Application of asymmetric reflections to the X-ray study of structural deformation of surfaces

A discussion is given of the use of X-ray diffraction in the structural characterization of surfaces and thin films with an arbitrary orientation with respect to the diffracted rays. The instrumental effects arising in a general diffraction geometry are considered in detail with reference to a Bragg-Brentano diffractometer. In particular, intensity variations, peak shifts and broadening are analysed as functions of the scattering angle and the surface orientation. These effects generally obscure the intrinsic structural properties so that an appropriate correction is required in an exhaustive X-ray investigation. Applications are discussed and examples are given in the study of the anisotropic structural properties of PbTe thin films prepared by sputtering onto various substrates.     

Asymmetric Fullerene Nanosurfactant: Interface Engineering for Automatic Molecular Alignments

Since the molecular self‐assembly of nanomaterials is sensitive to their surface properties, the molecular packing structure on the surface is essential to build the desired chemical and physical properties of nanomaterials. Here, a new nanosurfactant is proposed for the automatic construction of macroscopic surface alignment layer for liquid crystal (LC) molecules. An asymmetric nanosurfactant (C₆₀NS) consisted of mesogenic cyanobiphenyl moieties with flexible alkyl chains and a [60]fullerene nanoatom is newly designed and precisely synthesized. The C₆₀NS directly introduced in the anisotropic LC medium is self‐assembled into the monolayered protrusions on the surface because of its amphiphilic nature originated by asymmetrically programmed structural motif of LC‐favoring moieties and LC‐repelling groups. The monolayered protrusions constructed by the phase‐separation and self‐assembly of asymmetric C₆₀NS nanosurfactant in the anisotropic LC media amplify and transfer the molecular orientational order from surface to bulk, and finally create the automatic vertical molecular alignment on the macroscopic length scale. The asymmetric C₆₀NS nanosurfactant and its self‐assembly described herein can offer the direct guideline of interface engineering for the automatic molecular alignments.     

Polarization properties of a nematic liquid-crystal spatial light modulator for phase modulation

The polarization properties of a nematic zero-twist liquid-crystal (NLC) spatial light modulator (SLM) were studied. A large ratio between the liquid-crystal (LC) layer thickness and the pixel pitch combined with spatial variations in the applied electric field causes fringing fields between pixels. Depending on the LC alignment, the electric field components within the LC layer can result in a twist deformation. The produced inhomogeneous optical anisotropy affects the polarization of light propagating through the device. We experimentally examined polarization effects in different diffraction orders for both binary and blazed phase gratings. Simulations of the LC deformation together with finite-difference time-domain simulations for the optical propagation were used to calculate the corresponding far-field intensities. It was demonstrated how rigorous simulations of the NLC SLM properties can be used to understand the polarization features of different diffraction orders.     

Diffraction patterns of binary liquid crystal gratings in homeotropic and hybrid geometries

We report on the diffraction properties of the binary liquid crystal (LC) gratings consisting of alternating homeotropic and hybrid domains. The measured diffraction patterns agree well with theoretical results in a comprehensive model with a linearly approximated phase profile around the domain boundary. It is found that the linearly distorted length is independent of the grating period and depends only on the cell thickness, the surface anchoring energy, and the elastic constants of the LC material. For the LC cell thickness of 5.9 μm, the linearly distorted length is determined as 2.12 μm. The binary LC grating devices are expected to play an important role in the area of optical data storage and optical information processing.     

Three-dimensional coupled wave study for finite-sized anisotropic volume holographic gratings under ultrashort pulsed beam readout

The three-dimensional coupled wave theory is extended to systematically investigate the diffraction properties of finite-sized anisotropic volume holographic gratings (VHGs) under ultrashort pulsed beam (UPB) readout. The effects of the grating geometrical size and the polarizations of the recording and readout beams on the diffraction properties are presented, in particular under the influence of grating material dispersion. The wavelength selectivity of the finite-sized VHG is analyzed. The wavelength selectivity determines the intensity distributions of the transmitted and diffracted pulsed beams along the output face of the VHG. The distortion and widening of the diffracted pulsed beams are different for different points on the output face, as is numerically shown for a VHG recorded in a LiNbO3 crystal. The beam quality is analyzed, and the variations of the total diffraction efficiency are shown in relation to the geometrical size of the grating and the temporal width of the readout UPB. In addition, the diffraction properties of the finite-sized and one-dimensional VHG for pulsed and continuous-wave readout are compared. The study shows the potential application of VHGs in controlling spatial and temporal features of UPBs simultaneously.     

