Birefringent Samples Research Articles

Structural anisotropy of silica hydrogels prepared under magnetic field

Birefringence measurements have been carried out on the Pb-doped silica hydrogels prepared under various magnetic fields up to 5T. The silica gels prepared at 5T were used as a medium of crystal growth of PbBr 2, whose result implied the structural anisotropy; an aligned array of crystallites was obtained by transmission electron microscopy. While the samples prepared at 0, 1, and 3T have no birefringence, we found that the samples have negative birefringence on the order of magnitude 10 − 6 as if the direction of the magnetic field is the optic axis of a uniaxal crystal. To the authors' knowledge, the birefringent silica hydrogels were obtained by gelation under magnetic field for the first time. Also, scanning microscopic light scattering experiments have been performed. The results indicate that the characteristic length distribution for birefringent samples is narrower than that for non-birefringent ones.

Birefringence measurement using a confocal setup with a pair of birefringent lenses

A confocal microscopic setup with a pair of birefringent lenses having their crystal axes crossed is used to measure birefringence of thin samples. The method can be used to measure phase down to the order of /10. The fast and slow axes of the object also can be identified. The principle of the method along with the experimental setup is described. Some first-order experimental results are also presented.

Metal−Ligand-Coordinated Vesicles and Vesicle-Assisted Preparation of Calcium Oxalate

A Ca(2+) -ligand-coordinated vesicle phase was prepared from a mixture of tetradecyldimethylamine oxide (C14DMAO) and calcium tetradecylamidomethyl sulfate [(CH3(CH2)13NHCOCH2OSO3)2Ca] in aqueous solution. At the appropriate mixing ratios, Ca(2+) -ligand coordination results in the formation of molecular bilayers because Ca(2+) can firmly bind to the head groups of C14DMAO and (CH3(CH2)13NHCOCH2OSO3)2Ca by complexation which reduces the area of head group. In this system, no counterions in aqueous solution exist because of the Ca(2+) -ligand coordination, and the bilayer membranes are not shielded by salts, i.e., a salt-free but charged molecular bilayer. The structures of the birefringent solutions of (CH3(CH2)13NHCOCH2OSO3)2Ca and C14DMAO mixtures were determined by transmission electron microscopy (TEM) images and rheological measurements, demonstrating that the birefringent sample solutions consist of vesicles. The Ca(2+) -ligand complex vesicle phase was used as a microreactor to prepare calcium oxalate (CaC2O4) crystals. Dimethyl oxalate, as a precursor, can hydrolyze to oxalic acid and methanol. Oxalic acid should precipitate Ca(2+) ions binding to the head groups of C14DMAO and (CH3(CH2)13NHCOCH2OSO3)2Ca to produce CaC2O4 crystals (Ca(2+) + H2C2O4 --> CaC2O4 (downward arrow) + 2H+). The obtained particles were CaC2O4 monohydrate, which were dominated by (020) faces. CaC2O4 precipitates were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) analysis. After removal of CaC2O4 precipitates, a new cationic and anionic (catanionic) vesicle phase was constructed through electrostatic interaction between cationic C14DMAOH+ (C14DMAO + H+ --> C14DMAOH+) and anionic CH3(CH12)13 NHCOCH2OSO3-.

High-speed polarization sensitive optical frequency domain imaging with frequency multiplexing

Polarization sensitive optical coherence tomography (PS-OCT) provides a cross-sectional image of birefringence in biological samples that is complementary in many applications to the standard reflectance-based image. Recent ex vivo studies have demonstrated that birefringence mapping enables the characterization of collagen and smooth muscle concentration and distribution in vascular tissues. Instruments capable of applying these measurements percutaneously in vivo may provide new insights into coronary atherosclerosis and acute myocardial infarction. We have developed a polarization sensitive optical frequency domain imaging (PS-OFDI) system that enables high-speed intravascular birefringence imaging through a fiber-optic catheter. The novel design of this system utilizes frequency multiplexing to simultaneously measure reflectance of two incident polarization states, overcoming concerns regarding temporal variations of the catheter fiber birefringence and spatial variations in the birefringence of the sample. We demonstrate circular cross-sectional birefringence imaging of a human coronary artery ex vivo through a flexible fiber-optic catheter with an A-line rate of 62 kHz and a ranging depth of 6.2 mm.

