Self-organized arrays of dislocations in thin smectic liquid crystal films.

Combining optical microscopy, synchrotron X-ray diffraction and ellipsometry, we studied the internal structure of linear defect domains (oily streaks) in films of a smectic liquid crystal 8CB with thicknesses in the range of 100-300 nm. These films are confined between air and a rubbed PVA polymer substrate which imposes hybrid anchoring conditions (normal and unidirectional planar, respectively). We show how the presence or absence of dislocations controls the structure of highly deformed thin smectic films. Each domain contains smectic layers curved in the shape of flattened hemicylinders to satisfy both anchoring conditions, together with grain boundaries whose size and shape are controlled by the presence of dislocation lines. A flat grain boundary normal to the interface connects neighboring hemicylinders, while a rotating grain boundary (RGB) is located near the axis of curvature of the cylinders. The RGB shape appears such that dislocation lines are concentrated at its summit close to the air interface. The smectic layers reach the polymer substrate via a transition region where the smectic layer orientation satisfies the planar anchoring conditions over the entire polymer substrate and whose thickness does not depend on that of the film. The strength of planar anchoring appears to be high, larger than 10(-2) mJ m(-2), compensating for the high energy cost of creating an additional 2D defect between a horizontal smectic layer and perpendicular ones of the transition region. This 2D defect may be melted, in order to avoid the creation of a transition region structure composed of a large number of dislocations. As a result, linear defect domains can be considered as arrays of oriented defects, straight dislocations of various Burger vectors, whose location is now known, and 2D nematic defects. The possibility of easy variation between the present structure with a moderate amount of dislocations and a structure with a large number of dislocations is also demonstrated.

Demonstration of full 4×4 Mueller polarimetry through an optical fiber for endoscopic applications.

A novel technique to measure the full 4 × 4 Mueller matrix of a sample through an optical fiber is proposed, opening the way for endoscopic applications of Mueller polarimetry for biomedical diagnosis. The technique is based on two subsequent Mueller matrices measurements: one for characterizing the fiber only, and another for the assembly of fiber and sample. From this differential measurement, we proved theoretically that the polarimetric properties of the sample can be deduced. The proof of principle was experimentally validated by measuring various polarimetric parameters of known optical components. Images of manufactured and biological samples acquired by using this approach are also presented.

Hemodynamic comparison between Trifecta and Freestyle valves implanted in small aortic roots. One-year echocardiographic results from a prospective randomized study.

BACKGROUND AND AIM OF THE STUDY: Aortic valve replacement in patients with a small aortic root may be associated to high residual gradients. In such patients, both stentless valves and aortic annulus enlargement can reduce these residual gradients. Several studies have reported that Trifecta valves yield very good hemodynamic results. The aim of the present study was to compare the hemodynamic performance of Trifecta vs. Freestyle valves at one year in patients with an aortic annulus ≤ 2.3 cm. METHODS: Between September 2011 and September 2013, 40 patients with a native aortic annulus diameter ≤ 2.3 cm and average age of 81 ± 4 years, were randomized to receive either a St-Jude Trifecta stented prosthesis (20 patients) or a Medtronic Freestyle stentless prosthesis (20 patients). RESULTS: No differences between Trifecta and Freestyle were found at one year in mean gradient s: 6.1 ± 3 mmHg and 6.6 ± 3 mmHg (p = 0.796); effective ori fice area: 1.82 ± 0.3 mmHg and 1.76 ± 0.4 mmHg (p = 0.676) or regression of left ventricular mass: - 25% ± 14 vs. -19% ± 16 (p = 0.204), respectively. Only moderate patient -pro sthesis mismatch was found, which affected 3 patient s in each group. CONCLUSION: At one year both stentless and stented prostheses yielded comparable hemodynamic results. These data suggest that Trifecta implantation is a valid means of avoiding patient -prosthesis mismatch in aortic valve replacement in elderly patients with a small native aortic annulus.

Differential Mueller matrix of a depolarizing homogeneous medium and its relation to the Mueller matrix logarithm.

The different z-dependence and non-commutativity of the two components of the differential Mueller matrix of a homogeneous depolarizing medium prevent its formal identification with the Mueller matrix logarithm. By using a classic linear differential equation expansion, we advance a procedure for the extraction of the elementary polarization properties, in terms of mean values and variances-covariances, from the Mueller matrix logarithm. The approximate solution, based on the immediate identification of the differential matrix with the matrix logarithm, turns out to remain satisfactory up to relatively high depolarization levels. Physically interpreted experimental examples from the literature illustrate the formal developments.