Direct electrochemical formation of alloyed AuPt nanostructured electrocatalysts for the oxidation of formic acid

The formation of highly anisotropic AuPt alloys has been achieved via a simple electrochemical approach without the need for organic surfactants to direct the growth process. The surface and bulk properties of these materials were characterised by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDX) and electrochemically by cyclic voltammetry to confirm alloy formation. It was found that AuPt materials are highly active for both the model hydrogen evolution reaction and the fuel cell relevant formic acid oxidation reaction. In particular for the latter case the preferred dehydrogenation pathway was observed at AuPt compared to nanostructured Pt prepared under identical electrochemical conditions which demonstrated the less preferred dehydration pathway. The enhanced performance is attributed to both the ensemble effect which facilitates CO_(ads) removal from the surface as well as the highly anisotropic nanostructure of AuPt.     

Dynamic Control of Light Direction Enabled by Stimuli‐Responsive Liquid Crystal Gratings

The ability to control light direction with tailored precision via facile means is long‐desired in science and industry. With the advances in optics, a periodic structure called diffraction grating gains prominence and renders a more flexible control over light propagation when compared to prisms. Today, diffraction gratings are common components in wavelength division multiplexing devices, monochromators, lasers, spectrometers, media storage, beam steering, and many other applications. Next‐generation optical devices, however, demand nonmechanical, full and remote control, besides generating higher than 1D diffraction patterns with as few optical elements as possible. Liquid crystals (LCs) are great candidates for light control since they can form various patterns under different stimuli, including periodic structures capable of behaving as diffraction gratings. The characteristics of such gratings depend on several physical properties of the LCs such as film thickness, periodicity, and molecular orientation, all resulting from the internal constraints of the sample, and all of these are easily controllable. In this review, the authors summarize the research and development on stimuli‐controllable diffraction gratings and beam steering using LCs as the active optical materials. Dynamic gratings fabricated by applying external field forces or surface treatments and made of chiral and nonchiral LCs with and without polymer networks are described. LC gratings capable of switching under external stimuli such as light, electric and magnetic fields, heat, and chemical composition are discussed. The focus is on the materials, designs, applications, and future prospects of diffraction gratings using LC materials as active layers.     

Dynamic response of a slab of elastic-viscoplastic material that exhibits induced plastic anisotropy

Various 2-dimensional problems of the dynamic loading of a slab are solved for a material characterization that is elastic-viscoplastic and exhibits anisotropic work-hardening. The governing constitutive equations are based on a unified formulation which requires neither a yield criterion nor loading or unloading conditions. They include multi-dimensional anisotropic effects induced by the plastic deformation history. The theory also considers plastic compressibility which depends on the extent of the anisotropy. A numerical procedure for solving the equations is developed which incorporates the history dependent anisotropic hardening effects. Cases considered are the dynamic penetration of a slab by a rigid cylindrical indenter, and a distributed force rapidly applied over part of the slab surface. Both conditions of fixed and free rear surfaces of the slab are examined. A uniaxial problem is also considered in which different bases for the anisotropic hardening law are examined.     

Characterization of stainless steel parts by Laser Metal Deposition Shaping

Laser Metal Deposition Shaping (LMDS) is a new rapid manufacturing technology, which can build fully-dense metal components directly from the information transferred from a computer file by depositing metal powders layer by layer with neither mould nor tool. Typically, performed with stainless steel (SS) 316 powder, the orthogonal experiments combining with the ideal overlapping model were applied to ascertain the optimal processing parameters. Then the characteristics of microstructure, composition and phase of as-deposited cladding layers were analyzed through Scanning Electron Microscope (SEM) and X-ray diffraction (XRD), as well as relative model. Furthermore, the cooling rate and the solidification velocity during LMDS were evaluated based on empirical method. With the optimal parameters, some parts were fabricated without obvious defects, and then the mechanical properties of them were tested. Finally, the influencing regularities of critical parameters on microstructure and properties were concluded by comparison. The results prove that the microstructure of SS 316 deposits is composed of the slender dendrites growing epitaxially from the substrate, the mechanical properties are favorable and anisotropic, and the composition is uniform. Besides, the microstructure morphology and the mechanical properties are affected by the varied processing parameters at different degrees. Among them, the scanning speed shows the most remarkable effects on microstructure morphology, characteristic microscale, mechanical properties, as well as geometric shape of as-deposited parts.     

