Search

By means of plastic deformation and subsequent annealing of a Fe-Mn alloy, strain-induced ɛ martensite laths transform into austenite nano-scale twins in the form of bundles. This provides a potential approach to prepare bulk nano-twinned materials that are expected to exhibit high strength and good ductility. The transformation mechanism from ɛ martensite laths to austenite nano-scale twins is discussed.

We simulate boron on Pb (1 1 0) surface by using ab initio evolutionary methodology. Interestingly, the two-dimensional (2D) Dirac Pmmn boron can be formed because of good lattice matching. Unexpectedly, by increasing the thickness of 2D boron, a three-bonded graphene-like structure ( P 2 1 / a boron) was revealed to possess double anisotropic Dirac cones. It is 20 meV/atom lower in energy than the Pmmn structure, indicating the most stable 2D boron with particular Dirac cones. The puckered structure of P 2 1 / a boron results in the peculiar Dirac cones, as well as substantial mechanical anisotropy. The calculated Young’s modulus is 320 GPa·nm along zigzag direction, which is comparable with graphene.

The quantum chemical calculation method is used in this paper to study the energy difference between the highest occupied molecular orbitals and lowest unoccupied molecular orbitals of three bisphenol F (BPF) isomers. The results show that the minimum energy difference occurs at 4,4'-dihydroxydiphenylmethane (4,4′-BPF); the middle energy difference occurs at 2,4'-dihydroxydiphenylmethane (2,4′-BPF); and the maximum energy difference occurs at 2,2'-dihydroxydiphenyl- methane (2,2′-BPF). Based on these results, the electron transition in 4,4′-BPF is easier than that in 2,4′-BPF, while the electron transition in 2,2′-BPF is the most difficult. This paper investigates the effect of the hydroxyl group, specifically concerning the spectroscopic properties of BPF isomers by observing the solvent effect, hydroxyl group amount and the element substitution of an oxygen atom in the hydroxyl group. The solvent effect results show that infrared spectroscopy intensity increases significantly for 4,4′-BPF in the polar solvent (methanol and water), but does not increase proportionally to the electric constant increase of the polar solvent. The multi hydroxyl substitution of BPF isomers results show that with an increase in the hydroxyl group amount, new characteristic peaks occur in FTIR spectra and Raman spectra, while the characteristic peaks in UV spectra are smoother in the range of 240–300 nm. The element substitution results show that the red shifts occur in a different element substituted 4,4′-BPF isomer in UV spectra, and the red shifts order is opposite of that involving substitution element electronegativity.

Nano-twinned austenite grains were characterized in an austenitic stainless steel by using nanoindentation. The nano-twinned austenite grains exhibit identical elastic modulus with the micro-sized recrystallized austenitic grains. Nanoindentation hardness of the nano-twinned grains is about 4.5 GPa, ∼1.0 GPa higher than that of the recrystallized grains, but lower than that of the nano-sized grains with the comparable size owing to the presence of twinning partial dislocations.

We study the nonparaxial propagation behavior of Laguerre–Gaussian (LG) beams under the tight focusing condition by using the Rayleigh–Sommerfeld integrals. We obtain an analytical formula and show the focal shift with respect to the geometric focus of a high numerical-aperture objective. An analytical expression for the focal shift of a tightly focused LG beam increases with the focal length while decreases with the beam waist. This approach can be extended to deal with the tight focusing field and the focal shift of LG vector fields with space-variant polarization distributions or other focusing behaviors such as the 4π focusing and the Fresnel zone plates.

a b s t r a c t The nudged elastic band (NEB) method, as a popular technique for studying reaction paths due to its efficiency, has not been extensively used in solid state physics because of the need to deal with the variation of the unit cell during solid-solid transformations. Here we present an extended technique— a variable-cell NEB (VC-NEB) technique combined with the ab initio method, implemented in the USPEX code. Our technique is applied to reconstructive solid-solid phase transitions of GaN: from wurtzite to rocksalt (B4 → B1) and from zincblende to rocksalt (B3 → B1). As a more challenging application, we study the mechanism of the recently predicted insulator-metal phase transition of BH. The results reveal that the VC-NEB technique is an efficient and general method and should have wide applications for studying the paths and mechanisms of reconstructive phase transitions. Results of the VC-NEB method can be considered as a starting point for a more sophisticated treatment using the transition path sampling method, the main prerequisite of which is to have an initial transformation trajectory.

