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Metamaterial based near perfect absorbers have a wide range of potential applications in sensors, photovoltaic devices, thermal imaging and other fields due to their unique absorption properties. However, the absorption characteristics of traditional absorbers are based on their static structure. When the structural parameters is fixed, their absorption characteristics is structure locked. In this work, we design a broadband dynamic tunable absorber based on pyramid array structure using phase change material VO 2 . The absorption spectrum of the structure in the range of 0.4 ~ 8um is simulated by 3D finite difference time domain method. By controlling the phase transition of VO 2 and structure parameters of pyramid array, the dynamic tunable absorber based on VO 2 shows excellent switching characteristics, and the difference of absorptiion before and after phase transition in 3 ~ 8um band can reach more than 0.75.

Metasurface-based near perfect absorbers exhibit a wide range of potential applications in the fields of solar energy harvesting, thermal images and sensors due to their unique absorption regulation function. However, absorption characteristics of devices are locked by the device structure, leading to the limitation in real-time dynamic applications. In this work, we integrate the phase change material VO

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This letter reports the design of a magneto-elastic metasurface composed of arrayed Terfenol-D pillars deposited on a homogeneous aluminum plate, aiming to realize the tunability of flexural wave anomalous propagation without altering the structure. Considering the magneto-mechanical coupling of magnetostrictive materials, the phase shift and transmission of the functional unit can be calculated. The anomalous refraction of incident flexural wave (i.e. negative refraction) can be accomplished by adjusting the magnetic field and pre-stress properly, the refraction angle is remarkably affected by magnetic distribution. The proposed metasurface provides a method for flexible tunability of elastic waves in the fields of vibration/noise control.

Double-layered nanoporous silver is fabricated by dealloying an electrodeposited AgCu double layer with different compositions in each layer. The pore/ligament size and porosity of each layer can be conveniently tailored by controlling the applied voltage profile when electrodepositing the AgCu double-layer precursors. Therefore, nanoporous Ag double layers with a tailor-made porous profile along the film thickness can be easily fabricated. The Ag structures thus obtained are particularly attractive as novel multifunctional enhancement substrates for surface-enhanced Raman spectroscopy (SERS) applications. When a higher porosity is created in the top layer, the double layer can trap more light because of the antireflection effect, enabling stronger SERS enhancement. On the other hand, with smaller pores formed in the top layer, the double layer readily works as a size-screening SERS substrate that can help distinguish SERS signals from a mixture of reagents with different sizes. The theoretical simulation shows good agreement with the experimental observation.

Direct synthesis of three-dimensional Ag structures on solid substrates for the purposes of producing reproducible and recyclable surface-enhanced Raman scattering (SERS) applications remains challenging. In this work, flower-like Ag structures with concave surfaces (FACS) were successfully electrodeposited onto ITO glass using the double-potentiostatic method. The FACS, with an enhancement factor of the order of 10 8 , exhibited a SERS signal intensity 3.3 times stronger than that measured from Ag nanostructures without concave surfaces. A cleaning procedure involving lengthy immersion of the sample in ethanol and KNO 3 was proposed to recycle the substrate and confirmed by using rhodamine 6G, adenine, and 4-aminothiophenol as target molecules. The findings can help to advance the practical applications of Ag nanostructure-based SERS substrates.

Dealloying is an important industrial technique for generating nanoporous metallic structures by selectively leaching out the more reactive metal component from an alloy material. A constant voltage is often applied to facilitate the dealloying process. Here we report the first study on dealloying with the application of a voltage waveform—specifically, pulsed voltage waveforms are applied for dealloying Ni–Cu alloys. It is found that pulsed dealloying voltage waveforms can exert a strong impact on the dealloying process by 1) significantly lowering the compositional threshold of the more reactive metal component for the dealloying reaction to take place, 2) more thoroughly removing the more reactive metal component and thus producing a porous metal of higher purity and higher porosity (volume fraction of voids), and 3) greatly affecting the morphology of the generated porous metal structure (e.g., leading to significantly thinner ligaments). The nanoporous metallic materials obtained by the pulsed voltage waveform enable supercapacitor electrodes of significantly better performance than the counterpart dealloyed with a constant voltage.