Optical anisotropy of fibrous biological tissues: analysis of the influence of structural properties

The results of theoretical analysis of the optical anisotropy of multiply scattering fibrillar biological tissues based on the model of an effective anisotropic medium are compared with the experimental in vivo birefringence data for the rat derma obtained earlier in spectral polarisation measurements of rat skin samples in the visible region. The disordered system of parallel dielectric cylinders embedded into an isotropic dielectric medium was considered as a model medium. Simulations were performed taking into account the influence of a partial mutual disordering of the bundles of collagen and elastin fibres in derma on birefringence in samples. The theoretical optical anisotropy averaged over the spectral interval 550–650 nm for the model medium with parameters corresponding to the structural parameters of derma is in good agreement with the results of spectral polarisation measurements of skin samples in the corresponding wavelength range.

Influence of Molecular Orientation and Melt Relaxation Processes on Glassy Stress−Strain Behavior in Polystyrene

This paper explores the relationship between the level of preferred molecular orientation at different length scales and the subsequent glassy stress/strain behavior of polystyrene. To produce samples with a range of orientation states, samples of a commercial polydisperse polystyrene (M w = 274 000 g/mol, polydispersity = 2.74) were first drawn to a set draw ratio and then annealed for a range of times: subsequent to this the glassy stress-strain behavior of the various annealed samples was then measured. Appropriate annealing times, and temperatures, were informed by the molecular based theories for the melt state. In particular, annealing times were chosen to span the range between the entanglement time, τ e , and the typical reptation time, τ d . Differences in the deformation behavior were linked to measurements of the birefringence and thermal shrinkage, reflecting short-range orientation and orientation of the order of the entanglement length, respectively. Results showed that for short annealing times (∼τ e ) the birefringence fell almost immediately and that the true stress-strain curves measured for these samples could be superposed by shifting along the true strain axis by an amount which correlated with the sample birefringence. For longer annealing times, of the order of the Rouse time τ R , the stress/strain curves were found to no longer superpose in the higher true strain region. The shape of the strain hardening curve was found to correlate very well with the measurement of thermal shrinkage of the samples obtained after annealing. Samples with different initial draw ratios (and hence different initial birefringence and shrinkage), but then annealed to have the same shrinkage, were found to show the same shaped strain hardening curve: that is, the higher strain region of the curves superposed.

Spectroscopic polarimetry using channeled spectroscopic polarization state generator (CSPSG)

A novel method for the spectropolarimetric measurement of materials using the channeled spectrum is described. A pair of high order retarders are incorporated into the illuminating optics of a broadband spectropolarimeter, so that the sample under measurement is illuminated with the light that is modulated in the spectral-dependence of its polarization. The Fourier analysis of the channeled spectrum obtained from the spectropolarimeter allows determining the four spectrally-resolved polarimetric parameters of the sample simultaneously. This approach has a feature that it requires neither mechanically- nor electrically-controllable components for polarization modulation, similar to the previous method for the channeled spectropolarimetry in which the high-order retarders are placed in the receiving optics. The new method can offer the same information about the sample as has been obtained by the previous method, provided that all the optical components satisfy the principle of reciprocity. Furthermore, the new method has an additional advantage over the previous method that it is less susceptible to the sample-induced fluctuations of the wavefront or ray-direction. The effectiveness of this method is experimentally demonstrated with the measurement of a birefringent sample.

Corneal birefringence compensation for polarization sensitive optical coherence tomography of the human retina

In previous publications we have reported on polarization-sensitive optical coherence tomography (PS-OCT) systems that measure and image retardation and axis orientation of birefringent samples with only a single input polarization state. This method requires that the sample is illuminated by circularly polarized light. In the case of retinal imaging, the retina is measured through the birefringent cornea, which causes a deviation of the sampling beam from the circular polarization state. To obtain undistorted birefringence patterns of the retina by PS-OCT, the corneal birefringence has to be compensated. We report on a software-based corneal birefringence compensation that uses the polarization state of the light backscattered at the retinal surface to measure the corneal birefringence. This information is used to numerically compensate the corneal birefringence. Contrary to hardware-based solutions, our method accounts for local variations of the corneal birefringence. We implemented the method in a state of the art spectral domain PS-OCT system and demonstrate it in a test sample and human retina in vivo.

