Guozhen Yang

Switchable diode effect and ferroelectric resistive switching in epitaxial BiFeO3 thin films

Current-voltage hysteresis and switchable rectifying characteristics have been observed in epitaxial multiferroic BiFeO3 (BFO) thin films. The forward direction of the rectifying current can be reversed repeatedly with polarization switching, indicating a switchable diode effect and large ferroelectric resistive switching. With analyzing the potential barriers and their variation with ferroelectric switching at the interfaces between the metallic electrodes and the semiconducting BFO, the switchable diode effect can be explained qualitatively by the polarization-modulated Schottky-like barriers.

Gerchberg–Saxton and Yang–Gu algorithms for phase retrieval in a nonunitary transform system: a comparison

A detailed comparison of the original Gerchberg-Saxton and the Yang-Gu algorithms for the reconstruction of model images from two intensity measurements in a nonunitary transform system is presented. The Yang-Gu algorithm is a generalization of the Gerchberg-Saxton algorithm and is effective in solving the general amplitude-phase-retrieval problem in any linear unitary or nonunitary transform system. For a unitary transform system the Yang-Gu algorithm is identical to the Gerchberg-Saxton algorithm. The reconstruction of images from data corrupted with random noise is also investigated. The simulation results show that the Yang-Gu algorithm is relatively insensitive to the presence of noise in data. In all cases studied the Yang-Gu algorithm always resulted in a highly accurate recovered phase.

All-optical switching in subwavelength metallic grating structure containing nonlinear optical materials

All-optical switching based on a subwavelength metallic grating structure containing nonlinear optical materials has been proposed and numerically investigated. Metal-dielectric composite material is used in the switching for its larger third-order nonlinear susceptibility (approximately 10(-7)esu) and ultrafast response properties. The calculated dependence of the signal light intensity on the pump light intensity shows a bistable behavior, which results in a significant switch effect. It rests on a surface plasmons enhanced intensity-dependent change of the effective dielectric constant of Kerr nonlinear media, corresponding to a transition of the far-field transmission from a low- to high-transmission state. The study of this switching structure shows great advantages of smaller size, lower requirement of pump light intensity, and shorter switching time at approximately the picosecond level.

Effects of shapes and orientations of scatterers and lattice symmetries on the photonic band gap in two-dimensional photonic crystals

The photonic band structures of two-dimensional photonic crystals consisting of lattices with different symmetries and scatterers of various shapes, orientations, and sizes are studied numerically. Specifically, four types of lattices (triangular, hexagonal, square, and rectangular) and five different shapes of scatterers (hexagon, circle, square, rectangle, and ellipse) are considered. The scatterers are either dielectric rods in air, or air rods in dielectric media. The lattice symmetry and all these properties of the scatterers can affect the band gap size. Given a lattice symmetry, the largest absolute photonic band gap is achieved by selecting a scatterer of the same symmetry; e.g., hexagonal rods in triangular or honeycomb lattices, square rods in square lattices, and rectangular rods in rectangular lattices. The band gap can be further maximized by adjusting the orientation and size of the scatterers; but no simple, systematic rules can be drawn.

Thickness and oxygen pressure dependent structural characteristics of BaTiO3 thin films grown by laser molecular beam epitaxy

A series of BaTiO3 thin films with various thicknesses from 10 to 400 nm were epitaxially grown under various oxygen pressures from 2×10−4 to 12 Pa on SrTiO3 (001) substrates using laser molecular beam epitaxy. Being confirmed by reflection high energy electron diffraction, atomic force microscopy, x-ray diffraction, and high resolution transmission electron microscopy the epitaxial single crystal BaTiO3 thin films are highly c-axis or a-axis oriented with a root-mean-square surface roughness of 0.14 nm. The observed thickness and oxygen pressure dependent structural characteristics of the BaTiO3 thin films are discussed by taking into account both the misfits in thermal expansion and lattice constants between BaTiO3 films and SrTiO3 substrates, and the effect of the energy of the sputtered particles, which is consistent with the established strain relaxation mechanism. An abnormal expansion of lattice volume of a BaTiO3 unit cell is found and attributed to the effect of oxygen vacancies in the BaTiO3 films.

