Anisotropy of the angle resolved electron attenuation at crystal surfaces

Photoemission multiple scattering theory is used to describe the electron transport in the surface region of a crystal. Intensities of photoemission from core levels of atoms situated in subsurface atomic layers are calculated as a function of the emitter distance from the surface. The electron angle resolved attenuation length (ARAL) is extracted from the exponential fitting of the intensity decays of photoemission into different directions. Substantial anisotropy of the electron ARAL is found for the Cu(1 1 1) surface in Mg Kα photoexcitation of Cu 2p_3/2 levels and correlated with the orientation of highly packed atomic rows. Enhanced photoemission contributions from specific subsurface layers, caused by electron forward focusing effects, are reported.     

Intra-cavity second harmonic generation with Nd:YVO4/BIBO laser at 542 nm

We report, for the first time, an efficient intra-cavity second-harmonic generation (SHG) at 1084 nm in a nonlinear optical crystal, BiB₃O₆(BIBO) at the direction of (θ, ?) = (170.1°, 90°), performed with a LD end-pumped cw Nd:YVO₄ laser. With 590 mW diode pump power, a continuous-wave (cw) SHG output power of 19 mW at 542 nm yellow-green color has been obtained using a 1.5 mm-thick BIBO crystal. The optical conversion efficiency was 3.22%. It was found that the output wavelength could be 532 nm, 537 nm or 542 nm according to regulating the angle of BIBO.     

The chemical potential in surface segregation calculations: AgPd alloys

We put forward a technique for calculating the surface segregation profile in substitutional disordered alloys. The surface internal energy and the effective bulk and surface chemical potentials are calculated using the full charge density exact muffin-tin orbitals method, combined with the coherent potential approximation. The application of our approach is demonstrated to the close-packed surface of Ag_cPd_1−c random alloys with 0 < c < 1. The surface concentration profile, surface energy and segregation energy are investigated as functions of bulk composition. The present results are compared with former theoretical and experimental data. It is found that at low temperature, Ag segregates to the surface layer for the entire bulk composition range. At 0 K, the subsurface layer contains 100% Pd for c ? 0.4, and somewhat more than (2c − 1) Ag in alloys with c > 0.5. The temperature dependence of the segregation profile is significant for Pd rich alloys and for alloys with intermediate concentrations. At temperatures ?600 K, the subsurface layer is obtained to be almost bulk like.     

Crystallographic and magnetic properties of SrM thin films on Pt underlayer prepared at various substrate temperatures

Strontium ferrite (SrM) thin films deposited on thermally oxidized silicon wafer (SiO₂/Si) and single crystal sapphire with (0 0 l) orientation (Al₂O₃(0 0 l)) substrate using Pt underlayer were prepared by DC magnetron sputtering system. It was found that the intensity of (1 1 1) line for Pt and that of (0 0 l) diffraction line for SrM increases with increasing substrate temperature, T_u. The c-axis dispersion angle, Δθ₅₀, of SrM(0 0 8) depends on that of Pt underlayer. Both dispersion angle of Pt(1 1 1) and SrM(0 0 8) decrease with increasing temperature. It was observed that the saturation magnetization of SrM/Pt deposited on SiO₂/Si is higher than that of Al₂O₃ substrate. The coercivity and remanent squareness ratio in perpendicular direction are higher than that in in-plane direction. The maximum of coercivity in perpendicular direction of SrM/Pt films deposited on single crystal Al₂O₃ is about 4.2 kOe.     

Novel procedure for the simultaneous determination of the Debye length and electro-optic coefficient for an optically active photorefractive Bi12SiO20 crystal