Ca2+– and Ba2+–Ligand Coordinated Unilamellar, Multilamellar, and Oligovesicular Vesicles

Ca(2+)- and Ba(2+)-coordinated vesicle phases were prepared in mixed aqueous solutions of tetradecyldimethylamine oxide (C(14)DMAO) and calcium oleate (Ca(OA)(2)) or barium oleate (Ba(OA)(2)). At the right mixing ratios, metal-ligand coordination between Ca(OA)(2) or Ba(OA)(2) and C(14)DMAO results in the formation of molecular bilayers due to the reduction in area per head group. Ca(2+) and Ba(2+) tightly associate to the head groups of surfactants and in this system the bilayer membranes are not shielded by excess salts. The structures of the birefringent samples of the Ca(OA)(2)/C(14)DMAO/H(2)O and Ba(OA)(2)/C(14)DMAO/H(2)O systems were determined by freeze-fracture transmission electron microscopy (FF-TEM), small-angle X-ray scattering (SAXS), and rheological measurements to consist of unilamellar, multilamellar, and oligovesicular vesicles. The coordination between C(14)DMAO and Ba(OA)(2) or Ca(OA)(2) plays an important role in the formation of the vesicles, which was easily confirmed by studying the phase behavior of the KOA/C(14)DMAO/H(2)O system in which only the L(1) phase forms, due to the absence of coordination between KOA and C(14)DMAO. A mechanism is proposed that accounts for the formation of these new metal-ligand coordinated vesicles.

Transmission imaging polarimetry for a linear birefringent medium using a carrier fringe method

We present an imaging polarimeter in transmission mode that is based on a carrier frequency method and allows a spatially resolved polarimetric description of nondichroic linear birefringent media. The apparatus incorporates a generator of polarization states in the incoming pathway and a Wollaston prism and a linear polarizer as the analyzer unit. A series of two fringe pattern images of the birefringent sample under study, corresponding to two independent polarization states of the generator unit, were recorded. From these images and by using Fourier analysis, the 2D distribution of azimuth angle and retardation were calculated. Two alternative generator units were used: (i) a linear polarizer combined with a rotatory quarter-wave plate and (ii) a liquid-crystal variable retarder. A uniform quarter-wave plate at different orientations was measured with both generator units to demonstrate the effectiveness and the accuracy of the method. The mean absolute deviations were 1.8 degrees and 4.1 degrees for the azimuth and the retardation, respectively, with the first generator unit, and 2.9 degrees and 4.4 degrees for the second one. Moreover, some nonuniform birefringent samples presenting wider ranges of azimuth and retardation were also tested.

Simultaneous observation of phase-stepped photoelastic fringes using a pixelated microretarder array

An instantaneous phase stepping and subsequent phase analysis method, using a charge-coupled device (CCD) camera with a pixelated form-birefringent microretarder array, is proposed for 2-D birefringence distribution measurement. A birefringent sample placed behind a polarizer and a quarter-wave plate is analyzed by the proposed method. Light emerging from the sample is recorded using a CCD camera that has a pixelated microretarder array on the CCD plane. This microretarder array has four different principal directions. That is, an image obtained by the CCD camera contains four data corresponding to four different optical axes of the retarder. The four images separated from the image recorded by the CCD camera are reconstructed using gray-level interpolation. Then, the distributions of the Stokes parameters that represent the state of polarization are calculated from the four images. The birefringence parameters, that is, the angle of the principal axis and the phase retardation, are then obtained from these Stokes parameters. This method is applicable to real-time inspection of optical elements as well as the study of the mechanics of time-dependent materials, because multiple exposures are unnecessary for sufficient data acquisition in the completion of data analysis.