Structural behavior of thin BaTiO3 film grown at different conditions by pulsed laser deposition

Thin epitaxial BaTiO3 (BTO) films having a high crystallinity and uniformity of grains were deposited on the SrTiO3 (STO) substrates. When the oxygen pressure is from 0.04 to 70Pa the lattice constant decreases from 4.1100 to 3.9972 Angstrom and the orientation normal to the substrate changes from (001) to (100). The surface topography changes from Bat to hilly with a surface roughness (rms) changing between 1.2 nm at 0.7 Pa, 3.4 nm at 7 Pa and 6.4 nm at 70 Pa. At the substrate temperature of 550, 750 and 850 degrees C the surface topography of the films varies from corrugated structure to rectangular grain and the surface roughness increases from 1.2, 3.4 to 3.4 nm. The epitaxial BTO films were also deposited on the YBCO/LaAlO3 (YBCO/LAO) substrates at 750 degrees C and 7 Pa. The films have a rough surface (rms=29.1 nm), but a good uniformity of grains. The c-axis oriented BTO films with a poor crystallinity were deposited on the MgO substrates at 750 degrees C and 7 Pa. The films have a smooth surface (rms=1.0 nm), but a poor uniformity of grains. The interfaces between the BTO films and the substrates were determined by transmission electron microscopy (TEM).

High sensitivity of positive magnetoresistance in low magnetic field in perovskite oxide p–n junctions

Large positive magnetoresistance (MR) and high MR sensitivity in low magnetic fields have been discovered in the Sr-doped LaMnO3 and Nb-doped SrTiO3 p-n junctions fabricated by laser molecular-beam epitaxy. The MR ratios, defined as DeltaR/R-0, DeltaR = R-H-R-0, are observed as large as 11 % in 5 Oe, 23 % in 100 Oe, and 26 % in 1000 Oe at 290 K; 53% in 5 Oe, 80 % in 100 Oe, and 94 % in 1000 Oe at 255 K. The MR sensitivities are 85 Omega/Oe at 290 K, 246 Omega/Oe at 255 K, and 136 Omega/Oe at 190 K, respectively, with the applied magnetic field changed from 0 to 5 Oe. The positive MR ratios and high MR sensitivities of the p-n junctions are very different from that of the LaMnO3 compound family

Linear and nonlinear optical properties of Ag nanocluster/BaTiO3 composite films

To investigate the optical properties of metallic nanocluster/oxide composite films, Ag nanocluster/BaTiO3 composite films were synthesized on MgO (100) substrates by co-depositing the Ag and BaTiO3 targets using pulsed laser deposition. The x-ray diffraction results demonstrated that the Ag and BaTiO3 were well crystallized. The x-ray photoelectron spectroscopy analysis indicated that the samples were composed of nano-metal Ag embedded in the BaTiO3 matrices. The optical absorption properties were measured from 300 to 800 nm, and the absorption peaks due to the surface plasmon resonance of Ag particles were observed. With increasing the Ag concentration, the peak absorption increased and shifted to longer wavelength (redshift). Furthermore, the z-scan results showed that the films exhibited large optical nonlinearities and the signs of the nonlinear absorption (β) and nonlinear refractive index (n2) changed with increasing the Ag concentration.

Dember effect induced photovoltage in perovskite p-n heterojunctions

An unusual and rather large transient lateral photovoltage (LPV) has been observed in La0.9Sr0.1MnO3∕SrNb0.01Ti0.99O3 and La0.7Sr0.3MnO3∕Si heterojunctions under the nonuniform irradiation of pulsed laser. The irreversible LPVs on both sides of a p-n junction challenge the well established model for LPV in conventional semiconductor p-n junctions, which can be well explained by the Dember effect. Much larger LPV is observed in La0.7Sr0.3MnO3∕Si than that in La0.9Sr0.1MnO3∕SrNb0.01Ti0.99O3. Similar results measured from both substrates of SrNb0.01Ti0.99O3 and Si also support such a Dember effect. Much larger LPVs in heterojunctions than those in simple samples (SrNb0.01Ti0.99O3 or Si) suggest a potential application of Dember effect in heterostructures.

BEAM SHAPING IN THE FRACTIONAL FOURIER TRANSFORM DOMAIN

A new design approach for the diffractive phase elements (DPE’s) that implement beam shaping in the fractional Fourier transform (FRFT) domain is presented. The new algorithm can successfully achieve the design of DPE’s for beam shaping in both unitary and nonunitary transform systems. The unitarity transform condition of the FRFT domain is discussed. Modeling designs of the DPE’s are carried out for several fractional orders and different parameters of the beam for converting a Gaussian profile into a uniform beam. Our approach can realize beam shaping well for a nonunitary transform system in the FRFT domain.