Crystal Retarder Research Articles

High-speed inline holographic Stokesmeter imaging

We demonstrate a high-speed inline holographic Stokesmeter that consists of two liquid crystal retarders and a spectrally selective holographic grating. Explicit choices of angles of orientation for the components in the inline architecture are identified to yield higher measurement accuracy than the classical architecture. We show polarimetric images of an artificial scene produced by such a Stokesmeter, demonstrating the ability to identify an object not recognized by intensity-only imaging systems.

LIQUID CRYSTAL BASED ROTATING ORTHOGONAL POLARIZATION IMAGING SYSTEM

Rotating orthogonal polarization imaging provides images of the polarization properties of scattering media which are free from surface reflections. Previously the technique has been demonstrated using manually rotated Glan–Thompson polarizers to control and analyze the polarization state of the light entering and emerging from the tissue. This paper describes a system that performs these functions using liquid crystal retarders. The system is tested using a polarizing target embedded within a scattering medium and is compared with Monte Carlo simulations. The results compare well with those obtained with manual rotation of polarizers. The liquid crystal based approach has advantages over the previous system in terms of ease of use, speed, and repeatability and is therefore an important step towards taking the technique into routine clinical use.

Liquid crystal Lyot tunable filter with extended free spectral range

Extension of the dynamic range of liquid crystal tunable Lyot filter is demonstrated by incorporating with it a liquid crystal variable retarder as an eliminator for the third and fourth order peaks. The filter is continuously tunable in the range 500 nm to 900 nm with a nominal width in the range 50nm-100nm. Design procedure is described including the exact solution to the LC director profile and the suitability for biomedical optical imaging applications. Flexibility in the design is proposed by applying different voltages to the different liquid crystal retarders thus compensating for small thickness deviations from the nominal values and obtaining the high dynamic range.

Separation algorithm for a 2D refractive index distribution and thickness profile of a phase object by laser diode-based multiwavelength interferometry

A separation algorithm that uses three wavelengths for the simultaneous measurement of the two-dimensional refractive index distribution and thickness profile of transparent samples is proposed. The optical system is based on a Mach-Zehnder interferometer with laser diode-based multiwavelength sources. A liquid crystal retarder is used to obtain interference images at four states with phases, and the optical phase of the object is then calculated with a four-bucket algorithm. A glass rod and several samples, including a slide glass, a glass wafer, and a cover glass are used to obtain experimental results at wavelengths of 635, 660, and 675 nm. The refractive indices of the sample are distributed with an accuracy of less than 0.0003 and the thickness profile is calculated on the basis of the measured refractive index. This result demonstrates that the proposed algorithm can be used to separate the refractive index distribution and thickness profile of samples in two dimensions.

Retardation of Ice Crystallization by Short Peptides

The effect of peptides on the growth of ice crystals are studied using molecular dynamics simulations. The growth of the ice crystal is simulated at a supercooling of 14 K, and the effect of a single tetrapeptide on the growth rate is calculated. For pure ice the simulated crystal grows at a rate comparable to experiment. When a peptide molecule is added near the interface, the growth rate is diminished significantly, by up to a factor of 5 for Gly-Pro-Ala-Gly and a factor of 3 for Gly-Gly-Ala-Gly. The retardation occurs via the binding of the peptide to the ice surface, suppression of ice growth near the binding site, and eventual growth of the crystal around the bound peptide. The peptide with a proline residue is more effective in retarding the crystal growth, and this can be understood from the conformation of the peptide within the frozen ice phase after overgrowth. The simulations suggest that short peptides can be effective antifreeze agents.

Formation of mesoporous alumina via hydrolysis of modified aluminum isopropoxide in presence of CTAB cationic surfactant

Formation of mesoporous alumina at room temperature via combined utilization of acetic acid (as an organic modifier) and cetyl trimethyl ammonium bromide (CTAB) as a cationic surfactant has been investigated in n-heptane medium. The reactants molar ratio was aluminum isopropoxide:acetic acid:water:CTAB:n-heptane, as 1:1:3: x:15, where x was 1 or 3. The calcined materials (at 600 °C for 3 h) were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and N 2 adsorption/desorption techniques. Thus, amorphous (weakly crystalline γ-phase) mesoporous aluminas of possible hexagonal symmetry, high surface area and relatively narrow pore size distribution were obtained. The enhancement of surface area and retardation of crystallization were found to increase with the increasing of CTAB ration in the reaction mixture.

