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We report that planar chiral structures affect the polarization state of light in a manner similar to three-dimensional chiral (optical active) media. In experiments with artificial metal-on-silicon chiral planar gratings of 442 wallpaper group symmetry, containing millions of chiral elements per square centimeter, we observed rotation of the polarization azimuth in excess of 30 of light diffracted from it. The rotation was found to change its sign for two enantiomeric forms of the media and to have components associated with both the structural arrangement and the chirality of individual structural elements. DOI: 10.1103/PhysRevLett.90.107404 PACS numbers: 78.67.-n, 78.20.Ek The ability of left-right asymmetrical (chiral) threedimensional molecules to rotate the polarization state of light known as optical activity is one of the most remarkable effects in optics that has been extensively studied since its discovery at the beginning of the 19th century. An optical active medium (for example, a medium consisting of randomly oriented helixlike molecules) will show opposite signs of polarization azimuth rotation for the two mirror-symmetric (enantiomeric) forms of the constituting molecule. The general concept of chirality also exists in two dimensions [1][2][3][4], where a planar object is said to be chiral if it cannot be brought into congruence with its mirror image unless it is lifted from the plane. One could therefore envisage a planar chiral medium that consists of ''flat'' chiral elements possessing no line of symmetry in the plane. So far, there have been only a few theoretical publications on the optical manifestations of such planar chirality. Hecht and Barron predicted incoherent circular differential Rayleigh and Raman light scattering from an ensemble of planar chiral molecules [5]. They showed that genuine strong chiral scattering phenomena could be generated through pure electric dipole interactions (in comparison with the much weaker processes involving magneto-dipole interaction in threedimensional chirality), while Arnaut and Davis calculated the scattered fields from the two-dimensional chiral structure of a metallic wire gammadion and found rotation of the polarization azimuth of the scattered field [6]. However, there have as yet been no reports of experimental observations of any optical manifestations of planar chirality, apart from the observation of a random chiral component in a highly localized near-field polarization effect in metallic fractal aggregates [7]. Consequently, it has yet to be shown whether planar chiral media could affect the far-field polarization state of light scattered on it in a manner similar to three-dimensional chiral media when the polarization effect is sensitive to the handedness of the structure. Here we report that we have manufactured left-and right-handed forms of a regular artificial medium consisting of microscopic chiral metallic objects distributed regularly in a plane, with a density of several millions per square centimeter. In this artificial medi...

Liquid crystal 'blue phases' are highly fluid self-assembled three-dimensional cubic defect structures that exist over narrow temperature ranges in highly chiral liquid crystals. The characteristic period of these defects is of the order of the wavelength of visible light, and they give rise to vivid specular reflections that are controllable with external fields. Blue phases may be considered as examples of tuneable photonic crystals with many potential applications. The disadvantage of these materials, as predicted theoretically and proved experimentally, is that they have limited thermal stability: they exist over a small temperature range (0.5-2 degrees C) between isotropic and chiral nematic (N*) thermotropic phases, which limits their practical applicability. Here we report a generic family of liquid crystals that demonstrate an unusually broad body-centred cubic phase (BP I*) from 60 degrees C down to 16 degrees C. We prove this with optical texture analysis, selective reflection spectroscopy, Kössel diagrams and differential scanning calorimetry, and show, using a simple polarizer-free electro-optic cell, that the reflected colour is switched reversibly in applied electric fields over a wide colour range in typically 10 ms. We propose that the unusual behaviour of these blue phase materials is due to their dimeric molecular structure and their very high flexoelectric coefficients. This in turn sets out new theoretical challenges and potentially opens up new photonic applications.

A systematic study on the parameter space of graphene CVD on polycrystalline Cu foils is presented, aiming at a more fundamental process rationale in particular regarding the choice of carbon precursor and mitigation of Cu sublimation. CH 4 as precursor requires H 2 dilution and temperatures ≥1000°C to keep the Cu surface reduced and yield a high quality, complete monolayer graphene coverage. The H 2 atmosphere etches as-grown graphene, hence maintaining a balanced CH 4 /H 2 ratio is critical. Such balance is more easily achieved at low pressure conditions, at which however Cu sublimation reaches deleterious levels. In contrast, C 6 H 6 as precursor requires no reactive diluent and consistently gives similar graphene quality at 100-150°C lower temperatures. The lower process temperature and more robust processing conditions allow the problem of Cu sublimation to be effectively addressed. Graphene formation is not inherently self-limited to a monolayer for any of the precursors. Rather, the higher the supplied carbon chemical potential the higher the likelihood of film inhomogeneity and primary and secondary multilayer graphene nucleation. For the latter, domain boundaries of the inherently polycrystalline CVD graphene offer pathways for a continued carbon supply to the catalyst. Graphene formation is significantly affected by the Cu crystallography, i.e. the evolution of microstructure and texture of the catalyst template form an integral part of the CVD process.

A promising approach to the fabrication of materials with nanoscale features is the transfer of liquid-crystalline structure to polymers. However, this has not been achieved in systems with full three-dimensional periodicity. Here we demonstrate the fabrication of self-assembled three-dimensional nanostructures by polymer templating blue phase I, a chiral liquid crystal with cubic symmetry. Blue phase I was photopolymerized and the remaining liquid crystal removed to create a porous free-standing cast, which retains the chiral three-dimensional structure of the blue phase, yet contains no chiral additive molecules. The cast may in turn be used as a hard template for the fabrication of new materials. By refilling the cast with an achiral nematic liquid crystal, we created templated blue phases that have unprecedented thermal stability in the range -125 to 125 °C, and that act as both mirrorless lasers and switchable electro-optic devices. Blue-phase templated materials will facilitate advances in device architectures for photonics applications in particular.