Non linearly induced self waveguiding structure in dye doped nematic liquid crystals confined in capillaries

We report on experiments dealing with the propagation of a collimated laser beam in a dye doped nematic confined in a capillary of optical fiber size. The nematic is aligned in a such a way that the source beam is self focused. The behavior of the beam - focusing, multifocus regime, filamentation and undulation - already observed in larger cylindrical geometry and pure nematic is shown here to be reproduced in the dye doped medium, at much lower powers. Another feature is reported here: a stable regime looking like a waveguide appears in certain conditions, in which the beam to propagate in a narrow tube. This regime is simply modeled in terms of saturated reorientation of the nematic

Modeling planar degenerate wetting and anchoring in nematic liquid crystals

We propose a simple surface potential favoring the planar degenerate anchoring of nematic liquid crystals, i.e., the tendency of the molecules to align parallel to one another along any direction parallel to the surface. We show that, at lowest order in the tensorial Landau-de Gennes order-parameter, fourth-order terms must be included. We analyze the anchoring and wetting properties of this surface potential. In the nematic phase, we find the desired degenerate planar anchoring, with positive scalar order-parameter and some surface biaxiality. In the isotropic phase, we find, in agreement with experiments, that the wetting layer may exhibit a uniaxial ordering with negative scalar order-parameter. For large enough anchoring strength, this negative ordering transits towards the planar degenerate state

Dispersion curve measurement using Talbot bands

Talbot bands, looking like a channelled spectrum, are produced when a phase plate is partly inserted in the field of a spectroscope. The position and the spacing of the bands in the spectrum of the source directly depends on the optical thickness and therefore the refractive index of the phase plate. In the case of a transmission grating spectroscope, the bands appear only in spectral orders on the same side as the phase plate. The bands can be observed also in case of a two stepped plate, their position being related to the difference of the two refractive indices of the plate, they appear in spectrum orders on the side where the higher index half plate have been inserted in. Using this early known phenomenon in conjunction with modem technology, we propose a method based upon an accurate determination of the positions of these dark bands for computing the dispersion curve of any transparent liquid or solid material, the curve being built up from only one experiment whereas several are required usually. In this paper, after the method have been described, experimental results obtained with known glasses are reported proving the efficiency and the reliability of the technique. Also this method is extended to birefringent materials, it is shown that the extraordinary and ordinary dispersion curve can be obtained simultaneously. Experimental results on liquid crystals materials are reported.Les bandes de Talbot, ressemblant à un spectre cannelé, sont observables lorsqu'une lame de phase est partiellement insérée dans le champ optique d'un spectroscope. Leur position dans le spectre de la source est directement liée au retard optique introduit par la lame de phase et donc à son indice de réfraction. Dans le cas d'un spectroscope à réseau, les bandes n'existent que dans les ordres de diffraction situés du côté par lequel la lame a été insérée. Les bandes apparaissent également lorsque la lame de phase est constituée de deux parties d'indices différents, du côté de la lame d'indice le plus élevé. En utilisant des technologies modemes de mesures sur ce phénomène connu de longue date, nous proposons une nouvelle méthode pour obtenir la courbe de dispersion de matériaux transparents. Elle est basée sur une détermination précise de la position des bandes dans le spectre associée à une manipulation numérique des données conduisant à l'obtention de la courbe de dispersion et ce, en une seule expérience alors que la technique classique en nécessite autant que de points sur la courbe. Dans cette publication, après avoir décrit la méthode, nous reportons des résultats expérimentaux obtenus sur des substances tests, démontrant ainsi la faisabilité et la fiabilité de la méthode. Ensuite elle est étendue aux matériaux biréfringents. Il est montré qu'il est possible de mesurer les courbes de dispersions ordinaires et extraordinaires simultanément. Des résultats expérimentaux concernant les cristaux liquides sont également reportés

Experimental determination of nematic director distribution in the vicinity of the interface by reflectivity measurements

Reflectivity of a nematic cell has been measured round about the total internal reflection that occurs either at the glass-nematic interface or within the nematic bulk should the material be distorded. The used set-up is described in the paper : it allows to record the reflected intensity against the incidence angle. In addition to the usual sharp limit between the total reflection and the transmission regimes at the glass-nematic interface, these curves show also a fringe pattern in the transmission regime. Actually, interferences occur between the beam reflected at the glass-nematic interface and the beam reflected either at the upper interface (homogeneous material) or totally reflected within the nematic bulk (heterogeneous material). Both cases must yield to some differences in the fringe pattern : this question is considered in a theoretical part. Using a simple geometrical optics approach, it is shown that the fringe distance behaves differently for homogeneous and heterogeneous materials. As a result, the way to decipher this fringe pattern is given, allowing to decide whether a cell is homogeneous. This method has been applied to nematic films and experimental results are presented. Different boundary conditions have been considered : homogeneous homeotropic and planar or hybrid and results are consistent with the simple presented model.La réflectivité d'un film nématique est mesurée pour des angles d'incidence proches de la réflexion totale qui intervient soit à l'interface verre-nématique, soit à l'intérieur même du matériau si celui-ci est distordu. Le montage expérimental est présenté : il permet de mesurer l'intensité réfléchie par le film en fonction de l'angle d'incidence. En plus de l'angle limite entre les régions de réflection totale et de transmission, les courbes présentent dans le régime transmission une figure d'interférences. Il s'agit d'interférences entre le faisceau réfléchi à l'interface verre-nématique et soit à l'interface supérieur (milieu homogène), soit à l'intérieur même du nématique (milieu hétérogène). Ces deux situations doivent aboutir à des différences dans la figure d'interférences : ce point est considéré dans la partie théorique. En utilisant un simple modèle d'optique géométrique, il est montré que l'interfrange se comporte différemment suivant que le milieu est homogène ou non. Ce résultat est mis à profit pour donner une méthode expérimentale simple permettant de savoir si un film est homogène ou non. Cette méthode est appliquée aux films nématiques et les résultats expérimentaux sont présentés. Les géométries homogène homéotrope et planaire et hétérogène hométrope-planaire ont été considérées. Les résultats obtenus sont en accord avec le modèle mis en œuvre

Experimental determination of nematic director distibution in the vicinity of interface by reflectivity mesurements

The formula (4) must be replaced by :
$${\phi}/{2\pi} = {1}/{\lambda} \int_{0}^{e}n\cos r dz - 1/{\lambda} \int_{0}^{e}n'\cos r' dz \; \; \; (4)$$
Where the first integral corresponds to the ray travelling along with the $z$ direction and the second to the ray going in the reverse direction. 
In the same way, the formula (12) must be replaced by : 
$$\phi/{2\pi} = {h}/{\lambda} [n\cos r + n'\cos r'] \; \; \; (12)$$
where $n'\cos r'$ is deduced from $n\cos r$ by changing $\theta$ in $-\theta$. 
As a result, the term $-k\ln({k^2}/{n_e^2})$ in the formula (16) must be disregarded and the 1 in the formula (17) as well.

From bulk Janossy effect to nonlinear self waveguiding or spatial soliton in dye doped liquid crystals

Invited Publicatio

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