High Birefringence Values Research Articles

Molecular Design of Sexiphenyl-Based Liquid Crystals: Towards Temperature-Stable, Nematic Phases with Enhanced Optical Properties.

This research introduces a novel liquid crystal molecular design approach based on the para-sexiphenyl (6P) structure. Six new liquid crystalline materials were synthesized, incorporating an alkyl terminal and lateral substitutions of the sexiphenyl core to achieve temperature-stable and broad nematic phases. The synthetic pathway involved cross-coupling, resulting in derivatives with strong nematogenic characteristics. Optical investigations demonstrated that the tested material had high birefringence values, making it promising for optical and electronic applications. These results open up new avenues of research and offer potential practical applications in electronics, photonics, optoelectronics and beyond.

Ultra high birefringent dispersion flattened fiber in terahertz regime

Abstract In the current study, a novel Zeonex based porous core photonic crystal fiber (PC-PCF) is presented for polarization-maintaining and dispersion flattened in the terahertz (THz) region. For minimizing the Effective Material Loss (EML), an array of three rectangular and six triangular air holes are surrounded by hexagonal-shaped cladding. Finite Element Method (FEM) is employed through Comsol V5.3a software to design and examine the essential features of the proposed porous core fiber which revealed that it has an extremely small EML of 0.04 cm−1 at 1.2 THz and has almost zero flattened dispersion of 0.8 ± 0.08 ps/THz/cm in 1.0–1.4 THz frequency spectrum. Moreover, the optimum designing parameters offer an extremely high value of birefringence (0.043 at 1.2 THz). Besides, other major features notably bending loss, effective area, and confinement loss are also found to be precise and relatively low. For effective, adaptable and fitting transmission characteristics, this type of design would lay the foundations for broadband THz radiation wide variety of usage in the THz regime.

A high birefringence liquid crystal for lenses with large aperture

This work presents the application of an experimental nematic liquid crystal (LC) mixture (1929) in a large aperture lens. The LC material is composed of terphenyl and biphenyl derivatives compounds with an isothiocyanate terminal group and fluorinated lateral substituents. The substitution with a strongly polar isothiocyanate group and an aromatic rigid core provides pi-electron coupling, providing high birefringence (Delta n = 0.3375 at 636 nm and 23 °C) and low viscosity (eta = 17.03 mPa s). In addition, it also shows high values of birefringence at near infrared (0.318 at 1550 nm). The synthesis process is simple when comparing materials with high melting temperatures. The excellent properties of this LC mixture are demonstrated in a large aperture LC-tunable lens based on a transmission electrode structure. Thanks to the particular characteristics of this mixture, the optical power is high. The high birefringence makes this LC of specific interest for lenses and optical phase modulators and devices, both in the visible and infrared regions.

Design of a broadband dispersion compensated ultra-high nonlinear photonic crystal fiber

A photonic crystal fiber (PCF) with four circular rings of air holes expanded toward the cladding region is proposed. Four circular tiny air hole rings have been used between the air holes in a regular circular PCF to achieve low dispersion and confinement loss. Additionally, the core region is perforated with a rectangular-shaped hole filled with an extremely nonlinear material, gallium phosphide, to achieve the desired level of nonlinearity. We achieved extremely high nonlinearity and birefringence values of 4.6104 W−1 km−1 and 0.078 at the 1.55 µm telecommunication window by doing so. Further, we observed the structure with varying pitch (Ʌ) values and found a significant reduction in dispersion and confinement loss, as well as a decrease in effective material loss. Thus, at 1.55 µm, an ultra-high negative dispersion of − 8000 ps/nm km is achieved, particularly with Ʌ = 1.8 µm, along with extremely low confinement and material losses of 10–9 dB/km and 0.017 cm−1, respectively. Similarly, other critical parameters such as the power fraction, numerical aperture, and effective area have been examined. Hence, owing to these enhanced optical properties, the proposed PCF is capable of effectively compensating for dispersion, generating supercontinuum, and maintaining polarization.

Computational refocusing of Jones matrix polarization-sensitive optical coherence tomography and investigation of defocus-induced polarization artifacts.