Modeling the optical properties of self-organized arrays of liquid crystal defects.

Local full Mueller matrix measurements in the Fourier plane of a microscope lens were used to determine the internal anisotropic ordering in periodic linear arrays of smectic liquid crystal defects, known as 'oily streaks'. We propose a single microstructure-dependent model taking into account the anisotropic dielectric function of the liquid crystal that reproduces the smectic layers orientation and organization in the oily streaks. The calculated Mueller matrix elements are compared to the measured data to reveal the anchoring mechanism of the smectic oily streaks on the substrate and evidence the presence of new type of defect arrangement. Beyond the scientific inquiry, the understanding and control of the internal structure of such arrays offer technological opportunities for developing liquid-crystal based sensors and self-assembled nanostructures.

Determination of collagen fiber orientation in histological slides using Mueller microscopy and validation by second harmonic generation imaging.

We studied the azimuthal orientations of collagen fibers in histological slides of uterine cervical tissue by two different microscopy techniques, namely Mueller polarimetry (MP) and Second Harmonic Generation (SHG). SHG provides direct visualization of the fibers with high specificity, which orientations is then obtained by suitable image processing. MP provides images of retardation (among other polarimetric parameters) due to the optical anisotropy of the fibers, which is enhanced by Picrosirius Red staining. The fiber orientations are then assumed to be those of the retardation slow axes. The two methods, though fully different from each other, provide quite similar maps of average fiber orientations. Overall, our results confirm that MP microscopy provides reliable images of dominant fiber orientations at a much lower cost that SHG, which remains the "gold standard" for specific imaging of collagen fibers using optical microscopy.

Polarimetric imaging of uterine cervix: a case study.

We present a preliminary investigation of macroscopic polarimetric imaging of uterine cervix. Orthogonal state contrast (OSC) images of healthy and anomalous cervices have been taken in vivo at 550 nm. Four ex vivo cervix samples have been studied in full Muller polarimetry, at 550 nm and 700 nm, and characterized in detail by standard pathology. One sample was totally healthy, another one carried CIN lesions at very early stage (CIN1) in its visible exocervical region, while for the other two samples more advanced (CIN3) lesions were present, together with visible glandular epithelium (ectropion). Significant birefringence has been observed in the healthy regions of all six samples, both in vivo and ex vivo. Standard treatments of the Mueller images of the ex vivo samples allowed to quantify both retardation and depolarization. Retardation reached 60° in healthy regions, and disappeared in the anomalous regions of the other three ex vivo samples. The depolarization power was largest in healthy regions, and lower in CINs and ectropion. Possible origins of the observed effects are briefly discussed.

Effect of speckle on APSCI method and Mueller Imaging.

The principle of the polarimetric imaging method called APSCI (Adapted Polarization State Contrast Imaging) is to maximize the polarimetric contrast between an object and its background using specific polarization states of illumination and detection. We perform here a comparative study of the APSCI method with existing Classical Mueller Imaging(CMI) associated with polar decomposition in the presence of fully and partially polarized circular Gaussian speckle. The results show a noticeable increase of the Bhattacharyya distance used as our contrast parameter for the APSCI method, especially when the object and background exhibit several polarimetric properties simultaneously.

Ex-vivo characterization of human colon cancer by Mueller polarimetric imaging.

Cancerous and healthy human colon samples have been analyzed ex-vivo using a multispectral imaging Mueller polarimeter operated in the visible (from 500 to 700 nm) in a backscattering configuration with diffuse light illumination. Three samples of Liberkühn colon adenocarcinomas have been studied: common, mucinous and treated by radiochemotherapy. For each sample, several specific zones have been chosen, based on their visual staging and polarimetric responses, which have been correlated to the histology of the corresponding cuts. The most relevant polarimetric images are those quantifying the depolarization for incident linearly polarized light. The measured depolarization depends on several factors, namely the presence or absence of tumor, its exophytic (budding) or endophytic (penetrating) nature, its thickness (its degree of ulceration) and its level of penetration in deeper layers (submucosa, muscularis externa and serosa). The cellular density, the concentration of stroma, the presence or absence of mucus and the light penetration depth, which increases with wavelength, are also relevant parameters. Our data indicate that the tissues with the lowest and highest depolarizing powers are respectively mucus-free tumoral tissue with high cellular density and healthy serosa, while healthy submucosa, muscularis externa as well as mucinous tumor probably feature intermediate values. Moreover, the specimen coming from a patient treated successfully with radiochemotherapy exhibited a uniform polarimetric response typical of healthy tissue even in the initially pathological zone. These results demonstrate that multi-spectral Mueller imaging can provide useful contrasts to quickly stage human colon cancer ex-vivo and to distinguish between different histological variants of tumor.