The far-field and near-field properties of a spherical nematic liquid crystal (NLC) coated metal nanoparticle (NPs) have been investigated in an external field, basing on the quasistatic theory. The resonant wavelength is tunable by varying metallic material of core, anisotropy extent and thickness of liquid crystals (LCs). The field enhancement is along the incident polarization near the outer surface of the shell. The direction of field is reverse in the inner surface comparing with the one if outer shell. In contrast to isotropy shell, the surface plasmon resonance (SPR) shows an obvious red shift and field enhancement near outer surface of the shell always is stronger.

We theoretically investigated on the focal shift of flat-topped beams (FTBs) passing through a lens system with or without aperture. We find that the position of on-axis peak intensity of focused FTBs strongly depends on the Fresnel number, the flatness order of FTBs, the truncation of the aperture, as well as the relative incident distance. The results indicate that the focal shift away from the geometrical focus of the focused FTBs appears when the relative incident distance does not equal to 1 and increases with the decreasing of the Fresnel number or the beam flatness order for both the apertured and unapertured lens systems. In the lens system with aperture, the focal shift effect decreases with the increase of the truncation parameter.

An austenitic Fe-25Mn steel with a low stacking fault energy was subjected to dynamic plastic deformation (DPD) followed by thermal annealing. The as-DPD sample is structurally characterized by a mixed nanostructure consisting of nanosized grains with an average size of 43 nm and bundles of nanoscale twins (with an average twin/matrix lamella thickness of 5 nm), as well as some dislocation structures, which exhibits a high yield strength of about 1470 MPa but a limited tensile ductility. Thermal annealing leads to static recrystallization (SRX) of the nanostructures forming a hierarchical structure of nanotwinned grains embedded in microsized SRX grains, owing to the higher thermal stability of the nanotwinned bundles than that of nanosized grains. With an increasing number of SRX grains the yield strength and ultimate tensile strength drop while the tensile ductility increases. The calculated yield strength of the nanotwinned grains is about 1.5 GPa, much lower than that determined from Hall-Petch strengthening extrapolated to the nanoscale. Work hardening rates of the nanotwin grains, comparable with that of the microsized grains, are higher than that of the original coarse grained sample. The micrograined austenitic Fe-Mn samples strengthened by nanotwinned grains exhibit enhanced strength ductility synergy in comparison with the deformed samples. A combination of a 977 MPa yield strength with a uniform elongation of 21% is achieved in the annealed samples, well above that of the deformed samples. (C) 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

We report a theoretical investigation of the open-aperture Z-scan technique for an optically thin medium with simultaneous saturation of both two- and three-photon absorption. By using the Adomian decomposition method (ADM), we obtain the analytical expression for the normalized transmittance of the open-aperture Z-scan traces. Furthermore, the analytical results are good agreement with the numerical simulations. The results indicate that ADM is an effective yet quickly approach to evaluate the saturable properties originating from both two- and three-photon absorption.

The modified Z-scan technique using the quasi-one-dimensional slit (QODS) beam for characterizing the third-order optical nonlinearity has been reported recently. In the present work, we investigate the effect of the finite length of the slit in the QODS beam Z-scan for the practical experimental conditions, and then give an empirical expression that allows the direct estimation of nonlinear refraction coefficient from the measured normalized peak-valley transmittance difference. In particular, we explore relatively in detail the influences of the sample imperfection on the Z-scan traces, such as the hollow hole of the sample and the nonuniform nonlinearity. Compared with the other Z-scan techniques, such as the top-hat and Gaussian-beam Z-scans, we find that the QODS beam Z-scan has the great improvement for the sample imperfections. The results suggest that the QODS beam Z-scan is a more promising and useful technique for characterizing the optical nonlinearity of an imperfect sample.

We analyze the propagation of Laguerre–Gaussian (LG) beams in a cubic–quintic nonlinear medium by using the variational method. For low intensities, the results are in good agreement with the results of a numerical simulation. The characteristics of LG beams in the nonlinear medium are explored and the electric field distribution in the far field after passing through the nonlinear medium is calculated.