We report a study on the introduction of the coupling effect of nanostructures into the metal-insulator-metal (MIM) based absorber to enhance the intensity of incident optical signal. A system including square-shaped holes with an embedded gold nanorods structure is elaborately designed and integrated into the MIM based absorber. Through the fine-tuning of size, shape and material inside the system, a strong coupling effect is formed among the void plasmons resonance mode of nanoholes, the localized surface plasmons resonance mode of nanorods and the Fabry-Perot resonance of the middle layer cavity. And the maximum strength of electric field intensity is enhanced to 4000 times at 'hotspots' in the visible region. The proposed absorber here can realize an enhanced coupling effect as well as keep the high absorption efficiency, showing a great application potential in the field of effective optical signal amplification.

Absorber is an important component in various optical devices. Here we report a novel type of asymmetric absorber in the visible and near-infrared spectrum which is based on lossy Bragg stacks. The lossy Bragg stacks can achieve near-perfect absorption at one side and high reflection at the other within the narrow bands (several nm) of resonance wavelengths, whereas display almost identical absorption/reflection responses for the rest of the spectrum. Meanwhile, this interesting wavelength-selective asymmetric absorption behavior persists for wide angles, does not depend on polarization and can be ascribed to the lossy characteristics of the Bragg stacks. Moreover, interesting Fano resonance with easily tailorable peak profiles can be realized using the lossy Bragg stacks.

We report on Ni-modified TiO2 nanotubes with improved photocatalytic properties. Using the as-anodized TiO2 nanotubes as templates, Ni was electrodeposited using pulsed current waveforms. It was found that the Ni deposition was first initiated at the bottoms of the intertubular voids and then grew upward, resulting in a Ni/TiO2 coaxial nanostructure with Ni wrapping around the TiO2 nanotubes. Moreover, the tube inside was kept empty and tube openings unclogged for the fabricated Ni/TiO2 nanocomposites. Further photodegradation tests using methyl red revealed that the fabricated Ni/TiO2 nanocomposites possess higher photocatalytic efficiency than the counterparts of pristine TiO2 nanotubes. The observed improved photocatalytic efficiency is ascribed to the Schottky barriers formed between Ni and TiO2.

Metallic rugate filters are theoretically investigated for the first time. Our model builds on randomly dispersed metallic nanorod arrays with sinusoidally varying porosities along the film thickness. It was found that metallic rugate filters can display strong optical response in the visible and near-infrared regions for both TE and TM polarizations and for a wide range of incident angles. Moreover, the optical features of a metallic rugate filter can be fine tuned by adjusting its structural features. Interestingly, multiple-peak rugate filters can be conveniently achieved by combining different periodicities in one rugate structure. The interesting optical behaviors of the novel metal-based reflectors, combined with the many desirable properties of metals (e.g. electric, thermal, magnetic, mechanical, and chemical properties) are bound to lead to various multi-functional optical devices.

Self-organized TiO2 nanomaterials grown by anodic oxidation of Ti foils have attracted broad scientific interest due to their wide potential applications. The majority of anodic TiO2 nanostructures studied to date are in the form of self-ordered nanotube arrays. Here we report an exotic type of multilayered nanoporous TiO2 films that are conveniently fabricated by anodizing Ti foils of high imperfection levels using a novel multipulse anodization method. The fabricated TiO2 films feature tens of well-defined layers whose long-range structural periodicity leads to photonic band gaps in the visible wavelengths and vivid film colors. Moreover, the optical responses (or the film color) of the fabricated TiO2 multilayers not only are sensitive to environmental chemicals but also can be electrically switched on and off repeatedly, demonstrating their novel potential applications for, e.g., colored smart windows or electric display boards.

We present the Terahertz wave extraordinary transmission through thin metal film with periodic arrays of subwavelength rectangular holes. By designing metallic hole arrays with different periods along x and y direction, the transmission spectrum with a non-Fano type is obtained. The roles of shape resonances and Rayleigh anomalies in extraordinary transmission are clarified. Our results demonstrate that the enhanced transmission can be attributed to the shape resonance, and the position of transmission minimum comes from the Rayleigh anomaly which is related to the period of the holes array. Shape resonance in hole arrays is different from the classical waveguide resonance, instead, the resonance behaves a surface plasmon polariton-like character but it is irrelevant to the period of the hole arrays. Our finding is applicable to exploit terahertz field localization which can be used to study terahertz nonlinear spectroscopy of materials.

The anomalous transmission through one-dimensional lamellar metallic gratings was investigated in terahertz (THz) regime. The extraordinary optical transmission (EOT) is identified to originate from two possible ways: coupling of incident light with waveguide resonances and coupling of surface plasmon polaritons (SPPs) at the upper and lower interfaces of metal grating. The dual effects of SPPs have been clarified in this study: (i) the excitation of SPP modes at each individual interface results in the weakness of the THz wave transmission; and (ii) the coupling of SPP modes at two interfaces of metal grating is attributed to enhancement of THz wave transmission. The enhanced transmission is dominated by the coupling of incident light with transverse waveguide resonances. Numerical simulation based on finite-difference time-domain (FDTD) agrees well with experimental results.

