N. B. Ming

Larger polarization and weak ferromagnetism in quenched BiFeO3 ceramics with a distorted rhombohedral crystal structure

Single-phase insulating BiFeO3 ceramics have been synthesized by a simple but effective method that conventional solid state reaction is followed immediately by quenching processing. At room temperature, the ceramics show a metastable, distorted rhombohedral phase and the refined structure parameters are presented based on x-ray diffraction. It is revealed that the formations of Fe2+ and oxygen deficiency are greatly suppressed by the quenching processing. A well-saturated ferroelectric hysteresis loop with a large remnant polarization (2Pr=23.5μC∕cm2) is observed with an applied field of 155kV∕cm. Temperature-dependent magnetic property is investigated and weak ferromagnetism with a remnant magnetization of 4×10−6μB∕Fe at 10K is established. These results may have implications for further studies on multiferroics.

Preparation of monodisperse silica particles with controllable size and shape

The effects of dilution of tetraethyl orthosilicate (TEOS) with ethanol on the shape and monodispersity of silica particles were investigated. The results indicated that the dilution of TEOS with ethanol can depress the formation of new nuclei and the aggregation or adhesion of particles and make the distillation of TEOS unnecessary to achieve monodispersed silica spheres. A seeded growth technique using continuous drop addition of TEOS diluted with 4× volume of ethanol was developed to improve monodispersity and spheric shape and increase the size of silica particles. The monodisperse silica particles (150 nm ± 5%−1.2 μm ± 1%) with fine spheric shape were successfully synthesized by the seeded growth technique. Using the homemade 280 nm ± 2.8% silica spheres, we prepared opals of high quality which showed periodically ordered packing and a photonic band-gap effect.

Ferroelectric properties of La and Zr substituted Bi4Ti3O12 thin films

Thin films of A-site substituted, B-site substituted, and both A- and B-sites cosubstituted Bi4Ti3O12 (BTO) by La3+ and Zr4+, i.e., Bi3.25La0.75Ti3O12 (BLT), Bi4Ti2.8Zr0.2O12 (BTZ), and Bi3.25La0.75Ti2.8Zr0.2O12 (BLTZ), were fabricated on Pt/Ti/SiO2/Si substrates by pulsed laser deposition. Structures of the films are investigated by x-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Compared to the well known BLT films, both the BTZ and BLTZ films have larger remanent polarization (Pr) but smaller coercive field (Ec). It is shown experimentally that the oxygen vacancy is the predominant factor determining ferroelectric fatigue. The effects of substitution on structural and ferroelectric properties of BTO are discussed in detail. As a result, the A- and B-sites cosubstitution might be one of the promising ways to improve ferroelectric properties of BTO.

Electrical and magnetic properties of La0.5Sr0.5CoO3 thin films

Highly conductive perovskite La0.5Sr0.5CoO3 thin films were grown on (012) LaAlO3 by pulsed-laser deposition at different substrate temperatures. Magnetoresistance (MR) measurements show only a negative effect for the films grown at a temperature of 800u200a°C. However, a positive MR effect was found for the films grown at lower temperatures of 400 and 500u200a°C. The sign of the magnetoresistance changed from positive to negative near 150 K under an applied magnetic field of 5.0 T, and the largest positive MR of 2.6% was obtained at a temperature of 4.2 K for the film grown at 500u200a°C. Low-temperature magnetic hysteresis of the films was also studied, and it is suggested that the scattering of the carrier at the domain boundary may lead to the positive-MR effect.

Optical response in two‐dimensional optical superlattices with Kerr nonlinearity

We present an analysis of the steady state optical response of two‐dimensional optical superlattices with Kerr nonlinearity. We consider the effects of the modification of the refractive index modulation strengths in the superlattices and the angular deviation of incoming radiation from Bragg conditions. We find that under Bragg incidence the system displays optical bistability only when the modulation ratio exceeds a certain threshold. We also predict that optical bistability can disappear under non‐Bragg incidence. Physical considerations are presented and discussed on how to optimize the chances for observing optical bistability in such a superlattice.

Aperiodic optical superlattices engineered for optical frequency conversion

An aperiodic optical superlattice is designed. The designing method is universal and can be applied to all frequency conversion processes by using the coupling of quasiphase matching, without any limitations to special materials and to given fundamental wavelengths.

Multiple-wavelength second-harmonic generation in aperiodic optical superlattices

We present a kind of aperiodic structure for the generation of multiple-wavelength second harmonic in a quasi-phase-matching scheme. In order to confirm its efficiency, a LiTaO3 superlattice with such an aperiodic domain-inverted structure was designed and fabricated. The second-harmonic generation at four present wavelengths was experimentally demonstrated from the superlattice with high and nearly equal conversion efficiencies. The tested result is in good agreement with theoretical consideration. The method may be used for the design of optical superlattices to construct multiple-wavelength lasers and wavelength converters for all-optical network.

Fabrication and optical properties of two-dimensional ZnO hollow half-shell arrays

Two-dimensional optically active ZnO shells on polystyrene (PS) templates have been fabricated using a sputtering technique. Periodic ZnO hollow half-shell structure has been obtained by removing PS templates. It exhibits several anomalous dips in transmission spectra that have not been reported. The dips can be tuned up by changing the shell thickness and PS sphere size. The PL spectra confirm that the structure has enhanced defect emission of ZnO.

Reflectivity behavior of two-dimensional ordered array of metallodielectric composite particles at large incidence angles

Optical reflectance of a two-dimensional (2D) metallodielectric (MD) photonic crystal composed of hexagonal array of close-packed silver-shell polystyrene-core composite particles has been studied at off-normal incidence under both P and S polarizations. A dramatic change in reflectivity behavior of the 2D MD film from a 2D colloidal crystal template and a homogeneous silver film has been observed. The phenomenon of strong reflectance bands of the 2D MD film was explained as a selective coupling of the incident light with surface plasmon of the 2D textured structure via the 2D grating.

Efficient generation of red light by frequency doubling in a periodically-poled nearly-stoichiometric LiTaO3 crystal

An efficient generation of red light in a periodically-poled nearly-stoichiometric LiTaO3 (PPSLT) by extracavity single-pass frequency doubling of a diode-pumped, Q-switched Nd:YVO4 laser at 1342nm was realized. An average power of 1.4W of the 671nm red light is obtained at the fundamental power of ∼2.8W with the conversion efficiency of 50%. The high conversion efficiency and steady output of red light indicate that the thick PPSLT is a competitive candidate for frequency conversion in order to construct a compact all-solid-state red laser.