Temperature-tunable terahertz metamaterial device based on VO2 phase transition principle.

Terahertz devices play an irreplaceable role in the development of terahertz technology. However, at present, it is difficult for most natural materials to respond in the terahertz band, making the devices made of them perform poorly. In order to realize the diversity and tunability of device functions, we designed a terahertz metamaterial device composed of the thermally-induced phase change material VO2. The device structure is composed of a Au bottom layer, a SiO2 dielectric layer and a VO2 top layer. Through software simulation, we found that when T = 313 K, the device has complete reflection ability in the whole terahertz band. When T = 342 K, the average absorptivity is above 95% in the ultra-wide band range of 4.71-9.41 THz, and the absorptivity reaches an amazing 0.99999 at 6.31 THz. Thus, the maximum thermal modulation range of the device is 0.001-0.99999. The Bruggeman effective medium theory clarifies the phase transition characteristics of vanadium dioxide. The Drude model establishes the functional relationship between the conductivity of vanadium dioxide and temperature. The basic principle of high absorption was described using the impedance matching theory. We also drew the electric field intensity diagram during the temperature rise of the device to further confirm the reason for the change in the device performance. In addition, the influence of the absence of different structural layers on the absorptivity was simulated, which reflected the role of each layer structure more intuitively. We also explored the influence of the geometric size of the device on the absorptivity, which provided a certain reference value for practical application. In short, we have designed a tunable terahertz device with simple structure, high absorptivity, and wide absorption bandwidth, which can be used in the fields of energy collection, electromagnetic stealth, and modulation.

Three peak metamaterial broadband absorbing materials based on ZnSe-Cr-InAs stacked disk arrays.

Metamaterial absorbers show great potential in many scientific and technological applications by virtue of their sub-wavelength and easy-to-adjust structure, with bandwidth as an important standard to measure the performance of the absorbers. In this study, our team designed a new broadband absorber, which consists of an indium arsenide (InAs) disk at the top, a zinc selenide (ZnSe)-chromium (Cr) stacked disk in the middle and a metal film at the bottom. Simulation results show that the absorber has remarkable absorptivity properties in the mid-long infrared band. In a wavelength range of 5.71-16.01 µm, the average absorptivity is higher than 90%. In the band of 5.86-15.49 µm, the absorptivity is higher than 95%. By simulating the electromagnetic field diagram at each resonant frequency, the reason for high broadband absorptivity is obtained. We also constructed Poynting vector diagrams to further elucidate this phenomenon. Next, we analyzed the influence of different materials and structural parameters on absorptivity properties and tested spectral response at different polarization angles and oblique incidence of the light source in the TM and TE modes. When the source is normally incident, the absorber shows polarization insensitivity. When the angle is 40°, absorptivity is still high, indicating that the absorber also possesses angle insensitivity. The broadband absorber proposed by us has good prospects in infrared detection and thermal radiators.

Dual-Tuned Terahertz Absorption Device Based on Vanadium Dioxide Phase Transition Properties.

In recent years, absorbers related to metamaterials have been heavily investigated. In particular, VO2 materials have received focused attention, and a large number of researchers have aimed at multilayer structures. This paper presents a new concept of a three-layer simple structure with VO2 as the base, silicon dioxide as the dielectric layer, and graphene as the top layer. When VO2 is in the insulated state, the absorber is in the closed state, Δf = 1.18 THz (absorption greater than 0.9); when VO2 is in the metallic state, the absorber is open, Δf = 4.4 THz (absorption greater than 0.9), with ultra-broadband absorption. As a result of the absorption mode conversion, a phenomenon occurs with this absorber, with total transmission and total reflection occurring at 2.4 THz (A = 99.45% or 0.29%) and 6.5 THz (A = 90% or 0.24%) for different modes. Due to this absorption property, the absorber is able to achieve full-transmission and full-absorption transitions at specific frequencies. The device has great potential for applications in terahertz absorption, terahertz switching, and terahertz modulation.

Multilayer stacked ultra-wideband perfect solar absorber and thermal emitter based on SiO2-InAs-TiN nanofilm structure.