Terahertz (THz) wave transmission through a thin metal film with periodic arrays of subwavelength rectangular holes is investigated systematically. The roles of the waveguide resonances and surface plasmon polaritons (SPPs) in extraordinary transmission are also clarified. The transverse waveguide resonances (hole-shape dependence) dominates the position and width of maximum transmission. In addition, the periodicity of the hole arrays dominates the position of minimum transmission due to the excitation of SPP, which acts as a band-stop filter in the THz transmission spectra. Our results clearly demonstrate that the extraordinary transmission in THz frequency is originated from the transverse waveguide resonances that behave with a SPP-like character. Simulation based on finite-difference time-domain (FDTD) agrees well with the experimental results.

We report the numerical study of a selective thermal emitter based on a metallic multilayered structure consisting of a graded antireflection top layer, a middle layer with uniform porosity (i.e., volume fraction of voids), and a nonporous substrate layer. Simulation results show that the proposed emitters feature an emission edge in near-IR where the emissivity drops from over 0.9 to below 0.1, for both the TE and TM polarizations. Moreover, these desired emission characteristics persist for a wide range of emission angles with the emission edge nearly nonshifted, making the proposed emitters promising for achieving isotropic thermal emission. The designed emitters are particularly attractive for the thermal-photovoltaic applications by suppressing emission below the photovoltaic material bandgap, which is normally in near-IR.

Metallic rugate structures are theoretically investigated for achieving near-perfect absorption in the visible and near-infrared regions. Our model builds on nanoporous metal films whose porosity (volume fraction of voids) follows a sinewave along the film thickness. By setting the initial phase of porosity at the top surface as 0, near-perfect absorption is obtained. The impacts of various structural parameters on the characteristic absorption behaviors are studied. Furthermore, multiple peaks or bands with high absorption can be achieved by integrating several periodicities in one structure. The rugate absorbers show near-perfect absorption for TE and TM polarizations and large incident angles.

A facile electrochemical method to selectively remove the outer walls of anodic TiO(2) nanotubes by leaving the as-anodized nanotubes in the same electrolyte and applying an electric field parallel to the anodic film for several minutes is reported. The better-separated single-walled TiO(2) nanotubes thus obtained show significantly improved photocatalytic efficiency compared with their non-etched counterparts.

We report a theoretical study on a novel type of absorber that can achieve near perfect absorption in the visible and near-infrared regions by utilizing the Fabry-Perot and the surface plasmon polariton (SPP) effects. The absorber consists of an Al/dielectric/Al triple-layered structure with the top Al layer consisting of an array of holes. The absorption features can be easily controlled by tuning the structural parameters, particularly the porous features of the top Al layer. When the porous features in the top Al layer are significantly smaller than the wavelength, light absorption is enabled through the Fabry-Perot effect. On the other hand, when the porous features in the top layer are at the subwavelength scale, new absorption peaks emerge due to the SPP effect. Furthermore, when the top Al layer consists of an array of hollow rings, the electric field at the interface between the top Al layer and the middle dielectric layer is greatly enhanced due to the plasmonic effect, indicating that the absorber reported here may be suitable for novel applications, e.g., the surface-enhanced Raman spectroscopy (SERS) substrates.

This paper presents a combinational technique of approximate wavelet transform and time-frequency analysis for detection of moving targets in SAR signals. Approximate wavelet transform is based on the matched filtering and its inverse process. It offers an approach of suppressing background clutters and noises for time-frequency analysis in terms of the differences of the rate of frequency change between the moving target signals and the stationary signals. We develop also an improved Wigner-Ville Distribution (WVD) for time-frequency analysis in which the cross terms are suppressed efficiently.

A simple absorber design which enables near-perfect absorption in the visible and near-infrared regions is presented. The absorber is an unpatterned metal/dielectric/metal triple-layer, e.g., a 20 nm-thick metal film as the top layer, a 250 nm-thick dielectric film as the middle layer, and a 200 nm-thick metal film as the bottom layer. It was found that the high-efficiency absorption at specific wavelengths is mainly due to the Fabry-Perot (FP) resonances in the dielectric middle layer which result in trapping of the resonant light in the middle layer and thus enhanced absorption efficiency.