The third-order nonlinear optical properties of polyurethane-urea/multiwalled carbon nanotube composites (PU/MWNT) films with different MWNT concentrations are investigated by the use of the Z-scan technique at a wavelength of 532 nm with a pulse duration of 8 ns. The results reveal that the nonlinear refraction and absorption coefficients are linearly dependent on the MWNT concentration. The negative nonlinear refraction effect is validated from the closed-aperture Z-scan measurements. We find that PU/MWNT films are promising nonlinear optical materials, and the nonlinear coefficients can be controlled.

By using the Adomian decomposition method, we present the transmitted optical intensity of a nonlinear medium with the concurrence of saturable absorption (SA) and two-photon absorption (TPA) processes. We obtain the analytical expression of the open-aperture Z-scan and investigate the interplay between SA and TPA in the Z-scan trace. Through Z-scan measurements at different levels of laser intensities, both the saturable intensity and TPA coefficient could be obtained quickly yet unambiguously.

Alumina nanowire array standing on porous anodic alumina (PAA) membrane has been directly and efficiently synthesized in oxalic acid solution by one-step electrochemical anodic oxidation with changing anodic voltages. The morphology of the nanowires on PAA was observed by scanning electron microscopy (SEM). The different patterns on the surface of the sample were synthesized due to the reaction time. The fine structure of the patterns has shown that the pattern is composed of the alumina nanowire array and the morphologies of the patterns can mainly be attributed to the differences in the length of the alumina nanowires. The method of selected area electron diffraction (SAED) for transmission electron microscopy (TEM) has shown that these nanowires are amorphous. An etching model was given to explain the formation mechanism of the amorphous alumina nanowires.

We propose a new approach to generate a controllable three-dimensional (3D) optical cage by double-mode cylindrical vector beam in the vicinity of focus, by controlling the polarization state of beam. The simulation results show that the 3D optical cage of spheroid surface shape with a sharp dark region surrounded by a uniform optical barrier with a maximum deviation of 2% is achieved. The finite-difference time-domain calculation of optical force validates that such a kind of 3D optical cage has the trapping capability of the low-refractive-index particles with the size being much smaller than the light wavelength.

The hardness of α- and β-Si3 − nCnN4 (n = 0, 1, 2, 3) crystals were calculated by using our microscopic model of hardness combined with first principles calculations. The results show that the hardness progressively increased with the increase of carbon content. Both C-rich α- and β-SiC2N4 phases are novel superhard materials whose hardness is at the range of 57–61 GPa, which can rival to that of c-BN. The two Si-rich SiC2N4 phases are also superhard materials, but their hardness is just higher than 40 GPa. The C–N bonds with short length and lower ionicity play key role to the increase of hardness. The Si–N bonds in the presented SiCN phases have shorter bond length than the binary silicon nitride and also contribute to the increase of hardness.

The effects of near-surface airflow on aeolian deposits across the Shapotou region's railway-protection system were studied. We found that iso-speed lines in the protective belt gradually become less dense with increasing distance along the wind direction due to the inclined topographic relief in that direction and to changes in surface properties (particularly dune undulation). In addition, wind speed at each elevation tends to decrease, with a negative velocity variation coefficient. This coefficient was positive on windward sides of fixed dunes and smaller than that on mobile dunes; its values were negative on the leeward slope of fixed dunes, averaging −1.48% m −1 below a height of 250 cm. Windward wind profiles followed a logarithmic relationship but deviated from this relationship leeward. The mean aerodynamic roughness length ( z 0 ) of artificially vegetated surfaces was 0.00543 m, versus 0.000118 m for mobile sands. The average air drag coefficient ( C d ) of the artificial vegetation belt was 1.7 times that of mobile sands. The near-surface airflow field and aerodynamic characteristics control clay and silt contents in surface soils, which increase greatly (with decreasing median particle size) from the mobile sands upwind of the protective belt to the artificial vegetation further downwind within the belt, and sorting becomes increasingly poor. Silt content was negatively correlated with, and median particle size and sorting were positively correlated with the relative surface elevation in the protective belt. The relationship between clay content and landform was unclear. Silt content was high and sorting poor in the surface 5 cm; below this depth, silt content decreased and sorting increased. The particle-size profile varied among landforms. Estimated aeolian deposition rates around the crest of fixed dunes average 0.147 cm per year, versus more than 0.588 cm on the leeward side and in inter-dune depressions.

