imaging. In this work, we demonstrate a continuous-wave (CW) single-frequency DFG in a ZnGeP2 (ZGP) crystal driven by all-fiber near-infrared (NIR) fiber lasers, for the first time to our knowledge. The all-fiber NIR laser sources consist of a 1.5 µm erbium-doped fiber amplifier seeded by a CW tunable fast scanning single-frequency laser and a 1.9 µm CW tunable single-frequency thulium-doped fiber laser. Taking advantage of the high nonlinear coefficient and large birefringence of the ZGP crystal, single-frequency DFG in ZGP achieves a broad spectral tuning range from 6.7 to 8.1 µm, with an output power at the 10 µW level. Precise detection of C2H2 gas by continuously scanning the DFG source across a spectral range of 1.1 THz (∼34 cm−1) is also presented, highlighting the potential of the tunable DFG source for high-resolution optical spectroscopy applications. We anticipate this work will provide what we believe to be a new platform for spectroscopy in the molecular fingerprint spectral region.

Electronic, optical, elastic, piezoelectric and vibrational properties of P63 KLiSO4

Abstract KLiSO4 of the P63 symmetry is a well refined crystal at room temperature, which is a pyroelectric material with a large second harmonic generation response. However, its fundamental physical properties are still not well studied. In this work, first principles calculations are performed to study its electronic, optical, elastic, piezoelectric and vibrational properties. The results indicate that it is an ionic crystal with a large indirect band gap. Calculated optical properties imply that P63 KLiSO4 has little optical anisotropy at low frequencies. Obtained elastic constants reveal that it is mechanically stable but anisotropic, as illustrated by the directional bulk and shear moduli. Piezoelectric coefficients, dielectric constants, and Born effective charges (BECs) are computed using the density functional perturbation method. Studies disclose that it has a greater piezoelectric coefficient along the c axis. The ions have more contribution to the total dielectric constants than the electrons. The S atoms have the largest BECs. The phonon vibrational modes at the Brillouin zone center are analyzed by the factor group theory. Its infrared and Raman spectra are simulated. The causation for the vanishment of some infrared peaks in the computed infrared spectrum is uncovered. Additionally, elastic related moduli, hardness, melting point and electromechanical coupling coefficients of P63 KLiSO4 are also predicated.

Imaging‐Based Lensless Polarization‐Sensitive Fluid Stream Analyzer for Automated, Label‐Free, and Cost‐Effective Microplastic Classification

The presence of microplastics in the environment is of significant concern, yet the exact extent of this pollution remains largely unknown. The ocean is of particular interest as the monitoring of microplastics presents a challenge in that in situ fluid stream solutions are not readily available and traditional sampling methods are labor‐intensive and costly. This study introduces an imaging‐based lensless polarization‐sensitive fluid stream analyzer (FSA) for automated, label‐free, and cost‐effective detection and classification of microplastics. The FSA incorporates digital in‐line holography and birefringence computation, enabling quantitative polarization‐sensitive imaging and machine‐learning‐based activities including sample classification. Birefringent textures of synthetic polymers are investigated owing to their optical anisotropy. A microplastic classifier is developed for the FSA and integrated to form an end‐to‐end workflow capable of sampling fluid streams and determining marine and microplastic particle presence. Cultures of two phytoplankton species form a simplified marine environment for FSA evaluation. The device is tested in a two‐class configuration for marine microorganisms and microplastics, as well as a five‐class configuration for marine microorganisms and four individual microplastic types (polyethylene, polyethylene terephthalate, polypropylene, and polystyrene). The results demonstrate high classification accuracy, supported by experiments in the simulated marine environment that validate the proposed implementation's efficacy.

Enhanced Coplanarity and Giant Birefringence in Hydroxypyridinium Nitrate via Hydrogen Bonding between Planar Donors and Planar Acceptors.

Birefringent crystals, which possess optical anisotropy, are important optical components. However, designing and synthesizing birefringent crystals faces the challenge of achieving anisotropic structures, especially coplanar geometries. Herein, we achieve a significant birefringence in an ionic compound (C5H6ON)+(NO3)-, (4-hydroxypyridinium nitrate, 4HPN) by hydrogen bonding between planar donors and planar acceptors. We demonstrate that the interactions between the planar hydrogen bond donor ((C5H6ON)+) and planar hydrogen bond acceptor ((NO3)-) ensure the coplanarity during the crystal packing, generating the desired giant optical anisotropy. On two manually cut crystal chips, we observe a = 0.494 (= 0.593), which is the largest among nitrates, or hydroxypyridinium derivatives. This value already surpasses those of the benchmark crystals, e.g., YVO4 and CaCO3, commonly used in the UV to visible and near IR spectral range. 4HPN also exhibits a strong second harmonic generation response (9.55 × KDP). This strategy offers a promising avenue for the design and development of birefringent crystals with potential applications in optical communication, sensing and signal processing devices.

