Binary phase gratings for couplers used in fiber-optic communications

Star couplers and bidirectional couplers for use in fiber optic networks have been constructed using binary phase transmission gratings. The principles of operation and the performance of these devices are discussed. Experimental results are shown which agree with the theoretical predictions to an accuracy of 0.5 dB.     

Cyclic cluster growth in ferroelectric perovskites

We prove that point defect clustering in perovskite ferroelectrics and the subsequent loss in switchable polarization can occur only if the depolarizing fields are locally unscreened during certain fractions of time in each of the multiple bipolar electric field cycles.     

Synthesis and self-assembly of a rigid exotopic bisphenanthroline macrocycle: surface patterning and a supramolecular nanobasket

The synthesis and characterisation of a rigid nanoscale macrocycle with two exotopic phenanthroline binding sites is reported. Scanning tunnelling microscopy (STM) at the solid-liquid interface reveals the formation of highly ordered monolayers of macrocycles with dimensions that are in good agreement with the calculated structure. Using the HETPHEN concept several bisheteroleptic coordination complexes with other phenanthrolines and a nanoscale basket assembly were prepared in presence of copper(I) ions. NMR spectroscopy, mass spectrometric data and elemental analysis point to three distinct isomers of the basket assembly in solution. A silver basket was prepared and readily converted to its copper analogue. Electrospray ionisation (ESI)-MS and spectrophotometric investigations provided additional mechanistic insight into the assembly process. Hence, the exotopic bisphenanthroline macrocycle in combination with HETPHEN concept proves to be very effective in controlling the compositional aspects of multicomponent self-assembly.     

Fully extended polyelectrolyte-amphiphile complexes adsorbed on graphite

In a polyelectrolyte-amphiphile complex, where the amphiphilic component is known to self-assemble on graphite, the polyelectrolyte adsorbs in a predictable way as straight, fully extended macromolecule.     

The self-assembly of lipophilic guanosine derivatives in solution and on solid surfaces

The self-assembly of lipophilic deoxyguanosine derivatives 1 and 2 has been studied in solution by NMR spectroscopy and ESI-MS (electrospray ionization mass spectrometry). NMR data show the existence of two types of self-assembled, ribbonlike structures (A and B), which are connected at the guanine moieties through two different H-bonded networks. The first species (A), which is stable in the solid state and characterised by cyclic NH(2)-O(6) and NH(1)-N(7) hydrogen bonds, is detected soon after dissolving the polycrystalline powder in rigorously anhydrous CDCl3. In solution it slowly undergoes a structural transition towards a thermodynamically stable ribbon characterised by NH(1)-O(6) and NH(2)-N(3) cyclic hydrogen bonds (B). On the other hand, at surfaces, self-assembled ribbon nanostructures have been grown from solutions of derivative 1 both on mica and at the graphite-solution interface. They have been investigated by means of tapping mode scanning force microscopy (SFM) and scanning tunnelling microscopy (STM), respectively. SFM revealed dry, micrometer-long nanoribbons with a molecular cross-section. while STM imaging at submolecular resolution indicates a molecular packing of type A, like the one detected in the solid state. This indicates that, upon adsorption at the solid-liquid interface, the guanosine moieties undergo a structural rearrangement from a B-type to an A-type ribbon.     

Asymptotics of the Smallest Singular Value of a Class of Toeplitz-generated Matrices II

Square matrices of the form ${X_n = T_n + f_n(T_n^{-1})^*}$ , where T _n is a ${n \times n}$ invertible banded Toeplitz matrix and f _n some positive sequence are considered. Convergence via an order estimate is proven for the difference of ${\|X_n^{-1}\|}$ and a function depending only on f _n . Fredholmness of the infinite counterpart of T _n is shown to greatly affect this result. A correction of a proof in the paper on which the current research is based, is appended as well.     

Light-orchestrated macromolecular "accordions": reversible photoinduced shrinking of rigid-rod polymers

Light can play: Irradiation causes dramatic changes in the shape of rigid-rod polymers incorporating azobenzene photochromes in the main chain. The embedded photoswitches act as hinges, which upon light-induced isomerization lead to reversible shrinking and stretching of the polymer backbone (see scheme), resembling light-orchestrated macromolecular accordions.     

Predicting polarization and nonlinear dielectric response of arbitrary perovskite superlattice sequences

We carry out first-principles calculations of the nonlinear dielectric response of short-period ferroelectric superlattices. We compute and store not only the total polarization, but also the Wannier-based polarizations of individual atomic layers, as a function of the electric displacement field, and use this information to construct a model capable of predicting the nonlinear dielectric response of an arbitrary superlattice sequence. We demonstrate the successful application of our approach to superlattices composed of SrTiO3, CaTiO3, and BaTiO3 layers.