Yewen Zhang

Bio: Yewen Zhang is an academic researcher from Tongji University. The author has contributed to research in topics: Space charge & Metamaterial. The author has an hindex of 22, co-authored 255 publications receiving 2321 citations.

Omnidirectional gap and defect mode of one-dimensional photonic crystals containing negative-index materials

Abstract: We show theoretically that a one-dimensional photonic crystal containing a negative-index material has an omnidirectional gap, owing to the mechanism of zero (volume) averaged refractive index. In contrast to the Bragg gap, the edge of such a zero-n gap is insensitive to incident angle and polarization. When an impurity is introduced, a defect mode appears inside the zero-n gap with a very weak dependence on incident angle and invariant with scaling.

Properties of one-dimensional photonic crystals containing single-negative materials.

Abstract: The transmission properties of a one-dimensional photonic crystal containing two kinds of single-negative (permittivity- or permeability-negative) media are studied theoretically. We show that this structure can possess a type of photonic gap with zero effective phase $({\ensuremath{\phi}}_{\text{eff}})$. The zero-${\ensuremath{\phi}}_{\text{eff}}$ gap distinguishes itself from a Bragg gap in that it is invariant with a change of scale length and is insensitive to thickness fluctuation. In contrast to a photonic gap corresponding to zero averaged refractive index, the zero-${\ensuremath{\phi}}_{\text{eff}}$ gap can be made very wide by varying the ratio of the thicknesses of two media. An equivalent transmission-line model is utilized to explain the properties. A photonic quantum-well structure based on zero-${\ensuremath{\phi}}_{\text{eff}}$ gaps is proposed as a multiple channeled filter that is compact and robust against disorder.

Subwavelength Substrate-Integrated Fabry-Pérot Cavity Antennas Using Artificial Magnetic Conductor

Abstract: This paper presents the high-gain low-profile subwavelength substrate-integrated Fabry-Perot (FP) cavity antennas with artificial magnetic conductor (AMC) sheets. A partially reflective planar AMC sheet and a ground plane are used as the two reflectors of the FP-type resonant cavity for an ultra-thin planar design. The cavity is fully filled with dielectric substrate for further reduction of thickness of the antenna and easy integration. A microstrip patch antenna is embedded into the cavity as a feed. As design examples, the antennas are designed to operate at 10 GHz with a fixed overall thickness of λ0/9 (where λ0 is the operating wavelength in free space) and an aperture of 2λ0 × 2λ0. The losses caused by both dielectric and conductors are analyzed, which are critical for a fully dielectric substrate antenna design. The via-walls surrounding the radiating aperture are introduced to improve radiation patterns and gain by suppressing the surface waves, which are another critical loss for a thin fully dielectric substrate antenna design. Measured results show that such dielectric-integrated subwavelength cavity antennas feature the high gain of 12.5 dBi, low profile, easy integration into circuit board, and mechanical robustness, which makes them suitable for low-cost mass production.

Tunneling modes of photonic heterostructures consisting of single-negative materials

Abstract: We show theoretically that heterostructures consisting of single-negative materials can possess tunneling modes inside forbidden gaps, owing to the resonant coupling of the evanescent-wave-based interface modes The tunneling modes appear when the heterostructure becomes nihility They are independent of incident angles and polarizations and have zero phase delay, which can be utilized to design zero-phase-shift omnidirectional filters

Significant suppression of surface charge accumulation on epoxy resin by direct fluorination

Abstract: Prepared epoxy sheets were surface fluorinated in a laboratory vessel using a F2/N2 mixture with 12.5% F2 by volume at 50 oC and 0.1 MPa (1000 mbar) for 10 min to suppress surface charge accumulation on the epoxy sheet. Attenuated total reflection infrared analyses indicate that the fluorination led to substantial changes in chemical composition and structure of the sheet surface layer. The thickness of the fluorinated layer was determined to be 0.42 μm by SEM observation of the cross-section of the fluorinated sheet, and its SEM image shows that the fluorination also resulted in an increase in surface roughness. As a result, the deposited corona charge cannot be stored on the fluorinated surface even at room temperature, compared with a stable surface charge of the nonfluorinated (original) epoxy sample which has deep surface charge traps as indicated by the open-circuit thermally stimulated discharge current measurement. The measurements of surface conductivity and contact angle and the calculation of surface energy reveal that the fluorination gave rise to dramatic increases in surface conductivity and surface wettability and polarity. A very likely substantial decrease in depth of charge traps in the fluorinated surface layer and the adsorbed water on the fluorinated surface are responsible for the high surface conductivity of the fluorinated epoxy sheet. Surface charging current measurements further show a large steady state current flowing along the fluorinated surface during corona charging, compared with the almost zero steady state current of the original sample. This therefore suggests a lower steady state surface potential and a smaller dynamic surface charge accumulation of the fluorinated sample during the charge.