Wavelength dependence of linear birefringence and linear dichroism of Bi2−xPbxSr2CaCu2O8+δ single crystals

Copper oxide high-temperature superconductors, such as Bi2Sr2CaCu2O8+δ (Bi2212), have garnered extensive research interest due to their high critical temperatures (Tc) surpassing the Bardeen–Cooper–Schrieffer (BCS) limit. The two-dimensional CuO₂ plane is widely regarded as the most crucial element of high-Tc cuprate superconductors. The anisotropy of this CuO₂ layer remains a topic of ongoing interest. Although a few experimental results have reported strong optical anisotropy in both ab and ac-planes of Bi2212 through optical “reflectivity” measurements, there is a lack of studies focusing on the optical anisotropy of these materials using optical “transmittance” measurements by utilizing ultraviolet and visible light. Using a generalized high-accuracy universal polarimeter, we observed significant linear birefringence and linear dichroism peaks in the UV region at room temperature. To investigate the origin of the significant optical anisotropy, single crystals of Bi2−xPbxSr2CaCu2O8+δ with different lead contents (x = 0, 0.4, and 0.6) were grown using the floating zone method, and the wavelength dependencies of linear birefringence and linear dichroism along the c axis were measured. The insights gained into the optical anisotropy of Bi2−xPbxSr2CaCu2O8+δ from this study are significant for discussing its origin of the mechanism of high-Tc superconductivity.

Metallic Ca Aggregates Formed Along Ion Tracks and Optical Anisotropy in CaF2 Crystals Irradiated with Swift Heavy Ions

It is known that swift heavy ion (SHI) irradiation induces the shape elongation of metal nanoparticles (NPs) embedded in transparent insulators, which results in anisotropic optical absorption. Here, we report another type of the optical anisotropy induced in CaF2 crystals without including intentionally embedded metal NPs. The CaF2 samples were irradiated with 200 MeV Xe14+ ions with an incident angle of 45° from the surface normal. With the increasing fluence, an absorption band at ~550 nm, which is ascribed to Ca aggregates, increases both the intensity and the anisotropy. XTEM observation clarified the formation of the continuous line structures and the discontinuous NP chains parallel to the SHI beam. Numerical simulations of the optical absorption spectra suggested the NP chains but not the continuous line structures as the origin of the anisotropy. The optical anisotropy in CaF2 irradiated with SHIs is different from the shape elongation of NPs.

From B3O3(OH)3 to B3O3F3: Uncovering Series of New Functional Units Based on [BO2F] Beneficial for Deep‐Ultraviolet Birefringence

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

From β-KB3O4F2 to α-KB3O4F2: Phase Transition-Directed Great Changes in Structure and Optical Anisotropy.

Herein, a new fluorooxoborate α-KB3O4F2 is synthesized successfully in a closed system. It crystallizes in the P21/n space group and features the rare one-dimensional (1D) zigzag [B6O8F4]∞ chains built by fundamental building blocks (FBBs) [B6O9F4]. To the best of our knowledge, KB3O4F2 is a unique example of inorganic anhydrous borate whose two polymorphs show 1D B-O/F chains constructed by different FBBs but the same symmetry operation. Different from β-KB3O4F2, the inconsistent arrangement of π-conjugated [B2O5] units in α-KB3O4F2 makes it exhibit a small birefringence (0.011@546 nm). Further, α-KB3O4F2 owns a deep-ultraviolet (DUV) cutoff edge (<200 nm), suggesting that it has a potential application as the zero-order waveplate material in the DUV region.

Hong‐Ou‐Mandel Interference on a Lattice: Symmetries and Interactions

AbstractThe Hong‐Ou‐Mandel interference of two identical particles evolving on a 1D tight‐binding lattice where a potential barrier plays the role of a beam splitter is described. Careful consideration of the symmetries underlying the two‐particle interference effect allows to reformulate the problem in terms of ordinary wave interference in a Michelson interferometer. This approach is easily generalized to the case where the particles interact, and we compare the resulting analytical predictions for the bunching probability to numerical simulations of the two‐particle dynamics.

