Conoscopic Observation Research Articles

Elasto-optic properties and internal stress analysis for monoclinic and trigonal crystals

We study the elasto-optic behaviour of monoclinic and trigonal crystals. The eigenvalues and eigenvectors of the dielectric impermeability tensor under stress are evaluated by the means of the approximated formulae obtained by Sirotin. The parameters which characterize Bertin's surfaces and fringe patterns in conoscopic observation are determined as functions of either residual or applied stress. Later, the photoelastic constants, and the rotation of the optic plane and optic axes bisector are estimated in order to allow for the evaluation of the residual or applied stress by conoscopic measurements. Finally, we obtain the dependence of the refraction indices on the stress. The knowledge of these crystal properties is mandatory for applications in high physics energy, security, geological prospection and in crystals for photonics and medical applications. The evaluated photoelastic constants allow the development of methodologies for an easy and fast quality control via stress distribution detections by means of conoscopic observation, even for strongly anisotropic crystals. As a case study we apply these results to the cases of monoclinic Lu2(1-x)Y2xSiO5(LYSO) and trigonal LiNbO3 (Lithium Niobate).

Electronic intervalence Fe2+ + Ti4+ → Fe3+ + Ti3+ charge-transfer transition in ilmenite

The optical absorption spectrum of natural ilmenite was measured on a narrow translucent rim zone of a thin polished slide of thickness ca. 0.004 mm, mounted with epoxy resin upon a supporting glass plate. The color of the material in the zone is dark red. At conoscopic observation between crossed Nicols, it displays a very dark, vague, off-centered conoscopic figure. Unpolarized optical absorption spectrum of ilmenite, measured in the transparent zone, consists of a very intense high-energy absorption edge which covers a major part of the visible range and causes the dark-red color of ilmenite. On the edge tail, which extends far to the NIR range, there is an intense, broad, asymmetric, vaguely structured absorption band with maximum at around 7000 cm−1. On the account of high iron and titanium contents (0.82 and 0.86 apfu, respectively), it may confidently be attributed to Fe2+/Ti4+ IVCT transition, though its energy is unusually low compared to other Fe2+, Ti4+-bearing minerals studied so far. This most probably proved that the effect of lowering of energy of IVCT bands with increasing number of ion pairs, established by Amthauer and Rossman (Phys Chem Miner 11:37–51 1984), on Fe2+/Fe3+ IVCT bands also takes place in case of electronic Fe2+/Ti4+ IVCT transition in ilmenite.

Photoelastic sphenoscopic analysis of crystals.

Birefringent crystals are at the basis of various devices used in many fields, from high energy physics to biomedical imaging for cancer detection. Since crystals are the main elements of those devices, a great attention is paid on their quality and properties. Here, we present a methodology for the photoelastic analysis of birefringent crystals, based on a modified polariscope. Polariscopes using conoscopic observation are used to evaluate crystals residual stresses in a precise but time consuming way; in our methodology, the light beam shape, which impinges on the crystal surface, has been changed from a solid cone (conoscopy) to a wedge (sphenoscopy). Since the polarized and coherent light is focused on a line rather than on a spot, this allows a faster analysis which leads to the observation, at a glance, of a spatial distribution of stress along a line. Three samples of lead tungstate crystals have been observed using this technique, and the obtained results are compared with the conoscopic observation. The samples have been tested both in unloaded condition and in a loaded configuration induced by means of a four points bending device, which allows to induce a known stress distribution in the crystal. The obtained results confirm, in a reliable manner, the sensitivity of the methodology to the crystal structure and stress.

Conoscopic analysis of electric field driven planar aligned nematic liquid crystal.

This paper illustrates the conoscopic observation of a molecular reconstruction occurring across a nematic liquid crystal (NLC) medium in the presence of an external electric field. Conoscopy is an optical interferometric method, employed to determine the orientation of an optic axis in uniaxial crystals. Here a planar aligned NLC medium is used, and the topological changes with respect to various applied voltages are monitored simultaneously. Homogenous planar alignment is obtained by providing suitable surface treatments to the ITO coated cell walls. The variation in the conoscopic interferometric patterns clearly demonstrates the transition from planar to homeotropic state through various intermediate states.

Conoscopic observation of director reorientation during Poiseuille flow of a nematic liquid crystal

Director reorientation under pressure driven (Poiseuille) flow is observed conoscopically for the liquid crystal 5CB aligned at an azimuthal angle of 45° to the direction of flow. A polyimide surface treatment (AL 1254) is used to promote planar homogeneous alignment and rubbed to produce an initial azimuthal alignment angle ϕ0. Conoscopic interference figure rotation is documented as a function of flow rate and compared to that produced from numerical models using Leslie–Ericksen–Parodi theory. Model and data show excellent agreement.

