Optical Axis Parallel Research Articles

Reflected and transmitted powers from a planar isotropic chiral–uniaxial anisotropic chiral interface

In this paper, analysis and theoretical investigations are carried out to find the reflected and transmitted powers for a planar isotropic chiral–uniaxially anisotropic chiral interface with optical axis parallel to interface. The expressions of the reflected and transmitted powers for right-circularly polarized (RCP) and left-circularly polarized (LCP) incident waves are derived. Results of the reflected and transmitted powers for the RCP and LCP incident waves from materials with different isotropic chirality and uniaxial chirality values are presented. The effect of the uniaxial chirality on three other cases of dielectric constant combinations is also studied. The numerical results derived from the presented analytical expressions are found to be in good agreement with those obtained by computational techniques elsewhere.

New method to determine the optical rotatory dispersion of inorganic crystals applied to some samples of Carpathian Quartz

A new method to determine the optical rotatory dispersion (ORD) in the visible range, based on a channeled spectrum obtained with a uniax inorganic crystal introduced between two crossed polarizers with its optical axis parallel to the light propagation direction is detailed in this paper. When the studied inorganic crystals are transparent, this method permits the estimation of the optical rotatory dispersion in the visible range, for which the cheap polarizers are available. The speed of the measurements is very high, because the estimations are made from the channeled spectrum obtained for a single arrangement of the optical components. By using a computer, ORD is quickly determined for the visible range. The results obtained by this method for some Carpathian Quartz samples are consistent with those from literature. The proposed method can be also applied in UV and IR spectral ranges, when the anisotropic layers are transparent and the linearly polarized radiations can be obtained.

Reflection and transmission of electromagnetic waves on the interface of uniaxial chiral media

The reflection and transmission properties of plane waves on the interface of uniaxial chiral media with the optical axis parallel to the interface are investigated. The formulas of the reflected and transmitted power are derived. The curves of the group refractive angles, power of the reflected and transmitted waves for TE and TM incident waves are presented for three cases of dielectric constant combinations and for non-chiral, weak chiral and strong chiral media. Some new results are obtained, which are different from those in the uniaxial chiral media with the optical axis perpendicular to the interface.

Reflection and transmission characteristics of electromagnetic waves by the uniaxially anisotropic chiral slab

The reflection and transmission characteristics of electromagnetic waves by the uniaxially anisotropic chiral slab with the optical axis parallel to the interface are investigated. Formulas of the reflection and transmission coefficients (power) are derived. The curves of powers of the reflected and transmitted electromagnetic waves are presented for four cases of dielectric constants according to their signs. The effects of chirality parameter on the reflection and transmission are discussed. Especially, the dependences of pseudo-Brewster angles on the chirality parameter are plotted.

An analytical study of modal dispersion characteristics of helically clad crystal-cored optical fiber

The uniaxial crystal cored optical fiber, with optical axis parallel to the fiber axis, having helical cladding is proposed and a characteristic equation is derived. Employing helical boundary conditions, the characteristic equation is found and dispersion curves are drawn for different Kco (the ratio of longitudinal and transverse refractive index) values and for different modal order ν. The field components in the core region of the proposed fiber are found and a curve between effective-index vs. normalized frequency is drawn. Our studies show that the anisotropic parameter (Kco) influences the dispersion characteristics and modal cut-offs and the modal grouping occurs at different V number for different Kco values. It is also observed that the helix pitch angle ψ has no any pronounced effect on propagation characteristics of the proposed fiber.

Gradient-driven fluctuations in microgravity

Equilibrium fluctuations of thermodynamic variables, such as density or concentration, are known to be small and typically occur at a molecular length scale. In contrast, theory predicts that non-equilibrium fluctuations grow very large both in amplitude and spatial size. On earth, the presence of gravity and buoyancy forces severely limits the development of the fluctuations. We will present the results of a 14-year long international collaboration on an experiment on non-equilibrium fluctuations in a single liquid and in a polymer solution under microgravity conditions. Non-equilibrium conditions are generated by applying a temperature gradient across millimetre-size liquid slabs. Phase modulations introduced by fluctuations are measured using a quantitative shadowgraph method, with the optical axis parallel to the temperature gradient. Thousands of images are analysed and their two-dimensional power spectra yield the fluctuation structure function S(q), once data are reduced accounting for the instrumental transfer function T(q). The mean-squared amplitude of the fluctuations exhibits an impressive power-law dependence at larger q and a crossover at low q showing that the fluctuation size is limited by the sample thickness. The shape of the structure function, its increase due to removing gravity, and its dependence on applied gradient are in reasonable agreement with available theoretical predictions.

