Jiafu Wang

Bio: Jiafu Wang is an academic researcher from Xi'an Jiaotong University. The author has contributed to research in topics: Metamaterial & Surface plasmon polariton. The author has an hindex of 39, co-authored 307 publications receiving 5836 citations. Previous affiliations of Jiafu Wang include National University of Singapore.

Potassium–sodium niobate based lead-free ceramics: novel electrical energy storage materials

Abstract: The development of lead-free bulk ceramics with high recoverable energy density (Wrec) is of decisive importance for meeting the requirements of advanced pulsed power capacitors toward miniaturization and integration. However, the Wrec (<2 J cm−3) of lead-free bulk ceramics has long been limited by their low dielectric breakdown strength (DBS < 200 kV cm−1) and small saturation polarization (Ps). In this work, a strategy (compositions control the grain size of lead-free ceramics to submicron scale to increase the DBS, and the hybridization between the Bi 6p and O 2p orbitals enhances the Ps) was proposed to improve the Wrec of lead-free ceramics. (K0.5Na0.5)NbO3–Bi(Me2/3Nb1/3)O3 solid solutions (where Me2+ = Mg and Zn) were designed for achieving large Ps, and high DBS and Wrec. As an example, (1 − x)(K0.5Na0.5)NbO3–xBi(Mg2/3Nb1/3)O3 (KNN–BMN) ceramics were prepared by using a conventional solid-state reaction process in this study. Large Ps (41 μC cm−2) and high DBS (300 kV cm−1) were obtained for 0.90KNN–0.10BMN ceramics, leading to large Wrec (4.08 J cm−3). The significantly enhanced Wrec is more than 2–3 times larger than that of other lead-free bulk ceramics. The findings in this study not only provide a design methodology for developing lead-free bulk ceramics with large Wrec but also could bring about the development of a series of KNN-based ceramics with significantly enhanced Wrec and DBS in the future. More importantly, this work opens a new research and application field (dielectric energy storage) for (K0.5Na0.5)NbO3-based ceramics.

Significantly enhanced recoverable energy storage density in potassium–sodium niobate-based lead free ceramics

Abstract: Ceramic-based dielectric materials are regarded as the best candidates for advanced pulsed power capacitors because of their excellent mechanical and thermal properties. Nevertheless, lead-free bulk ceramics show relatively low recoverable energy storage density (Wrec < 2 J cm−3) owing to their low dielectric breakdown strength (DBS < 200 kV cm−1). In order to significantly increase Wrec, we proposed a strategy (compositions drive the grain size to submicrometer) to improve the DBS of lead-free ceramics. In this work, (1 − x)(K0.5Na0.5)NbO3–xSrTiO3 (KNN–ST) ceramics were chosen as a representative to verify the validity of this strategy. The (1 − x)KNN–xST ceramics (x = 0.15 and 0.20) with submicrometer grains (about 0.3 μm) were prepared using pressureless solid state sintering. A large Wrec (4.03 J cm−3) and DBS (400 kV cm−1 with a thickness of 0.2 mm) were achieved for 0.85KNN–0.15ST ceramics. The value of 4.03 J cm−3 is superior to all other Wrec in lead-free bulk ceramics and 2–3 times larger than that of other lead-free bulk ceramics. A large Wrec (3.67 J cm−3) and energy storage efficiency (72.1%) were simultaneously achieved for 0.80KNN–0.20ST ceramics. The results confirm that the (1 − x)KNN–xST ceramics (x = 0.15 and 0.20) are desirable materials for advanced pulsed power capacitors. The findings in this study could push the development of a series of KNN-based ceramics with enhanced DBS and Wrec in the future. On the other hand, this work could broaden the applications of KNN materials in a new field.

Ultra-wideband polarization conversion metasurfaces based on multiple plasmon resonances

Abstract: We propose to realize ultra-wideband polarization conversion metasurfaces in microwave regime through multiple plasmon resonances. An ultra-wideband polarization conversion metasurface is designed using a double-head arrow structure and is further demonstrated both numerically and experimentally. Four plasmon resonances are generated by electric and magnetic resonances, which lead to bandwidth expansion of cross-polarization reflection. The simulated results show that the maximum conversion efficiency is nearly 100% at the four plasmon resonance frequencies and a 1:4 3 dB bandwidth can be achieved for both normally incident x- and y-polarized waves. Experimental results agree well with simulation ones.

Deep Learning: A Rapid and Efficient Route to Automatic Metasurface Design.

Abstract: Metasurfaces provide unprecedented routes to manipulations on electromagnetic waves, which can realize many exotic functionalities. Despite the rapid development of metasurfaces in recent years, the design process of metasurface is still time-consuming and computational resource-consuming. Moreover, it is quite complicated for layman users to design metasurfaces as plenty of specialized knowledge is required. In this work, a metasurface design method named REACTIVE is proposed on the basis of deep learning, as deep learning method has shown its natural advantages and superiorities in mining undefined rules automatically in many fields. REACTIVE is capable of calculating metasurface structure directly through a given design target; meanwhile, it also shows the advantage in making the design process automatic, more efficient, less time-consuming, and less computational resource-consuming. Besides, it asks for less professional knowledge, so that engineers are required only to pay attention to the design target. Herein, a triple-band absorber is designed using the REACTIVE method, where a deep learning model computes the metasurface structure automatically through inputting the desired absorption rate. The whole design process is achieved 200 times faster than the conventional one, which convincingly demonstrates the superiority of this design method. REACTIVE is an effective design tool for designers, especially for laymen users and engineers.