Polarimetric studies of polyethylene terephtalate flexible substrates

Polymer sheets are currently used worldwide in a wide range of applications. The manufacturing process of these sheets involves extruding machines that stretch the material in both lateral and longitudinal directions with respect to the machine direction, thus inducing birefringence. In most cases, the film obtained is optically biaxial. Polarimetric spectroscopy (Ellipsometry and Mueller Matrix) combined with conoscopic observation are the methods of choice to study these properties. In this work we report an analysis of commercially available polyethylene terephtalate (PET) films used as substrate for food packaging as well as for embedded electronic devices such as solar cells or flexible displays. Initial observation of these films through polarizing microscope in conoscopic mode reveals first the trace of optical axis plane with respect to the film surface and second, whether the optical axis is acute or not. This preliminary study allows optimal sample positioning for further polarimetric studies. The measurements and modelling are done in both reflection and transmission mode on several spectroscopic polarimetric setups from UV to NIR. The models give as a main result, the dielectric tensor of the film as well as its orientation with respect to the laboratory reference frame.

Preparation and Characterization of Azobenzene Liquid-Crystalline Elastomer Films with Homeotropic Alignment

ABSTRACT Azobenzene liquid-crystalline elastomer (LCE) films with homeotropic alignment were prepared by in-situ photopolymerization of mixtures of an LC monomer and a cross linker, both of which contain azobenzene chromophores. Thermodynamic and mesomorphic properties of the monomers and the LCE films were determined by differential scanning calorimertry and polarizing optical microscopy. X-ray diffraction patterns revealed that the LCE films show a smectic A phase. Polarized UV spectra and conoscopic observation with a polarizing optical microscope demonstrated that the azobenzene mesogens are aligned homeotropically in the films.

Liquid-Crystalline Branched Polymers having Ionic Moieties

Ionic LC-dendrimers were obtained by proton-transfer reaction of alkanoic acid and primary amino groups of poly(amidoamine) dendrimer in a 1:1 molar ratio. The ion complexes containing stearate, palmitate or myristate ions showed a smectic A phase. However, the ion complexes containing laurate and decanoate ions exhibited no liquid-crystalline phase. Oilystreak, maltese, and homeotropic textures were observed in the smectic A phase. The formation of the homeotropic structure is characterized by conoscopic observation. The X-ray diffraction pattern in the smectic A temperatures consists of sharp inner reflections at the small angles and the wide-angle diffuse halo. Mixtures of the dendrimer and stearic acid [a molar ratio (x) of stearic acid against the primary amino groups of the dendrimer] were prepared; mixtures with x = 0.11–1.5 showed a smectic A phase. However, the mixture with x = 2.33 formed a disordered hexagonal columnar mesophase, characterized by an optical texture and X-ray diffraction measurement.

Conoscopic observation with naked eye

Conoscopic observation with naked eye

Tilt Orientation of p-Methoxyazobenzene Side Chains in Liquid Crystalline Polymer Films by Irradiation with Nonpolarized Light

Unambiguous observation of the reorientation of azobenzene chromophores toward the propagation direction of the actinic light has been performed. Prolonged irradiation of a spin-coated film of liquid crystalline poly[6-{4-(4‘-methoxyphenyl)azophenyloxy}hexyl methacrylate] (C6MeO) with 436 nm light for n−π* excitation led to a marked decrease of the π−π* absorption band. Spectral analysis in combination with conoscopic observation revealed that the reorientation of the azobenzene chromophores occurs in such a way that the longitudinal molecular axis of the azobenzene aligns in parallel with the propagation of light to form strong H-aggregates. The tilt orientation of the azobenzene was more readily achieved by irradiation with light at high temperatures near or above the glass transition temperature. Concerning the effect of spacer length on the three-dimensional orientation, the azobenzene groups showed a tendency to orient more homeotropically as the alkylene spacer length increases.

Ellipsometric study of freely-suspended films of an antiferroelectric liquid crystal mixture showing V-shaped switching

Ellipsometric studies have been conducted on freely-suspended films of an antiferroelectric liquid crystal mixture showing V-shaped switching. We observed a discontinuous change of the phase difference between s- and p- components of transmitted light Δ, at 46 °C indicating a phase transition from smectic-C* to the ferrielectric phase, although this transition is continuous in conoscopic observation. We also discussed the details of A as a function of an applied electric field in each phase.