Negative refraction in uniaxially chiral medium with optical axis parallel to the interface

The condition to realize the negative refraction in the uniaxially anisotropic chiral medium with an optical axis parallel to the interface has been investigated. The variations of refraction angles of two eigenwaves with the incident angle for different electromagnetic parameters and chirality parameters are also analyzed. The results show that the uniaxially anisotropic chiral medium is easy to realize the negative refraction, even though it has a small chirality parameter. And for the different permittivities, chirality parameters and incident angles, the phase refractive angles of the two eigenwaves in the uniaxially chiral medium are distinctly different. In a peculiar condition, one or two eigenwaves cannot propagate in the medium.

FDTD Modeling of Dispersive Bianisotropic Media Using Z-Transform Method

The finite-difference time-domain (FDTD) technique for simulating electromagnetic wave interaction with a dispersive chiral medium is extended to include the simulation of dispersive bianisotropic media. Due to anisotropy and frequency dispersion of such media, the constitutive parameters are represented by frequency-dependent tensors. The FDTD is formulated using the Z-transform method, a conventional approach for applying FDTD in frequency-dispersive media. Omega medium is considered as an example of bianisotropic media, the frequency-dependent tensors of which are based on analytical models. The extended FDTD method is used to determine the reflection and transmission coefficients of co- and cross-polarized electromagnetic waves from omega slabs, illuminated by normally incident plane waves. Three cases are simulated: 1) a slab of uniaxial omega medium with its optical axis parallel to the propagation vector; 2) a slab of rotated uniaxial omega medium with its optical axis not parallel to the propagation vector; and 3) a slab of biaxial omega medium. The results are validated by means of comparisons with analytical solutions.

Twisted-nematic liquid-crystal displays with small grayscale inversion and wide viewing angle

This work proposes a twisted-nematic liquid-crystal display (TN-LCD) with a small grayscale inversion and wide viewing angle. The electro-optical properties of a tilted TN cell are studied to develop the proposed TN-LCD. Calculations reveal that when the applied voltage exceeds a critical voltage, the effective optical property of the tilted TN cell is optically uniaxial with the effective optical axis parallel to the bisector of the twist angle. This bisector effect is used to improve the grayscale inversion of TN-LCDs. A viewing angle of over the entire 80° viewing cone of the proposed TN-LCD is also demonstrated theoretically and experimentally.

Orientation-dependent Casimir force arising from highly anisotropic crystals: Application toBi2Sr2CaCu2O8+δ

We calculate the Casimir interaction between parallel planar crystals of Au and the anisotropic cuprate superconductor ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (BSCCO), with BSCCO's optical axis either parallel or perpendicular to the crystal surface, using suitable generalizations of the Lifshitz theory. We find that the strong anisotropy of the BSCCO permittivity gives rise to a difference in the Casimir force between the two orientations of the optical axis, which depends on distance and is of order 10--20 % at the experimentally accessible separations $10\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}5000\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$.

Refractive index dispersion and anisotropy in NaGd(WO 4) 2 single crystal

Refractive index characteristics were measured in the tetragonal, scheelite-like, NaGd(WO 4) 2 (NGW) single crystal. The room temperature ordinary refractive-index dispersion was measured by the minimum-deviation method between 320 and 1342 nm. The refractive-index anisotropy was measured between 400 and 1860 nm using a combination of bireflection in an oriented single crystal prism, and birefringence modulation in an oriented plane-parallel plate. The crystal is positive uniaxial, with its optic axis parallel to the crystallographic c-axis. The dispersion was fitted to a simple two-parameter Sellemeier function. The anisotropy dispersion was also fitted to an empiric function. Taking the 632.8 nm wavelength (the He–Ne laser line) as a representative of the visible range, the ordinary refractive index is n o = 1.989(1), and the anisotropy ( n e − n o) = 0.00342(1). The refractive indices’ changes with temperature were measured between room temperature and ∼200 °C at λ = 632.8 nm, yielding d n o/d T = − 10.70 × 10 −5 °C −1 and d n e/d T = − 4.17 × 10 −5 °C −1.

The mechanism of multi-focusing of lasers into uniaxial crystals

We study the mechanism of multi-focusing of a Gaussian beam propagating from the isotropic medium into a uniaxial crystal with its optical axis parallel or orthogonal to the propagation direction. The analytical expressions are obtained by paraxial approximation and the different mechanisms of multi-focusing under different circumstances are revealed. Further, our experiments support our analytical and numerical analyses quite well.

