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

International audienceLocal 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

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

International audienceCombining optical microscopy, synchrotron X-ray diffraction and ellipsometry, we studied the internal structure of linear defect domains (oily streaks) in films of smectic liquid crystal 8CB with thickness 100-300 nm confined between air and a rubbed PVA polymer substrate which impose hybrid anchoring conditions (normal and unidirectional planar, respectively). We show how the presence or absence of dislocations control 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 condition over the entire polymer substrate and whose thickness does not depend on the one of the film. The strength of the planar anchoring appears to be high, larger than 10 −2 J/m 2 , compensating for the high energy cost of creating an additional 2D defect between an horizontal smectic layer and perpendicular ones. 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

Dispersions of Goethite Nanorods in Aprotic Polar Solvents

Colloidal suspensions of anisotropic nanoparticles can spontaneously self-organize in liquid-crystalline phases beyond some concentration threshold. These phases often respond to electric and magnetic fields. At lower concentrations, usual isotropic liquids are observed but they can display very strong Kerr and Cotton-Mouton effects (i.e., field-induced particle orientation). For many examples of these colloidal suspensions, the solvent is water, which hinders most electro-optic applications. Here, for goethite (α-FeOOH) nanorod dispersions, we show that water can be replaced by polar aprotic solvents, such as N-methyl-2-pyrrolidone (NMP) and dimethylsulfoxide (DMSO), without loss of colloidal stability. By polarized-light microscopy, small-angle X-ray scattering and electro-optic measurements, we found that the nematic phase, with its field-response properties, is retained. Moreover, a strong Kerr effect was also observed with isotropic goethite suspensions in these polar aprotic solvents. Furthermore, we found no significant difference in the behavior of both the nematic and isotropic phases between the aqueous and non-aqueous dispersions. Our work shows that goethite nanorod suspensions in polar aprotic solvents, suitable for electro-optic applications, can easily be produced and that they keep all their outstanding properties. It also suggests that this solvent replacement method could be extended to the aqueous colloidal suspensions of other kinds of charged anisotropic nanoparticles

Décoration de réseaux linéaires de défauts smectiques par des nanoparticules d'or

We create thin smectic films of topological defects, linearly oriented along a unique direction, on top of rubbed polymer substrate, thanks to our knowledge develop on crystalline substrate. First we can study the influence of the substrate on defects structure, and perform X-Rays diffraction in combining transmission and reflexion setup. Second by mixing the liquid crystal with gold nanoparticles (NPs), we provide anisotropy to the NPs self-assembly and to their optical properties, tunable under light polarization. At low NPs concentration, we create chains of individual nanoparticles trapped within smectic dislocations, parallel to each other and aligned along the defects. Inside a chain, depending on concentration NPs can be isolated or electromagnetically coupled. At high enough concentration , this leads to anisotropic properties of the collective localized surface plasmon resonance as large as 40 nm. Then the linear assembly turn into two dimensional organization of the GNPs, with dense packing along the defects and an anisotropy of the optical properties remains.Concerning pure liquid crystal matrix, we are strengthening the hypothesis of a structure made of a rotating grain boundary but with an eccentric curvature center.We identify two areas being able to trap the NPs: a preferential site should be one dislocation close to the substrate, without disorder induced by the presence of the NPs; a second site should be an assembly of dislocation localized on top of the rotating grain boundary close to the air interface, which reconstructs depending on the NPs concentration. This directed assembly of NPs opens the route for various hybrid system with NPs of larger size and anisotropic shape, in order to enhanced the anisotropic effect of their assemblies.Nous créons des films minces smectiques de défauts topologiques linéaires sur substrats de polymère grâce au savoir faire acquis sur substrats cristallins. Le transfert de ce système sur polymère permet d'étudier l'influence du substrat sur la structure du film, et de mener un nouveau type d'étude par diffraction des rayons X en adoptant deux configurations couplées en réflexion et en transmission. D'autre part, en mélangeant des nanoparticules (NPs) à ces films, on transmet leur anisotropie aux auto-assemblages NPs d'or pour obtenir des propriétés optiques définies et activables par variation de la polarisation de la lumière. Dans le cas de films hybrides faiblement concentrés, nous créons des chaînes de NPs individuelles piégées par les dislocations smectiques, sur plusieurs dizaines de microns. En fonction de la concentration, les NPs peuvent être isolées ou en condition de couplage électromagnétique, permettant d'obtenir une anisotropie de la résonance de plasmon jusqu'à 40 nm. A plus grande concentration, le réseau linéaire de NPs se transforme en réseau bidimensionnel, toujours anisotrope, dont une direction dense est imposée le long des défauts. Pour le film smectique pur, un modèle avec un joint de grain tournant est retrouvé et interprété comme minimisant la présence de dislocations. Nous identifions deux sites de dislocations capables de piéger les nanoparticules : le premier site, préférentiel, piège les NPs dans un environnement ordonné situé près du substrat ; le second est en haut du joint de grain tournant plus proche de l'interface avec l'air, il mobilise une assemblée de dislocations reconstruites en fonction de la concentration en NPs. Finalement, cette auto-organisation dirigée de nanoparticules, ouvre la voie pour l' étude de systèmes hybrides de particules plus grandes ou de formes anisotropes pour exalter les effets d'anisotropie de l'auto-assemblage