Co-Fe films: effect of Fe content on their properties

The characterizations of Co-Fe films electrodeposited on Ti substrates under potentiostatic conditions were investigated as a function of the Fe content in the films. The compositional analysis was carried out by energy dispersive X-ray spectroscopy. Scanning electron microscopy used to analyze the surface morphology of the films revealed that the film surface became rather smooth with the increase of the Fe contents. X-ray diffraction patterns showed that the crystal structure changed depending on the Co:Fe ratio in the films. It was observed that the crystal structure converted from the predominant face-centered-cubic to the body-centered-cubic with increasing Fe content. All films showed anisotropic magnetic resistance, but their magnitudes decrease as the Fe content increases. Magnetic data obtained from by vibrating sample magnetometer revealed that the changes observed in the saturation magnetization and coercivity values may arise from the Fe content of the films. The different magnetic and magnetotransport properties may come from the structural differences caused by the Fe content.     

Theory and characteristics of holographic polymer dispersed liquid crystal transmission grating with scaffolding morphology

We have performed a detailed characterization of the optical properties of a holographic polymer dispersed liquid crystal (LC) transmission grating with polymer scaffolding morphology, which was fabricated with conventional high-functionality acrylate monomer under low curing intensity. Temporal evolution of the grating formation was investigated, and the amount of phase-separated LC was determined by birefringence investigation. A grating model combined with anisotropic coupled-wave theory yielded good agreement with experimental data without any fitting parameter. The results in this study demonstrate the non droplet scaffolding morphology grating is characterized by a high degree of phase separation (70%), high anisotropy, low scattering loss (<6%), and high diffraction efficiency (95%).     

Diffraction minimization in SAW devices using wide aperture compensation

A relationship is outlined between the previously derived uniform asymptotic expansion (UAE) for surface acoustic waves on anisotropic substrates and the intuitive geometrical theory of diffraction (GTD). It is demonstrated that even for an anisotropic medium, the leading terms in the UAE have a direct interpretation in terms of the simple GTD. This model allows a physical explanation of the mechanism of ;wide aperture diffraction minimization', a simple technique that allows an almost complete elimination of diffractive effects in SAW devices by merely increasing the aperture of the unapodized transducer by a calculable amount. These techniques are illustrated with both theoretical and experimental results.     

Formation of anisotropic diffraction gratings in a polymer-dispersed liquid crystal by polarization modulation using a spatial light modulator

Anisotropic diffraction gratings based on a holographic polymer-dispersed liquid crystal (HPDLC) are realized by interferometric exposure using a spatial light modulator (SLM). The SLM is used in the HPDLC grating formation for anisotropic holographic recordings of two-dimensional polarization states for an incident light beam. The diffraction efficiency for P-polarization and the distinctive ratio of diffraction efficiency in P-polarization to that in S-polarization increases with the signal level applied to the SLM. The resulting volume gratings exhibit diffraction efficiency of more than 60% and a distinctive ratio of diffraction over 100. The microscopic origin of the anisotropic property is investigated by an optical polarizing microscope. The novel characteristics of the anisotropic diffraction properties of HPDLC are applied to an image reconstruction technique.     

Heat and Mass Transfer in Anisotropic Bodies

This review is aimed, on the one hand, at attracting the researchers' attention to the problems associated with heat transfer in anisotropic bodies and, on the other hand, at analyzing the scanty experience in investigating the heat transfer in anisotropic bodies that has been accumulated over the last 30 years. We treat conjugate heat transfer between boundary layers and anisotropic bodies, and heat transfer under conditions of simultaneous reciprocal influence of anisotropic thermal conductivity and anisotropic filtration upon film cooling of bodies with anisotropic properties. New effects are revealed during simulation of conjugate heat transfer; the law of nonlinear filtration in anisotropic bodies is identified; and the effect of individual components and of the orientation of the principal axes of thermal conductivity and penetrability tensors on stationary temperature fields and on the distribution of components of the filtration rate is investigated. The results of analytical investigation of heat transfer in anisotropic bodies enables one, under conditions of lack of adequate information, to exactly simulate nonstationary temperature in simplest anisotropic bodies, which may be used as test results. A number of recommendations are formulated for implementation of heat shielding of anisotropic bodies under conditions of their aerogasdynamic heating.     

Optical reconfiguration by anisotropic diffraction in holographic polymer-dispersed liquid crystal memory

Holographic polymer dispersed liquid crystal (HPDLC) memory is fabricated by a photoinduced phase separation comprised of polymer and liquid crystal (LC) phases using laser light interference exposures. The anisotropic diffraction induced by the alignment of LC in the periodic structure of the HPDLC memory is applied to reconstruct the configuration contexts for the optically reconfigurable gate arrays. Optical reconfiguration for various circuits under parallel programmability is implemented by switching the polarization state of incident light on the HPDLC memory using a spatial light modulator.     