In this paper, a broadband solar absorber is constructed and simulated based on the finite difference time domain method (FDTD). The modeled structure of the absorber consists of cyclic stacking of five absorber cells with different periods on refractory metal W, where a single absorber cell is composed of a three-layer SiO2-InAs-TiN square film. Due to the Fabry-Perot resonance and the surface plasmon resonance (SPR), an absorptivity greater than 90% within a bandwidth of 2599.5 nm was achieved for the absorber. Notably, one of these bands, 2001 nm, is a high-efficiency absorption with an absorption rate greater than 99%. The average absorption efficiency reaches 99.31% at an air mass of 1.5 (AM 1.5), and the thermal radiation efficiencies are 97.35% and 97.83% at 1000 K and 1200 K, respectively. At the same time, the structure of the absorber is also polarization-independent, and when the solar incidence angle is increased to 60°, it still achieves an average absorption of 90.83% over the entire wavelength band (280 nm to 3000 nm). The novelty of our work is to provide a design idea based on a unit structure with multiple cycles, which can effectively expand the absorption bandwidth of the absorber in the visible-near-infrared wavelengths. The excellent performances make the structure widely used in the field of solar energy absorption.

All layers patterned conical nanostructured thin-film silicon solar cells for light-trapping efficiency improvement.

Thin-film silicon solar cells (TSSC) has received great attention due to its advantages of low cost and eco-friendly. However, traditional single-layer patterned solar cells (SPSC) still fall short in light-trapping efficiency. This article presents an all layers patterned (ALP) conical nanostructured TSSC to enhance the low absorption caused by the thin absorption layers. The Finite-Difference Time-Domain result shows that a photocurrent density up to 41.27 mA/cm2 can be obtained for the structure, which is 31.39% higher than that of the SPSC. An electrical optimization simulation of doping concentration was carried out on the parameters of the optically optimal structure of the model. The power conversion efficiency is 17.15%, which is 1.72 times higher than that of the planar structure. These results demonstrate a success for the potential and prospect of the fully patterned nanostructures in thin-film photovoltaic devices.

Design of a Penta-Band Graphene-Based Terahertz Metamaterial Absorber with Fine Sensing Performance.

This paper presents a new theoretical proposal for a surface plasmon resonance (SPR) terahertz metamaterial absorber with five narrow absorption peaks. The overall structure comprises a sandwich stack consisting of a gold bottom layer, a silica medium, and a single-layer patterned graphene array on top. COMSOL simulation represents that the five absorption peaks under TE polarization are at fI = 1.99 THz (95.82%), fⅡ = 6.00 THz (98.47%), fⅢ = 7.37 THz (98.72%), fⅣ = 8.47 THz (99.87%), and fV = 9.38 THz (97.20%), respectively, which is almost consistent with the absorption performance under TM polarization. In contrast to noble metal absorbers, its absorption rates and resonance frequencies can be dynamically regulated by controlling the Fermi level and relaxation time of graphene. In addition, the device can maintain high absorptivity at 0~50° in TE polarization and 0~40° in TM polarization. The maximum refractive index sensitivity can reach SV = 1.75 THz/RIU, and the maximum figure of merit (FOM) can reach FOMV = 12.774 RIU-1. In conclusion, our design has the properties of dynamic tunability, polarization independence, wide-incident-angle absorption, and fine refractive index sensitivity. We believe that the device has potential applications in photodetectors, active optoelectronic devices, sensors, and other related fields.

The Effect of Nitrogen Annealing on the Resistive Switching Characteristics of the W/TiO2/FTO Memory Device.

In this paper, the effect of nitrogen annealing on the resistive switching characteristics of the rutile TiO2 nanowire-based W/TiO2/FTO memory device is analyzed. The W/TiO2/FTO memory device exhibits a nonvolatile bipolar resistive switching behavior with a high resistance ratio (RHRS/RLRS) of about two orders of magnitude. The conduction behaviors of the W/TiO2/FTO memory device are attributed to the Ohmic conduction mechanism and the Schottky emission in the low resistance state and the high resistance state, respectively. Furthermore, the RHRS/RLRS of the W/TiO2/FTO memory device is obviously increased from about two orders of magnitude to three orders of magnitude after the rapid nitrogen annealing treatment. In addition, the change in the W/TiO2 Schottky barrier depletion layer thickness and barrier height modified by the oxygen vacancies at the W/TiO2 interface is suggested to be responsible for the resistive switching characteristics of the W/TiO2/FTO memory device. This work demonstrates the potential applications of the rutile TiO2 nanowire-based W/TiO2/FTO memory device for high-density data storage in nonvolatile memory devices.

Self-assembly of Au nanocrystals into large-area 3-D ordered flexible superlattice nanostructures arrays for ultrasensitive trace multi-hazard detection.