Nanostructured silica spin–orbit optics for modal vortex beam shaping

Abstract Modality is a generic concept of wave-optics at the basis of optical information and communications. One of the challenges of photonics technologies based on optical orbital angular momentum consists in the production of a modal content for both the azimuthal and radial degrees of freedom. This basically requires shaping the complex amplitude of an incident light beam, which is usually made up from adaptive spatial light modulators or bespoke devices. Here, we report on the experimental attempt of a recent theoretical proposal [ Opt. Lett. 42 , 1966 (2017)] toward the production of various optical vortex modes of the Laguerre–Gaussian type relying on the spin–orbit interaction of light. This is done in the visible domain from optical elements made out of silica glass. The idea consists in exploiting the combined effects of azimuthally-varying geometric phase with that of radially-varying propagation features. The proposed approach can be readily extended to any wavelength as well as to other families of optical modes, although some dynamic phase problems remain to be solved to make it a turnkey technology

Trapping of gold nanoparticles within arrays of topological defects: evolution of the LSPR anisotropy

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Kinked row-induced chirality driven by molecule-substrate interactions

International audienceCombining STM measurements on three different substrates (HOPG, MoS2, and Au[111]), DFT calculations, and using a previously developed phenomenological model, we analyze the origin of the self-assembly of 4-Cyano-4′-n-decylbiphenyl (10CB), into kinked row structures. This molecule has an alkyl chain with 10 carbons and a cyanobiphenyl group of particularly large dipole moment. 10CB represents a toy model that we use here to unravel the relationship between the induced kinked structure, in particular, the corresponding chirality expression, and the balanced intermolecular/molecule-substrate interaction. We show that the local ordered structure is driven by the typical alkyl chain/substrate interaction for HOPG and Au[111] and the cyanobiphenyl group/substrate interaction for MoS2. The strongest molecule/substrate interactions are observed for MoS2 and Au[111]. These strong interactions should have led to non-kinked, commensurate adsorbed structures. However, this latter appears impossible due to steric interactions between the neighboring cyanobiphenyl groups that lead to a fan-shape structure of the cyanobiphenyl packing on the three substrates. As a result, the kink-induced chirality is particularly large on MoS2 and Au[111]. A further breaking of symmetry is observed on Au[111] due to an asymmetry of the facing molecules in the rows induced by similar interactions with the substrate of both the alkyl chain and the cyanobiphenyl group. We calculate that the overall 10CB/Au[111] interaction is of the order of 2 eV per molecule. The close 10CB/MoS2 interaction, in contrast, is dominated by the cyanobiphenyl group, being particularly large due to dipole-dipole interaction between the cyanobiphenyl groups and the MoS2 substrate

Electric Birefringence and Electric Dichroism of Goethite Colloidal Suspensions

International audienceAqueous colloidal suspensions of goethite have attracted much attention by their outstanding magnetic properties [1]. In both the isotropic and the nematic phase of these suspensions the goethite particles align either parallel to the field (at low field strength) or perpendicular to it (in high fields). The goethite particles orient easily also in electric fields [2], showing strong induced birefringence and short response times, making them attractive for potential applications.Here we study the electric field induced order in the isotropic phase of the goethite suspensions in water and other polar solvents. In our recently developed electro-optic setup [3] we measure the birefringence induced in the sample by short pulses of high frequency electric field. However, due to the strong absorption of the goethite particles and their high specific birefringence, the precision of the measurements is not satisfactory in the case of high volume fractions, >1%. To resolve this problem, we study also the field induced dichroism of the suspensions, using the same sample, sealed in a flat optical glass capillary, and the same experimental setup, mounted on a polarizing microscope. The good agreement between the induced order parameter values measured by the two techniques (see Figure 1) demonstrates that the dichroism is a useful alternative when a direct measurement of the birefringence is impractical. Moreover, in some cases the dichroism gives more information, because the two independent components of the absorption are measured separately, and not only their difference (as in the birefringence case). Indeed, the isotropic part of the absorption is a useful local probe for the volume fraction of the particles in inhomogeneous samples, and also it enables to measure the induced order parameter even when the light propagates parallel to the field (in polarised or unpolarised light). We discuss potential technological applications of the field induced order in the isotropic phase of colloidal suspensions and the required improvement of the materials for their practical realization.Acknowledgement: This work is supported by the ANR (France) through the grant NASTAROD. Figure 1: Field-induced order parameter in 1.0 % aqueous goethite suspension.References: [1]B. J. Lemaire, et al., Phys. Rev. Lett. 88, 125507 (2002).[2]B. J. Lemaire, et al., Eur. Phys. J. E 13, 309-319 (2004).[3]I. Dozov, et al., J. Phys. Chem. B 115, 7751-7765 (2011)

Reactive optical matter: light-induced motility in electrodynamically asymmetric nanoscale scatterers

From Newton’s third law, which is known as the principle of actio et reactio1, we expect the forces between interacting particles to be equal and opposite for closed systems. Otherwise, “nonreciprocal” forces can arise.2 This has been shown theoretically in the interaction between dissimilar optically trapped particles that are mediated by an external field.3 As a result, despite the incident external field not having a transverse component of momentum, the particle pair experiences a force in a direction that is transverse to the light propagation direction.3,4 In this letter, we directly measure the net nonreciprocal forces in electrodynamically interacting asymmetric nanoparticle dimers and nanoparticle structures that are illuminated by plane waves and confined to pseudo one-dimensional geometries. We show via electrodynamic theory and simulations that interparticle interactions cause asymmetric scattering from heterodimers. Therefore, the putative nonreciprocal forces are actually a consequence of momentum conservation. Our study demonstrates that asymmetric scatterers exhibit directed motion due to the breakdown of mirror symmetry in their electrodynamic interactions with external fields