Ultrafast X-ray experiments on structural changes in single crystals of polar molecules

Intramolecular charge transfer in aminobenzonitrile (DIABN) single crystals is studied by a combination of ultrafast techniques. The intramolecular dynamics is probed by time-resolved spectroscopy in the UV/Vis and mid-IR. The intermolecular structural response is investigated by transient X-ray scattering and transmission experiments. A theoretical analysis shows that diffuse scattering and dipole solvation determine the X-ray data. The observed dynamics results from charge transfer reaction leading to strong local changes of molecular dipole moments and related structural rearrangements in the crystal.     

Non-linear dissolution of amorphous arsenic sulfide-selenide photoresist films

The properties of non-linear inorganic chalcogenide photoresists fabricated by co-evaporation of Ag with amorphous arsenic sulfide-selenide materials are considered in detail. The influence of several factors, including Ag concentration, irradiation wavelength and composition of the developer on the photoresists characteristics was studied. Superlinear dissolution characteristics of the photoresists are explained in the frame of the so-called “percolation approach”. The advantages of superlinear photoresists, especially for the case of maskless photolithography, are briefly discussed.     

Enhanced Photoluminescence of InGaN/GaN Green Light-Emitting Diodes Grown on Patterned Sapphire Substrate

Green InGaN/GaN based light-emitting diodes （LEDs） are fabricated both on planar and wet-etched patterned sapphire substrates by metalorganic vapour phase epitaxy （MOVPE）. Their photoluminescence （PL） properties of the two samples are studied. The results indicate that the PL integral intensity of the green LED on the patterned substrate is nearly two times of that on the planar one within the whole measured temperature range. The enhanced PL intensity in the green LED on the patterned substrate is shown completely contributed from the extraction efficiency, but not from the internal quantum efficiency. The conclusion is supported by temperature-dependent PL analysis on the two samples, and the mechanisms axe discussed.     

An asymmetric imidazole derivative as potential fluorescent chemosensor for Fe in aqueous solution

A novel conjugated molecule, L, based on 2,4,5-triphenylimidazole and 6-phenyl-2,2′-bipyridine (HC^∧N^∧N) was synthesized in two steps. The molecule can recognize Fe³⁺ in aqueous solution (THF/H₂O, 1/1, v/v) by the appearance of new emission bands at 416 and 442 nm, which can be attributed to the emission of the newly formed L-Fe³⁺ complex. The binding constant of the complex was calculated to be (6.6±0.39)×10³ M⁻¹, and its formation was also confirmed by the appearance of isosbestic points at 312 and 381 nm in the UV-visible spectral titration experiment. While other transition and rare-earth metal ions, such as Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Pb²⁺, Eu³⁺ and Nd³⁺, can only cause some decrease of L's fluorescence, alkali and alkaline earth metal ions, such as Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺, almost have no effect on L's fluorescence. The fluorescence of L can be recovered by the addition of EDTA to the L-Fe³⁺ system just due to EDTA's stronger chelating ability than that of L.     

Photomodification of heteroleptic titanium-based, complex metal alkoxides

A heteroleptic titanium metal alkoxide (OPy)₂Ti(4MP)₂, where OPy = NC₅H₄(CH₂O)-2 and 4MP = OC₆H₄(SH)-4, was investigated as a candidate precursor for the solution-based (sol-gel) synthesis of titanium oxide via the photoactivation of intermolecular linking reactions (e.g., hydrolysis/condensation). The evolution of the electronic structure of the solution-based molecule arising from conventional (dark) chemical reaction kinetics was compared with that of samples exposed to ultraviolet (UV) radiation at wavelengths of λ = 337.1 nm and 405 nm using UV-visible absorption spectroscopy. Photoinduced changes in the spectra were examined as a function of both the incident wavelength of exposure and the total fluence. Experimental results confirm the UV-induced modification of spectral absorption features, attributed to ligand-localized and charge transfer transitions accompanied by structural changes associated with hydrolysis and condensation. The photoenhancement of reaction kinetics in these processes was confirmed by the increased modification of the absorption features in the solution spectra, which saturated more rapidly under UV-illumination than under dark conditions. Similar saturation behaviors were observed for both the 337.1 nm and the 405 nm incident wavelengths with the same total deposited energy density indicating a relative insensitivity of the photoinduced response to excitation energy for the wavelengths and fluences studied.     