Photoinduced Anisotropy in Thin Films of Azobenzene-Containing Liquid Crystalline Supramolecular Complexes of Various Polymer Architecture.

Light patternable colorless liquid crystalline (LC) polymers are promising materials for functional photonic devices with broad applications in optical communication, diffractive optics, and displays. This work reports photoinduced optical anisotropy in thin films of azobenzene-containing (Azo) LC block copolymer supramolecular complexes, which can be decolorized after light patterning providing colorless patterned birefringent polymer films. The supramolecular complexes are prepared via intermolecular pyridine-phenol hydrogen bonding between a low-molecular-weight Azo phenol and host LC AB diblock and ABA triblock copolymers consisted of LC phenylbenzoate (PhM) blocks and poly(vinylpyridine) units. The molecular architecture of the host polymers and the morphological pattern formed by the complexes can affect orientational behavior of Azo groups under irradiation with linearly polarized light. Photoorientation of hydrogen-bonded Azo groups is accompanied by the cooperative orientation of non-photochromic PhM units, which form individual microphases and stabilize the orientation of Azo groups. This effect is specific for block copolymer complexes and it is absent for random copolymer complex, which is used as a reference sample. Optical anisotropy induced in films of the block copolymer complexes can be amplified by heating above the glass transition temperature and subsequent rinsing with diethyl ether allows colorless birefringent polymer films to be prepared.

Ptychographic Mueller matrix imaging (PMMI): principle and proof-of-concept demonstration.

Quantitative characterization of optical anisotropies is extremely important for wide fields and applications. The Mueller matrix, providing all the polarization-related properties of a medium, is a powerful tool for the comprehensive evaluation of optical anisotropies. Here, we propose a ptychographic Mueller matrix imaging (PMMI) technique, which features the Mueller matrix polarization modulation being introduced into the ptychography. The ptychographic reconstruction is performed for each polarization state, and the Mueller matrix can be determined from the reconstructed polarization-modulated amplitude images. A proof-of-concept of the proposed PMMI is implemented, and both simulations and experiments are conducted to demonstrate the validity of the method. Results indicate that the imaging resolution of the home-built PMMI apparatus achieves 1.550 µm at the wavelength of 633 nm, which is of the highest level for the Mueller matrix imaging to the best of our knowledge. A customized birefringent specimen is characterized, and both retardance and axis azimuth are quantitatively evaluated.

Optical anisotropy of high-order harmonic generation by strong laser radiation in rectangular graphene quantum dot

Optical anisotropy of high-order harmonic generation by strong laser radiation in rectangular graphene quantum dot

Measuring the Electro-Optical Kerr Effect Against the Background of Electro-Absorption Modulation in Liquids.

A new approach to the dynamic polarimetric method is proposed, which allows for the decoupling of electro-optical Kerr effect measurements from the electro-absorption effect in partially transparent liquids. The method is illustrated by using the results of engine oil measurements as a function of temperature and modulating field frequency. It was shown that the birefringence induced in the sample, the modulation of the ordinary wave transmission, and the modulation of the extraordinary wave transmission in the sample can be shifted in phase with respect to the square of the applied alternating modulating field. Each of these three phase shifts can depend differently on the temperature and frequency. Neglecting the influence of electro-absorption on electro-optical measurements in liquids or considering electro-absorption as an effect correlated in phase with induced birefringence may lead to significant measurement errors. This indicates that the Kerr constant and the electro-absorption coefficients for an alternating electric field should be considered as complex quantities instead of real values, as they have been traditionally. The proposed approach fills an important gap in measurement techniques described in the literature, which may provide erroneous results for measurements of the Kerr constant in partially transparent liquids including many industrially important liquids.

Van der Waals GeSe with Strain- and Gate-Tunable Linear Dichroism for Wearable Electronics.