Retardation correction for photoelastic modulator-based multichannel reflectance difference spectroscopy

The wavelength dependence of the retardation induced by a photoelastic modulator (PEM) is a central issue in multichannel modulator-based spectroscopic ellipsometry and reflectance difference spectroscopy (RDS), where the optical signal is detected simultaneously at different wavelengths. Here we present a refined analysis of the modulator crystal's retardation and its effect on the signal quality. Two retardation correction schemes that take into account the actual wavelength dependence of the stress-optic coefficient are introduced. It is demonstrated experimentally that both methods provide a better correction than the procedure currently used in multichannel RDS. We define quality factors to evaluate the actual performance of the multichannel detection system as compared with a wavelength adaptive single-channel experiment. These quality factors thus provide a useful guideline for choosing the appropriate PEM retardation or reference wavelength in a multichannel experiment.

Fast near‐infra‐red spectroscopic Mueller matrix ellipsometer based on ferroelectric liquid crystal retarders

AbstractThe cover picture of this issue of physica status solidi (c) has been taken from the article [1]. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Product decomposition of measured Mueller matrices from the FLC based NIR Mueller Matrix Ellipsometer

AbstractThis paper reports on the application of the first near‐infrared spectroscopic Mueller Matrix Ellipsometer (MME) based on Ferro‐electric Liquid Crystal retarders (FLC) and fixed waveplates. Operation of the instrument is demonstrated by transmission Mueller matrix measurements, and product decomposition of an imperfect dichroic polarizer and a depolarizing rough glass surface. The decomposition of the measured matrices is qualtitatively discussed in terms of retardance, diattenuation, depolarization, and azimuthal orientation. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fast near‐infra‐red spectroscopic Mueller matrix ellipsometer based on ferroelectric liquid crystal retarders

AbstractThis paper reports on the design and construction of the first near‐infra‐red spectroscopic Mueller Matrix Ellipsometer (MME) based on Ferroelectric Liquid Crystal retarders (FLC) and fixed waveplates. The design was performed by assuming ideal optical polarizing components. In particular, the current FLC‐MME is optimised to operate in the near‐infra‐red (NIR) spectral range from 800 nm to 1800 nm. The FLC based Polarization State Generator (PSG) and Polarization State Analyzer (PSA) have been optimized by keeping the condition number of the PSG's modulation matrix and the PSA's analysis matrix close to an optimum over the spectral design range. The design is realized in a setup using the full Mueller matrix measurement formalism based on the Eigenvalue Calibration Method (ECM). The actual modulation matrix and analysis matrix are obtained from the ECM, and the condition numbers immediately indicates the quality of the design. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Crystallization of poly( N-methyldodecano-12-lactam) in blends with poly(styrene- stat-acrylic acid)

Crystallization behaviour of blends of poly( N-methyldodecano-12-lactam) (PMDL) with statistical copolymer poly(styrene- stat-acrylic acid) (PSAA) has been studied by the DSC and WAXD methods. The blend films prepared from dioxane solutions were crystallized at laboratory temperature for five days. Approximate crystallinities of as-prepared neat lower- PMDL 5 and higher-molecular weight PMDL 45 were 28% and 19%, respectively. With increasing PSAA content in the blends the crystallinities decreased sharply. The melting point of the primary crystalline structure of PMDL showed a decreasing dependence on PSAA content in the blends, confirming miscibility of the PMDL–PSAA pair. Recrystallization was strongly suppressed in the blends. The lower-melting endotherm appearing at about 10–15 °C above the crystallization temperature was attributed to melting to less perfect structures formed during secondary crystallization. In neat PMDL, the extent of secondary crystallization was approximately 5–10%. In the blends containing 20% PSAA approximate relative proportion of secondary crystallites on total crystallinity was 40% and 60% for the blends with PMDL 5 and PMDL 45, respectively. WAXD measurements did not reveal any change in crystal modification on blending. Increased T g in blends of flexible PMDL cannot play a significant role in suppression of primary in favour of secondary crystallization. This was attributed to low mobility of PMDL chains due to dilution effect and specific interactions with the amorphous copolymer component, and, in case of the higher-molecular-weight PMDL, a greater involvement of entanglements. Higher T g of blends was involved in retardation of non-isothermal crystallization on cooling and subsequent cold crystallization.

Fourier transform channeled spectropolarimetry in the MWIR

A complete Fourier Transform Spectropolarimeter in the MWIR is demonstrated. The channeled spectral technique, originally developed by K. Oka, is implemented with the use of two Yttrium Vanadate (YVO(4)) crystal retarders. A basic mathematical model for the system is presented, showing that all the Stokes parameters are directly present in the interferogram. Theoretical results are compared with real data from the system, an improved model is provided to simulate the effects of absorption within the crystal, and a modified calibration technique is introduced to account for this absorption. Lastly, effects due to interferometer instabilities on the reconstructions, including nonuniform sampling and interferograms translations, are investigated and techniques are employed to mitigate them.