Fiber-based single-channel polarization-sensitive spectral interferometry

We present a novel, to our knowledge, fiber-based single-channel polarization-sensitive spectral interferometry system that provides depth-resolved measurement of polarization transformations of light reflected from a sample. Algebraic expressions for the Stokes parameters at the output of the interferometer are derived for light reflected from a birefringent sample by using the cross-spectral density function. By insertion of a fiber-optic spectral polarimetry instrument into the detection path of a common-path spectral interferometer, the full set of Stokes parameters of light reflected from a sample can be obtained with a single optical frequency scan. The methodology requires neither polarization-control components nor prior knowledge of the polarization state of light incident on the sample. The fiber-based single-channel polarization-sensitive spectral interferometer and analysis are demonstrated by measurement of phase retardation and fast-axis angle of a birefringent mica plate.

Fast and Simultaneous Measurement of Both Birefringence and Azimuth Angle Using a y-Cut LiNbO3 Phase Modulator

For fast and simultaneous measurement of both birefringence and azimuth angle of samples, a technique using transverse phase modulator is proposed. The phase modulator includes y-cut z-propagation LiNbO3 crystal. Several experiments using rotating quarter-wave plates and LiNbO3 crystal in presence of direct voltage have been performed, and the measurement results agree well with predicted theoretical data.

Rayleigh scattering, absorption, and birefringence of large-size bulk single-crystal sapphire

While the thermomechanical properties of sapphire make it an excellent candidate of test mass for advanced laser interferometers, its optical quality is not well understood or well controlled. We have studied the results from high-resolution measurements of scattering, absorption, and birefringence in test-mass samples to better understand issues of quality. Samples show large-scale scattering structures clearly linked to the crystal-growth process. Samples characterized by the presence of point defects have significantly lower scattering (except at the point defects). In general on a large scale, high scattering also correlates with higher absorption and higher average birefringence inhomogeneity. However, on a smaller scale there is not a clear point-to-point correlation between scattering and absorption. Often a large-scale scattering structure is spatially displaced by tens of millimeters from a similar absorption structure, indicating that quite separate microscopic mechanisms give rise to scattering and absorption. The spatial displacements indicate that absorption centers and scattering centers are laid down during crystal growth at different distances from the solid-liquid interface. We suggest that absorption may be linked to F centers, while scattering may be linked to impurities such as iron.

Development of a Real-Time Corneal Birefringence Compensated Glucose Sensing Polarimeter

As is well known, in order to optimally manage diabetes mellitus, monitoring blood glucose levels several times daily is recommended so appropriate actions can be taken to maintain these levels within a near-normal physiologic range. One technique that shows promise is the use of optical polarimetry. This technique has the potential to noninvasively measure physiological glucose levels in the eye that are correlated to blood glucose concentrations. To date, the main factor limiting in vivo polarimetric glucose measurements is corneal birefringence, which tends to mask the glucose signature. In this investigation, a method to compensate for the effects of corneal birefringence is demonstrated, thus allowing for polarimetric glucose measurements in samples with time-varying birefringence contributions. In this paper, using a custom-designed laser-based optical polarimetry system with an integrated birefringence compensator, noninvasive glucose measurements in the physiological range are accurately measured within various birefringent samples similar in structure to the eye. Using the laser-based polarimetric approach, it is shown that glucose levels within the physiological range in the presence of significant varying birefringence can be accurately predicted with as low as 13.84 mg/dL error. The ability to compensate for corneal birefringence effects provides promise for the eventual development of a commercial home-based noninvasive polarimetric glucose monitor.

