Crystal Growth Transition from Flat-On to Edge-On Induced by Solvent Evaporation in Ultrathin Films of Polystyrene-b-Poly(ethylene oxide)

The transition of lamellar crystal orientation from flat-on to edge-on in ultrathin films of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) via solvent vapor (toluene) treatment Was investigated. When the as-prepared film was treated in saturated solvent vapor, breakout crystals could form quickly, and then they transformed from square single crystals (flat-on lamellae) to dendrites and finally to nanowire crystals (edge-on lamellae). Initially, heterogeneous nucleation tit the polymer/substrate interface dominated the structure evolution, leading to flat-on lamellar crystals orientation. And the transition from faceted habits to dendrites indicated a transition of underlying mechanism from nucleation-controlled to diffusion-limited growth. As the solvent molecules gradually diffused into the polymer/substrate interface, it will subsequently weaken the polymer-substrate interaction.

Characteristics of the conductive polyimide film surfaces induced by ultraviolet laser beam

A conducting layer with the conductivity of 1.2 Omega(-1)cm(-1) stripped in a solvent from KrF-laser-irradiated polyimide thin film is taken as a sample to determine the microstructure of the conducting layer. Fourier-transform infrared and X-ray photoelectron spectroscopies show the formation of the carbon-rich clusters after irradiation. The element analysis gives the atomic ratio of C:H:N:O for the carbon-rich cluster as 60:20:3:1. Wide-angle X-ray diffraction indicates that the conducting layer is mainly amorphous carbon with a small amount of the short-range ordered carbon-rich clusters. This study suggests a structural model with three-layer carbon sheets linked together in a random fashion for the short-range ordered carbon-rich clusters. The interplanar spacing is 3.87 Angstrom and the layer diameter 25 Angstrom. The transport model of variable-range hopping in three dimensions is used to explain the conducting behavior of the conducting layer. In our case, the short-range ordered carbon-rich clusters are assumed to be conducting islands dispersed in the amorphous carbon-rich cluster matrix.

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The surface sites of sulfated zirconia studied in situ by laser-induced fluorescence spectroscopy

The surface sites of sulfated zirconia were investigated in situ by laser-induced fluorescence spectroscopy using aniline as the probe molecule. Different from the cases for many other oxides, the aniline adsorbed on the unique active sites of sulfated zirconia at r.t. is changed into another species, which emits a characteristic fluorescence band at 422 nm. The results illustrate that the sulfate groups in sulfated zirconia are favorable for the generation of these unique active sites, which also rarely exist on pure zirconia composed of tetragonal and monoclinic phases but do not exist on pure zirconia composed of monoclinic phase. (C) 2004 Elsevier B.V. All rights reserved.

Metallographic Sample Prepared by Ion Beam Etching

Ion beam etching technique was used to reveal the metallograhpic microstructure and interface morphology of electroplating chromium coating, in particular, whose substrate surface layer was treated in advance by laser quenching. Chemical etchings were, also conducted for comparison. The reveal microstructures were observed and analyzed by scanning electron microscopy. The results show that ion beam etching can reveal well the whole microstructures of composite coating-substrate materials.

A thermal-elastic analysis on laser-induced reverse bulging and plugging in circular brass foil

A new failure mode is observed in circular brass foils induced by laser beam. The new failure is based on the following experimental facts : (1) the peripheries of the circular brass foils are fixed and the surfaces of the foils are radiated by laser beam ; (2) the laser beam used is considered to be non-Gaussian spatially, actually an approximately uniform distribution limited in a certain size spot ; (3) the pulse on time of laser beam should be 250 ��s, i.e. so called long duration pulse laser. The failure process consists of three stages ; i.e. thermal bulging, localized shear deformation and perforation by plugging. The word reverse in reverse bulging and plugging mode means that bulging and plugging occur in the direction of incident laser beam. To study the newly-discovered type of failure quantitatively, analytical solutions for the axisymmetric temperature field and deflection curve are derived. The calculated results show that the newly discovered failure mode is attributed to the spatial structure effect of laser beam indeed.

