Surface plasmon polaritons (SPPs) are localized surface electromagnetic waves that propagate along the interface between a metal and a dielectric. Owing to their inherent subwavelength confinement, SPPs have a strong potential to become building blocks of a type of photonic circuitry built up on 2D metal surfaces; however, SPPs are difficult to control on curved surfaces conformably and flexibly to produce advanced functional devices. Here we propose the concept of conformal surface plasmons (CSPs), surface plasmon waves that can propagate on ultrathin and flexible films to long distances in a wide broadband range from microwave to mid-infrared frequencies. We present the experimental realization of these CSPs in the microwave regime on paper-like dielectric films with a thickness 600-fold smaller than the operating wavelength. The flexible paper-like films can be bent, folded, and even twisted to mold the flow of CSPs. metamaterials | plasmonics | waveguiding S urface plasmon polaritons (SPPs) are highly localized surface waves (1) that propagate along the interface between two materials whose real parts of electric permittivity have opposite signs, and decay exponentially in the transverse direction. At optical frequencies, metals behave like plasma with negative permittivity, and thus SPPs exist on metal-air interfaces (2, 3). Owing to their ability to confine light in a subwavelength scale with high intensity, SPPs can be used to overcome the diffraction limit, miniaturize photonic components, and build highly integrated optical components and circuits. Thus, they have found (or have potential) applications in biomedical sensing, near-field microscopy, optoelectronics, photovoltaics, and nanophotonics (4-11).In the far-infrared, terahertz, and microwave frequency bands, metals behave akin to perfectly electrical conductors (PECs), and thus SPPs cannot be supported by a metal surface. Although some designs based on metal wires or strips are able to support surface leaky modes that have some degree of lateral confinement at terahertz frequencies (12, 13), the concept of plasmonic metamaterials has proven very useful in the production of highly confined surface electromagnetic (EM) waves at low frequencies (14-27). Early work in this area can be traced back to the 1950s and 1960s, when corrugated metal structures were used to generate surface EM waves at microwave frequencies (14, 15). Generally, plasmonic metamaterials consist of metal surfaces decorated with 1D arrays of subwavelength grooves, 2D arrays of subwavelength holes/dimples, or 3D metal wires in which a periodic array of radial grooves is drilled (16-26). Recently, an alternative "spoof" SPP structure using complementary split-ring resonators as the unit cell elements has been proposed theoretically (27). The surface EM modes decorated by all of these plasmonic metamaterials are called spoof SPPs, or designer SPPs, because their properties are very similar to those of SPPs at optical frequencies. An important advantage of this metamaterial approach ...

The conversion from spatial propagating waves to surface plasmon polaritons (SPPs) has been well studied, and shown to be very efficient by using gradient‐index metasurfaces. However, feeding energies into and extracting signals from functional plasmonic devices or circuits through transmission lines require the efficient conversion between SPPs and guided waves, which has not been reported, to the best of our knowledge. In this paper, a smooth bridge between the conventional coplanar waveguide (CPW) with 50 Ω impedance and plasmonic waveguide (e.g., an ultrathin corrugated metallic strip) has been proposed in the microwave frequency, which converts the guided waves to spoof SPPs with high efficiency in broadband. A matching transition has been proposed and designed, which is constructed by gradient corrugations and flaring ground, to match both the momentum and impedance of CPW and the plasmonic waveguide. Simulated and measured results on the transmission coefficients and near‐filed distributions show excellent transmission efficiency from CPW to a plasmonic waveguide to CPW in a wide frequency band. The high‐efficiency and broadband conversion between SPPs and guided waves opens up a new avenue for advanced conventional plasmonic integrated functional devices and circuits.

This paper presents a wide-angle polarization independent triple-band absorber based on a metamaterial structure for microwave frequency applications. The designed absorber structure is the combination of two resonators (resonator-I and resonator-II). The proposed absorber is ultra-thin in thickness (0.012λ o at lowest resonance frequency and 0.027λ o at highest resonance frequency). The proposed absorber structure offers three absorption bands with peak absorptivities of 99.95%, 95.32% and 99.47% at 4.48, 5.34 and 10.43 GHz, respectively. Additionally, it also offers the full width at half maximum (FWHM) bandwidth of 167.2 MHz (4.40-4.56 GHz), 178.1 MHz (5.25-5.43 GHz) and 393.8 MHz (10.24-10.63 GHz), respectively. The metamaterial property of the designed absorber structure has been discussed by using dispersion diagram plot. The designed absorber structure exhibits wide-angle absorption at various oblique incidence angle for both TM and TE polarizations. The absorption mechanism of the designed absorber structure has been analyzed through electric field and surface current distribution plots. The input impedance of the designed absorber (375.67 Ω at 4.48 GHz and 346.73 Ω at 10.43 GHz) nearly matches the free space impedance. The proposed absorber structure is fabricated and measured. Simulated and measured results are in good agreement with each other.