Novel polarization-sensitive micropulse lidar measurement technique

Polarization-sensitive detection of elastic backscattered light is useful for detection of cloud phase and depolarizing aerosols. The U.S. Department of Energy's Atmospheric Radiation Measurement Program has deployed micropulse lidar (MPL) for over a decade, but without polarized detection. Adding an actively-controlled liquid crystal retarder provides the capability to identify depolarizing particles by alternately transmitting linearly and circularly polarized light. This represents a departure from established techniques, which transmit exclusively linear polarization or exclusively circular polarization. Mueller matrix calculations yield simple relationships between the well-known linear depolarization ratio delta(linear), the circular depolarization ratio delta(circ), and this MPL depolarization ratio delta(MPL).

Polarization Stabilization in Optical Communications Systems

The control of the state of polarization (SOP) of light remains one of the open issues in optical communications. In particular, the achievement of a stabilization of the SOP can find many applications in advanced optical communication systems: from the mitigation of polarization-mode dispersion to the development of novel multilevel modulation formats. In this paper, theoretical and experimental aspects of polarization stabilization are dealt with, and a novel algorithm to overcome the issues related to the practical availability of finite-range birefringent components and to solve the requirement for endless stabilization is also presented. A complete analysis of the control algorithm, based on the Jones matrix formalism, is also presented. The practical implementation of the polarization stabilizer is discussed, and experimental demonstrations based on liquid crystal and magnetooptical retarders are shown

Mechanical properties of talc- and CaCO3-reinforced high-crystallinity polypropylene composites

Toughening mechanisms and mechanical properties of two high-crystallinity polypropylene (hcPP)-based composite systems, hcPP/talc and hcPP/CaCO3, are investigated. Significant improvement in tensile modulus is observed in the PP/talc composite, but only a moderate improvement is found for hcPP/CaCO3. The introduction of CaCO3 nanoparticles to hcPP helps nucleate a measurable amount of β-phase crystals and results in a significant drop in crystallization temperature, suggesting a possible retardation of hcPP crystallization. In addition, the hcPP/CaCO3 nanocomposite shows more pronounced damping characteristics than that of hcPP/talc, throughout the temperature range studied. A detailed investigation of fracture mechanisms suggests that well-dispersed, highly oriented talc particles cause embrittlement of hcPP. Only when the crack extends toward the edges of the specimen will the crack deflection/bifurcation and microcracking mechanisms initiate. In the case of hcPP/CaCO3, the CaCO3 nanoparticles help trigger massive crazing and shear yielding if the testing speed is in quasi-static. The presence of β-phase crystals around the CaCO3 particles could facilitate the formation of crazes throughout the hcPP matrix. Approaches for toughening hcPP are discussed.

バルク金属ガラスの局所構造と結晶化・ガラス転移、そして構造安定性(OS-4 基調講演)

The physical significance of the glass transition observed by differential scanning calorimetry (DSC) in the metallic glasses is explained in a study of measurements of the heating-rate(β) dependence of the glass transition temperature Tg and the crystallization temperature Tx in the Zr70Cu3O and Zr70Ni30 amorphous alloys and their structures in as-quenched and crystallized states. Zr70Cu30 exhibits the glass transition before crystallization in the normal DSC measurement and Zr70Ni30 immediately crystallizes. The heating rate β_c at the intersection of the two linear curves of Tg and Tx against logβ provides us a significant measure to determine the GFA and thermal stability of the metallic glasses. By heating at β larger than β_c, the crystallization is suppressed and the glass transition is observed even in Zr_<70>Ni_<30>. The thermal stability of the Zr_2Cu amorphous alloy is caused by retardation of crystallization due to the amorphous structure different from the Zr_2Cu crystalline phase. By contrast, the thermal instability of Zr_<70>Ni_<30> is attributed to the structural similarity to Zr_2Ni. Consequently, suppressing the crystallization is a key to gain the thermal stability of the present amorphous alloys. In the present paper, I will also introduce an example to evaluate a thermally stable cluster size near glass transition in Pd-based bulk metallic glasses.