Here we demonstrate a long-depth-of-focus imaging method using polarization sensitive optical coherence tomography (PS-OCT). This method involves a combination of Fresnel-diffraction-model-based phase sensitive computational refocusing and Jones-matrix based PS-OCT (JM-OCT). JM-OCT measures four complex OCT images corresponding to four polarization channels. These OCT images are computationally refocused as preserving the mutual phase consistency. This method is validated using a static phantom, postmortem zebrafish, and ex vivo porcine muscle samples. All the samples demonstrated successful computationally-refocused birefringence and degree-of-polarization-uniformity (DOPU) images. We found that defocusing induces polarization artifacts, i.e., incorrectly high birefringence values and low DOPU values, which are substantially mitigated by computational refocusing.

First-principles investigation of structural, electronic and optical properties of quasi-one-dimensional barium cadmium chalcogenides Ba2CdX3 (X = S, Se, Te) using HSE06 and GGA-PBE functionals

Structural, electronic and optical properties of quasi-one-dimensional barium cadmium chalcogenides Ba2CdS3, Ba2CdSe3 and Ba2CdTe3 are studied using density functional theory. Structural properties of these materials are investigated by incorporating Van der Waals correction with PBE exchange correlation functional. Results of structural analysis are in good agreement with the previously reported experimental observations and confirm the quasi-one-dimensional structure of the compounds. Ba2CdX3 chalcogenide structures crystallize in orthorhombic phase with space group Pnma-D2h16. Band structure calculations with HSE06 show that Ba2CdSe3 and Ba2CdTe3 are wide bandgap materials with direct bandgap of 2.56 eV for Ba2CdSe3 and 2.16 eV for Ba2CdTe3. Ba2CdS3 is an insulator with direct bandgap 3.16 eV. Optical properties are investigated by calculating the refractive index, birefringence, dichroism, dielectric function and absorption coefficient using the HSE06 method. Ba2CdX3 materials exhibit large optical anisotropy with considerable birefringence and dichroism. The comparatively high values of absorption coefficients, extinction coefficients, birefringence and dichroism make Ba2CdX3 materials suitable for applications as polarizers and sensing devices, while the large direct bandgaps indicate that they are promising candidates in photovoltaic applications.

Light-emitting high birefringence chlorinated bistolanes

Eight both symmetrical and non-symmetrical bistolanes with terminal alkyl-, alkoxy- and alkysulphanyl- chain and lateral chlorine atom have been synthesized via Sonogashira cross-coupling and their mesomorphic, optical and photoluminescence properties have been studied. All compounds exhibit enantiotropic nematic and exhibit high value of birefringence (>0.30). The photoluminescence properties both in the diluted solution, solid states and liquid crystal phase were evaluated. Measured compounds emit fluorescence in all states. The correlation between the compounds structure and their properties have been explained using molecular simulation.

Defect-induced modulation of magnetic, electronic, and optical properties of the double-perovskite oxide La2CoMnO6

Electron- and hole-doped La$_2$CoMnO$_6$(LCMO) are investigated using first principles DFT calculations. Hole and electron doping are achieved respectively by introducing Sr$^{2+}$ at La$^{3+}$ sites and by inducing O-site vacancies in LCMO. Electronic structure calculations suggest that hole doping alters the charge and valence state of Co ions, whereas electron doping influences the Mn ions. Introduction of defects is found to enhance antisite disorder(ASD) at Co/Mn site, which is expected to influence the magnetodielectric properties of the system. Our calculations suggest that while ASD and/or hole doping induces half-metallicity in LCMO, electron doping restores its insulating state. Mean-field calculations performed using exchange constants obtained by mapping DFT total energies of different collinear spin configurations onto the Ising Hamiltonian find that defects tend to reduce the Curie temperature ($T_C$). Interestingly, the calculated linear optical properties of the system suggest that the material becomes optically active with high values of birefringence in the presence of defects, a property that is highly sought-after in the optical communications and laser industry.

Photoactivated Refractive Index Anisotropy in Fluorescent Thiophene Derivatives.