We present a novel procedure based on an improved physical model and a versatile numerical fitting technique, to simultaneously determinate the Debye screening length and electro-optic coefficient using a thick sample of an optically active photorefractive crystal Bi₁₂SiO₂₀ (BSO). For the computation, experimental data of gain vs. grating spacing were obtained by a two-beam coupling arrangement. Unlike former calculation models, in our work, the general expression for the diffraction efficiency in the presence of self-diffraction is considered, and the influence of the optical activity in the coupling parameters is had into account for the calculation of the gain values. The fitting of the experimental data to the predicted theoretical behavior by our model is achieved by finding the closest theoretical curve to a set of data sampled from a spline-smoothed curve of the experimental data. Both, the Debye screening length l_s and the electro-optic coefficient r₄₁ are used as fitting parameters by searching in a rather wide range for each one of the parameters, so that, the estimation of their values is obtained in a more reliable and direct way from the same experiment. The calculations are performed in diffusion regimen and the procedure leads to l_s = 0.22 μm and r₄₁ = 4.5 × 10 ^− 12m/V. Because the optical activity can alter the maximum gain and self-diffraction effects influence the energy exchange, the procedure reveals to be physically appropriated for the simultaneous determination of these physical parameters when thick photorefractive crystals with high optical activity are considered.     

Electronic decoupling of surface layers from bulk and its influence in oxidation catalysis: A molecular beam study

Interactions between oxygen and Pd-surfaces have important implications, especially towards oxidation reactions, and influence of subsurface oxygen to oxidation reactions is the focus of the present study. In our efforts to understand the above aspects, CO oxidation reactions have been carried out with mixed molecular beam (MB), consisting CO and O₂, on Pd(1 1 1) surfaces under a wide variety of conditions (T = 400-900 K, CO:O₂ = 7:1 to 1:10). A new aspect of the above reaction observed in the transient kinetics regime is the evidence for oxygen diffusion into Pd subsurface layers, and its significant influence towards CO oxidation at high temperatures (≥600 K). Interesting information derived from the above studies is the necessity to fill up the subsurface layers with oxygen atoms to a threshold coverage (θ_O-sub), above which the reactive CO adsorption occurs on the surface and simultaneous CO₂ production begins. There is also a significant time delay (Γ) observed between the onset of oxygen adsorption and CO adsorption (and CO₂ production). Above studies suggest an electronic decoupling of oxygen covered surface and subsurface layers, which is slightly oxidized, from the metallic bulk, which induces CO adsorption at high temperatures and simultaneous oxidation to CO₂.     

Synthesis, growth, and characterization of a non-linear optical crystal-glycine lithium chloride

A new non-linear optical material, glycine lithium chloride, was synthesized and single crystals were grown by slow evaporation solution growth technique at constant temperature from its aqueous solution. Transparent and well-crystallized hexagonal prisms were obtained by controlled evaporation at a constant temperature of 45 °C. The grown crystals were characterized by X-ray diffraction methods, Fourier transforms infrared spectroscopy, and optical absorption spectrum. Single crystal X-ray diffraction analysis revealed that the crystal lattice of glycine lithium chloride is hexagonal with unit cell a = b = 7.023 Å, c = 5.478 Å, α = β = 90°, γ = 120°, V = 234 Å³. The dielectric response of the crystal with varying frequencies was studied. The second harmonic generation efficiency of the crystal was studied and is found to be larger than KDP.     

Pressure-induced structural phase transition in ReO3

We have investigated the pressure-induced structural phase transition in ReO₃ by neutron diffraction on a single crystal. We collected neutron diffraction intensities from the ambient and high pressure phases at P=7 kbar and refined the crystal structures. We have determined the stability of the high pressure phase as a function temperature down to T=2 K and have constructed the (P-T) phase diagram. The critical pressure is P_c=5.2 kbar at T=300 K and decreases almost linearly with decreasing temperature to become P_c=2.5 kbar at T=50 K. The phase transition is driven by the softening of the M₃ phonon mode. The high pressure phase is formed by the rigid rotation of almost undistorted ReO₆ octahedra and the Re-O-Re angle deviates from 180°. We do not see any evidence for the existence of the tetragonal (P4/mbm) intermediate pressure phase reported earlier.     