We present a precise data processing method, in the Z-scan experiments using a Gaussian beam and trimmed Airy beam, for directly extracting nonlinear refraction from the closed aperture Z-scan trace with the aid of the open aperture Z-scan trace when the materials exhibit nonlinear refraction and nonlinear absorption simultaneously. This method is still applicable when the nonlinear absorption is dominant and the closed aperture Z-scan curve degenerates into a single valley configuration, which is a salient advantage over other methods. In addition, we give gracefully empirical formulas with very high precision, which have good practicability for characterizing the optical nonlinearities of materials by use of the Gaussian beam and trimmed Airy beam Z-scan technique, respectively.

Vickers hardness of the cubic BC2N has been investigated using the microscopic hardness model, in which the parameters have been obtained using first principles calculations. Ionicities of the chemical bonds in the cubic BC2N depend on their surrounding chemical environments, which are included in the hardness calculations of the cubic BC2N using the population ionicity scale. For the four selected configurations of the cubic BC2N, the theoretical Vickers hardness has been found to lie between 70 and 72 GPa, consistent with the experimental value of 76 GPa. According to our calculations, it should be the cubic BC2N that ranks second among the superhard materials instead of the cubic BN.

We demonstrate high-efficiency diode-end-pumped multi-wavelength Nd:YAG lasers for continuous-wave and Q-switched operation. For the continuous-wave case, the Nd:YAG laser oscillates at 1.06 and 1.3 μm simultaneously; the maximum output power of 2.0 W ( M 2 = 1.3) and 3.6 W ( M 2 = 1.8) have been achieved at the incident pump power of 20.3 W, with the respective average slope efficiencies of 12.0% and 21.4%, for the lines of 1.06 and 1.3 μm, respectively. For the Q-switched operation, we achieve the average output power of 1.3 W ( M 2 = 2.7) at 1.06 μm and 2.0 W ( M 2 = 3.0) at 1.3 μm with the corresponding peak power of 10.2 and 4.2 kW under an incident pump power of 20.3 W.

Saturable Kerr nonlinearity is theoretically investigated by use of the top-hat beam Z-scan technique. The saturation intensity changes the nonlinear refractive profile and decreases the sensitivity of the Z-scan measurements, which were quantitatively analyzed. An empirical formula for the saturable Kerr nonlinearity, which gave the relationship between the light intensity and the peak–valley transmittance difference, was accomplished. A high-accuracy method to determine the nonlinear refractive index and the characteristic saturation intensity was proposed.

We report a method for generation of arbitrary shape and array of optical vortices by use of a superposition of coherent elementary vortices based on helical phase spatial filtering in spatial frequency domain. In this method, a helical phase spatial filter (HPSF) is placed in the spatial frequency plane of a 4-f imaging processing system. We demonstrated that the output field distribution represents the convolution between the input field and an elementary vortex field introduced by the HPSF, which results in a special shape or array of optical vortices determined by the “degenerate” properties of coherent elementary vortices and the distribution formats of the input field.

Based on the typical two step polyurethane – urea synthesis, self-crosslinkable polyurethane – urea compounded of poly(tetramethylene oxide), MDI (4,4 0 -diphenylmethane diisocyanate) and extended by AEAPS (aminoethylaminopropyltrimethoxysilane) were prepared. On the other hand, a C60-amine adducts by reacting C60 with excessive APES (aminopropyltrimethoxysilane) were synthesized and introduced into the polyurethane – urea to form the homogenous and transparent C60 containing polyurethane – urea films. FT-IR, UV, Electron Spectroscopy for Chemical Analysis, Wide-angle X-ray diffractions, dynamic mechanical thermal analysis and mechanical properties of samples were recorded. The optical limiting properties were also measured. The results show that the C60 containing polyurethane – urea is a kind of homogenous material with the ranging of C60 content. The solution state before processing makes it easy to be cast as variety configurations. With the difference in C60 content in polymers, the optical limiting properties are various and the required transmittance and transmitted energy can be altered with the C60 contents in polymers. q 2003 Elsevier Science Ltd. All rights reserved.

