Showing papers by "Zhen Gao published in 2018"

Spoof Plasmonics: From Metamaterial Concept to Topological Description

Abstract: Advances in metamaterials have offered the opportunity of engineering electromagnetic properties beyond the limits of natural materials. A typical example is "spoof" surface plasmon polaritons (SPPs), which mimic features of SPPs without penetrating into metal, but only with periodic corrugations on metal surfaces. They hold considerable promise in device applications from microwaves to the far infrared, where real SPP modes do not exist. The original spoof SPP concept is derived from the description of corrugated surfaces by a metamaterial that hosts an effective plasma frequency. Later, studies have attempted to describe spoof SPP modes with the band structure by strictly solving Maxwell's equations, which can possess band gaps from polaritonic anticrossing principle or Bragg interference. More recently, as inspired by the development of topological framework in condensed matter physics, the topological description of spoof SPPs is used to propose topologically protected waveguiding phenomena. Here, the developments of spoof SPPs from both practical and fundamental perspectives are reviewed.

Realization of a three-dimensional photonic topological insulator

Abstract: Confining photons in a finite volume is in high demand in modern photonic devices. This motivated decades ago the invention of photonic crystals, featured with a photonic bandgap forbidding light propagation in all directions. Recently, inspired by the discoveries of topological insulators (TIs), the confinement of photons with topological protection has been demonstrated in two-dimensional (2D) photonic structures known as photonic TIs, with promising applications in topological lasers and robust optical delay lines. However, a fully three-dimensional (3D) topological photonic bandgap has never before been achieved. Here, we experimentally demonstrate a 3D photonic TI with an extremely wide (> 25% bandwidth) 3D topological bandgap. The sample consists of split-ring resonators (SRRs) with strong magneto-electric coupling and behaves as a 'weak TI', or a stack of 2D quantum spin Hall insulators. Using direct field measurements, we map out both the gapped bulk bandstructure and the Dirac-like dispersion of the photonic surface states, and demonstrate robust photonic propagation along a non-planar surface. Our work extends the family of 3D TIs from fermions to bosons and paves the way for applications in topological photonic cavities, circuits, and lasers in 3D geometries.

Topological triply-degenerate point with double Fermi arcs

Abstract: Unconventional chiral particles have recently been predicted to appear in certain three dimensional (3D) crystal structures containing three- or more-fold linear band degeneracy points (BDPs). These BDPs carry topological charges, but are distinct from the standard twofold Weyl points or fourfold Dirac points, and cannot be described in terms of an emergent relativistic field theory. Here, we report on the experimental observation of a topological threefold BDP in a 3D phononic crystal. Using direct acoustic field mapping, we demonstrate the existence of the threefold BDP in the bulk bandstructure, as well as doubled Fermi arcs of surface states consistent with a topological charge of 2. Another novel BDP, similar to a Dirac point but carrying nonzero topological charge, is connected to the threefold BDP via the doubled Fermi arcs. These findings pave the way to using these unconventional particles for exploring new emergent physical phenomena.

Past, current, and future research on microalga-derived biodiesel: a critical review and bibliometric analysis

Abstract: Microalga-derived biodiesel plays a crucial role in the sustainable development of biodiesel in recent years. Literature related to microalga-derived biodiesel had an increasing trend with the expanding research outputs. Based on the Science Citation Index Expanded (SCI-Expanded) of the Web of Science, a bibliometric analysis was conducted to characterize the body of knowledge on microalga-derived biodiesel between 1993 and 2016. From the 30 most frequently used author keywords, the following research hotspots are extracted: lipid preparation from different microalga species, microalga-derived lipid and environmental applications, lipid-producing microalgae cultivation, microalgae growth reactor, and microalga harvest and lipid extraction. Other keywords, i.e., microalga mixotrophic cultivation, symbiotic system between microalga and other oleaginous yeast, microalga genetic engineering, and other applications of lipid-producing microalga are future focal points of research.

A bibliometric analysis of biodiesel research during 1991–2015

Abstract: A bibliometric analysis based on the Science Citation Index Expanded (SCI-EXPANDED) from the Web of Science was carried out to provide insights into research activities and tendencies of the global biodiesel from 1991 to 2015. The document type and language, characteristics of publication outputs, Web of Science categories, journals, countries, institutions, author keyword and most cited articles were emphasized. The results indicated that annual output of the related scientific articles increased steadily. The top six categories focus on different aspects of biodiesel research. Bioresource Technology and Fuel were the two most frequent journals in the field of biodiesel research. The USA took a leading position and had the highest h-index (108) out of 122 countries/territories, followed by China and Brazil. Finally, author keywords and most cited articles were analyzed, indicating that microalgae, Jatropha curcas, vegetable oil and waste cooking oil are the most general raw materials for biodiesel production.

Subwavelength wave manipulation in a thin surface-wave bandgap crystal

Abstract: It has been recently reported that the unit cell of wire media metamaterials can be tailored locally to shape the flow of electromagnetic waves at deep-subwavelength scales [Nat. Phys.9, 55 (2013)NPAHAX1745-247310.1038/nphys2480]. However, such bulk structures have a thickness of at least the order of wavelength, thus hindering their applications in the on-chip compact plasmonic integrated circuits. Here, based upon a Sievenpiper “mushroom” array [IEEE Trans. Microwave Theory Tech.47, 2059 (1999)IETMAB0018-948010.1109/22.798001], which is compatible with standard printed circuit board technology, we propose and experimentally demonstrate the subwavelength manipulation of surface waves on a thin surface-wave bandgap crystal with a thickness much smaller than the wavelength (1/30th of the operating wavelength). Functional devices including a T-shaped splitter and sharp bend are constructed with good performance.

Slowing designer surface plasmons in a surface-wave photonic crystal.

Abstract: We propose and experimentally demonstrate a broadband slow-wave system in a surface-wave photonic crystal based on a non-uniform line-defect waveguide with graded metallic pillars, whose dispersions and cutoff frequencies vary gradually along the waveguide. Since the group velocity of designer surface plasmons at the cutoff frequencies is zero, we show that surface waves can be slowed and stopped at different positions under different frequencies. Experimental evidence including transmission measurements and direct near-field profile imaging performed in the microwave frequencies validate the stopping of surface waves in a broadband range within the forbidden bandgap of the surface-wave photonic crystal. This proposal is a promising candidate for slow-wave devices implemented on a single metal surface in both the microwave and terahertz frequency ranges.