The direct detection of light polarization poses a crucial challenge in the field of optoelectronics and photonics. Herein, the tunable linear dichroism (LD) in GeSe-based polarized photodetectors is presented through electronic and structural asymmetry modulation, and demonstrate their application prospects in wearable electronics. An improvement in the dichroic ratio up to 34% is achieved under a gate voltage of 20V, and the improvement reaches 44% by applying a tensile strain along the zigzag direction. Theoretical calculations reveal that the gate regulation of barrier height between GeSe and Au electrodes is responsible for the electrical-tunable LD, while the anisotropic optical absorption in response to strains leads to the strain-tunable LD. Moreover, flexible GeSe transistors are developed for wearable applications including motion sensors and glucose monitors. This study offers viable approaches for modulating the optical anisotropy of low-dimensional materials and emphasizes the versatility of van der Waals materials for practical applications in wearable electronic devices.

Unprecedented Silver-Based Hybrid Pyroelectric Enabling Wide Spectral Photo-Pyroelectricity for Birefringence Photomodulation.

Birefringent crystal, which has the capacity to manipulate polarization light, holds an indispensable position in optics and optoelectronics, while it remains challenging to fulfill the modulation of birefringence. Here, we present wide spectral photo-pyroelectric effect in a silver-based hybrid pyroelectric, (N-CHM)Ag2I3 (N-CHM=N-cyclohexylmethylamine), serving as a feasible strategy to regulate birefringence through light stimuli. As the first silver-based hybrid pyroelectric, (N-CHM)Ag2I3 exhibits strong room-temperature photo-pyroelectricity with a large polarization of ~3.23 μC/cm2 and high voltage responsivity of ~0.96 m2/C across the UV-NIR spectral region. Strikingly, the photomodulation of its in-plane birefringence is established through pyroelectric effect, giving a saturation value of ~1.68×10-2 that is among the highest level achieved to date. This study on the birefringence photomodulation of lead-free hybrid pyroelectric is anticipated to boost future development of new smart optical and optoelectronic devices.

Double refractive particle tracking and sizing

We present a novel bifocal imaging method enabling three-dimensional particle tracking and size determination employing a single camera only. The method is based on double refraction causing a particle to be imaged twice, each particle image of different blur. From these double images, a linear calibration function can be derived allowing to determine the three-dimensional particle position unambiguously over the entire depth of measurement volume. As this calibration function is independent of the particle size used, the particle size can be determined simultaneously by relating size of the double images and depth position of the particle. To prove the applicability, a co-laminar flow of two particle suspensions with particles of 1.14 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu$$\\end{document}m and 2.47 μ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\upmu$$\\end{document}m in diameter was measured in a Y-shaped microchannel. While the laminar flow field was measured with very low uncertainty and independent of the particle size, the particle size distributions determined reproduced reliably the size distributions expected for the co-laminar flow applied, with a precision of about 98.6 %\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\%$$\\end{document} regarding the particle size discrimination. The progress for research is a new method readily to implement in common optical setups, promising, for example, valuable insights in polydisperse suspension flows—the vast majority of flows in fundamental research and applications.Graphical abstract

Imaging human coronary cholesterol/urate crystals with cross-polarized micro-optical coherence tomography.

Birefringent crystals such as monosodium-urate (MSU) and cholesterol crystals (CC) likely contribute to the progression of coronary artery disease (CAD) due to their potential to exacerbate inflammation through inflammatory cytokine activation. Here, we present cross-polarized micro-optical coherence tomography (CP-µOCT) for visualizing individual birefringent crystals in human coronary arteries. Human cadaver coronary arteries with a history of CAD with or without gout were dissected for CP-µOCT imaging. Specimens were processed for histological identification of birefringence under polarization light microscopy (PLM). CP-µOCT visualized needle-crystals that appeared as long projections in orthogonal planes, and PLM confirmed that CP-µOCT-delineated needle-crystals demonstrated negative birefringence. The needle-crystals were dissolved after immersion in uricase (p < 0.05), and thus were MSU. CP-µOCT was three-dimensionally volume-rendered for counting MSU and CCs in 79 regions of interest sized [750 (x) × 500 (y) × 400 (z) µm]. Crystal counts were normalized by the total coronary length utilized. The relationship between CP-µOCT-delineated MSU counts and those seen in corresponding histology, and the difference in coronary MSU amongst gout vs. non-gout patients was analyzed. CP-µOCT-delineated MSU counts were significantly correlated with MSU counted by PLM-based histology (R = 0.98, p < 0.01), and with histology-derived intimal thickening (R = 0.51, p < 0.01). MSU and CCs were both significantly greater in gout patients compared with non-gout patients (p < 0.05). These results demonstrate a significant increase in CP-µOCT-delineated crystals in gout vs. non-gout patients, suggesting that this technology can be used to improve our understanding of crystal-driven coronary pathogenesis.