Physics of negative refractive index materials

Abstract: In the past few years, new developments in structured electromagnetic materials have given rise to negative refractive index materials which have both negative dielectric permittivity and negative magnetic permeability in some frequency ranges. The idea of a negative refractive index opens up new conceptual frontiers in photonics. One much-debated example is the concept of a perfect lens that enables imaging with sub-wavelength image resolution. Here we review the fundamental concepts and ideas of negative refractive index materials. First we present the ideas of structured materials or meta-materials that enable the design of new materials with a negative dielectric permittivity, negative magnetic permeability and negative refractive index. We discuss how a variety of resonance phenomena can be utilized to obtain these materials in various frequency ranges over the electromagnetic spectrum. The choice of the wave-vector in negative refractive index materials and the issues of dispersion, causality and energy transport are analysed. Various issues of wave propagation including nonlinear effects and surface modes in negative refractive materials (NRMs) are discussed. In the latter part of the review, we discuss the concept of a perfect lens consisting of a slab of a NRM. This perfect lens can image the far-field radiative components as well as the nearfield evanescent components, and is not subject to the traditional diffraction limit. Different aspects of this lens such as the surface modes acting as the mechanism for the imaging of the evanescent waves, the limitations imposed by dissipation and dispersion in the negative refractive media, the generalization of this lens to optically complementary media and the possibility of magnification of the near-field images are discussed. Recent experimental developments verifying these ideas are briefly covered. (Some figures in this article are in colour only in the electronic version)

A review of piezoelectric polymers as functional materials for electromechanical transducers

Abstract: Polymer based MEMS and microfluidic devices have the advantages of mechanical flexibility, lower fabrication cost and faster processing over silicon based ones. Also, many polymer materials are considered biocompatible and can be used in biological applications. A valuable class of polymers for microfabricated devices is piezoelectric functional polymers. In addition to the normal advantages of polymers, piezoelectric polymers can be directly used as an active material in different transduction applications. This paper gives an overview of piezoelectric polymers based on their operating principle. This includes three main categories: bulk piezoelectric polymers, piezocomposites and voided charged polymers. State-of-the-art piezopolymers of each category are presented with a focus on fabrication techniques and material properties. A comparison between the different piezoelectric polymers and common inorganic piezoelectric materials (PZT, ZnO, AlN and PMN?PT) is also provided in terms of piezoelectric properties. The use of piezopolymers in different electromechanical devices is also presented. This includes tactile sensors, energy harvesters, acoustic transducers and inertial sensors.

Cascade-based attack vulnerability on the US power grid.

Abstract: The vulnerability of real-life networks subject to intentional attacks has been one of the outstanding challenges in the study of the network safety. Applying the real data of the US power grid, we compare the effects of two different attacks for the network robustness against cascading failures, i.e., removal by either the descending or ascending orders of the loads. Adopting the initial load of a node j to be Lj !" kj# Rm2Cj km$% a with kj and Cj being the degree of the node j and the set of its neighboring nodes, respectively, where a is a tunable parameter and governs the strength of the initial load of a node, we investigate the response of the US power grid under two attacks during the cascading propagation. In the case of a < 0:7, our investigation by the numerical simulations leads to a counterintuitive finding on the US power grid that the attack on the nodes with the lowest loads is more harmful than the attack on the ones with the highest loads. In addition, the almost same effect of two attacks in the case ofa ! 0:7 may be useful in furthering studies on the control and defense of cascading failures in the US power grid.