Zi-jian Wu

Self-polarizing terahertz liquid crystal phase shifter

Using sub-wavelength metallic gratings as both transparent electrodes and broadband high-efficiency polarizers, a highly-compact self-polarizing phase shifter is demonstrated by electrically tuning the effective birefringence of a nematic liquid crystal cell. The metal grating polarizers ensure a good polarizing efficiency in the range of 0.2 to 2 THz. Phase shift of more than π/3 is achieved in a 256 μm-thick cell with a saturation root mean square voltage of around 130 V in this integrated device.

Low-field magnetoresistance in nanosized La0.7Sr0.3MnO3/Pr0.5Sr0.5MnO3 composites

Nanosized La0.7Sr0.3MnO3/Pr0.5Sr0.5MnO3 (LSMO1−xPSMOx) ceramic composites are prepared using solid-sate sintering. Their microstructural, electro- and magnetotransport properties are characterized by means of various techniques. It is found that the antiferromagnetic/ferromagnetic coupling between PSMO/LSMO at low temperature and the weak ferromagnetic order of PSMO at high temperature results in enhanced low-field magnetoresistance (LFMR) of the composites. With increasing temperature up to 250 K the observed LFMR decays more slowly than that for pure LSMO and this behavior may be explained by the spin coupling near boundaries between LSMO and PSMO grains.

Highly effective and reproducible surface-enhanced Raman scattering substrates based on Ag pyramidal arrays

Close-packed Ag pyramidal arrays have been fabricated by using inverted pyramidal pits on Si as a template and used to generate plentiful and homogeneous surface-enhanced Raman scattering (SERS) hot sites. The sharp nanotip and the four edges of the Ag pyramid result in strong electromagnetic field enhancement with an average enhancement factor (EF) of 2.84 × 107. Moreover, the features of the close-packed Ag pyramidal array can be well controlled, which allows SERS substrates with good reproducibility to be obtained. The relative standard deviation (RSD) was lower than 8.78% both across a single substrate and different batches of substrates.Graphical abstract

Polarization independent liquid crystal gratings based on orthogonal photoalignments

The polarization independence of liquid crystal gratings with alternate orthogonal aligned regions is theoritically studied and demonstrated by means of photoalignment technique. The different alignments in adjecent regions are achieved by two-step photo exposure to guide orientations of sulfonic azo dye layers and further align the liquid crystal molecules. Both one-dimensional and two-dimensional switchable phase gratings have been demonstrated. Such polarizer-free gratings show very high transmittance (∼92%), diffraction efficiency (over 31%), and optical contrast (over 150) including low power consumption.

Partially crystallized La0.5Sr0.5MnO3 thin films by laser ablation and their enhanced low-field magnetoresistance

Amorphous, partial-crystallized, and epitaxial La0.5Sr0.5MnO3 thin films have been deposited at various temperatures of 200–650 °C on (001) SrTiO3 substrates using pulsed-laser deposition. The x-ray diffraction and high-resolution transmission electron microscopy indicate complete (001) orientation of the crystalline structures in these films. Enhanced low-field magnetoresistance effect has been observed for the partial-crystallized thin films where the nanosized ferromagnetic crystals are embedded in nonferromagnetic amorphous matrix. It is argued that the amorphous layer separating the neighboring nanocrystals behaves as the barrier for the spin-polarized tunneling and/or spin-dependent scattering, resulting in enhanced magnetoresistance at low magnetic field.

Nonlinear frequency conversion of fields with orbital angular momentum using quasi-phase-matching

We propose and investigate the quasi-phase matched (QPM) nonlinear optical frequency conversion of optical vortices in periodically poled Lithium Niobate (PPLN). Laguerre-Gaussian (LG) modes are used to represent the orbital angular momentum (OAM) states, characterized with the azimuthal and radial indices. Typical three-wave nonlinear interactions among the involved OAM modes are studied with the help of coupling wave equations. Being different from normal QPM process where the energy and quasi-momentum conservations are satisfied, both of the azimuthal and radial indices of the OAM states keep constant in most of the cases. However, abnormal change of the radial index is observed when there is asynchronous nonlinear conversion in different parts of the beams. The QPM nonlinear evolution of fractional OAM states is also discussed showing some interesting properties. In comparison with the traditional birefringent phase matching (BPM), the QPM technique avoids the undesired walk-off effect to reserve high-quality LG modes. We believe the QPM is an efficient way to convert, amplify and switch OAM states in various optical vortex related applications.

Tunable Fano resonance in hybrid graphene-metal gratings

Hybrid graphene-metal gratings with tunable Fano resonance are proposed and theoretically investigated in THz band. The grating contains alternately aligned metal and graphene stripes, which could be viewed as the superposition of two kinds of gratings with the same period. Due to different material properties, the resonance coupling between the metal and graphene parts forms typical Fano-type transmitting spectra. The related physical mechanism is studied by inspecting the induced dipole moment and local surface charge distributions at different wavelengths. Both of the resonance amplitude and frequency of the structure thus are adjustable by tuning graphenes Fermi energy and the gratings geometrical parameters. Furthermore, the Fano-type spectra are also quite sensitive to environmental indices, which supply another kind of tunability. All these features should have promising applications in tunable THz filters, switches, and modulators.

Comparative study of laser ablation techniques for fabricating nanocrystalline SnO2 thin films for sensors

Abstract The microstructure and sensing properties of SnO 2 thin films prepared by using SnO 2 and Sn targets at different substrate temperatures with and without post annealing were investigated systematically. It is found that nanocrystalline SnO 2 films with grain size of 4.0–5.2 nm can be achieved by two methods: (1) by crystallization of amorphous SnO 2 films at 400 °C; and (2) by oxidation of Sn films at 400 °C. The nanocrystalline SnO 2 films prepared with these methods exhibit much higher C 2 H 5 OH gas sensitivity in comparison with SnO 2 films prepared using SnO 2 and Sn targets at higher substrate temperatures with grain sizes of several tens of nm upon exposure to air containing 2000 ppm C 2 H 5 OH.

Pulsed laser deposition of (001) textured LiNbO3 films on Al2O3/SiO2/Si substrate

Abstract On SiO 2 /Si substrate (Si wafer with thermally oxidized SiO 2 coating), only polycrystalline LiNbO 3 thin films were normally obtained. In this work, we found that an ultrathin layer of predeposited Al 2 O 3 layer on SiO 2 /Si has greatly mediated the growth pattern of the subsequently deposited LiNbO 3 films. As a result, highly or completely (001) textured LiNbO 3 films were achieved depending on the substrate temperature and the thickness of Al 2 O 3 as well. The textured growth phenomenon is discussed in terms of surface energy relation.

Growth of LiNbO3 optical waveguide films by excimer laser ablation

Abstract Waveguiding LiNbO 3 films were prepared on sapphire substrates by excimer laser ablation. The as-grown films were characterized by XRD and SEM techniques, which revealed that epitaxial c -oriented and a -oriented LiNbO 3 films were achieved on (001) and (012) sapphire substrates, respectively. The optical waveguiding properties were demonstrated by measuring the TM and TE modes.