Crystallization behavior of polymer/montmorillonite nanocomposites. Part III. Polyamide-6/montmorillonite nanocomposites, influence of matrix molecular weight, and of montmorillonite type and concentration

Several series of polyamide-6 (PA-6) nanocomposites, differing in montmorillonite (MMT) type and content and PA-6 matrix molecular weight, were prepared by melt-extrusion and the associated PA-6 crystallization behavior and morphology was evaluated using (synchrotron) X-ray diffraction, transmission electron microscopy and differential scanning calorimetry. The nucleating ability of silicate layers is poor in PA-6 nanocomposites made by melt-extrusion because highly active, stable PA-6 crystallization precursors are generated during melt-extrusion. In most of the studied PA-6/MMT nanocomposites the dispersed silicate layers act as impurities and decrease rather than increase the overall crystallization kinetics of PA-6, especially at high MMT contents. Furthermore, at a given MMT concentration, the crystal growth retardation inflates with increasing degree of exfoliation, which dependents on the MMT type and which increases with increasing PA-6 molecular weight. One of the considered MMT types leads to a poorly exfoliated nanomorphology and as a result no retardation of crystal growth is observed. Furthermore, the disturbed crystal growth does not alter the PA-6 semicrystalline stack morphology. Moderate nucleation effects due to the presence of MMT can be observed when the particle load is low (low amount of MMT and/or poor degree of exfoliation) and provided the supercooling is sufficiently large.

Local Structure and Glass Transition in Zr-Based Binary Amorphous Alloys

The physical significance of the glass transition observed by differential scanning calorimetry (DSC) in the metallic glasses was considered through the measurements of the heating-rate, β, dependence of the glass transition temperature, Tg, and the crystallization temperature, T x , in the Zr 7 0 Cu 3 0 and Zr 7 Ni 3 0 amorphous alloys and X-ray study of their structures in as-quenched and crystallized states. Zr 7 0 Cu 3 0 exhibits the glass transition before crystallization, hut Zr 7 0 Ni 3 0 is immediately crystallized at heating rates of conventional time scale in the DSC measurement. The heating rate β c at the intersection of the two linear curves of T g and T x against log β provides us with a significant measure to determine the glass-forming ability or thermal stability of the metallic glasses. By heating at β larger than β c , the crystallization is suppressed and the glass transition is clearly observed even in Zr 7 0 Ni 3 0 . The thermal stability of the Zr 7 0 Cu 3 0 amorphous alloy is caused by retardation of crystallization due to the amorphous structure that is different from the Zr 2 Cu crystalline phase. In contrast, the thermal instability of Zr 7 0 Ni 3 0 is attributed to the structural similarity to the Zr 2 Ni crystalline phase. Thus, suppressing the crystallization is shown to be a key to enhance the thermal stability of the present amorphous alloys.

Acousto-optic-tunable-filter-based spectropolarimetric imagers for medical diagnostic applications—instrument design point of view

Compact optical imagers that can detect both spectral and polarization signatures are required in many biomedical applications. An acousto-optic-tunable-filter (AOTF)-based imager is ideally suited to provide both agile spectral and polarization signatures. Such an imager can be readily used for real-time in vivo medical diagnostic applications. We develop a family of small, robust, and programmable hyperspectral imagers operating from the ultraviolet (UV) to the long-wave IR (LWIR). Such imagers require minimal data processing because they can acquire images at only select wavelengths of interest. We use AOTFs made of KDP, TeO2, and TAS with Si-based CCD, InGaAs, InSb, and HgCdTe cameras to cover different spectral regions from the UV to the LWIR. Operation of each of these imagers and image acquisition is computer controlled. The most developed imager covers the visible to near-infrared (VNIR) region from 400 to 900 nm, with a 10-nm spectral resolution at 600 nm, it uses an electronically tunable TeO2 AOTF as a bandpass filter, and a nematic liquid crystal retarder to change polarization. We describe our concept in the development of these imagers and present new results obtained using the VNIR imager.

Crystallization Kinetics of Maleated Polypropylene/Clay Hybrids

The isothermal crystallization kinetics of maleic anhydride grafted polypropylene (mPP) in hybrids with unmodified or organically modified montmorillonite (MMT), which have been prepared by melt intercalation in a mixer, has been investigated by using time-resolved wide-angle X-ray scattering (WAXS) and differential scanning calorimetry (DSC). WAXS and transmission electron microscopy (TEM) showed that the mPP/unmodified MMT hybrids formed macrophase-separated mixtures rather than intercalated nanocomposites, while mPP/organically layered silicate (OLS) hybrids formed exfoliated nanocomposites. The crystallization rate of mPP/unmodified silicate hybrids became faster with the MMT addition because agglomerated silicates in the mPP matrix act as nucleating agents. On the other hand, the crystallization rate of the mPP/OLS hybrids was slowed with the OLS addition. This retardation of the mPP crystallization is probably due to the presence of well-dispersed exfoliated silicates as diffusion or displacement barriers for crystallization. Interestingly, these well-dispersed exfoliated silicates change the crystal orientation as well as the crystallization kinetics of mPP in mPP/OLS nanocomposites, showing larger (040) peak than (110) peak for the α form crystal in WAXS profiles.