The optical control of anisotropy in materials is highly advantageous for many technological applications, including the real-time modulation of another light signal in photonic switches and sensors. Here, we introduce three thiophene derivatives with a donor–acceptor structure, which feature different positions of an electron-acceptor nitrile group, and both photoalignment and luminescence properties. Quantum chemical calculations highlight the presence of trans-forms stable at room temperature and metastable cis-isomers. Besides photoluminescence peaked at 440–460 nm and 0.4 ns lifetime, the three nonlinear optical chromophores exhibit photoinduced anisotropy of the refractive index closely depending on the specific molecular structure, with higher values of birefringence at lower driving signal being obtained for ortho substitution of the nitrile group. All-optical modulation of an external light beam at rates of hundreds of hertz is demonstrated in the fluorescent systems. This finding opens an interesting route to multispectral photonic switches embedded in the active layers of light-emitting devices.

Synthesis and mesomorphic properties of 4,4”-dialkynyl-2’,3’-difluoro-p-terphenyls – the influence of C≡C acetylene linking bridge

ABSTRACTDielectrically negative liquid crystals (LC) based on the terphenyl molecular core are widely used class of rod-like mesogenes. One of the most important parameters from applications point of view is optical anisotropy at nematic phase. Hence some applications require high values of birefringence (optical anisotropy in the case of nematic medium), which can be ensured by mesomorphic molecules having high anisotropy of electronic polarisabilities Δα. The molecular design of presented terphenyls is based on the idea of elongation of conjugated π electronic system by terminal acetylene bridge (C ≡ C), as a linking element to terminal alkyl chains. In this work, we developed synthesis of 4,4’’-dialkynyl-2’,3’-difluoroterphenyls with different length of terminal alkyl chains. All synthesised compounds have wide nematic phase ranges. Synthetic problems of this work will be presented. The purity and structures of the final products and intermediates were confirmed by spectral methods: gas chromatography GC-MS, GC-FID, IR spectroscopy and nuclear magnetic resonance NMR. The phase transition temperatures, phase situation and enthalpy data were determined by polarising optical microscopy (POM) and differential scanning calorimetry.

Study of effective geometry parameter, order parameter, temperature gradients and crossover temperatures of three nematic liquid crystals having highly different clearing temperature

The temperature dependent properties of three liquid crystals (LC) namely effective geometry parameter, order parameter and temperature gradients of refractive indices of two fluorinated isothiocyanato nematic liquid crystal (NLC) compounds 4′-pentylcyclohexyl-3,5-difluoro-4-isothiocyanatobiphenyl, 4'−butylcyclohexyl−3, 5− difluoro− 4− isothiocyanato-biphenyl of high clearing temperature (TC) and low TC LC compound 4 (trans-4/-n-hexycyclohexyl) isothiocyanatobenzene (6CHBT) have been studied. In this paper, the effective geometry parameters of the three LC compounds are calculated. The order parameters of three LC compounds are also calculated from the effective geometry parameter. From the available data, it is found that the two fluorinated NLC compounds exhibit moderately high value of birefringence (∼0.24–0.26) and 6CHBT exhibits a lower value of birefringence (∼0.16). From the study, it is found that the order parameter is found to be higher in higher birefringence and higher TC LC compounds. The values of temperature gradient of ordinary refractive index (dno/dT) are found to be showing negative to positive value while increasing temperature which gives the crossover temperature (TCO). The crossover temperature has been calculated from the temperature gradient at the value (dno/dT) = 0. LC compounds with lower TC are found to have larger TCO value which gives narrower temperature between TCO and TC. The TCO value of compound having larger alkyl group is found to be smaller. The Lorentz field factors for both compound are also reported which can be used for order parameter calculation by different aspects.

Synthesis and mesomorphic properties of novel high birefringence azobenzene liquid crystals with lateral fluorosubstituent

ABSTRACTIn order to obtain liquid crystals with high birefringence (Δn), four azobenzene compounds based on lateral fluorinated bistolane moieties with trifluoromethyl terminal groups were synthesized with Hagihara–Sonogashira cross-coupling reaction. The molecular structures of the compounds were confirmed by Fourier transform infrared, 1H-nuclear magnetic resonance. The transition temperatures, optical textures, and optical anisotropies were determined by differential scanning calorimetry, polarizing optical microscopy. The effect of the alkyl substituents on liquid crystal behavior is discussed in detail. The compounds have showed higher birefringence values.

