Y.-Y. Chang

Photorefractive effect in nematic-clay nanocomposites.

The orientational photorefractive effect was observed in an organic-inorganic nanocomposite of nematic liquid crystal hybridized with montmorillonite clay. Both the self-diffraction and beam-coupling effects were evaluated in a two-wave-mixing experiment in conjunction with an externally applied dc field. The experimental results indicate that photoinduced generation was enhanced by the addition of smectite clay with adequate concentration. Physically, the drifting ion charges were trapped by clay layers and separated by interlayer cations, creating an internal, spatially modulated space-charge field, which led to nematic molecular orientation and, then, refractive-index modulation via the electro-optical response. The diffraction efficiency as well as the beam-coupling ratio of the phase gratings recorded in the cells of the nematic liquid crystal hybridized with montmorillonite clay was found to be two to three times higher than that in the pristine nematic cell.

Electrical and Electro-Optical Properties of Nematic-Liquid-Crystal–Montmorillonite-Clay Nanocomposites

We report a new organic–inorganic hybrid composite comprising both calamitic liquid crystal and nanoscale montmorillonite clay. The electrical and electro-optical properties of such a nanocomposite confined in a homogeneously-aligned cell were observed by voltage–capacitance, voltage–transmittance, and time–transmittance measurements when dc voltage was applied across the cell thickness. Experimental evidence suggested that the addition of a small amount of pristine clay substantially improves the electro-optical characteristics of pure liquid crystals by trapping impurity ion charges in the bulk and by promoting orientational order.

Diffraction-Enhanced Beam-Focusing for X-rays in Curved Multi-Plate Crystal Cavity

Unusual x-ray focusing effect is reported for parabolic curved multi-plate x-ray crystal cavities of silicon consisting of compound refractive lenses (CRL). The transmitted beam of the (12 4 0) back reflection near 14.4388 keV from these monolithic silicon crystal devices exhibits extraordinary focusing enhancement, such that the focal length is reduced by as much as 18% for 2-beam and 56% for 24-beam diffraction from the curved crystal cavity. This effect is attributed to the presence of the involved Bragg diffractions, in which the wavevector of the transmitted beam is bent further when traversing several curved crystal surfaces.

Inclined-incidence hard-X-ray resonator with ultrahigh efficiency and resolution.

We report a high-efficiency hard-X-ray resonator with inclined-incidence geometry. A beam incident at 36.87° with respect to [3 1 0] excites Bragg back diffraction along (12 4 0) at 14.4388 keV for resonance in a Si-based resonator to produce intense resonance fringes. The experimental results showed the visibility enhanced by nearly 30 times compared with normal incidence. Also numerical calculations of the inclined-incidence resonator demonstrate ultrahigh efficiency and extremely narrow resolving power (sub-meV) with low background. This geometry surpasses the intrinsic limits of normal-incidence crystal-based resonators and enables ultrahigh-resolution X-ray optics for X-ray diffraction, spectroscopy, and imaging applications.

Dynamical diffraction effect in a curved multi-plate crystal cavity

A modified algorithm for X-ray dynamical diffraction theory is presented for curved boundary crystals and a detailed description of the numerical procedure is given. The simulated and experimental results both show an anomalous focusing behavior in a curved multi-plate crystal cavity of silicon under the (12,4,0) back-diffraction condition at a photon energy of 14.4388 keV. The focusing effects are analyzed, within the framework of the dynamical theory of X-ray diffraction, from the excitation of the dispersion surface and modification of the index of refraction with respect to the non-plane-parallel boundaries of a curved crystal cavity.

Beam suppression and focusing in multi-plate crystal cavity with compound refractive lenses

Beam suppression and focusing in multi-plate crystal cavity with compound refractive lenses Sung-Yu Chen, Y.-Y. Chang, M.-T. Tang, Yu. Stetsko, M. Yabashi, H.-H. Wu, Y.-R. Lee, B.-Y. Shew, S.-L. Chang National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsinchu, Taiwan 30013, R.O.C., Hsinchu, Taiwan, 30013, Taiwan, National Synchrotron Radiation Research Center, Hsinchu, Taiwan, R.O.C. 300, Spring-8/RIKEN Mikazuki, Hyogo, Japan, E-mail:d913303@oz.nthu. edu.tw