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Liquid-crystalline side chain polymers have recently been used to store optical data by a local optical heating of a preoriented polymer film into the isotropic phase. The resulting macroscopically unoriented light scattering spots were frozen-in by subsequent cooling. This technique is referred to as thermorecording'**).In our studies we found a completely different method for optical data storage in liquid-crystalline polymers. It consists in inducing optically a birefringence pattern in a layer of such a polymer. A phase object results due to spatial refractive index modulation. The stored optical information is therefore invisible to the naked eye. fX-@; !--~12~-. f g 43 sA 94 n 104 i N c e N = N + A 1The liquid-crystalline polyester 1 with transitions glassy to smectic A, nematic, and isotropic as indicated (in "C), containing mesogenic groups of the azobenzene type in the side chain was used as storage medium. Its dielectric anisotropy is strongly positive due to the cyano dipoles linked to the mesogenic units. This offers the possibility to orient the liquid-crystalline polymers 1 with the director parallel to an externally applied electric field. Experimental partThe liquid-crystalline polymer 13) was oriented with the director parallel to an externally applied electric field. A cell, consisting of two conductive glass plates, separated by 7 prn polyimide spacers, was prepared and the polymer was subsequently filled-in. A uniform homeotropic orientation was achieved by applying an appropriate electric field at temperatures above the glass temperature of the polymer. A completely clear monodomain film resulted in each case.

Control of thermal radiation at high temperatures is vital for waste heat recovery and for high-efficiency thermophotovoltaic (TPV) conversion. Previously, structural resonances utilizing gratings, thin film resonances, metasurfaces and photonic crystals were used to spectrally control thermal emission, often requiring lithographic structuring of the surface and causing significant angle dependence. In contrast, here, we demonstrate a refractory W-HfO2 metamaterial, which controls thermal emission through an engineered dielectric response function. The epsilon-near-zero frequency of a metamaterial and the connected optical topological transition (OTT) are adjusted to selectively enhance and suppress the thermal emission in the near-infrared spectrum, crucial for improved TPV efficiency. The near-omnidirectional and spectrally selective emitter is obtained as the emission changes due to material properties and not due to resonances or interference effects, marking a paradigm shift in thermal engineering approaches. We experimentally demonstrate the OTT in a thermally stable metamaterial at high temperatures of 1,000 °C.

Modes of photonic crystal (PC) line-defect waveguides can have small group velocity away from the Brillouin zone edge. This property can be explained by the strong interaction of the modes with the bulk PC. An anticrossing of “index guided” and “gap guided” modes should be taken into account. To control dispersion, the anticrossing point can be shifted by the change of the PC waveguide parameters. An example of a waveguide is presented with vanishing second- and third-order dispersion.

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We show that the structure demonstrated by Feng et al. (Reports, 5 August 2011, p. 729) cannot enable optical isolation because it possesses a symmetric scattering matrix. Moreover, one cannot construct an optical isolator by incorporating this structure into any system as long as the system is linear and time-independent and is described by materials with a scalar dielectric function.