Plasmonic nanoparticles that self-assemble into highly ordered superlattice nanostructures hold substantial promise for facilitating ultra-trace surface-enhanced Raman scattering (SERS) detection. Herein, we propose a boiling-point evaporation method to synthesize ordered monocrystal-like superlattice Au nanostructures (OML-Au NTs) with a polyhedral morphology. Combined with thermal nanoimprint technology, OML-Au NTs were directly transferred to impact-resistant polystyrene (IPS) flexible SERS substrates, the obtained flexible substrates (donated as OML-Au NTs/IPS) detection limit for R6G molecules as low as 10-13 M. These results were confirmed by simulating the electromagnetic field distribution of ordered/unordered two-dimensional single-layer and three-dimensional aggregated gold nanostructures. The OML-Au NTs/IPS substrates were successfully used to detect and quantify three commonly-used agricultural pesticides, achieving detection limits as low as 10-11 M and 10-12 M, and in situ real-time detection limit reached 0.24 pg/cm2 for thiram on apple peels, which was 3 orders of magnitude lower than the current detection limit. In addition, the Raman intensity from multiple locations showed a relative standard deviation lower than 7 %, exhibiting the reliability necessary for practical applications. As a result, this research demonstrates a highly reproducible method to enable the development of plasmonic nanomaterials with flexible superstructures.

Tunable Antireflection Properties with Self-Assembled Nanopillar and Nanohole Structure.

Nanostructure engineering has proven to be one of the most effective strategies to improve the efficiency of photoelectric devices. Herein, we numerically investigate and experimentally demonstrate a self-assembled silicon-based nanopillars and nanoholes structures, to improve the light absorption of photoelectric devices by an antireflection enhancement. The nanopillars and nanoholes structures are fabricated by the air-liquid interface self-assembly method based on polystyrene (PS) nanospheres. Additionally, the tunable antireflective properties with the different operation wavelength and nanostructures parameters have been discussed based on the Finite-Difference Time-Domain (FDTD) method. The experimental result shows that the self-assembled silicon-based nanopillars and nanoholes structures can achieve the lowest reflectivity of 1.42% (nanopillars) and 5.83% (nanoholes) in the wavelength range of 250-800 nm, which reduced 95.97% and 84.83%, respectively, compared with the plane silicon. The operation mechanism of the tunable antireflective property of self-assembled nanopillars and nanoholes structures is also analyzed in the simulation. Our study suggests that the self-assembled nanopillars and nanoholes structures are potentially attractive as improving efficiency of photoelectric devices.

Individually tunable array reflector for amplitude and phase modulation.

Based on graphene's phase modulation property and vanadium dioxide's amplitude modulation property, we developed an array reflector for terahertz frequencies that is individually adjustable. Starting with a theoretical analysis, we look into the effects of voltage on the Fermi level of graphene and temperature on the conductivity of vanadium dioxide, analyze the beam focusing characteristics, and finally link the controllable quantities with the reflected beam characteristics to independently regulate each cell in the array. The simulation findings demonstrate that the suggested array structure can precisely manage the focus point's position, intensity, and scattering degree and that, with phase compensation, it can control the wide-angle incident light. The array structure offers a novel concept for adjustable devices and focusing lenses, which has excellent potential for study and application.

Dynamically Tunable and Multifunctional Polarization Beam Splitters Based on Graphene Metasurfaces.

Based on coupled-mode theory (CMT) and the finite-difference time-domain (FDTD) approach, we propose a graphene metasurface-based and multifunctional polarization beam splitter that is dynamically tunable. The structure, comprising two graphene strips at the top and bottom and four triangular graphene blocks in the center layer, can achieve triple plasma-induced transparency (PIT). In a single polarization state, the computational results reveal that synchronous or asynchronous six-mode electro-optical switching modulation may be performed by modifying the Fermi levels of graphene, with a maximum modulation degree of amplitude (MDA) of 97.6% at 5.148 THz. In addition, by varying the polarization angle, a polarization-sensitive, tunable polarization beam splitter (PBS) with an extinction ratio and insertion loss of 19.6 dB and 0.35 dB at 6.143 THz, respectively, and a frequency modulation degree of 25.2% was realized. Combining PIT with polarization sensitivity provides a viable platform and concept for developing graphene metasurface-based multifunctional and tunable polarization devices.

High Anti-Reflection Large-Scale Cup-Shaped Nano-Pillar Arrays via Thin Film Anodic Aluminum Oxide Replication.