Optimization of optical parametric chirped pulse amplification with different pump wavelengths

Optical parametric chirped pulse amplification with different pump wavelengths was investigated using LBO crystal, at signal central wavelength of 800 nm. According to our theoretical simulation, when pump wavelength is 492.5 nm, there is a maximal gain bandwidth of 190 nm centered at 805 nm in optimal noncollinear angle using LBO. Presently, pump wavelength of 492.5 nm can be obtained from second harmonic generation of a Yb:Sr₅(PO₄)₃F laser. The broad gain bandwidth can completely support ∼6 fs with a spectral centre of seed pulse at 800 nm. The deviation from optimal noncollinear angle can be compensated by accurately tuning crystal angle for phase matching. The gain spectrum with pump wavelength of 492.5 nm is much better than those with pump wavelengths of 400, 526.5 and 532 nm, at signal centre of 800 nm.     

X-ray diffraction study of molybdenum diselenide to 35.9 GPa

In situ high-pressure angle dispersive synchrotron X-ray diffraction studies of molybdenum diselenide (MoSe₂) were carried out in a diamond-anvil cell to 35.9 GPa. No evidence of a phase transformation was observed in the pressure range. By fitting the pressure-volume data to the third-order Birch-Murnaghan equation of state, the bulk modulus, K_0T, was determined to be 45.7±0.3 GPa with its pressure derivative, K′_0T, being 11.6±0.1. It was found that the c-axis decreased linearly with pressure at a slope of −0.1593 when pressures were lower than 10 GPa. It showed different linear decrease with the slope of a −0.0236 at pressures higher than 10 GPa.     

Application of obliquely interfering TE10 modes for electron energy gain

Two fundamental TE₁₀ modes are considered to interfere at a small angle θ and then propagate along the z-axis in an evacuated rectangular waveguide. The electron trajectory in the resultant field and the expressions for energy gain and the acceleration gradient are obtained, when the electron is injected along the z-axis. A 50 keV electron gains 718 keV energy in a 4.0 cm × 2.5 cm waveguide, when the microwave with intensity of 1 × 10¹⁰ W/cm² and frequency 5.577 GHz is used and the modes superpose at an angle of 10°; here the maximum acceleration gradient is obtained as 251 MeV/m. The energy gain and acceleration gradient are decreased with increasing width of the waveguide and microwave frequency. Higher gradient and larger energy gain are obtained for the higher microwave intensity, smaller angle of superposition and also when the electron is injected with larger initial energy.     

Molecular dynamics simulation of subsurface deformed layers in AFM-based nanometric cutting process

Three-dimensional molecular dynamics simulations of AFM-based nanometric cutting monocrystalline copper with pin tool radius of 0.713 nm are performed to investigate the effect of uncut chip thicknesses (0.1805 nm, 0.361 nm, 0.5415 nm, 0.722 nm, 0.9025 nm, 1.0875 nm, and 1.268 nm) on the depth of subsurface deformed layers. The EAM potential and Morse potential are utilized respectively to compute the interactions between workpiece atoms, the interactions between workpiece atoms and tool atoms. The single-atom potential energy variations of the workpiece atoms within the subsurface regions during the cutting process are obtained and analyzed through a deformation criterion to determine the deformation behaviors of subsurface atoms. The simulation results reveal that the depth of subsurface deformed layers is affected by the AFM pin tool's rake angle. At each uncut chip thickness, the AFM pin tool presents different negative rake angles, consequently different degrees of deformation in the subsurface take place.     

Structural phase transitions at high-temperature in double perovskite Sr2GdRuO6

The crystal structure evolution of the Sr₂GdRuO₆ complex perovskite at high-temperature has been investigated over a wide temperature range between 298 K≤T≤1273 K. Powder X-ray diffraction measurements at room temperature and Rietveld analysis show that this compounds crystallizes in a monoclinic perovskite-type structure with P2₁/n (#14) space group and the 1:1 ordered arrangement of Ru⁵⁺ and Gd³⁺ cations over the six-coordinate M sites, with lattice parameters a=5.81032(8) Å, b=5.82341(4) Å, c=8.21939(7) Å, V=278.11(6) Å³ and angle β=90.311(2)^o. The high-temperature analysis shows that this material suffers two-phase transitions. At 373 K it adopts a monoclinic perovskite structure with I2/m space group, and lattice parameters a=5.81383(2) Å, b=5.82526(4) Å, c=8.22486(1) Å, V=278.56(2) Å³ and angle β=90.28(2)^o. Above of 773 K, it suffers a phase transition from monoclinic I2/m to tetragonal I4/m, with lattice parameters a=5.84779(1) Å, c=8.27261(1) Å, V=282.89(5) Å³ and angle β=90.02(9)^o. The high-temperature phase transition from monoclinic I2/m to tetragonal I4/m is characterized by strongly anisotropic displacements of the anions.     