Summary With nanosecond scale at a 532 nm wavelength, we firstly measured the nonlinear optical absorption and refraction coefficients of Pd(po)2 complex by using Z-scan technique, here Hpo=1-hydioxy-2-pyridone. We describe an empirical expression for the case when nonlinear refraction is accompanied by nonlinear absorption to separately evaluate the nonlinear refraction and the nonlinear absorption by performing straightforward measurements with the aperture removed. The nonlinear optical response of Pd(po)2 was determined by the linear decreasing irradiance-dependence. The nonlinear absorption originates from the near resonant two-photon absorption while the mechanism of the nonlinear refraction is the near resonant two-photon absorption transition enhancement. The linear increasing dependences of the optical nonlinearities on the incident irradiance arise from the population redistribution due to the near resonant two-photon absorption.

The mid-infrared Christiansen effect was found to occur in the KBr pellets in which powder of the dispersive material α-SiC was dispersed. The KBr pellet technique is commonly used for infrared transmission (or absorption) spectroscopy of powders, and when we dispersed in KBr powder a moderate amount of a dispersive material, such as α-SiC, and pressed them into a pellet, a transmission spectrum with a sharp peak was obtained. The phenomenon is very interesting, and can be explained with the well-known Lorentz dispersion model accurately. Also, it is very useful for making a new type of mid-infrared band-pass filter, e.g. a mid-infrared Christiansen filter. After performing combinations of some other dispersive materials, such as MgO, with some transparent refractive index matching matrices, such as KCl etc, another aspect of the application of this important classical effect is shown, that is, determining the refractive indices of some mid-infrared transparent materials accurately.

The features of the closed-aperture (CA) Z-scan transmittance curves have been studied in detail when the third-order nonlinear refraction and absorption, both of which can be independently positive or negative, are simultaneously present. We found that whether the peak–valley or valley–peak structure exists or not, depends on the ratio ρ of the imaginary part χI(3) to the real part χR(3) of the third-order nonlinear susceptibility χ(3) which are caused by nonlinear refraction and absorption respectively. This communication also gives the ρ dependences of the distance between the peak position and valley position, the difference between peak transmittance and the valley transmittance, the ratio of peak height to the valley depth for CA Z-scan, and especially the exact critical value ρc which determines whether the peak–valley or valley–peak structure exists or not, with Gaussian beams for the first time to our knowledge.

A simultaneous generation of red light at 631 nm and blue light at 460 nm was realized, in single-pass quasi-phase-matched scheme, from two periodical optical superlattices in the same LiTaO3 crystal in series. Using a 532 nm laser as pump, the red light was generated at 631 nm as the signal of optical parametric process in the first superlattice, and the blue light at 460 nm was the sum-frequency mixing the idler and the pump in the second superlattice. Along with the remnant pump at 532 nm, red light, green light and blue light can be obtained from the superlattice system simultaneously. This may be an emulous scheme to realize a compact three-fundamental-colors; laser in future. (C) 2001 Elsevier Science Ltd. All rights reserved.

We demonstrate a novel photorefractive incoherent-to-coherent optical converter in which there is a ∼45° angle between the propagation direction of the incoherent image and the grating vector. Thus the number of the grating periods in one pixel is large even for a very high resolution. When the output image has the same direction with the incoherent incident beam, the resolution does not depend on the interaction length between the incoherent beam and the grating. A resolution as high as 283 line pairs/mm, which is the resolution of out projection system, is obtained for a one-dimensional resolution target. Two-dimensional incoherent-to-coherent conversion is also demonstrated.

We numerically investigated the diffraction properties of an image-bearing beam from a fixed index grating in a photorefractive medium at steady-state, in which the field of the image-bearing beam is decomposed into a combination of Fourier plane-wave components, and the newly induced photorefractive gratings written by the pairs of the incident and its diffracted plane-wave components are taken into account. The numerical results are presented and discussed.

