An RNAi screen picks Tuba out of the GTPase exchange factor (GEF) orchestra as a regulator of cell polarity in epithelial morphogenesis. (See also a companion paper from Rodriguez-Fraticelli et al., in this issue.)

We have designed and fabricated a novel nonlinear optical superlattice of LiTaO 3 in which two antiparallel 180 ± domains building blocks A and B were arranged as a Fibonacci sequence. We measured the quasi-phase-matched second-harmonic spectrum of the superlattice. The second-harmonic blue, green, red, and infrared light generation, with energy conversion efficiencies of ϳ5% 20%, was demonstrated experimentally, which efficiencies are comparable with those of a periodic superlattice. Destruction of self-similarity and extinction phenomenon have also been observed in the spectrum. The experiment results are in good agreement with theory. [S0031-9007(97)02784-1] PACS numbers: 42.65. Ky, 77.80.Dj, An important development in condensed-matter physics is the discovery of quasicrystalline structure [1]. Much effort has been devoted to the studies of structure and physical properties of quasicrystal [2,3]. A quasiperiodic superlattice is an analog of one-dimensional quasicrystal. The first quasiperiodic semiconductor superlattice was fabricated by Merlin et al. by molecular-beam epitaxy in 1985 [4]. Since then metallic and dielectric Fibonacci superlattices have been produced by various techniques [5][6][7]. These superlattices have shown many unusual physical properties depending on their composition and layer thickness.In dielectric crystals, the most important physical processes are the excitation and the propagation of classical waves, including optical waves and acoustic waves. Ultrasonic excitation and propagation in the quasiperiodic acoustic superlattices have been studied both theoretically and experimentally [8]. More recently, the localization of optical waves in a quasiperiodic optical superlattice (QPOS) of SiO 2 and TiO 2 has been reported [9]. For second-order nonlinear optical effects of the QPOS, some preliminary theoretical work has been carried out [10]. It has been discovered that the second harmonic spectrum of a QPOS is different from that of a periodic optical superlattics (POS) due to its lower space-group symmetry. According to the theory of quasiphase-matching (QPM) proposed by Armstrong et al. [11], the phase matching condition in the second harmonic process of a QPOS can be written intowhere k 2v , k v are the wave vectors of the second harmonic and fundamental waves, respectively, G m,n is the reciprocal vector (called the "grating wave vector" in nonlinear optics) which depends on the structure parameter of a QPOS. In a Fibonacci system, two incommensurate periods with ratio t are superimposed. The indexing of G m,n requires two integers m, n, which is different from the POS's reciprocal vector G n indexed with only one integer. Therefore, a QPOS can provide more reciprocal vectors to the QPM optical parametric process, which results in the second harmonic spectrum of a QPOS showing more plentiful spectrum structure than that of a POS. This characteristic of the QPOS may be used in multiwavelength laser frequency conversion application. However, up to now, this has not been experimentally ...

We have developed a facile and compatible method to in situ fabricate uniform metal nanowire networks on substrates. The as-fabricated metal nanowire networks show low sheet resistance and high transmittance (2.2 Ω sq(-1) at T = 91.1%), which is equivalent to that of the state-of-the-art metal nanowire networks. We demonstrated that the transmittance of the metal networks becomes homogeneous from deep-ultraviolet (200 nm) to near-infrared (2000 nm) when the size of the wire spacing increases to micrometer size. Theoretical and experimental analyses indicated that we can improve the conductivity of the metal networks as well as keep their transmittance by increasing the thickness of the metal films. We also carried out durability tests to demonstrate our as-fabricated metal networks having good flexibility and strong adhesion.