Chiral plasmonic-dielectric coupling enables strong near-infrared chiroptical responses from helicoidal core-shell nanoparticles

Helicoid plasmonic nanoparticles with intrinsic chirality are an emerging class of artificial chiral materials with tailorable properties. The ability to extend their chiroplasmonic responses to the near-infrared (NIR) range is critically important for biomedical and nanophotonic applications, yet the rational design of such materials remains challenging. Herein, a strategy employing chiral plasmon-dielectric coupling is proposed to manipulate the chiroptical responses into the NIR region with high optical anisotropy. Through this strategy, the helicoid Au@Cu2O nanoparticles with structural chirality are designed and synthesized with tunable and enriched NIR chiroptical responses. Specially, a high optical anisotropy (g-factor) with a value of 0.35 is achieved in the NIR region, and multi-band chiroptical behaviors are observed. Spectral and electromagnetic simulations elucidate that strong coupling between chiroplasmonic core and chiral dielectric shell with high refractive index contributes to the rich and strong chiroptical responses, which are related to the interplay between various emerged and enhanced electric and magnetic multipolar resonance modes proved by multipole expansion analysis. Moreover, the helicoid Au@Cu2O nanoparticles display greater polarization rotation capability than the helicoid Au nanoparticles. This work offers mechanistic insights into chiral plasmon-dielectric coupling and suggests a general approach of creating NIR chiroplasmonic materials.

A Novel Polarized Light Microscope for the Examination of Birefringent Crystals in Synovial Fluid

Background: The gold standard for crystal arthritis diagnosis relies on the identification of either monosodium urate (MSU) or calcium pyrophosphate (CPP) crystals in synovial fluid. With the goal of enhanced crystal detection, we adapted a standard compensated polarized light microscope (CPLM) with a polarized digital camera and multi-focal depth imaging capabilities to create digital images from synovial fluid mounted on microscope slides. Using this single-shot computational polarized light microscopy (SCPLM) method, we compared rates of crystal detection and raters’ preference for image. Methods: Microscope slides from patients with either CPP, MSU, or no crystals in synovial fluid were acquired using CPLM and SCPLM methodologies. Detection rate, sensitivity, and specificity were evaluated by presenting expert crystal raters with (randomly sorted) CPLM and SCPLM digital images, from FOV above clinical samples. For each FOV and each method, each rater was asked to identify crystal suspects and their level of certainty for each crystal suspect and crystal type (MSU vs. CPP). Results: For the 283 crystal suspects evaluated, SCPLM resulted in higher crystal detection rates than did CPLM, for both CPP (51%. vs. 28%) and MSU (78% vs. 46%) crystals. Similarly, sensitivity was greater for SCPLM for CPP (0.63 vs. 0.35) and MSU (0.88 vs. 0.52) without giving up much specificity resulting in higher AUC. Conclusions: Subjective and objective measures of greater detection and higher certainty were observed for SCPLM over CPLM, particularly for CPP crystals. The digital data associated with these images can ultimately be incorporated into an automated crystal detection system that provides a quantitative report on crystal count, size, and morphology.

(C10N2H10)(HI2O6)(HIO3)(IO3): A Birefringent Material Featuring Large π-Conjugated Organic Cation and Two Types of Iodate Anions.

Birefringent crystals, which can modulate and polarize light, play a vital role in the development of optical sensors and optoelectronic devices. Herein, we synthesized a new birefringent crystal [(C10N2H10)(HI2O6)(HIO3)(IO3)] with large birefringence by integrating the large π-conjugated 4,4'-bipyridine group with two types of iodates anions ([IO3]- and [HI2O6]-) characterized by high polarizability anisotropy and strong stereochemically active lone pairs (SCALP). The crystal exhibits a large measured birefringence of 0.171 at 550 nm, surpassing most organic-inorganic hybrid iodate birefringent materials reported in the literature. Theoretical analyses reveal that the optical properties of the crystal arise primarily from the synergistic interactions between the π-conjugated 4,4'-bipyridine group and I-O groups. This research presents an effective method for the strategic integration of large π-conjugated organic cations into iodates, facilitating the design of novel birefringent materials and encouraging further exploration of superior birefringent substances.