Developing nonprecious hydrogen evolution electrocatalysts that can work well at large current densities (e.g., at 1000 mA/cm: a value that is relevant for practical, large-scale applications) is of great importance for realizing a viable water-splitting technology. Herein we present a combined theoretical and experimental study that leads to the identification of α-phase molybdenum diboride (α-MoB) comprising borophene subunits as a noble metal-free, superefficient electrocatalyst for the hydrogen evolution reaction (HER). Our theoretical finding indicates, unlike the surfaces of Pt- and MoS-based catalysts, those of α-MoB can maintain high catalytic activity for HER even at very high hydrogen coverage and attain a high density of efficient catalytic active sites. Experiments confirm α-MoB can deliver large current densities in the order of 1000 mA/cm, and also has excellent catalytic stability during HER. The theoretical and experimental results show α-MoB's catalytic activity, especially at large current densities, is due to its high conductivity, large density of efficient catalytic active sites and good mass transport property.

We demonstrate the design, characterization, and interference-theory interpretation of a terahertz triple-band metamaterial absorber (MA). The experiments show that the fabricated MA has three distinctive absorption peaks at 0.5, 1.03, and 1.71 THz with absorption rates of 96.4%, 96.3%, and 96.7%, respectively. We use the multi-reflection interference theory to investigate the physical insight of the proposed triple-band terahertz MA, which provides a design guideline for MA of such type. The theoretical predictions of the interference model have excellent agreements with experimental results. The designed multiband absorber is easy to manufacture and insensitive to incident polarizations with high absorption, which is favorable for various applications.

The studies of topological phases of matter have been extended from condensed matter physics to photonic systems, resulting in fascinating designs of robust photonic devices. Recently, higher-order topological insulators (HOTIs) have been investigated as a novel topological phase of matter beyond the conventional bulk-boundary correspondence. Previous studies of HOTIs have been mainly focused on the topological multipole systems with negative coupling between lattice sites. Here we experimentally demonstrate that second-order topological insulating phases without negative coupling can be realized in two-dimensional dielectric photonic crystals (PCs). We visualize both one-dimensional topological edge states and zero-dimensional topological corner states by using nearfield scanning technique. To characterize the topological properties of PCs, we define a topological invariant based on the bulk polarizations. Our findings open new research frontiers for searching HOTIs in dielectric PCs and provide a new mechanism for light-manipulating in a hierarchical way.Introduction.-One of the most enchanting developments of condensed matter physics over the past few decades has been the discovery of topological phases of matter primarily found in electronic systems [1,2] and recently extended to bosonic systems such as photonics [3][4][5][6][7][8][9][10][11][12][13][14] and phononics [16][17][18][19][20][21]. A key feature of the topological insulators is the backscattering-immune edge states which are robust against perturbations and provide potential designs of various topological devices [3-6, 18, 20]. Typically, n-dimensional (nD) topological insulators (TIs) have (n − 1)D edge states which is defined as the bulk-boundary correspondence (BBC) [15]. However, a new kind of TIs defined as the higher-order topological insulators (HOTIs), have been recently proposed in tight-binding models in electronic systems which go beyond the traditional BBC description [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36]. Concretely, the mth-order TIs have nD gapped bulk states and (n − 1)D, (n − 2)D, ..., (n − m − 1)D gapped edge states while having (n − m)D gapless edge states. The arising of these lower-dimensional topological edge states can either stem from the quantization of quadrupole moments such as the topological quadrupole insulators [22] which have been realized in mechanics [23], microwave systems [24] and topolectrical circuits [25], or stem from the quantization of the dipole moments [22] such as the HOTIs in 2D breathing kagome lattice [29] which have been realized in sonic crystals [33-35] and a waveguide array [32].

Here, we introduce the concept of magnetic localized surface plasmons (LSPs), magnetic dipole modes that are supported by cylindrical metal structures corrugated by very long, curved grooves. The resonance wavelength is dictated by the length of the grooves, allowing us to tune it to values much larger than the size of the particle. Moreover, magnetic LSPs also exist for extremely thin metal disks and, therefore, they could be used to devise metasurfaces with magnetic functionalities. Experimental evidence of the existence of these magnetic LSPs in the microwave regime is also presented, although the concept is very general and could be applied to terahertz or infrared frequencies.

The multipolar spoof localized surface plasmons (LSPs) on a planar textured metallic disk are proposed and experimentally demonstrated at microwave frequencies. Based on ultrathin metal film printed on a thin dielectric substrate, the designed plasmonic metamaterial clearly shows multipolar plasmonic resonances, including the dipole, quadrupole, hexapole, octopole, decapole, dodecapole, and quattuordecpole modes. Both numerical simulations and experiments are in good agreement. It is shown that the spoof LSP resonances are sensitive to the disk's geometry and local dielectric environments. Hence, the ultrathin textured metallic disk may be used as plasmonic sensors and find potential applications in the microwave and terahertz frequencies.