Negative refraction at various crystal interfaces

In an anisotropic positive index media the tangential components of Poynting vector S and that of wavevector k can have the opposite signs under certain conditions. The phenomenon is called as the anomalous negative refraction. The negative refraction phenomenon at planar interfaces between isotropic medium-crystal and crystal-crystal media has been analyzed. The crystals can be both uniaxial axis and biaxial axis and the azimuth of optical axis can be arbitrary. The negative refraction phenomenon under various azimuth of optical axis is analyzed. The phenomena can be very evident if the optical axis angle is arranged appropriately. The optimal conditions are obtained. For strong anisotropy crystal, the maximum incident angle that yields the negative refraction and the bending angle could be very large. For the interface of biaxial crystal the anisotropy parameter of crystal u₁ = n_z/n_x plays a dominant role in effect on negative refraction.     

1 × N add-drop filter structures using one dense wavelength division multiplexing thin-film filter

This paper proposes 1 × N add-drop filter structures in which only one thin-film filter (TF) is used. Our key idea is based on a combination of an angle-multiplexing concept and the flexibility of the optical fiber to allow a multiwavelength optical beam hit the TF several times, each time at a different angle but same position. Due to the TF angle sensitivity, the desired wavelength optical beam corresponding to the incident angle is therefore spatially isolated from the main optical beam. Our first TF-based 1 × N add-drop filter structure is arranged in a reflective design in which N wavelength optical beams can be dropped out from the main channel. For our transmissive architecture, N − 2 channels are directed to their associated output terminals while the remaining λ_N−1 and λ_N wavelength optical beams are sent out at the same port. Experimental proof of concept for our reflective TF-based 1 × 3 add-drop filter using one off-the-shelf TF, a triple fiber-optic collimator, and an optical circulator separates two wavelength optical beams with their channel spacing of 0.8 nm from the main channel. In this case, measured optical losses of 0.67 dB, 1.66 dB, and 2.59 dB are obtained for the first, the second, and the remaining dropped wavelength optical beams, respectively. Optical crosstalk and polarization dependent loss of <−18 dB and <0.08 dB are also investigated, respectively.     

Highly efficient Nd:GdVO4/BiBO laser at 456 nm under direct 880 nm diode laser pumping

The continuous-wave high-efficiency laser emission of Nd:GdVO₄ at the second-harmonic of 456 nm obtained by intracavity frequency doubling with an BiB₃O₆(BiBO) nonlinear crystal is investigated under pumping by diode laser at 880 nm into emitting level ⁴F_3/2. About 3.8 W at 456 nm with M² = 1.4 was obtained from a 5 mm-thick 0.4 at.% Nd:GdVO₄ laser medium and a 12 mm-long BiBO nonlinear crystal in a Z-type cavity for 13.9 W absorbed pump power. An optical-to-optical efficiency with respect to the absorbed pump power was 0.274. Comparative results obtained for the pump with diode laser at 808 nm, into the highly-absorbing ⁴F_5/2 level, are given in order to prove the advantages of the 880 nm wavelength pumping.     

Substrate orientation-induced distinct ferromagnetic moment in Co:ZnO films

Dilute (3.8 at.%) cobalt-doped ZnO thin films are deposited on LiTaO₃ (LT) substrates with three different orientations [LT(1 1 0), LT(0 1 2) and LT(0 1 8)] by direct current reactive magnetron co-sputtering. The experimental results indicate that Co atoms with 2+ chemical valence are successfully incorporated into the ZnO host matrix on various oriented substrates, and the substrate orientations have a profound influence on the crystal growth and magnetization of Co:ZnO films. A large magnetic moment of 2.42μ_B/Co at room temperature is obtained in the film deposited on LT(0 1 2), while the corresponding values of the other films deposited on LT(1 1 0) and LT(0 1 8) are 1.21μ_B/Co and 0.65μ_B/Co, respectively. Furthermore, the crystal growth mode of Co:ZnO films on various oriented LT, the relationship between the microstructures and corresponding ferromagnetic properties are also discussed.     