A mutual selection equilibrium theory (MSET) for rainbow-shaped mutually pumped phase conjugation is proposed. In fact, this theory can be applied to any mutually pumped phase conjugation that has two interaction regions interconnected by only fanning beams, and in principle it can also be extended to the case of many interaction regions. We consider that the fanning beam intensities at the first-edges of the two interaction regions are non-zero. The theoretical results show that the calculated phase conjugate reflectivity by the MSET is greater than that of the coupled-plane wave theory (CPWT), however this difference asymptotically vanishes if the coupling strength is enormously large.

The interaction of fullerenes C60 and C70 films with metal substrates Au, Ag, In and Sn has been studied by UV-visible absorption spectra, IR, Raman, photoluminescence and second-order nonlinear optics. The absorption spectra show new bands for films on Au, Ag, In and Sn film substrates. For C60 on Au and Ag, new bands appear, peaking at 723 and 566 nm, respectively. For C60 film on Sn, several new bands are found at 975.4, 1218, 1557, 2000 and 3570 nm in the NIR and IR regions. Raman spectra reveal an additional peak at 350 cm−1 for C60 film on Au and five new peaks at 350, 532, 564, 1692 and 1850 cm−1 for the film on Sn. An enhancement of the Raman peak intensity is observed in the latter case. A peak blue shift of photoluminescence band from 749 to 735 nm for C60 on Sn has also been found. Moreover, a second-order nonlinear optics study at 1.06,μm yields a ten times greater secondary harmonic generation signal intensity for C60 films on Ag than that of Ag, while no signal is observed for C60 film on glass. For C70 on Sn, UV absorption bands broaden and new bands are found at 747, 984 and 1738 nm. The above results suggest a structural distortion of C60 and C70 molecules, induced by charge transfer from these metals to the molecules.

Magneto-optical Kerr rotation (MOKR) spectra of single films Nd-Fe and bilayers NdFe/Cu, NdFe/Al made on glass substrates by vacuum evaporation were measured. It is found that there is a broad enhancement on MOKR spectra for bilayers NdFe/Cu and NdFe/Al and a Kerr peak for NdFe/Cu bilayers.

Fe/Pd compositionally modulated films (CMF's) of various thicknesses prepared by vacuum evaporation were studied by VSM, FMR and MOKE spectra measurements. The room temperature magnetization of Fe layers and the polarization of Pd layers are deduced. At short wavelength the Kerr rotation of Fe/Pd CMFs increases with increasing polarization of the Pd layers. At long wavelengths the Kerr rotation, which is mainly contributed by the Fe layers, increases with Fe content.

In the magneto-optical polar Kerr rotation spectra of Fe/Au-Cu bilayers made by vacuum evaporation with the thickness of Fe layer less than the light penetration depth, a broad enhancement of the Kerr rotation spectra occurs near the absorption edge in the reflectivity spectra of Au-Cu alloys and shifts towards the longer wavelength side when the content of Cu in the Au-Cu reflector increases. For Fe/Al-Cu bilayers made by rf magnetron sputtering with a small content of Al in the Al-Cu reflector a peak occurs in its Kerr rotation spectrum. When the content of Al becomes larger the Kerr rotation increases with the decrease of wavelength monotonically and an enhancement occurs in the short wavelength region.

The magneto-optical polar Kerr rotation and reflectivity spectra of Fe/ZnSe multilayers and bilayers prepared by vacuum evaporation have been studied. For bilayers the maximum Kerr rotation at 700 nm in a magnetic field of 7000 Oe is about 1.4° with the reflectivity being larger than 50%. On the short-wavelength with a sign reversal of the Kerr rotation appears, which is considered as being due to the influence of the optical constant of the ZnSe layer and the thickness of the Fe layers. For multilayers the Kerr rotation increases with increasing wavelengths on the long-wavelength side and does not change very much on the short-wavelength side. The influence of the non-magnetic ZnSe layers on the MOKE spectra is discussed.

Single frequency laser output has been obtained for the first time from a transverse excitation (TE)CO 2 laser by shortening the cavity length and decreasing the total gas pressure appropriately.