Surface anti-reflection (AR) with nanometer-scaled texture has shown excellent light trapping performance involving optical devices. In this work, we developed a simple and lithography-free structure replication process to obtain large scale surface cup-shaped nano-pillar (CSNP) arrays for the first time. A method of depositing was used for pattern transfer based on PMMA pre-coated through-hole anodic aluminum oxide (AAO) thin film (~500 nm), and eventually, the uniformity of the transferred nanostructures was guaranteed. From the spectrum (250 nm~2000 nm) dependent measurements, the CSNP nanostructured Si showed excellent AR performance when compared with that of the single-polished Si. Moreover, the CSNP was found to be polarization insensitive and less dependent on incidence angles (≤80°) over the whole spectrum. To further prove the excellent antireflective properties of the CSNP structure, thin film solar cell models were built and studied. The maximum value of Jph for CSNP solar cells shows obvious improvement comparing with that of the cylinder, cone and parabola structured ones. Specifically, in comparison with the optimized Si3N4 thin film solar cell, an increment of 54.64% has been achieved for the CSNP thin film solar cell.

Stable Resistive Switching in ZnO/PVA:MoS2 Bilayer Memristor.

Reliability of nonvolatile resistive switching devices is the key point for practical applications of next-generation nonvolatile memories. Nowadays, nanostructured organic/inorganic heterojunction composites have gained wide attention due to their application potential in terms of large scalability and low-cost fabrication technique. In this study, the interaction between polyvinyl alcohol (PVA) and two-dimensional material molybdenum disulfide (MoS2) with different mixing ratios was investigated. The result confirms that the optimal ratio of PVA:MoS2 is 4:1, which presents an excellent resistive switching behavior. Moreover, we propose a resistive switching model of Ag/ZnO/PVA:MoS2/ITO bilayer structure, which inserts the ZnO as the protective layer between the electrode and the composite film. Compared with the device without ZnO layer structure, the resistive switching performance of Ag/ZnO/PVA:MoS2/ITO was improved greatly. Furthermore, a large resistive memory window up to 104 was observed in the Ag/ZnO/PVA:MoS2/ITO device, which enhanced at least three orders of magnitude more than the Ag/PVA:MoS2/ITO device. The proposed nanostructured Ag/ZnO/PVA:MoS2/ITO device has shown great application potential for the nonvolatile multilevel data storage memory.

Spectral-temporal luminescence properties of Colloidal CdSe/ZnS Quantum Dots in relevant polymer matrices for integration in low turn-on voltage AC-driven LEDs.

This work employs spectral and spectral-temporal Photoluminescence (PL) spectroscopy techniques to study the radiative mechanisms in colloidal CdSe/ZnS Quantum Dot (QD) thin films without and with 1% PMMA polymer matrix embedding (QDPMMA). The observed bimodal transient-spectral PL distributions reveal bandgap transitions and radiative recombinations after interdot electron transfer. The PMMA polymer embedding protects the QDs during the plasma-sputtering of inorganic layers electroluminescent (EL) devices, with minimal impact on the charge transfer properties. Further, a novel TiO2-based, all-electron bandgap, AC-driven QLED architecture is fabricated, yielding a surprisingly low turn-on voltage, with PL-identical and narrow-band EL emission. The symmetric TiO2 bilayer architecture is a promising test platform for alternative optical active materials.

Deep-subwavelength spoof magnetic localized surface plasmon waveguiding over arbitrary bending angles.

A deep-subwavelength metal spiral structure (MSS) waveguide with arbitrary bending angles was proposed and demonstrated to propagate magnetic localized surface plasmons (MLSPs) in theoretical, simulated and experimental ways. The uniform coupling strengths and frequencies for adjacent MSSs with different azimuthal angles represent a significant advancement in the development of structures supporting MLSPs over arbitrary bending angles. The consistency among spectra, dispersion, and field distributions for five MSSs indicates that backward propagation of MLSPs over arbitrary bending angles is possible. In addition, a long S-chain consisting of adjacent MSSs at various angles holds promise for applications involving long-distance MLSPs waveguides.

Narrow-band asymmetric transmission based on the dark mode of Fano resonance on symmetric trimeric metasurfaces.