Optical,spectral and thermal properties of organic nonlinear optical single crystal: 2,3-Dimethoxy-10-oxostrychnidinium hydrogen oxalate dihydrate

The potential organic nonlinear optical single crystal of 2,3-dimethoxy-10-oxostrychnidinium hydrogen oxalate dihydrate has been grown by slow evaporation solution growth technique (SEST) using ethanol–water solution at room temperature. The powder X-ray diffraction study reveals that the crystal belongs to orthorhombic system with non-centrosymmetric space group P2₁2₁2₁ and the cell parameters are a = 7.61 Å, b = 10.73 Å, c = 29.49 Å, V = 2410.75 Å³. The functional groups of the synthesized compound have been identified by FT-Raman and FTIR analyses. Photoluminescence spectroscopy study is determined to explore its efficacy towards device fabrications. Birefringence measurement has been carried out in order to analyze the optical homogeneity of the grown crystal. The optical constants such as reflectance (R) and extinction coefficient (K) have been determined from the transmittance data. The relative second harmonic efficiency of the compound is found to be 4 times greater than that of KDP. DTA-DSC measurements indicate that the crystal is thermally stable up to 174 °C.     

Carbon sputtering yield measurements at grazing incidence

In this investigation, carbon sputtering yields were measured experimentally at varying angles of incidence under Xe⁺ bombardment. The measurements were obtained by etching a coated quartz crystal microbalance (QCM) with a low energy ion beam. The material properties of the carbon targets were characterized with a scanning electron microscope (SEM) and Raman spectroscopy. C sputtering yields measured under Ar⁺ and Xe⁺ bombardment at normal incidence displayed satisfactory agreement with previously published data over an energy range of 200 eV-1 keV. For Xe⁺ ions, the dependence of the yields on angle of incidence θ was determined for 0° ≤ θ ≤ 80°. Over this range, an increase in C sputtering yield by a factor of 4.8 was observed, with the peak in yield occurring at 70°. This is a much higher variation compared to Xe⁺ → Mo yields under similar conditions, a difference that may be attributed to higher scattering of the incident particles transverse to the beam direction than in the case of Xe⁺ → C. In addition, the variation of the yields with θ was not strongly energy dependent. Trapping of Xe in the surface was observed, in contrast to observations using the QCM technique with metallic target materials. Finally, target surface roughness was characterized using atomic force microscope measurements to distinguish between the effects of local and overall angle of incidence of the target.     

Optical and electrical properties of SnO2:Sb thin films deposited by oblique angle deposition

The antimony doped tin oxide (SnO₂:Sb) (ATO) thin films were prepared by oblique angle electron beam evaporation technique. X-ray diffraction, field emission scanning electron microscopy, UV-vis-NIR spectrophotometer and four-point probe resistor were employed to characterize the structure, morphology, optical and electrical properties. The results show that oblique angle deposition ATO thin films with tilted columns structure are anisotropic. The in-plane birefringence of optical anisotropy is up to 0.035 at α = 70°, which means that it is suitable as wave plate and polarizer. The electrical anisotropy of sheet resistance shows that the sheet resistance parallel to the deposition plane is larger than that perpendicular to the deposition plane and it can be changed from 900 Ω/□ to 3500 Ω/□ for deposition angle from 40° to 85°, which means that the sheet resistance can be effectively tuned by changing the deposition angle. Additionally, the sandwich structure of SiO₂ buffer layer plus normal ATO films and oblique angle deposition ATO films can reduce the resistance, which can balance the optical and electrical anisotropy. It is suggested that oblique angle deposition ATO thin films can be used as transparent conductive thin films in solar cell, anti-foggy windows and multifunctional carrier in liquid crystal display.