Intensity modulation in two Mach–Zehnder interferometers using plasma dispersion in silicon-on-insulator

We present comparative measurements of two Mach–Zehnder interferometers, one with Y-junction couplers and the other with MMI couplers, both developed in silicon-on-insulator technology and using plasma dispersion effect for light phase modulation. Measurements of fiber-to-fiber losses, absorption coefficient, output intensity vs. time and extinction ratio vs. frequency have been performed at λ=1.3 μm and at λ=1.55 μm. Results are reported and discussed in this paper. Received: 18 May 2001 / Revised version: 24 September2001 / Published online: 30 October 2001     

Surface modification of a polymer film by cryogenic laser ablation of organosilicon compounds

An organosilicon compound, hexamethylcyclotrisilazane (HMCTS), was photolyzed in a solid film at 84 K with an ArF excimer laser. A polymeric film was prepared as a result of polymerization and/or crosslinking using a reactive species such as 1-(methylsilyl)methanimine, generated by photolysis. At room temperature in air, the film changed into a polymer having siloxane units. However, fragments generated by laser irradiation of HMCTS in the frozen film made a similar polymer film having a siloxane structure on a PVA film in air. The polymeric film with siloxane units prepared by the cryogenic laser ablation method displayed hydrophobic properties. Received: 23 May 2001 / Accepted: 30 May 2001 / Published online: 30 August 2001     

Catalytic and non-catalytic growth of amorphous silica nanowires and their photoluminescence properties

A simple method based on the thermal oxidation of Si wafers in presence of a mixture of MgO and graphite powder was developed for large-scale synthesis of very long amorphous silica nanowires. The synthesis was done with and without gold as the catalyst. Almost aligned uniform nanowires with diameters within 60-90 nm and length up to few hundred micrometers were obtained using gold as the catalyst while bicycle chain like nanowires were obtained in absence of the catalyst. The growth sequence of the nanowires was observed through scanning electron microscope. Both forms of the nanowires emitted blue lights at 414 nm (3 eV) under excitation at 250 nm.     

Differential group delay monitoring using an all-optical signal spectrum-analyser

We demonstrate an all-optical device that monitors differential group delay (DGD) degradation of picosecond optical pulses. This device is based on an ultrafast all-optical signal analyser that uses nonlinear effects (cross-phase modulation) to transfer rapid temporal fluctuations into frequency domain effects that can be measured on an conventional optical spectrum analyser (incorporating a slow-detector). This monitoring scheme will enable rapid dynamic monitoring and compensation of DGD in ultrafast optical networks, at 160 Gb/s data rates and beyond, where electronic monitoring techniques cannot operate. We discuss the required signal polarisation condition.     

Torsional barriers and nonlinear optical properties of 2-, 3-, 4-phenylpyridine molecules

The torsional barriers and nonlinear optical properties for all phenylpyridine molecules were calculated by using Hartree-Fock (HF) theory and Becke three-parameter functional (B3LYP) hybrid approaches within the density functional theory framework with the 6-31++G(d, p) basis set, and via the GAUSSIAN 98W. The torsional barrier computations show that dihedral angle between the two rings increases with the number of H-H vicinal interactions and torsional barriers with dihedral angles for 3-, 4-phenylpyridines are too similar for both HF and B3LYP level calculations. Also, HOMO-LUMO energy gaps, polarizabilities, anisotropy of polarizabilities, and static hyperpolarizabilities are calculated as a function of dihedral angle between benzene and pyridine rings. The study reveals that the phenylpyridines show very low nonlinear optical properties. The calculated torsional barrier, equilibrium dihedral angle and molecular dipole moment results for these molecules were compared with available experimental and other results determining from different computational methods.