Acetylene-containing highly birefringent rod-type reactive liquid crystals based on 2-methylhydroquinone

ABSTRACTNew highly birefringent reactive liquid crystal materials based on the 2-methylhydroquinone core were designed and synthesised. Rod-type liquid crystal compounds bearing photo-crosslinkable reactive group of acryloyl, methacryloyl, cinnamoyl, furylacryloyl group were synthesised by introducing acetylene groups via Sonogashira coupling to obtain high birefringence, and lateral groups such as fluoro and methyl to adjust the temperature of the liquid crystal phase. The synthesised compounds were characterised using nuclear magnetic resonance spectroscopy, mass spectrometry and elemental analysis. In addition, their thermal behaviour was investigated using differential scanning calorimetry and polarised optical microscopy. After aligning the synthesised compounds, liquid crystal films were prepared by photo-irradiation. Photo-elastic modulator results showed that the obtained liquid crystal films had high birefringence (Δn) values of 0.32–0.40.

The effect of fluorine substitutions on the refractive index properties for π-conjugated calamitic nematic materials

ABSTRACTIn order to reveal the effect of fluorine substitutions on the refractive index properties for calamitic nematic materials, we carried out a comparative study with respect to non-fluorinated and two types of laterally fluorinated 1,4-bis[4-(hexyloxy)phenyl]ethynylbenzene molecules. Phase transition behaviours were investigated by differential scanning calorimetry and polarised optical microscopy. Additionally, extraordinary and ordinary refractive index and birefringence were evaluated from each single component system. All the analogues exhibited high birefringence values beyond 0.3 at 550 nm, of which an analogue with a fluorine substitution at the central benzene ring showed the highest Δn-value of 0.43. With respect to an analogue with the highest level of fluorination, Δn as well as ne and no values were declined due to decreased order parameter and diluted molecular density. Not only the mesomorphic behaviours but also optical properties strongly relied on the manner of fluorine substitution including the number and position.

Birefringence and temperature-gradient refractive index study of a laterally Fluorinated Terphenyl LC compound.

The temperature behaviour of the refractive indices in the nematic range of a laterally fluorinated terphenyl LC compound, viz, 2,3-difluoro-I,4 bis(4’,4”-dipropyl phenyl) benzene was determined using the thin prism experiment. Birefringence of this LC compound was calculated from the measured refractive indices. Using the four parameter model, the temperature variation of the refractive indices and birefringence were theoretically fitted. The compound is found to have high value of birefringence. The temperature gradients of refractive indices i.e, and dne/dT and dne/dT were determined, as a result, a cross over temperature was also found.The values of order parameter were obtained in two ways one by direct calculation using the effective geometry parameter αeg. and the other by using the modified Vuks’ method. Both show the same trend. It has been observed also αeg changes almost linearly with the birefringence.

Mach-Zehnder interferometric photonic crystal fiber for low acoustic frequency detections

Low frequency under-water acoustic signal detections are challenging, especially for marine applications. A Mach-Zehnder interferometric hydrophone is demonstrated using polarization-maintaining photonic-crystal-fiber (PM-PCF), spliced between two single-mode-fibers, operated at 1550 nm source. These data are compared with standard hydrophone, single-mode and multimode fiber. The PM-PCF sensor shows the highest response with a power shift (2.32 dBm) and a wavelength shift (392.8 pm) at 200 Hz. High birefringence values and the effect of the imparted acoustic pressure on this fiber, introducing the difference between the fast and slow axis changes, owing to the phase change in the propagation waves, demonstrate the strain-optic properties of the sensor.

Correlation between polarization sensitive optical coherence tomography and second harmonic generation microscopy in skin.

