Qiugu Wang

From Flexible and Stretchable Meta-Atom to Metamaterial: A Wearable Microwave Meta-Skin with Tunable Frequency Selective and Cloaking Effects.

This paper reports a flexible and stretchable metamaterial-based “skin” or meta-skin with tunable frequency selective and cloaking effects in microwave frequency regime. The meta-skin is composed of an array of liquid metallic split ring resonators (SRRs) embedded in a stretchable elastomer. When stretched, the meta-skin performs as a tunable frequency selective surface with a wide resonance frequency tuning range. When wrapped around a curved dielectric material, the meta-skin functions as a flexible “cloaking” surface to significantly suppress scattering from the surface of the dielectric material along different directions. We studied frequency responses of multilayer meta-skins to stretching in a planar direction and to changing the spacing between neighboring layers in vertical direction. We also investigated scattering suppression effect of the meta-skin coated on a finite-length dielectric rod in free space. This meta-skin technology will benefit many electromagnetic applications, such as frequency tuning, shielding, and scattering suppression.

Tunable meta-atom using liquid metal embedded in stretchable polymer

Reconfigurable metamaterials have great potential to alleviate complications involved in using passive metamaterials to realize emerging electromagnetic functions, such as dynamical filtering, sensing, and cloaking. This paper presents a new type of tunable meta-atoms in the X-band frequency range (8–12 GHz) toward reconfigurable metamaterials. The meta-atom is made of all flexible materials compliant to the surface of an interaction object. It uses a liquid metal-based split-ring resonator as its core constituent embedded in a highly flexible elastomer. We demonstrate that simple mechanical stretching of the meta-atom can lead to the great flexibility in reconfiguring its resonance frequency continuously over more than 70% of the X-band frequency range. The presented meta-atom technique provides a simple approach to dynamically tune response characteristics of metamaterials over a broad frequency range.

Ultrathin amorphous silicon thin-film solar cells by magnetic plasmonic metamaterial absorbers

Efficient solar harvesting for ultrathin amorphous silicon (α-Si) films with a thickness of less than 100 nm is critical to the performance of solar cells, since the very short carrier-diffusion length of α-Si and the Staebler–Wronski effect restrict their thickness. In this work, we numerically investigate energy harvesting in metamaterial-based solar cells, in which an ultrathin α-Si film is sandwiched between a silver (Ag) substrate and a square array of Ag nanodisks, and combined with an indium tin oxide (ITO) anti-reflection layer. It is found that only a 20 nm-thick α-Si film is able to absorb over 50% solar energy in the spectral range from 300 to 800 nm at normal incidence, and the amount of absorbed light is equivalent to a photocurrent of about 13.4 mA cm−2. This broadband absorption is achieved by the spectral design on the overlapped absorption peaks which are caused by the excitations of two lowest-order Fabry–Perot (FP) resonances in the α-Si and ITO layers and a magnetic resonance arising from the plasmon hybridization between Ag disks and the substrate. The absorption performance of our structure is less dependent on the incident angle θ and polarization of light when θ 70° (20°) for P (S) polarization.

Tunable Optical Nanoantennas Incorporating Bowtie Nanoantenna Arrays with Stimuli-Responsive Polymer.

We report on a temperature-responsive tunable plasmonic device that incorporates coupled bowtie nanoantenna arrays (BNAs) with a submicron-thick, thermosensitive hydrogel coating. The coupled plasmonic nanoparticles provide an intrinsically higher field enhancement than conventional individual nanoparticles. The favorable scaling of plasmonic dimers at the nanometer scale and ionic diffusion at the submicron scale is leveraged to achieve strong optical resonance and rapid hydrogel response, respectively. We demonstrate that the hydrogel-coated BNAs are able to sense environmental temperature variations. The phase transition of hydrogel leads to 16.2 nm of resonant wavelength shift for the hydrogel-coated BNAs, whereas only 3 nm for the uncoated counterpart. The response time of the device to temperature variations is only 250 ms, due to the small hydrogel thickness at the submicron scale. The demonstration of the ability of the device to tune its optical resonance in response to an environmental stimulus (here, temperature) suggests a possibility of making many other tunable plasmonic devices through the incorporation of coupled plasmonic nanostructures and various environmental-responsive hydrogels.

Electrically Tunable Quasi-3-D Mushroom Plasmonic Crystal

This paper reports an electrically tunable plasmonic crystal incorporating a nematic liquid crystal (LC) layer on the top surface of quasi-3-D mushroom plasmonic nanostructures. The presented plasmonic crystal is formed by an array of polymeric mushroom nanoposts with gold disks at the top and perforated nanoholes in a gold thin film at the bottom. The coupling between surface plasmon polariton (SPP) and Rayleigh anomaly (RA) is observed in experiments with quasi-3-D plasmonic crystals, and verified by simulations. The coupled SPP-RA resonance mode has its electric field vector prominently normal to the surface of the plasmonic nanostructures, and extends into the surrounding medium. This feature makes the coupled resonance sensitive to molecular reorientation in LC, and thus, is useful for designing index modulation-based tunable plasmonic crystal devices. Therefore, by applying external voltages across the LC layer, the SPP-RA resonance mode shows a redshift of 8 nm with a 35% change in the amplitude.