Experimental validation of the symmetric decomposition of Mueller matrices.

We experimentally assess the validity of the symmetric decomposition of Mueller matrices [R. Ossikovski, J. Opt. Soc. Am A 26, 1109-1118 (2009)] into a sequence of five factors consisting of a diagonal depolarizer between two retarder and diattenuator pairs. The raw data were Mueller images of combinations of polarization components which were individually measured and then assembled in different combinations. The possibility to recover all the elements is discussed, including the experimentally relevant cases of "degenerate" depolarizers, with two equal eigenvalues, which were not explicitly considered in the general theory.

Mueller matrix imaging of human colon tissue for cancer diagnostics: how Monte Carlo modeling can help in the interpretation of experimental data.

Colon samples with both healthy and cancerous regions have been imaged in diffuse light and backscattering geometry by using a Mueller imaging polarimeter. The tumoral parts at the early stage of cancer are found to be less depolarizing than the healthy ones. This trend clearly shows that polarimetric imaging may provide useful contrasts for optical biopsy. Moreover, both types of tissues are less depolarizing when the incident polarization is linear rather than circular. However, to really optimize an optical biopsy technique based on polarimetric imaging a realistic model is needed for polarized light scattering by tissues. Our approach to this goal is based on numerical Monte-Carlo simulations of polarized light propagation in biological tissues modeled as suspensions of monodisperse spherical scatterers representing the cell nuclei. The numerical simulations were validated by comparison with measurements on aqueous polystyrene sphere suspensions, which are commonly used as tissue phantoms. Such systems exhibit lower depolarization for incident linear polarization in the Rayleigh scattering regime, i.e. when the sphere diameters are smaller than the wavelength, which is obviously not the case for cell nuclei. In contrast, our results show that this behaviour can also be seen for "large" scatterers provided the optical index contrast between the spheres and the surrounding medium is small enough, as it is likely to be the case in biological tissues.

Scattered light measurements on textured crystalline silicon substrates using an angle-resolved Mueller matrix polarimeter.

Dry plasma etching is a promising technique for crystalline silicon surface texturing aimed at improving solar cell efficiencies by reducing incident light reflection and backscattering at the cell front surface. In this work we present a new optical characterization technique for textured surfaces based on a Mueller polarimeter coupled with a high numerical aperture microscope operated either in real or in angular spaces. This tool provides both the specularly reflected and the angle-resolved backscattered intensities in a very efficient manner, due to the absence of moving parts. Three different silicon samples were etched in a standard reactor with SF(6)/O(2) plasmas at various RF powers, resulting in different textures that were characterized by scanning electron microscopy, standard reflectometry, and by our tool. The three techniques yielded consistent results. However, reflectometry could not take into account the backscattered light from highly textured surfaces, leading to significant underestimation of the overall amount of the reflected light. In contrast, our tool has demonstrated the potential to measure both reflected and backscattered light quickly and efficiently for all samples, paving the way for a new characterization technique for textured solar cells both at the development and at the production stage.

Adapted polarization state contrast image.

We propose a general method to maximize the polarimetric contrast between an object and its background using a predetermined illumination polarization state. After a first estimation of the polarimetric properties of the scene by classical Mueller imaging, we evaluate the incident polarized field that induces scattered polarization states by the object and background, as opposite as possible on the Poincar e sphere. With a detection method optimized for a 2-channel imaging system, Monte Carlo simulations of low flux coherent imaging are performed with various objects and backgrounds having different properties of retardance, dichroism and depolarization. With respect to classical Mueller imaging, possibly associated to the polar decomposition, our results show a noticeable increase in the Bhattacharyya distance used as our contrast parameter.

Sources of possible artefacts in the contrast evaluation for the backscattering polarimetric images of different targets in turbid medium.

It is known that polarization-sensitive backscattering images of different objects in turbid media may show better contrasts than usual intensity images. Polarimetric image contrast depends on both target and background polarization properties and typically involves averaging over groups of pixels, corresponding to given areas of the image. By means of numerical modelling we show that the experimental arrangement, namely, the shape of turbid medium container, the optical properties of the container walls, the relative positioning of the absorbing, scattering and reflecting targets with respect to each other and to the container walls, as well as the choice of the image areas for the contrast calculations, can strongly affect the final results for both linearly and circularly polarized light.

Experimental evidence for naturally occurring nondiagonal depolarizers.