Asymmetric transmission (AT) is useful for polarization manipulation. We report narrowband AT that utilizes a triple-layered symmetric trimeric metasurface with near-field coupling of the dark mode of the Fano resonance. The coupling strength of the dark mode was tuned by using a mid-layer to break the dim AT between two slit layers. The peak transmission of linearly polarized waves and percentage bandwidth reached 0.7719 and 1.26% (numerical simulations) and 0.49 and 1.9% (experiments), respectively. Coupled-mode theory and field patterns are utilized to explain the underlying physical mechanisms of the mid-layer assisted field coupling. These results are useful for Fano-resonance-based devices.

High-brightness spectral beam combining of diode laser array stack in an external cavity.

We demonstrate the spectral beam combining of a diode laser stack, which contains three 970nm Mini-Bars along the fast-axis direction, in an external cavity. At the pump current of 60 A, the output power of 127 W, the spectral bandwidth of 12 nm and the Electro-optical conversion efficiency of 48.35% are achieved. The measured beam qualities after the spectral beam combining are M(2) ≈10.2 along the slow axis and M(2) ≈11.5 along the fast axis. Under a maximum injection current of 75A, the laser output power of more than 159W is achieved. The beam quality deteriorated slightly with the rising of the current from 60A to 75A, but it is enough to be coupled into a 50µm core / 0.22NA fiber.

Direct Tailoring the Si Substrate for Antireflection via Random Nanohole Nanoimprint.

In this paper, a 2 inch random nanohole Si template with hole diameter of 36-97 nm is employed for direct tailoring the Si substrate for antireflection. The random nanohole Si template is fabricated from the natural self-organization process and can be used repeatedly in nanoimprint lithography (NIL). The surface roughness induced from the nanohole structured surface enhanced the antiadhesion property (contact angle of 128°) of the Si template for high accuracy soft mold replication. The random nanohole structured polymer/Si substrate has a surface fluctuation of ~3 nm, which ensures a uniform and effective pattern transfer from resist to substrate. The reflectivity of the random nanohole structured Si substrate decreases from around 34% to less than 5% with the hole aspect ratio within 3.0 in the wavelength region of 400-800 nm. This method is simple, cheap, repeatable in large area and compatible with the high volume production lines.

Accurate characterization of nanoimprinted resist patterns using Mueller matrix ellipsometry.

In order to control nanoimprint lithography processes to achieve good fidelity, accurate characterization of structural parameters of nanoimprinted resist patterns is highly desirable. Among the possible techniques, optical scatterometry is relatively ideal due to its high throughput, low cost, and minimal sample damage. Compared with conventional optical scatterometry, which is usually based on reflectometry and ellipsometry and obtains at most two ellipsometric angles, Mueller matrix ellipsometry (MME) based scatterometry can provide up to 16 quantities of a 4 × 4 Mueller matrix in each measurement and can thereby acquire much more useful information about the sample. In addition, MME has different measurement accuracy in different measurement configurations. It is expected that much more accurate characterization of nanoimprinted resist patterns can be achieved by choosing appropriate measurement configurations and fully using the rich information hidden in the measured Mueller matrices. Accordingly, nanoimprinted resist patterns were characterized using an in-house developed Mueller matrix ellipsometer in this work. We have experimentally demonstrated that not only more accurate quantification of line width, line height, sidewall angle, and residual layer thickness of nanoimprinted resist patterns can be achieved, but also the residual layer thickness variation over the illumination spot can be directly determined, when performing MME measurements in the optimal configuration and meanwhile incorporating depolarization effects into the optical model. The comparison of MME-extracted imprinted resist profiles has also indicated excellent imprint pattern fidelity.

Polymer-templated electrodeposition of Ag nanosheets assemblies array as reproducible surface-enhanced Raman scattering substrate.

Position-configurable, reproducible, vertically aligned nanosheets assemblies (ANAs) arrays are fabricated by polymer-templated electrodeposition method at room temperature. Here, nanoimprint lithography is utilized to fabricate polymer template on the fluorine-doped tin oxide substrate for the purpose of evenly tuning the location of Ag nanostructures. Subsequently, vertically aligned ANAs can be achieved at the bottom of each hole via electrodeposition in a mixed aqueous solution of AgNO3 and citric acid. To obtain uniform ANAs array, we have systematically investigated the factors that influenced the electrodeposition. It was found that the formation of uniform ANAs arrays is strongly depended on the seeding layer, citric acid concentration, electrodeposition potential and time. The as-synthesized ANAs array exhibited a remarkable SERS activity and Raman signal reproducibility to rhodamine 6G, a concentration down to 10(-13) M can be identified. Our results revealed that the ANAs array is a highly desirable candidate as the reliable enhancer for high performance SERS analysis.