Numerical Study of an Ultra-Broadband and Polarization Independence Metamaterial Cross-Shaped Fractal Absorber

A three-dimensional cross-shaped fractal metamaterial absorber with ultra-wide wavelength band, polarization-independence and wide-angle, is numerically investigated by the finite-difference time-domain method. In this absorber, the solar energy is trapped by the cross-shaped fractal of the upper layer, and the Si-ring filled with iron in the middle layer and the wavelength band can be broadened by the self-similarity of fractal structure. The absorber exhibits absorptivity higher than 91% for the wavelengths from 400 to 2000 nm and an absorption bandwidth of about 133%. Furthermore, the proposed absorber realizes polarization independence, and the maximum incident angle is 76°. However, as the iron material applied in the nano-metamaterial absorber (NMA) can be easily oxidized and rusted, it is replaced by nickel with characteristics such as corrosion resistance and high-temperature resistance; thus, an improved NMA is obtained. The improved absorber not only eliminates the corrosion-prone defects of the above proposed structure but also maintains polarization independence and high absorption and widens the angle of incidence up to 79° and thereby can be applied in many areas, such as solar energy harvesting.

     

A Simple Multi-band Metamaterial Absorber with Combined Polarization Sensitive and Polarization Insensitive Characteristics for Terahertz Applications

This paper presents a simple multi-band metamaterial absorber for terahertz applications. The unit cell of the proposed structure consists of a single square ring having gaps at the centers on three of its sides. The proposed absorber produces three absorption bands for all polarizations and hence the design can be considered as insensitive to polarization variation. It provides an average absorption of 96.92% for the TE polarization with a peak absorption of 99.44% at 3.87 THz and for the TM polarization, it provides an average absorption of 98.4% with a peak absorption of 99.86% at 3.87 THz. An additional absorption peak is observed for the TE polarization at 1.055 THz that gradually diminishes with the increase in polarization angle and completely vanishes for the TM polarization. Thus, the structure displays a hybrid polarization response with polarization insensitivity in three bands and polarization sensitivity in one band. Parametric analysis has been carried out validating the optimal selection of the design parameters. The simplicity of the design and its combined polarization sensitive and polarization insensitive absorption characteristics can find tremendous applications in the field of terahertz imaging and sensing.

     

A Tunable Ultra-Broadband Metamaterial Absorber with Multilayered Structure

In this article, we demonstrate a tunable ultra-broadband metamaterial absorber (TUMA) in terahertz (THz) band which is based on the multilayered structure composed of an Au reflective layer, polyimide dielectric layers, and vanadium dioxide (VO₂) periodic structures, respectively. We gain the tunable absorption spectra because of the room temperature phased-changed character of VO2. The relative bandwidth reaches to 81.2% and the absorption rate is over 90% at the frequency range of 1.63–3.86 THz when the temperature (t₁) is 350 K, but when t₁ = 300 K, the presented absorber is acted as a reflector whose absorption is small besides the frequency points of 9.75 THz and 9.81 THz. For the sake of comprehending the physical mechanism in-depth, the electric field (E-field) diagrams, the surface current distributions and the power loss density (PLD) of the TUMA are investigated. The influences of structural arguments and incident angle (θ) on the absorption are also analyzed. The emulated consequences show that the absorption spectrum can be regulated by changing structural parameters and incident angle and the tunable absorption regions can be obtained by altering the external temperature.

     

A Compact Design of Multiband Terahertz Metamaterial Absorber with Frequency and Polarization Tunability

A new and simple design of quad-band metamaterial absorber for terahertz frequency has been proposed. The unit cell of the absorber is composed of a top metallic patch having H-shaped slot and a ground metallic plane, both separated by a dielectric layer. The proposed design is capable of providing four distinct absorption peaks over at 0.81, 1.98, 3.25, and 3.50 THz. Our design is a step ahead of the previously proposed terahertz absorbers for its simplistic design approach which removes the fabrication difficulty. Interestingly, rather placing multiple resonators in a single unit cell, we able to accommodate multiple orders of resonances in the proposed design using only a single metallic structure to achieve multiband absorbance. The sensing performance of the absorber in terms of surrounding index is also analyzed. Moreover, we have shown how the proposed structure can be easily converted into a frequency tunable absorber using a simple stub without changing the overall geometry of the absorber. This fast and easy frequency tunability feature is an additional advantage over the simple design of the structure. Also, we lead our work to its upgradation into a polarization tunable absorber where the absorption frequencies are controllable by the polarization of the incident light. The vibrant design of the proposed absorber is expected to find application in detection, imaging, radar cross-section (RCS) reduction, and sensing-related activities.     

An Octaband Polarization Insensitive Terahertz Metamaterial Absorber Using Orthogonal Elliptical Ring Resonators

Plasmonics - Terahertz metamaterial absorbers are the latest developments that have tremendous applications in terahertz spectroscopy and terahertz imaging. Especially, the research progressed in...     

Polarization Conversion and Absorption of Multifunctional All-dielectric Metamaterial Based on Vanadium Dioxide

Plasmonics - In this paper, we propose a kind of multi-functional all-dielectric metamaterial (ADMM), which is composed of vanadium dioxide (VO2), silicon, and silicon dioxide. Two functions of...     