An improved Jones matrix for a birefringent plate with optical axis parallel to the surface

Light propagation around the back surface of a birefringent plate was studied with Stokes treatment. An improved Jones matrix for the plate was deduced in a more strict way, in which the splitting between O and E waves and all factors at the boundaries were considered more rigorously. The given new matrix is a more extensive one which can be reduced to other previous matrices by using corresponding approximations. Furthermore, various matrices were divided into three basic types, and numerical calculation and comparison were done with different crystals. It demonstrates that, for crystals with large differences between no and ne, the splitting between O and E waves obviously affects the transmission character and should not be neglected in Jones matrices.

Transparent Image Sensors Using an Organic Multilayer Photodiode

The most fascinating advantages of organic photodiodes in the context of information-conversion applications are their ability for large-area surface light detection, the potentially wide selection of materials that allows color tuning, and the possibility for low-cost fabrication of flexible devices. The photosensitivity of organic photodiodes, either multilayer-heterojunction type or bulk-heterojunction type, is quite high, and internal photon-to-electron conversion quantum efficiency in some cases approaches unity. Thus, we have a good opportunity to utilize organic photodiodes for high-performance photosensors, even though power-conversion efficiency of organic photovoltaic solar cells under solar light irradiation remains less than 5 %. Fabrications of organic photodiodes with good photoresponse, such as fast-response photodetectors, full-color image sensors, and position-sensitive photodetectors, have been reported. The use of anisotropy of optical absorption results from molecular orientation is one of the most fascinating advantages of organic photodiodes. The combination of transparent electrodes and polarized light detection that results from optical anisotropy of molecules provides unique image-sensing devices that capture images under light illumination behind them. Figure 1 demonstrates the basic concept for reading a black and white picture using our transparent photodiodes with polarization-detection capability. Let us assume that white light or monochromatic light is transmitted through a linear polarizer to the polarization-sensitive photodetector. At present, for simplicity, we assume that the photodetector is composed of oriented molecules with large optical anisotropy. When polarized light perpendicular to the transition moments of the oriented molecules irradiates the photodetector, light passes through the detector with little optical absorption. Thus, a very small photocurrent is detected by the photodiode when the incident light is perpendicularly polarized. The transmitted polarized light is reflected back according to the reflectivity of the object. When white paper is used, a large fraction of the light is reflected back and depolarized by scattering. Thus, the light component with parallel polarization reaches the photodetector and a large photocurrent signal is detected. On the other hand, when the transmitted polarized light reaches a black object, all the light is absorbed and no photocurrent is detected. Thus, the transparent and polarization-sensitive photodetectors are useful for the detection of reflectivity of objects. We note that this principle is applicable to grayscale sensing as well as simple black and white detection. When addressable crossed-array photodiodes are used or 2D scanning of incident light is available, 2D picture images are obtainable. We have successfully developed polarization-sensitive photodiodes using a pn-heterojunction-type structure composed of 3,4,9,10-perylenetetracarboxylic-bis-benzimidazole (PTCBI) for the n-type layer and titanyl phthalocyanine (TiOPc) for the p-type layer. In this report, we demonstrate the feasibility of our transparent photodetector composed of a well-aligned PTCBI and a non-aligned TiOPc layer for image sensing. As shown in Figure 2a, the device configuration was indium tin oxide (ITO)/indium/aligned PTCBI/TiOPc/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/ Au. A well-aligned PTCBI layer with the optical axis parallel to the rubbing direction was grown. A large dichroic ratio of optical absorption was obtained at the peak wavelength of 540 nm. Figure 2b shows absorption spectra of both aligned PTCBI and non-aligned TiOPc films. An aligned PTCBI showed a dichroic ratio of 4.7:1 at the peak wavelength of 540 nm, while non-aligned TiOPc showed isotropic absorpC O M M U N IC A TI O N S

Investigation of the optical anisotropy of PET and PEN films by VIS-FUV to IR spectroscopic ellipsometry