Fabrication of two-dimensional periodic nanostructures by two-beam interference of femtosecond pulses

Two-dimensional periodic nanostructures on ZnO crystal surface were fabricated by two-beam interference of 790 nm femtosecond laser. The long period is, as usually reported, determined by the interference pattern of two laser beams. Surprisingly, there is another short periodic nanostructures with periods of 220-270 nm embedding in the long periodic structures. We studied the periods, orientation, and the evolution of the short periodic nanostructures, and found them analogous to the self-organized nanostructures induced by single fs laser beam. (C) 2008 Optical Society of America.

Femtosecond laser-induced periodic surface structure on ZnO

Three different ZnO nanostructures include nanoparticles, ripples and regular nanogratings were successfully prepared by femtosecond laser irradiation under different experimental conditions. The in-situ observation of the second harmonic generation (SHG) excited in ZnO crystals before, during, and after the formation of the nanostructures was investigated. The obtained results show that the formed nanostructures contribute to the enhancement of the SHG. We propose that the second harmonics in the sample surface plays an important role in the formation of nanostructures. (c) 2007 Published by Elsevier B.V.

Relativistic channel-coupling focusing enhanced nonlinear guiding of an intense laser beam in a plasma channel

We employ the variational method to study the optical guiding of an intense laser beam in a preformed plasma channel without using the weakly relativistic approximation. Apart from the dependence on the laser power and the nonlinear channel strength parameter, the beam focusing properties is shown also to be governed by the laser intensity. Relativistic channel-coupling focusing, arising from the coupling between relativistic self-focusing and linear channel focusing, can enhance relativistic self-focusing but its strength is weaker than that of linear channel focusing. (C) 2008 Elsevier B.V. All rights reserved.

Mechanisms of ultrafast laser-induced deep-subwavelength gratings on graphite and diamond

Deep-subwavelength gratings with periodicities of 170, 120, and 70 nm can be observed on highly oriented pyrolytic graphite irradiated by a femtosecond (fs) laser at 800 nm. Under picosecond laser irradiation, such gratings likewise can be produced. Interestingly, the 170-nm grating is also observed on single-crystal diamond irradiated by the 800-nm fs laser. In our opinion, the optical properties of the high-excited state of material surface play a key role for the formation of the deep-subwavelength gratings. The numerical simulations of the graphite deep-subwavelength grating at normal and high-excited states confirm that in the groove the light intensity can be extraordinarily enhanced via cavity-mode excitation in the condition of transverse-magnetic wave irradiation with near-ablation-threshold fluences. This field enhancement of polarization sensitiveness in deep-subwavelength apertures acts as an important feedback mechanism for the growth and polarization dependence of the deep-subwavelength gratings. In addition, we suggest that surface plasmons are responsible for the formation of seed deep-subwavelength apertures with a particular periodicity and the initial polarization dependence. Finally, we propose that the nanoscale Coulomb explosion occurring in the groove is responsible for the ultrafast nonthermal ablation mechanism.

Intense laser beam guiding in self-induced electron cavitation channel in underdense plasmas

In underdense plasmas, the transverse ponderomotive force of an intense laser beam with Gaussian transverse profile expels electrons radially, and it can lead to an electron cavitation. An improved cavitation model with charge conservation constraint is applied to the determination of the width of the electron cavity. The envelope equation for laser spot size derived by using source-dependent expansion method is extended to including the electron cavity. The condition for self-guiding is given and illuminated by an effective potential for the laser spot size. The effects of the laser power, plasma density and energy dissipation on the self-guiding condition are discussed.

Coupling efficiency of laser beam to multimode fiber

The coupling efficiency of laser beam to multimode fiber is given by geometrical optics, and the relation between the maximum coupling efficiency and the beam propagation factor M-2 is analyzed. An equivalent factor M-F(2) for the multimode fiber is introduced to characterize the fiber coupling capability. The coupling efficiency of laser beam to multimode fiber is calculated in respect of the ratio M-2/M-F(2) by the overlapping integral theory. The optimal coupling efficiency can be roughly estimated by the ratio of M-2 to M-F(2) but with a large error range. The deviation comes from the lacks of information on the detail of phase and intensity profile in the beam factor M-2. (C) 2007 Elsevier B.V. All rights reserved.