Pulse shaping properties of volume holographic gratings in anisotropic media

Based on a modified coupled wave theory, the pulse shaping properties of volume holographic gratings (VHGs) in anisotropic media VHGs are studied systematically. Taking photorefractive LiNbO(3) crystals as an example, the combined effect that the grating parameters, the dispersion and optical anisotropy of the crystal, the pulse width, and the polarization state of the input ultrashort pulsed beam (UPB) have on the pulse shaping properties are considered when the input UPB with arbitrary polarization state propagates through the VHG. Under the combined effect, the diffraction bandwidth, pulse profiles of the diffracted and transmitted pulsed beams, and the total diffraction efficiency are shown. The studies indicate that the properties of the shaping of the o and e components of the input UPB in the crystal are greatly different; this difference can be used for pulse shaping applications.     

Recent Progress on Stability and Passivation of Black Phosphorus

From a fundamental science perspective, black phosphorus (BP) is a canonical example of a material that possesses fascinating surface and electronic properties. It has extraordinary in‐plane anisotropic electrical, optical, and vibrational states, as well as a tunable band gap. However, instability of the surface due to chemical degradation in ambient conditions remains a major impediment to its prospective applications. Early studies were limited by the degradation of black phosphorous surfaces in air. Recently, several robust strategies have been developed to mitigate these issues, and these novel developments can potentially allow researchers to exploit the extraordinary properties of this material and devices made out of it. Here, the fundamental chemistry of BP degradation and the tremendous progress made to address this issue are extensively reviewed. Device performances of encapsulated BP are also compared with nonencapsulated BP. In addition, BP possesses sensitive anisotropic photophysical surface properties such as excitons, surface plasmons/phonons, and topologically protected and Dirac semi‐metallic surface states. Ambient degradation as well as any passivation method used to protect the surface could affect the intrinsic surface properties of BP. These properties and the extent of their modifications by both the degradation and passivation are reviewed.     

Evaluation of crystallinity in TiO2 films with mixed structures grown on MgO (001) substrates by argon-ion beam sputtering based on infrared reflection-absorption spectra

TiO2 films with thicknesses (d) above 15 nm were grown on optically polished surfaces of MgO (001) substrates held at 400 degrees C by sputtering a Ti target with an argon-ion beam when the partial pressure of O2 was kept at 1.1 x 10(-2) Pa. X-ray diffraction patterns show that TiO2 films with d < 56 nm are composed of an a-axis anatase-type structure, whereas those with d > 56 nm are composed of a mixture of phases with the c-axis parallel to the film surface. The thickness dependence of the infrared reflection-absorption spectra shows that TiO2 films with d < 56 nm are composed of both anatase and amorphous phases, whereas those with d > 56 nm are composed of anatase, rutile, and amorphous phases. The crystallinity in TiO2 films is also evaluated from the infrared reflection-absorption spectra by comparison of the observed and the calculated results determined from the dielectric function of anisotropic TiO2 bulk single crystal.     

Multi-Gaussian Beam Modeling for Multilayered Anisotropic Media,II: Numerical Examples of Slowness Surface and Geometry Effects

A multi-Gaussian beam model is used to simulate an immersion transducer radiating into an anisotropic solid through a curved fluid–solid interface. It is shown that the characteristics of the beam as it propagates in the solid are controlled by the properties of the slowness surface and the interface geometry. Methods are discussed for efficiently extracting the slowness surface properties including the development of an explicit expression for the curvature of the slowness surface. A number of numerical examples are presented to demonstrate the effects that both the slowness surface and interface geometry have on an ultrasonic beam.     

Transformation and spectrum properties of partially coherent beams in the fractional Fourier transform plane

An analytical and concise formula is derived for the fractional Fourier transform (FRT) of partially coherent beams that is based on the tensorial propagation formula of the cross-spectral density of partially coherent twisted anisotropic Gaussian-Schell-model (GSM) beams. The corresponding tensor ABCD law performing the FRT is obtained. The connections between the FRT formula and the generalized diffraction integral formulas for partially coherent beams passing through aligned optical systems and misaligned optical systems are discussed. With use of the derived formula, the transformation and spectrum properties of partially coherent GSM beams in the FRT plane are studied in detail. The results show that the fractional order of the FRT has strong effects on the transformation properties and the spectrum properties of partially coherent GSM beams. Our method provides a simple and convenient way to study the FRT of twisted anisotropic GSM beams.