Realizing high comprehensive energy storage performance in lead-free bulk ceramics via designing an unmatched temperature range

Abstract: The study of lead-free dielectric ceramics for capacitors has become one of the most active academic research areas in advanced functional materials owing to the environmental regulations. A large recoverable energy storage density (Wrec), a high energy storage efficiency (η) and good temperature stability in lead-free dielectric ceramics are highly desired simultaneously to meet the requirements of light weight and integration of dielectric capacitors in pulsed power devices. Unfortunately, a large Wrec of lead-free dielectric ceramics is usually achieved at the cost of η, and vice versa, hindering their practical applications. More importantly, despite the considerable efforts made so far to develop a large amount of lead-free bulk ceramics for dielectric capacitor applications, there is still a lack of scientific and feasible guidelines on how to design new material systems with a large Wrec, a high η and excellent thermal stability. Herein, we propose a new strategy to tailor the temperature region between the temperature corresponding to the maximum dielectric permittivity (Tm) and the Burns temperature (TB) of relaxor ferroelectrics to room temperature via composition optimization, to explore lead-free bulk ceramics with high comprehensive energy storage properties. A large Wrec of 3.51 J cm−3 and a high η of 80.1% are simultaneously obtained in 0.86NaNbO3–0.14(Bi0.5Na0.5)HfO3 (0.86NN–0.14BNH) ceramics under an electric field of 350 kV cm−1, leading to an excellent comprehensive energy storage performance in lead-free bulk ceramics. Furthermore, both the Wrec and η of 0.86NN–0.14BNH ceramics show good temperature stability over 20 °C to 200 °C at 280 kV cm−1, which is superior to that of other lead-free bulk ceramics. Most importantly, this work provides significant guidelines for designing new high-performance bulk ceramics for electrical energy storage applications.

A review of metasurfaces: physics and applications.

Abstract: Metamaterials are composed of periodic subwavelength metal/dielectric structures that resonantly couple to the electric and/or magnetic components of the incident electromagnetic fields, exhibiting properties that are not found in nature. This class of micro- and nano-structured artificial media have attracted great interest during the past 15 years and yielded ground-breaking electromagnetic and photonic phenomena. However, the high losses and strong dispersion associated with the resonant responses and the use of metallic structures, as well as the difficulty in fabricating the micro- and nanoscale 3D structures, have hindered practical applications of metamaterials. Planar metamaterials with subwavelength thickness, or metasurfaces, consisting of single-layer or few-layer stacks of planar structures, can be readily fabricated using lithography and nanoprinting methods, and the ultrathin thickness in the wave propagation direction can greatly suppress the undesirable losses. Metasurfaces enable a spatially varying optical response (e.g. scattering amplitude, phase, and polarization), mold optical wavefronts into shapes that can be designed at will, and facilitate the integration of functional materials to accomplish active control and greatly enhanced nonlinear response. This paper reviews recent progress in the physics of metasurfaces operating at wavelengths ranging from microwave to visible. We provide an overview of key metasurface concepts such as anomalous reflection and refraction, and introduce metasurfaces based on the Pancharatnam-Berry phase and Huygens' metasurfaces, as well as their use in wavefront shaping and beam forming applications, followed by a discussion of polarization conversion in few-layer metasurfaces and their related properties. An overview of dielectric metasurfaces reveals their ability to realize unique functionalities coupled with Mie resonances and their low ohmic losses. We also describe metasurfaces for wave guidance and radiation control, as well as active and nonlinear metasurfaces. Finally, we conclude by providing our opinions of opportunities and challenges in this rapidly developing research field.

A broadband achromatic metalens for focusing and imaging in the visible.

Abstract: A key goal of metalens research is to achieve wavefront shaping of light using optical elements with thicknesses on the order of the wavelength. Such miniaturization is expected to lead to compact, nanoscale optical devices with applications in cameras, lighting, displays and wearable optics. However, retaining functionality while reducing device size has proven particularly challenging. For example, so far there has been no demonstration of broadband achromatic metalenses covering the entire visible spectrum. Here, we show that by judicious design of nanofins on a surface, it is possible to simultaneously control the phase, group delay and group delay dispersion of light, thereby achieving a transmissive achromatic metalens with large bandwidth. We demonstrate diffraction-limited achromatic focusing and achromatic imaging from 470 to 670 nm. Our metalens comprises only a single layer of nanostructures whose thickness is on the order of the wavelength, and does not involve spatial multiplexing or cascading. While this initial design (numerical aperture of 0.2) has an efficiency of about 20% at 500 nm, we discuss ways in which our approach may be further optimized to meet the demand of future applications. Controlling the geometry of each dielectric element of a nanostructured surface enables frequency-dependent group delay and group delay dispersion engineering, and the fabrication of an achromatic metalens for imaging in the visible in transmission.