摘要:

Scanning shear‐force microscopy, which is usually employed to track scanning near‐field optical microscopy probes across samples, was used to observe the sub‐surface smectic layer structure through the thickness of a film of the alkyl cyanobiphenyl liquid crystal 8CB on a highly oriented pyrolytic graphite substrate from force‐distance measurements. With the noncontact imaging shear‐force microscopy supplies, it was possible to follow, in the topography, the movement of a monolayer of the 8CB molecules. This has enabled the diffusion of molecules in a precursor film of 8CB to be observed without significantly disrupting the free liquid surface. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363252

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Accurate stopping powers of polyimide have been determined for mean energies (E) in the range of 200 keV≤E≤3200 keV for4He ions and 550 keV≤E≤1550 keV for1H ions, using the transmission method for foils with areal densities of 30–225 &mgr;g/cm2. The overall uncertainties of ∼2% are mainly due to the foil thickness determination. The present data have been parametrized and compared to results obtained by previous authors in the upper energy range and to various parametrizations found in the literature. The best description of the data is given by Bragg’s rule calculations using parametrized elemental stopping powers that were derived from large data sets (deviations <2%). No support is found for the cores‐and‐bonds model showing deviations of 3%–5%, that may be due to the neglect of phase state effects. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363277

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Submicron orientational patterns were created in thin liquid crystal polymer films. A rubbed polyimide layer was locally modified by hatching it with the small tip of a simplified atomic force microscope. A nematic liquid crystal mixture, that can be spin‐coated and polymerized at room temperature was used to transfer the lithography from the polyimide into the liquid crystal polymer. Gratings as small as 240 nm periods were fabricated. The domain boundaries, which are nothing else than twist disclination lines attached to the substrate surface, influence the topography on a distance much larger than the film thickness. This topographic effect, which consists of a groove and a wall parallel to the boundary, was investigated as a function of the scratching angle, the film thickness and the size of the structure. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363253

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We investigated both time and temperature dependence of chromophore movements by means ofinsitusecond‐harmonic generation (SHG) measurements. Using real‐time detection of the SHG intensities during and after poling of various nonlinear‐optical polymer films we observed differences between main‐chain and side‐chain polymers regarding the nonlinear coefficientsd33and the glass transition dynamics. We present experimental data on new main‐chain polymers with polar stilbene chromophores attached in the most common transverse position to the backbones (MC‐T) or incorporated in a linear fashion where they are a part of the main‐chain (MC‐L). The side‐chain polymer (SC) presented here has been developed for photorefractive applications and is a copolymer containing Disperse Red 1 and a photoconducting carbazole unit in the side chain. The influence of the macromolecular structure on the chromophore dynamics is striking. This is verified by means ofinsitucorona poling experiments and thermal experiments based on temperature ramps on poled polymers. MC‐T and especially MC‐L polymers show a better thermal stability of the chromophore orientation relative to the glass transition temperature than SC polymers; however, chromophores of main‐chain polymers are, as can be expected, less mobile during the poling process. We obtained resonance enhanced nonlinear optical coefficientsd33of 100 pm/V in a SC polymer, 90 pm/V in a MC‐T polymer, and 30 pm/V in a MC‐L polymer. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363254

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Modified rhodamine 6G molecules with polymerizable double bonds have been copolymerized with methacrylic monomers and the resulting polymers have been pumped at 337 nm with a N2laser in a transversal configuration. The preparation of these new materials is described in detail and their lasing properties are evaluated. Important increases in photostability, with lasing efficiencies similar to those found for the parent dye rhodamine 6G in ethanol solution, have been obtained for some of these materials. Lifetimes (measured as an 80% efficiency drop) in excess of 20 000 shots at repetition rates of 2 Hz have been demonstrated. Strong dependence on pump repetition rate was observed. Possible mechanisms and processes responsible for the behavior of these materials are discussed. By using a rotating system where the sample is scanned in a continuous way, the laser output remained stable, with no sign of degradation, after 500 000 shots. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363255