We report on two Stokes nondiagonalizable Mueller matrices experimentally observed in a biological and in an organic sample. These matrices are examples of naturally occurring nondiagonal depolarizers whose unique property is to preserve the degree of polarization of all but one totally polarized light state. The description of the experimental matrices within the theory of Bragg scattering on cholesteric liquid crystals, as well as their interpretation in physical and structural terms, are likewise addressed.

Angle resolved Mueller polarimetry with a high numerical aperture and characterization of transparent biaxial samples.

We present a polarimetric instrument suitable for the simultaneous measurement of angle resolved normalized Mueller matrices for polar angles ranging from 0 degrees to 60 degrees and all azimuths. The polarimetric modulation and analysis are performed by means of an optimized polarization state generator and analyzer based on nematic liquid crystals. A high numerical aperture (0.95) microscope objective is used in double pass to illuminate the sample, with its rear focal plane imaged on a low noise CCD. This polarimeter can be used either in reflection, with the sample set in the objective front focal plane, or in transmission, for thin transparent samples. This latter configuration, which involves an additional spherical mirror with its center of curvature at the objective front focus, is described in detail. This instrument was used to accurately determine the directions of the optic axes and the angular dependence of the retardation of a biaxial polyethylene terephthalate (PET) plastic substrate in spite of the strong depolarization essentially due to the source 10 nm spectral width or the limitation in angular resolution due to the pixels distribution of the CCD combined with the sample large retardation.

Registration scheme suitable to Mueller matrix imaging for biomedical applications.

Most Mueller matrix imaging polarimeters implement sequential acquisition of at least 16 raw images of the same object with different incident and detected light polarizations. When this technique is implemented in vivo, the unavoidable motions of the subject may shift and distort the raw images to an extent such that the final Mueller images cannot be extracted. We describe a registration algorithm which solves this problem for the typical conditions of in vivo imaging, e.g. with spatially inhomogeneous medium to strong depolarization. The algorithm, based on the so called "optical flow," is validated experimentally by comparing the Mueller images of a pig skin sample taken in static and in dynamic conditions.

Metrology of replicated diffractive optics with Mueller polarimetry in conical diffraction.

The feasibility of metrological characterization of the one-dimensional (1D) holographic gratings, used in the nanoimprint molding tool fabrication step, by spectroscopic Mueller polarimetry in conical diffraction is investigated. The studied samples correspond to two different steps of the replicated diffraction grating fabrication process. We characterized master gratings that consist of patterned resist layer on chromium-covered glass substrate and complementary (replica) gratings made of nickel. The profiles of the gratings obtained by fitting the experimental spectra of Mueller matrix coefficients taken at different azimuthal angles were confirmed by atomic force microscopy (AFM) measurements. The calculated profiles of corresponding master and replica gratings are found to be complementary. We conclude that the Mueller polarimetry, as a fast and non-contact optical characterization technique, can provide the basis for the metrology of the molding tool fabrication step in the nanoimprint technique.

Unexpected Radiation-Induced Aortic Wall Thickening Requiring Composite Graft Technique during Off-Pump Coronary Artery Bypass Grafting.

Mediastinal radiation is commonly used to treat Hodgkin's and non-Hodgkin's lymphoma, lung and breast cancer. Cardiac complications after radiation therapy are well described, although rare. A large spectrum of injuries can occur, causing long term morbidity among survivors. We describe a case of post-actinic ascending aortic wall thickening that prevented saphenous vein proximal anastomosis and was successfully managed with aortic no-touch off-pump coronary artery bypass grafting (OPCAB), 25 years after radiation therapy for Hodgkin's lymphoma.

Physical interpretation of polarization-encoded images by color preview.

In the framework of Stokes parameters imaging, polarizationencoded images have four channels which makes physical interpretation of such multidimensional structures hard to grasp at once. Furthermore, the information content is intricately combined in the parameters channels which involve the need for a proper tool that allows the analysis and understanding this kind of images. In this paper we address the problem of analyzing polarization-encoded images and explore the potential of this information for classification issues and propose ad hoc color displays as an aid to the interpretation of physical properties content. The color representation schemes introduced hereafter employ a technique that uses novel Poincaré Sphere to color spaces mapping coupled with a segmentation map as an a priori information in order to allow, at best, a distribution of the information in the appropriate color space. The segmentation process relies on the fuzzy C-means clustering algorithms family where the used distances were redefined in relation with our images specificities. Local histogram equalization is applied to each class in order to bring out the intra-class's information smooth variations. The proposed methods are applied and validated with Stokes images of biological tissues.