Anomalous Phase Sequence in Binary Mixture of 3MC2PCPOPB and TFMHPOBC

Dielectric measurement, conoscopic observation and polarizing microscopic observation have been performed on the binary mixture of 4′-3(-methoxycarbonyl-2-propoxycarbonyl) phenyl 4-(4-(n-octyloxy) phenyl) benzoate (3MC2PCPOPB) and 4-(1,1,1-trifluoromethyl-heptyloxycarbonyl) phenyl 4′-octyloxybiphenyl-4-carboxylate (TFMHPOBC). The chiral smectic Cα ( SmCα*) and chiral smectic Cγ ( SmCγ*) phases have been observed between the smectic A (SmA) and chiral smectic CA ( SmCA*) phases. Another SmCγ* phase has been observed below the SmCA* phase and the appearance of reentrant SmCγ* phase has been confirmed. An anomalous temperature (T)-electric field (E) phase diagram, in which two SmCγ* phases observed above and below the SmCA* phase are connected to each other over the SmCA* phase, has been obtained.

Conoscopic study of the Scα∗ phase and the Devil's staircase in an antiferroelectric liquid crystal

Abstract The molecular orientational structure of the Scα∗ phase has been studied by means of X-ray, dielectric and conoscopic measurements for 4-(1-methylheptyloxycarbonyl)phenyl 4′-octylcarbonyloxybiphenyl-4-carboxylate. It was found by X-ray scattering that the Scα∗ phase is tilted. According to the conoscopic observation under a low field, the optic plane of Scα∗ is parallel to the field direction similar to that of the Scα∗ phase of 4-(1-methylheptyloxycarbonyl)phenyl 4′-octyloxybiphenyl-4-carboxylate. It was also found that the molecular orientation of the Scα∗ phase is antiferroelectric in the high temperature region, while in the low temperature region it is ferrielectric and changes with temperature, suggesting the so-called Devil's staircase. The critical temperature, where the biaxiality and the tilt angle of the average optic axis have maximum values, was assigned to the ferrielectric state with the largest spontaneous polarization.

Optical anisotropy of polyimide

AbstractOptical anisotropy of polyimide films prepared from polyamic acid solutions was investigated by conoscopic observation and refractive index measurements. The conoscopic figure (crossed pattern) characteristic of a negative uniaxial crystal with its optic axis perpendicular to the film plane was observed for samples imidized on substrates. Uniaxial drawing of a polyamic acid cast film with 40 wt % solvent gave a highly oriented sample with an elongation of more than 80%. This sample had a conoscopic figure characteristic of a positive uniaxial crystal with its optic axis equivalent to the molecular c axis (parallel to the film plane). A large optical anisotropy (bire‐fringence 0.18) was observed in the film plane. The TE‐mode refractive index (parallel to the film plane) ranged from 1.64 to 1.82 at a wavelength of 633 nm according to the sample rotation. These values corresponded to the refractive indices perpendicular and parallel to the c axis, respectively. The optical system change of the polyimide film was explained in terms of the uniaxial orientation of the in‐plane oriented molecular c axis.

THEORETICAL INTERPRETATION OF THE ENHANCEMENT OF NEUTRON DIFFRACTION IN AN α-LiIO<sub>3</sub> SINGLE CRYSTAL UNDER THE ACTION OF A DC FIELD

We have noticed in α-LiIO3 single crystals the following experimental facts: (1) Using the conoscopic observation method, it was found the contrast of layer imperfections increasing with the strength of a dc field. (2) Changes take place in the x-ray topography under the action of a dc field. (3) The grown-in spatial variation of lattice parameters were found and measured by an x-ray double-crystal spectrometer. Based on these observations, we infer that space charges (carriers, impurity ions and vacancies) are accumulated at the defects of macroscopic scale, which in turn produces a gradient of lattice parameters under the action of a dc field. We calculated the neutron Bragg scattering cross section for a distorted single crystal using the Born approximation and taking into account the extinction at the same time. Therefrom, the enhancement of diffraction in an α-LiIO3 crystal under the action of a dc field were qualitatively interpreted with good agreement.

Temperature-Dependent Tilt Angle in the SmecticCPhase of a Liquid Crystal

Liquid crystal smectic C phase temperature dependent tilt angle, using conoscopic observation and circularly polarized light

Temperature and Optical Frequency Dependence of the D. C. Electro-Optic Constant r22T of LiNbO3

Electro-optic constant r22T of lithium meta-niobate was measured up to 200°C over the wavelength range from 436 to 632.8 mµ with a conoscopic observation. At room temperature, r22T was found to be -6.4×10-12 m/v at 632.8 mµ and to decrease with decreasing wavelength. It exhibited a clear decrease above 100°C and the temperature dependence was discussed in connection with ferroelectric polarization.