Zhang Ye-Wen

Formation and migration of space charge packet in low density polyethylene

Pressure wave propagation (PWP) method was utilized to observe the space charges packet-like behavior in sheet samples of low-density polyethylene (LDPE) under the relative low electric field. The experimental results show that space charge packets are always generated at the anode and move across the sample to the cathode regardless of the polarity of DC voltage. This paper discusses the migration of space charge packets at 30–40°C, the possible charge resource of space charge packets carriers and trap level of the space charge.

Omnidirectional Gaps of One-Dimensional Photonic Crystals Containing Single-Negative Materials

We show that a one-dimensional photonic crystal containing two kinds of single-negative materials can possess an omnidirectional gap whose edges are insensitive to incident angle and polarization, owing to the unusual field configuration in the structure. In contrast to an omnidirectional gap based on a photonic gap that corresponds to zero averaged refractive index, it can be made very wide by varying the ratio of the thicknesses of two media. Moreover, it is independent of polarization under special parameters. These properties may provide a mechanism to design a broadband omnidirectional reflector with fixed bandwidth.

Electrochromic mechanism study of corona poled electro-optic polymer films

In the study of electro-optic polymers with nonlinear optical activity, the phenomena of electrochromic shifts and intensity decreases of charge-transfer absorption bands in the polymer film after corona poling have been used for real-time monitoring of the corona-poling process and poling stability. In this paper, the structure changes of the chromophore dipoles doped or chemically attached to polymer macromolecules before and after poling were investigated by measuring UV-VIS and IR spectra. The electrochromic mechanism was discussed. The results indicated that the main reason for the electrochromic shifts was the deformation of the conjugate /spl pi/ system in the chromophore dipole. The solvent used during specimen preparation affects the material structure.

Effective permittivity and permeability of one-dimensional dielectric photonic crystal within a band gap

We take a finite dielectric photonic crystal as a homogeneous slab and have extracted the effective parameters. Our systematic study shows that the effective permittivity or permeability of dielectric photonic crystal is negative within a band gap region. This means that the band gap might act as -negative materials (ENMs) with 0, or μ-negative materials (MNMs) with > 0 and μ < 0. Moreover the effective parameters sensitively rely on size, surface termination, symmetry, etc. The effective parameters can be used to design full transmission tunnelling modes and amplify evanescent wave. Several cases are studied and the results show that dielectric photonic band gap can indeed mimic a single negative material (ENM or MNM) under some restrictions.

Optical Tamm States in Dielectric Photonic Crystal Heterostructure

We investigate one-dimensional dielectric photonic crystal and optical Tamm modes formed by superposition of two band gaps and find that this kind of mode can be explained by the single negative materials tunnelling effect. A finite-size dielectric photonic band gap can mimic one kind of effective single negative material and this property sensitively depends on the frequency location in stop-band regions and surface termination and so on. The effective impedance match and effective phase match give the precise position of the optical Tamm mode. Complete transparency via tunnelling is achieved by two opaque media and demonstrates the validity of our approach.

Formation and Distribution of Space-Charge in Cross-Linked Polyethylene

The formation and distribution of space-charge in a cross-linked polyethylene (XLPE) sample are investigated by means of pressure wave propagation, infrared spectroscopy and electrostatic force microscopy (EFM). The related mechanism of space-charge distribution and the structure of XLPE are discussed. The EFM images show that quite large quantitative space-charges locate at the surface of spherulites.

Experimental Study of Tunneling modes in Photonic Crystal Heterostructure Consisting of Single-Negative Materials

The pair structure and the photonic crystal heterostructure consisting of -negative and μ-negative materials are successfully fabricated by using the transmission line approach. We experimentally investigate the tunneling mode properties by simulating and measuring the scattering parameters and phase shifts. It is shown that the pair structure and the photonic crystal heterostructures possess tunneling modes when the general zero average permittivity and zero average permeability condition are satisfied. At the tunneling frequency, the field (voltage) mainly concentrates in the center of the constructed structures based on the amplification of the evanescent wave. Moreover, the tunneling mode of the photonic crystal heterostructure has zero phase delay. The characteristics have potential applications in design of zero phase delay filters.

Electric Field Analysis of Space Charge Injection from a Conductive Nano-Filler Electrode

A simulation on the electric field distribution near the electrode is proposed to explain the reason for using nanosized carbon black mixed with ethylene vinyl acetate, as the electrode could lead to more charge injection into the polymer than using a deposited metal electrode. The electrode is simplified to a layer of conductive semi-spheres with fixed size and constant electric potential. By using the finite element method, it is found that both the size of the semi-spheres and the distance between adjacent semi-spheres could dramatically influence the electric field near the surface of the spheres; these are considered to be the two decisive factors for the charge injecting rate at electrodes of various materials.

Negative Refraction in One-Dimensional Photonic Crystals with Tilted Interface

We investigate the wave propagation through the tilted interface of one-dimensional photonic crystals. Negative refraction can be realized by excitation of the Bloch states in the extended Brillouin zone with suppressed reflection. Equi-frequency surface analysis shows that the positive refraction, negative refraction or birefringence in this configuration can be achieved under a proper incident angle, which is confirmed by finite-difference time-domain simulations. The results may be useful in applications in the new devices based on one-dimensional photonic or optical waveguide arrays.

Localized Mode Enhanced Coupler Based on Quasi-One-Dimensional Photonic Crystal Microstrip

We propose a novel localized mode enhanced (LME) coupler based on quasi-one-dimensional photonic crystal microstrips, which is promising to be applied in wavelength division multiplexed microwave communication systems. Compared to the traditional microstrip coupler, the LME structure has two advantages: high efficiency and frequency selectivity. Even in a relatively far coupling distance, this structure can still achieve a high efficiency about 50%. The frequency selectivity can be realized by simply tuning the distance between two transmission lines.