Automatic measurement of the absolute CTE of thin polymer samples. II. Effect of chain orientation on thermal expansion of drawn polymer films

AbstractMore details about the coefficient of thermal expansion (CTE) and thermal expansion anisotropy, Δα, of drawn films from polypropylene (PP) and high density polyethylene (HDPE) are discussed. CTE of the selected polymers are measured along different directions relative to the drawing direction (MD). The coefficient of thermal expansion parallel to drawing direction (α∥) was less than that in the transverse direction for both of the used polycrystalline polymers. Also α∥ for PP and HDPE for different draw ratios are measured. The morphology of measured samples is investigated by using an optical polarizing microscope and from the 2D‐plot of the angular distribution of the absorption coefficient of light through a polymer sample between two crossed polarizers. An agreement between obtained figures was found. Birefringence of samples was measured photometrically. The orientation of drawn polymer chains is estimated from the birefringence measurement. The set up of the used optical system is explained. The CTE variations of drawn polycrystalline polymer are explained on bases of the associated morphological variations. A recommendation was given to produce an all‐PP‐composite with satisfactory dimension stability at elevated temperature. Construction and operation of a simple drawing machine is given too. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4452–4460, 2006

Continuous Nondemolition Measurement of the Cs Clock Transition Pseudospin

We demonstrate a weak continuous measurement of the pseudospin associated with the clock transition in a sample of Cs atoms. Our scheme uses an optical probe tuned near the D1 transition to measure the sample birefringence, which depends on the component of the collective pseudospin. At certain probe frequencies the differential light shift of the clock states vanishes, and the measurement is nonperturbing. In dense samples the measurement can be used to squeeze the collective clock pseudospin and has the potential to improve the performance of atomic clocks and interferometers.

Determination of anisotropic optical parameters of thin films with allowance for multiple reflections

The intensity of light transmitted through a system consisting of a polarizer, a sample, and an anlyzer is calculated with due regard for multiple reflections at the sample boundaries. The intensities of the transmitted light calculated with and without allowance for multiple reflections are compared in the presence and the absence of absorption. It is shown that multiple reflections considerably change the transmitted-light characteristics. The computational errors are evaluated for sample birefringence and dichroism with due regard for multiple reflection.

Programmable spectral interferometric microscopy

A programmable optical sensor is proposed based on spectrally programmable heterodyne optical interferometric confocal microscopy implemented via an ultrastable in-line acousto-optic tunable filter (AOTF) based interferometer using double anisotropic acousto-optic Bragg diffraction. The design uses a tunable laser as the light source and anisotropic diffractions in an AOTF to generate two near-collinear orthogonal linear polarization and slightly displaced beams that both pass via the test sample to deliver highly sensitive sample birefringence or material optical retardation measurements. A spherical lens is used to form focused spots for high resolution confocal spatial sampling of the test object. Thus the instrument also forms a classic interferometric confocal microscope via the use of single mode fiber optics for the receive light. The laser and AOTF tuning allows birefringence measurements taken at different wavelengths, one at a time with minimal interwavelength crosstalk. Experimental demonstration of the instrument is achieved using a 1550nm center 100nm band tunable laser and variable birefringence liquid crystal and fixed retardance birefringent materials, all showing accurate retardation measurements to within a 0.5° rf phase accuracy. The instrument shows a fiber-in to fiber-out loss of 8.5dB. An alternate design via a transmissive beam generation design provides collinear co-located beams on the sample plane for superaccurate measurements.

Dual-wavelength polarimetry for monitoring glucose in the presence of varying birefringence

In a continuing effort to develop a noninvasive means of monitoring glucose levels using the aqueous humor of the eye, a dual-wavelength system is developed to show that varying birefringence, similar to what is seen with a moving cornea, can be compensated. In this work, a dual-wavelength, closed-loop system is designed and a model is developed to extract the glucose concentration information. The system and model are tested using various concentrations of glucose in a birefringent test cell subject to motion artifact. The results show that for a static, nonmoving sample, glucose can be predicted to within 10 mg/dl for the entire physiologic range (0 to 600 mg/dl) for either laser wavelength (523 or 635 nm). In the presence of moving birefringence, each individual wavelength produces standard errors on the order of a few thousand mg/dL. However, when the two wavelengths are combined into the developed model, this error is less than 20 mg/dL. The approach shows that multiple wavelengths can be used to drastically reduce the error in the presence of a moving birefringent sample and thus may have the potential to be used to noninvasively monitor glucose levels in vivo in the presence of moving corneal birefringence.