VO2 based dynamic tunable absorber and its application in switchable control and real-time color display in the visible region.

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 VO2 thin film into the metal-insulator-metal structured metasurface based absorber, and design a fully visible band switchable dynamically tunable absorber (DTA). By controlling the phase transition of VO2, the DTA can realize a novel switch function in the full band of visible light (400 ∼ 780 nm), with absorption contrast ranges from 42% to 60%. Furthermore, via accurate structural parameter control, the vivid cyan, magenta, and yellow pixels based on the VO2 DTA are designed and proposed in the real-time optical anti-counterfeiting, exhibiting outstanding characteristics of anti-glare interference and real-time encryption ability. The absorption spectrum and local electric field are simulated and analyzed to study the internal operation mechanism of DTA. The dynamic absorption adjustable function is attributed to the synergistic effect of insulator-metal transition of VO2 and Fabry-Pérot resonance of absorber.

Electrochemically Synthesized Porous Ag Double Layers for Surface-Enhanced Raman Spectroscopy Applications.

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.

Greatly enhanced electric field by the improved metal-insulator-metal structure in the visible region.

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.

Omnidirectional Absorber by the Void Plasmon Effect in the Visible Region with Greatly Enhanced Localized Electric Field.

We propose and investigate a wide-angle and high-efficiency absorber by using the void plasmon (VP) effect in a Fabry-Perot (FP)-like system, which consists of a perforated metal film and a ground metal plane separated by a dielectric spacer. A hybrid FP/VP resonance mode contributes to the high absorption efficiency. Besides the increased absorption, greatly enhanced localized electric-field intensity at "hot spots" (~ 2284 times) can be achieved. In addition, by varying the thickness of the perforated metal layer and the environmental refractive index, the position of resonance peak can be easily controlled. The proposed absorber can also work as a sensor for detecting the surrounding dielectric constant with the maximum value of the figure of merit (FOM) achieving 3.16 in theory. This work creates an alternative design for high-efficiency absorption devices.

Localized Surface Plasmon Resonance-Mediated Charge Trapping/Detrapping for Core-Shell Nanorod-Based Optical Memory Cells.

For following the trend of miniaturization as per Moore's law, increasing efforts have been made to develop single devices with versatile functionalities for Internet of Things (IoT). In this work, organic optical memory devices with excellent dual optoelectronic functionality including light sensing and data storage have been proposed. The Au@Ag core-shell nanorods (NRs)-based memory device exhibits large memory window up to 19.7 V due to the well-controlled morphology of Au@Ag NRs with optimum size and concentration. Furthermore, since the extinction intensity of Au@Ag NRs gradually enhance with the increase in Ag shell thickness, the phototunable behaviors of memory device were systematically studied by varying the thickness of Ag shell. Multilevel data storage can be achieved with the light assistant. Finally, the simulation results demonstrate that the phototunable memory property is originated from the multimode localized surface plasmon resonance (LSPR) of Au@Ag NRs, which is in consistent with the experimental results. The Au@Ag core-shell NRs-based memories may open up a new strategy toward developing high-performance optoelectronic devices.

Wide angle and narrow-band asymmetric absorption in visible and near-infrared regime through lossy Bragg stacks.

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.

Triple-layer Fabry-Perot/SPP aluminum absorber in the visible and near-infrared region.

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.

Triple-layer Fabry-Perot absorber with near-perfect absorption in visible and near-infrared regime.

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.

Selective removal of the outer shells of anodic TiO2 nanotubes.

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.

Porous metal-based multilayers for selective thermal emitters.

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 for near-perfect absorbers in visible and near-infrared regions.

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.