Enhancement of nematic order and global phase diagram of a lattice model for coupled nematic systems

We use an infinite-range Maier-Saupe model, with two sets of local quadrupolar variables and restricted orientations, to investigate the global phase diagram of a coupled system of two nematic subsystems. The free energy and the equations of state are exactly calculated by standard techniques of statistical mechanics. The nematic-isotropic transition temperature of system A increases with both the interaction energy among mesogens of system B, and the two-subsystem coupling $J$. This enhancement of the nematic phase is manifested in a global phase diagram in terms of the interaction parameters and the temperature $T$. We make some comments on the connections of these results with experimental findings for a system of diluted ferroelectric nanoparticles embedded in a nematic liquid-crystalline environment.Comment: 11 pages, 3 figures, to appear in Volume 42 of the Brazilian Journal of Physic

Spatially oriented plasmonic ‘nanograter’ structures

One of the key motivations in producing 3D structures has always been the realization of metamaterials with effective constituent properties that can be tuned in all propagation directions at various frequencies. Here, we report the investigation of spatially oriented “Nanograter” structures with orientation-dependent responses over a wide spectrum by focused-ion-beam based patterning and folding of thin film nanostructures. Au nano units of different shapes, standing along specifically designated orientations, were fabricated. Experimental measurements and simulation results show that such structures offer an additional degree of freedom for adjusting optical properties with the angle of inclination, in additional to the size of the structures. The response frequency can be varied in a wide range (8 μm–14 μm) by the spatial orientation (0°–180°) of the structures, transforming the response from magnetic into electric coupling. This may open up prospects for the fabrication of 3D nanostructures as optical interconnects, focusing elements and logic elements, moving toward the realization of 3D optical circuits

On the origin of pure optical rotation in twisted-cross metamaterials

We present an experimental and computational study of the response of twisted-cross metamaterials that provide near dispersionless optical rotation across a broad band of frequencies from 19 GHz to 37 GHz. We compare two distinct geometries: firstly, a bilayer structure comprised of arrays of metallic crosses where the crosses in the second layer are twisted about the layer normal; and secondly where the second layer is replaced by the complementary to the original, i.e. an array of cross-shaped holes. Through numerical modelling we determine the origin of rotatory effects in these two structures. In both, pure optical rotation occurs in a frequency band between two transmission minima, where alignment of electric and magnetic dipole moments occurs. In the cross/cross metamaterial, the transmission minima occur at the symmetric and antisymmetric resonances of the coupled crosses. By contrast, in the cross/complementary-cross structure the transmission minima are associated with the dipole and quadrupole modes of the cross, the frequencies of which appear intrinsic to the cross layer alone. Hence the bandwidth of optical rotation is found to be relatively independent of layer separation.The authors wish to thank Dr. Simon Horsley and Prof. Roy Sambles for their helpful discussions. A.D.-R., J.C. and J.S.-D. acknowledge the support by the Ministerio de Economica y Competitividad of the Spanish government, and the European Union FEDER through project TEC2014-53088-C3-1-R. L.E.B. and A.P.H. acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for Doctoral Training in Electromagnetic Metamaterials (Grant No. EP/L015331/1). E.H. wishes to acknowledge support from the EPSRC (Grant No. EP/K041215/1).Barr, LE.; Díaz Rubio, A.; Tremain, B.; Carbonell Olivares, J.; Sánchez-Dehesa Moreno-Cid, J.; Hendry, E.; Hibbins, AP. (2016). On the origin of pure optical rotation in twisted-cross metamaterials. Scientific Reports. 6:30307-30307. https://doi.org/10.1038/srep30307S30307303076Li, Z. et al. Coupling effect between two adjacent chiral structure layers. Opt. Exp. 18, 5375–5383 (2010).Rogacheva, A. V., Fedotov, V. A., Schwanecke, A. S. & Zheludev, N. I. Giant gyrotropy due to electromagnetic-field coupling in a bilayered chiral structure. Phys. Rev. Lett. 97, 177401 (2006).Barron, L. D. Molecular Light Scattering and Optical Activity 2nd Ed, Cambridge University Press (2004).Li, Z. et al. Chiral metamaterials with negative refractive index based on four “U” split ring resonators. App. Phys. Lett. 97, 081901 (2010).Monzon, C. & Forester, D. W. Negative refraction and focusing of circularly polarized waves in optically active media. Phys. Rev. Lett. 95, 123904 (2005).Pendry, J. B. A chiral route to negative refraction. Science 306, 1353–1355 (2004).Gao, W. & Tam, W. Y. Optical activities in complementary double layers of six-armed metallic gammadion structures. J. Opt. 13, 015101 (2011).Dong, J., Zhou, J., Koschny, T. & Soukoulis, C. Bi-layer cross chiral structure with strong optical activity and negative refractive index. Opt. Exp. 17, 14172–14179 (2009).Zhou, J. et al. Negative refractive index due to chirality. Phys. Rev. B. 79, 121104 (2009).Decker, M. et al. Strong optical activity from twisted-cross photonic metamaterials. Opt. Lett. 34, 2501–2503 (2009).Li, Z., Alici, K. B., Colak, E. & Ozbay, E. Complementary chiral metamaterials with giant optical activity and negative refractive index. App. Phys. Lett. 98, 161907 (2011).Hannam, K., Powell, D. A., Shadrivov, I. V. & Kivshar, Y. S. Dispersionless optical activity in metamaterials. App. Phys. Lett. 102, 201121 (2013).Hannam, K., Powell, D. A., Shadrivov, I. V. & Kivshar, Y. S. Broadband chiral metamaterials with large optical activity. Phys. Rev. B. 89, 125105 (2014).Li, Y. & Hung, Y. Dispersion-free broadband optical polarisation based on helix photonic metamaterials. Opt. Exp. 23, 16772 (2015).Zhu, W., Rukhlenko, I. D., Huang, Y., Wen, G. & Premaratne, M. Wideband giant optical activity and negligible circular dichroism of near-infrared chiral metamaterial based on a complementary twisted configuration. J. Opt. 15, 125101 (2013).ANSYS Electromagnetics Suite Release 15.0, ANSYS Inc., Pittsburgh, USA URL http://www.ansys.com .Luk’yanchuk, B. et al. The Fano Resonance in Plasmonic Nanostructures and Metamaterials. Nat. Mat. 9, 707–715 (2010).Kenanakis, G., Economou, E. N., Soukoulis, C. M. & Kafesaki, M. Controlling THz and Far-IR Waves with Chiral and Bianisotropic Metamaterials. EPJ Appl. Meta. 2, 1–12 (2015).Genet, C. & Ebbesen, T. W. Light in Tiny Holes. Nature 445, 39–46 (2007).Grigorenko, A. N., Nitkin, P. I. & Kabashin, A. V. Phase jumps and interferometric surface plasmon resonance imaging. App. Phys. Lett. 75, 3917–3919 (1999).Gorkunov, M. V. et al. Implications of the causality principle for ultra chiral metamaterials. Sci. Rep. 5, 1–5 (2015)