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Item type:Publication, Access status: with files , Anisotropic confinement of chromophores Induces second-order nonlinear optics in a nanoporous photonic metamaterial(Soc., 2021-02-15); ; ; ; Second-order nonlinear optics is the base for a large variety of devices aimed at the active manipulation of light. However, physical principles restrict its occurrence to non-centrosymmetric, anisotropic matter. This significantly limits the number of base materials exhibiting nonlinear optics. Here, we show that embedding chromophores in an array of conical channels 13 nm across in monolithic silica results in mesoscopic anisotropic matter and thus in a hybrid material showing second-harmonic generation (SHG). This non-linear optics is compared to the one achieved in corona-poled polymer films containing the identical chromophores. It originates in confinement-induced orientational order of the elongated guest molecules in the nanochannels. This leads to a non-centrosymmetric dipolar order and hence to a non-linear light-matter interaction on the sub-wavelength, single-pore scale. Our study demonstrates that the advent of large-scale, self-organised nanoporosity in monolithic solids along with confinement-controllable orientational order of chromophores at the single-pore scale provides a reliable and accessible tool to design materials with a nonlinear meta-optics.Publicationtype: Journal Article68 5 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , A ferroelectric liquid crystal confined in cylindrical nanopores : reversible smectic layer buckling, enhanced light rotation and extremely fast electro-optically active Goldstone excitations(Royal Society of Chemistry (RSC), 2017-11-27); ; ; ; The orientational and translational order of a thermotropic ferroelectric liquid crystal (2MBOCBC) imbibed in self-organized, parallel, cylindrical pores with radii of 10, 15, or 20 nm in anodic aluminium oxide monoliths (AAO) are explored by high-resolution linear and circular optical birefringence as well as neutron diffraction texture analysis. The results are compared to experiments on the bulk system. The native oxidic pore walls do not provide a stable smectogen wall anchoring. By contrast, a polymeric wall grafting enforcing planar molecular anchoring results in a thermal-history independent formation of smectic C* helices and a reversible chevron-like layer buckling. An enhancement of the optical rotatory power by up to one order of magnitude of the confined compared to the bulk liquid crystal is traced to the pretransitional formation of helical structures at the smectic-A*-to-smectic-C* transformation. A linear electro-optical birefringence effect evidences collective fluctuations in the molecular tilt vector direction along the confined helical superstructures, i.e. the Goldstone phason excitations typical of the para-to-ferroelectric transition. Their relaxation frequencies increase with the square of the inverse pore radii as characteristic of plane-wave excitations and are two orders of magnitude larger than in the bulk, evidencing an exceptionally fast electro-optical functionality of the liquid-crystalline-AAO nanohybrids.Publicationtype: Journal ArticleScopus© Citations 25 397 533 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Orientational order in liquids upon condensation in nanochannels: an optical birefringence study on rodlike and disclike molecules in monolithic mesoporous silica(American Physical Society (APS), 2010-12-02); ; ; We present high-resolution optical birefringence measurements upon sequential filling of an array of parallel-aligned nanochannels (14~nm mean diameter) with rod-like (acetonitrile) and disc-like (hexafluorobenzene) molecules. We will demonstrate that such birefringence isotherms, when performed simultaneously with optically isotropic and index-matched counterparts (neopentane and hexafluoromethane), allow one to characterize the orientational state of the confined liquids with a high accuracy as a function of pore filling. The pore condensates are almost bulk-like, optically isotropic liquids. For both anisotropic species we find, however, a weak orientational order (of a few percent at maximum) upon film-condensation in the monolithic mesoporous membrane. It occurs upon formation of the second and third adsorbed layer, only, and vanishes gradually upon onset of capillary condensation. Presumably, it originates in the breaking of the full rotational symmetry of the interaction potential at the cylindrical, free liquid-vapor interface in the film-condensed state rather than at the silica-liquid interface. This conclusion is corroborated by comparisons of our experimental results with molecular dynamics simulations reported in the literature.Publicationtype: Journal Article13 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Continuous paranematic-to-nematic ordering transitions of liquid crystals in tubular silica nanochannels(American Physical Society (APS), 2008-10-02); ; ; ; The optical birefringence of rod-like nematogens (7CB, 8CB), imbibed in parallel silica channels with 10 nm diameter and 300 micrometer length, is measured and compared to the thermotropic bulk behavior. The orientational order of the confined liquid crystals, quantified by the uniaxial nematic ordering parameter, evolves continuously between paranematic and nematic states, in contrast to the discontinuous isotropic-to-nematic bulk phase transitions. A Landau-de Gennes model reveals that the strength of the orientational ordering fields, imposed by the silica walls, is beyond a critical threshold, that separates discontinuous from continuous paranematic-to-nematic behavior. Quenched disorder effects, attributable to wall irregularities, leave the transition temperatures affected only marginally, despite the strong ordering fields in the channels.Publicationtype: Journal Article11