Tunable bioelectrodes with wrinkled-ridged graphene oxide surfaces for electrochemical nitrate sensors

The paper reports on controlled formation of microscale wrinkles and ridges on the surface of a bioelectrode via mechanical stretching to tune and optimize the electrochemical sensing performances of graphene oxide (GO) based nitrate ion sensors. The bioelectrode consists of GO nanosheets drop-coated on a gold (Au) layer with a pre-stretched elastomer substrate. Enzyme nitrate reductase is used for covalent immobilization on the wrinkled-ridged GO surface. Upon relaxation from the pre-stretch, wrinkles or ridges are formed in the GO layer. As the pre-stretch increases, the sinusoidal wrinkles transform to localized ridges on the surface of bioelectrodes. Such morphological transitions, realized by simple mechanical stretching and relaxing, allow optimizing of the electrochemical current and sensing characteristics of the nitrate sensor. The sensing performances of the bioelectrodes at different pre-stretches are investigated. In addition to an increased electroactive surface area, the predominant localized ridges with small sinusoidal wrinkles formed on the GO surface provide a favorable spatial feature, enabling efficient radial diffusion of nitrate ions from surrounding analyte solutions onto the surface of the textured bioelectrode. At the pre-stretch of 8%, the nitrate sensor using the wrinkled-ridged bioelectrode exhibits a considerably high sensitivity of 0.224 μA L mol−1 cm−2 in response to nitrate ions, which is five times higher than that provided by the planar counterpart. Also, the textured bioelectrode shows high selectivity even in the presence of other inferring ions. The present nitrate sensor has potential applications in nitrate detection in sustainable agriculture, environmental monitoring, food analysis, and pharmaceutical industries.

Surface-Plasmon-Polaritons-Assisted Enhanced Magnetic Response at Optical Frequencies in Metamaterials

We theoretically study the coupling of magnetic plasmon polaritons (MPPs) with propagating surface plasmon polaritons (SPPs) in a system composed of an array of metal nanowires close to a metal film with a dielectric spacer. Strong coupling between MPPs and SPPs is observed, manifested by the anticrossing behavior of the resonant positions in the reflection spectra. It creates narrow-band hybridized MPPs with Rabi-type splitting as large as 250 meV. Moreover, we also found that the coupling between the MPPs and the SPPs can be tailored by the period of the metal nanowire array to affect the magnetic response of the plasmonic structure. Above the resonant wavelength of the MPPs, coupling between two kinds of resonance modes can lead to a 20-fold enhancement of the magnetic fields in the dielectric spacer, as compared with the pure magnetic resonance upon the excitation of the hybridized MPPs, whereas below it, coupling cannot lead to a magnetic field enhancement. We suggest that this feature could offer a feasible way to achieve huge magnetic field enhancement at optical frequencies and hold promising potential applications in magnetic nonlinearity and sensors.

Toroidal Dipolar Response in Metamaterials Composed of Metal–Dielectric–Metal Sandwich Magnetic Resonators

Toroidal metamaterials have been drawing increasing interest recently because of their unusual electromagnetic properties and a variety of potential applications. In this work, we have investigated numerically toroidal dipolar response at optical frequency in metamaterials whose unit cell includes three magnetic resonators. The magnetic resonators are metal-dielectric-metal sandwich nanostructures, which are composed of two Ag rods and a SiO2 spacer. They have the same shape and dimension, but they are placed at different positions to break the space-inversion symmetry. The near-field plasmon coupling between magnetic resonators leads to the excitation of a toroidal dipolar mode, which is characterized by a head-to-tail distribution of magnetic dipoles within magnetic resonators. In our designed toroidal metamaterials, space-inversion symmetry breaking is needed only in the polarization direction of incident light, and light can be normally incident on the toroidal metamaterials.

Strain-tunable plasmonic crystal using elevated nanodisks with polarization-dependent characteristics

This paper reports on the mechanical tuning of optical resonances of a flexible plasmonic crystal. The device is structured with a square lattice nanopost array standing out of an elastomer substrate and coated with a gold thin film. The gold nanodisks residing on top of the nanoposts support a surface plasmon polariton (SPP) Bloch wave mode at the gold-air interface. By applying a strain along a planar direction of the substrate, the period of the elevated nanodisk array changes, thus altering the SPP resonance wavelength. Because the applied strain breaks period symmetry of the nanodisk array, the original single resonance mode is split into two polarized resonance modes. For the incident light polarized parallel with and perpendicular to the direction of the applied strain, the corresponding resonance modes are shifted in opposite directions at a rate of 1.6 ± 0.1 nm for every 1% change in strain. During stretching and compressing the substrate, the applied strains only change the period between nearby...

Toroidal Dipolar Excitation in Metamaterials Consisting of Metal nanodisks and a Dielectrc Spacer on Metal Substrate

We have investigated numerically toroidal dipolar excitation at optical frequency in metamaterials whose unit cell consists of three identical Ag nanodisks and a SiO2 spacer on Ag substrate. The near-field plasmon hybridization between individual Ag nanodisks and substrate forms three magnetic dipolar resonances, at normal incidence of plane electromagnetic waves. The strong coupling among three magnetic dipolar resonances leads to the toroidal dipolar excitation, when space-inversion symmetry is broke along the polarization direction of incident light. The influences of some geometrical parameters on the resonance frequency and the excitation strength of toroidal dipolar mode are studied in detail. The radiated power from toroidal dipole is also compared with that from conventional electric and magnetic multipoles.