In the last years, a significant amount of research is being performed in the field of polymer research for novel applications, such as flexible electronic devices, photovoltaic cells, high performance optics, data storage, etc. Toward this direction, in this work, the optical anisotropy of biaxially stretched poly(ethylene terephthalate) (PET) and poly(ethylene naphthalate) (PEN) films has been extensively investigated. The optical properties of the films have been studied in terms of their optical, electronic and vibrational response, by Fourier transform IR spectroscopic ellipsometry (FTIRSE) (900–3500 cm −1) and Vis-fUV variable angle SE (1.5–6.5 eV) techniques. The films optical anisotropy is the result of the stretching procedure during their fabrication, which results to the structural rearrangement of the macromolecular chains parallel to the stretching direction and to a higher structural symmetry. During the SE spectra analysis, the films have been approximated as uniaxial materials with their optic axis parallel to the sample/ambient interface leading to the accurate determination of the principal components ɛ ||( ω) and ɛ ⊥( ω) of the dielectric function ɛ( ω). The detailed study of the electronic transitions has been performed in the Vis-fUV region, where the characteristic features corresponding to the n → π* electronic transitions of the carbonyl –C O group and the 1A 1g → 1B 1u transition due to the π → π* excitation of the π-electron structures have been identified and analysed. Furthermore, the FTIRSE spectra allowed the accurate identification and assignment of the features of ɛ( ω) to the vibrational modes of the various bonding structures characteristic of the PET and PEN macromolecular chains.

On the optical anisotropy of poly(ethylene terephthalate) and poly(ethylene naphthalate) polymeric films by spectroscopic ellipsometry from visible-far ultraviolet to infrared spectral regions

The optical anisotropy of biaxially stretched poly(ethylene terephthalate) and poly(ethylene naphthalate) films has been investigated, by visible-far ultraviolet to IR spectroscopic ellipsometry (SE). The consistency of the SE spectra analysis results between the two energy regions, in a full rotational scan of the angle θ between the plane of incidence and the stretching direction, justified the approximation of the films as uniaxial materials with their optic axis parallel to the surface.

Orientation of optically trapped nonspherical birefringent particles

While the alignment and rotation of microparticles in optical traps have received increased attention recently, one of the earliest examples has been almost totally neglected--the alignment of particles relative to the beam axis, as opposed to about the beam axis. However, since the alignment torques determine how particles align in a trap, they are directly relevant to practical applications. Lysozyme crystals are an ideal model system to study factors determining the orientation of nonspherical birefringent particles in a trap. Both their size and their aspect ratio can be controlled by the growth parameters, and their regular shape makes computational modeling feasible. We show that both external (shape) and internal (birefringence) anisotropy contribute to the alignment torque. Three-dimensionally trapped elongated objects either align with their long axis parallel or perpendicular to the beam axis depending on their size. The shape-dependent torque can exceed the torque due to birefringence, and can align negative uniaxial particles with their optic axis parallel to the electric field, allowing an application of optical torque about the beam axis.

P-144: Low Voltage, High Contrast Ratio, and Wide Viewing Angle TN LCD's for Microdisplay, Mobile Phone and PDA Mobile Video Applications

The modeled contrast ratio of a twisted nematic liquid crystal display (TN/LCD) was improved by over a factor of 2 to greater than 500:1 at applied voltages of 2 to 4 volts via the addition of a thin negative birefringence optical film with a retardation value up to 50nm and the optical axis parallel to the surface. Combining wide-viewing-angle compensation films with this thin negative birefringence optical film can lead to significantly improve viewing angle as well. This performance improvement is important for high performance microdisplay HMD's, mobile phone, PDA mobile video TV's, and 3D applications.

Engineering colloids with optical and geometrical anisotropies: de-coupling size monodispersity and particle properties.

This paper gives an overview of our recent work on liquid crystal emulsions and their use as starting point towards making monodisperse suspensions of colloidal entities with optical and geometrical anisotropies. The procedure utilizes a monodisperse emulsion of a photopolymerizable liquid crystal that is polymerized after mechanical deformation of the drops. We have engineered solid spheres as well as oblate and prolate spheroids presenting a bipolar anisotropic character at the micrometer scale. These materials respond to optical and external electric fields by aligning their mean optic axis parallel to the field, thus constituting excellent systems for fundamental studies and applied uses.

Orthoconic antiferroelectric liquid crystalline materials

New LC materials based on chain fluorinated homologous ester series and exhibiting high tilt angles of the synclinic and anticlinic smectic C phases to the layer surface are synthesized. Their mixtures in surface-stabilized geometry show an unusually huge change of optical properties after application of an electric field during the transition from the antiferroelectric to the ferroelectric state. In the antiferroelectric state they are optically negative uniaxial liquid crystals with the optic axis perpendicular to the cell plates, while in the ferroelectric state they become optically positive biaxial liquid crystals with optic axis parallel to the glass plates. The results of electrooptical characterization of the mixtures are presented. © 2005 Optical Society of America