Both polarization sensitive optical coherence tomography (PS-OCT) and second harmonic generation (SHG) microscopy are 3D optical imaging methods providing information related to collagen in the skin. PS-OCT provides birefringence information which is due to the collagen composition of the skin. SHG microscopy visualizes collagen fibers in the skin based on their SHG property. These two modalities have been applied to the same skin pathologies associated with collagen changes, but their relationship has not been examined. In this study, we tried to find the relationship by imaging the same skin samples with both modalities. Various parts of the normal rat skin and burn damaged skin were imaged ex vivo, and their images were analyzed both qualitatively and quantitatively. PS-OCT images were analyzed to obtain tissue birefringence. SHG images were analyzed to obtain collagen orientation indices by applying 2D Fourier transform. The skin samples having higher birefringence values had higher collagen orientation indices, and a linear correlation was found between them. Burn damaged skin showed decreases in both parameters compared to the control skins. This relationship between the bulk and microscopic properties of skin may be useful for further skin studies.

The photoinduced birefringence and mass transport in azo compound K-D-2

Azobenzene containing compounds are among light polarization sensitive materials – the moieties may align relative to the electric field vector of light, leading to anisotropy and birefringence in the sample. Another phenomenon which can be observed in azo compounds under influence of light is macroscopic movement of the material. In this work photoinduced processes in low molecular weight organic glass – bis-azobenzene containing compound K-D-2 were experimentally studied. Birefringence was induced with linearly polarized laser light (473, 532 and 635 nm) and measured at 633 nm wavelength. Polarization holography with recording beam configuration +45°/-45° was used to induce mass motion. Dependence of the surface relief depth on the recording laser wavelength in the visible spectrum (375 – 671 nm) was obtained. Formation of the SRG was observed with all used wavelengths and high birefringence values were obtained. Certain correlation between the absorption of the wavelength and photoinduced mass transport and birefringence is yet to be confirmed.

Self-Assembly and Photoinduced Optical Anisotropy in Dendronized Supramolecular Azopolymers

Herein we report the preparation and characterization of dendronized supramolecular polymers composed of a carboxy-terminated azodendron, dAZO, and two different vinylpyridine-containing polymers: poly(4-vinylpyridine) (P4VP) and polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) block copolymer. P4VP can selectively complex dAZO through hydrogen-bonding interactions, thus resulting in liquid crystalline materials. Additionally, this strategy is also applicable to the preparation of dendronized supramolecular block copolymers (BCs). Lamellar, cylindrical, and spherical morphologies are observed for the BC complexes depending on the dAZO to vinylpyridine repeating unit ratio. Photoinduced orientation of the azobenzene moieties is obtained in films of the H-bonded materials—both P4VP and PS-b-P4VP based complexes—by using 488 nm linearly polarized light and characterized through birefringence and dichroism measurements. High and stable values of birefringence are obtained for polymers with azobenzene content a...

Study of optical parameters of two fluorinated isothiocyanato nematic liquid crystals exhibiting high birefringence

AbstractOptical studies have been carried out on two fluorinated isothiocyanato nematic liquid crystal (LC) compounds 4′-butylcyclohexyl-3, 5-difluoro-4-isothiocyanatobiphenyl and 4′-pentylcyclohexyl-3, 5-difluoro-4-isothiocynatobiphenyl. Transition temperatures of the two samples were confirmed using a polarizing microscope. The two LC compounds were found to exhibit fairly high clearing temperatures. Measurements of refractive indices of the two compounds were done by using thin prism method with He-Ne laser beam of wavelength 630 nm. Birefringence of the two LC compounds was calculated from the measured refractive indices. Both the compounds are found to display fairly high values of birefringence. Validation of a modified four-parameter model, based on Vuks equation describing the temperature dependence of refractive indices of the two liquid crystals, is also presented in this paper. The model is validated by fitting the experimentally measured values of refractive indices, birefringence and average refractive indices of the two nematic LCs with the theoretical values. In this paper, the calculation of order parameters of the LCs is presented by using two methods: direct extrapolation method based solely on the birefringence data and by using modified Vuks method based on Haller’s extrapolation. As observed from the obtained results, this procedure of calculating order parameter gives very reasonable results.