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

In a wavelength division multiplexed fiber system, where pulses on different wavelength beams may copropagate in a single mode fiber, the cross‐phase‐modulation (CPM) effects caused by the nonlinearity of the optical fiber are unavoidable. In other words, pulses on different wavelength beams can interact with and affect each other through the intensity dependence of the refractive index of the fiber. Although CPM will not cause energy to be exchanged among the beams, the pulse shapes and locations on these beams can be altered significantly. This phenomenon makes possible the manipulation and control of pulses copropagating on different wavelength beams through the introduction of a shepherd pulse at a separate wavelength. How this can be accomplished is demonstrated in this paper. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363256

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

A technique of impurity‐free vacancy‐induced disordering of GaAs/AlGaAs multiquantum wells (MQW) that is area selective, very reliable, and highly reproducible, has been developed. The localized compositional disordering is induced by rapid thermal annealing of the sample after it has been coated with a thin film of ‘‘spin‐on’’ glass and prebaked at 400 °C in a high purity nitrogen:oxygen (78:22) atmosphere. In order to self‐consistently determine the diffusion coefficient of the Al and Ga atoms, the photoluminescence peak is fitted to then=1 electron to heavy hole transition that corresponds to an error function potential profile caused by the diffusion. The process has been used to integrate two optical devices on a MQW structure. One is a nonlinear directional coupler all‐optical switch, and the other is an integrated Mach–Zehnder all‐optical modulator. The switching characteristics of the devices were measured using the conventional pump‐probe measurement technique. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363257

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

The third‐order nonlinear optical properties of sol–gel derived &agr;‐Fe2O3, &ggr;‐Fe2O3, and Fe3O4thin films have been investigated by the third‐harmonic generation method. Especially, the effects of the valence and coordination number of Fe ions on the third‐order nonlinear optical susceptibility, &khgr;(3), have been examined. The &khgr;(3)values of &agr;‐Fe2O3, &ggr;‐Fe2O3, and Fe3O4thin films were 5.8×10−11, 2.1×10−11, and 4.0×10−10esu, respectively, which are the highest values among inorganic oxides reported so far. It was considered that &khgr;(3)of &agr;‐Fe2O3and &ggr;‐Fe2O3was enhanced by the pair excitation process involving the simultaneous excitation of magnetically coupled two neighboring Fe3+ions as well as by virtual transition from O 2porbitals to Fe 3dorbitals while &khgr;(3)of Fe3O4was enhanced by both one‐ and three‐photon resonances. The higher second hyperpolarizability, &ggr;(Fex/yO), was obtained when Fe ions are 3+ rather than 2+ and octahedrally rather than tetrahedrally coordinated by oxygens. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363258

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

We present theoretical results for acoustic band structures for cubic arrays of spherical water balloons surrounded by a mercury host. Multiple, complete acoustic band gaps (i.e., stop bands) are found for the face‐centered cubic (fcc), body‐centered cubic (bcc), and simple‐cubic (sc) lattices. These stop bands are widest for a volume fraction of ∼24% and the corresponding gap/midgap ratios are about 0.83, 0.77, and 0.62, respectively, for fcc, bcc, and sc lattices. We stress that such a simple three‐dimensional (3D) inhomogeneous system of liquids exhibits the largest stop bands ever reported for 3D elastic as well as for dielectric composites. For mercury balloons surrounded by water the gaps obtained are surprisingly small. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363259

出版商:AIP    

年代:1996

数据来源: AIP

摘要:

Laser‐collision induced fluorescence (LCIF) is the emission of light from states that have been populated by laser excitation and a subsequent collision. By simultaneously measuring the LCIF from two different states, it is possible to determine both the electron density and temperature of the low energy bulk electrons within a plasma. This method is described in detail and has been applied in the determination of the total, temporally averaged, and spatially resolved electron density in a rf (13.56 MHz) helium discharge in the Gaseous Electronics Conference reference cell. The rf discharge was operated at pressuresP=33.3–133.3 Pa (0.25–1.0 Torr) and peak‐to‐peak voltages ofVpp=75–300 V were applied. We found the total electron density varied from 1.8×108cm−3atP=33.3 Pa andVpp=75 V to 4.0×1010cm−3atP=133.3 Pa andVpp=300 V. A comparison of results from different experiments has been made. ©1996 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.363260

出版商:AIP    

年代:1996

数据来源: AIP