Polarization Management of Terahertz Extraordinary Optical Transmission through Ultracompact L-Shaped Subwavelength Patterns on Metal Films

Compact and efficient terahertz (THz) polarization conversion components are of importance for applications where the small dimension of the laser device/system is critical. Here, we propose an ultracompact L-shaped subwavelength patterns on metal films to realize the THz polarization management. By optimizing the geometric parameters of single-layered and double-layered patterns, the linear-polarized THz incidence can be converted to elliptical polarized output or rotated by 90° efficiently due to the THz extraordinary optical transmission phenomenon. The physical mechanism is explored by mode analysis using numerical and analytical modeling.     

A Hybrid Terahertz Metamaterial Sensor Using a Hexagonal Ring Resonator with Bio-medical Applications

Plasmonics - In recent years, novel terahertz (THz) metamaterial sensors are being actively explored by the research community. These structures absorb the incident THz waves, and their absorption...     

Ultra-wideband Transmissive Water-Based Metamaterial Absorber with Wide Angle Incidence and Polarization Insensitivity

Plasmonics - A transmissive water-based metamaterial absorber(TWMA) is designed, simulated, and measured, which uses 3D-printing to manufacture. The TWMA uses a circular column and rectangular...     

A Dual-Band Polarization Insensitive Metamaterial Absorber with a Single Square Metallic Patch for Sensing Application

Plasmonics - We proposed a dual-band polarization-insensitive metamaterial absorber consisting of merely the square metallic patch and a continuous metallic ground separated by a middle dielectric...     

A Simple Design of a Multi-Band Terahertz Metamaterial Absorber Based on Periodic Square Metallic Layer with T-Shaped Gap

We present a multi-band terahertz absorber formed by periodic square metallic ribbon with T-shaped gap and a metallic ground plane separated by a dielectric layer. It is demonstrated that absorption spectra of the proposed structure consist of four absorption peaks located at 1.12, 2.49, 3.45, and 3.91 THz with high absorption coefficients of 98.0, 98.9, 98.7, and 99.6%, respectively. It is demonstrated that the proposed absorber has the tunability from single-band to broadband by changing the length of square metallic ribbon and we can also select or tune the frequencies which we want to use by changing polarization angles. Importantly, the quality factor Q at 3.91 THz is 30.1, which is 5.6 times higher than that of 1.12 THz. These results indicate that the proposed absorber has a promising potential for devices, such as detection, sensing, and imaging.

     

The Evolutionary Reduction Principle for Linear Variation in Genetic Transmission

The evolution of genetic systems has been analyzed through the use of modifier gene models, in which a neutral gene is posited to control the transmission of other genes under selection. Analysis of modifier gene models has found the manifestations of an “evolutionary reduction principle”: in a population near equilibrium, a new modifier allele that scales equally all transition probabilities between different genotypes under selection can invade if and only if it reduces the transition probabilities. Analytical results on the reduction principle have always required some set of constraints for tractability: limitations to one or two selected loci, two alleles per locus, specific selection regimes or weak selection, specific genetic processes being modified, extreme or infinitesimal effects of the modifier allele, or tight linkage between modifier and selected loci. Here, I prove the reduction principle in the absence of any of these constraints, confirming a twenty-year-old conjecture. The proof is obtained by a wider application of Karlin’s Theorem 5.2 (Karlin in Evolutionary biology, vol. 14, pp. 61–204, Plenum, New York, 1982) and its extension to ML-matrices, substochastic matrices, and reducible matrices. Dedicated to my doctoral advisor Marc Feldman on his 65th birthday, and to the memory of Marc’s doctoral advisor, Sam Karlin, who each laid the foundations necessary for these results; and to my mother Elizabeth Lee and to the memory of my father Roger Altenberg, who together laid the foundation necessary for me.     

Bayesian Insight into a Robust Test for Linear Contrast Based on Asymmetric Censored Samples

A robust test for linear contrast using modified maximum likelihood estimators based on symmetrically censored samples proposed by Tiku (1973, 1982a) is studied in this paper from the Bayesian point of view. The effects of asymmetric censoring on this testing procedure is investigated and a good approximation to its posterior distribution in this case is worked out. We also present an example which illustrates the method of obtaining the highest posterior density interval for the linear combination of the unknown location parameters.     

High-Performance Graphene Patch Antenna with Superstrate Cover for Terahertz Band Application

Plasmonics - Graphene-based patch antennas are rapidly gaining interests in communication technologies for high-speed data transmission due to the exciting properties of graphene material. Herein,...     

具时滞的人口模型的行波解

研究具时滞的人口模型的行波解存在性问题,利用[5]中的方法,鹕行波解的存在性问题转化为寻找上下解的问题。     

Quantifying the Likelihood of Co-existence for Communities with Asymmetric Competition

Trade-offs in performance of different ecological functions within a species are commonly offered as an explanation for co-existence in natural communities. Single trade-offs between competitive ability and other life history traits have been shown to support a large number of species, as a result of strong competitive asymmetry. We consider a single competition-fecundity trade-off in a homogeneous environment, and examine the effect of the form of asymmetry on the likelihood of species co-existing. We find conditions that allow co-existence of two species for a general competition function, and show that (1)?two species can only co-exist if the competition function is sufficiently steep when the species are similar; (2)?when competition is determined by a linear function, no more than two species can co-exist; (3)?when the competition between two individuals is determined by a discontinuous step function, this single trade-off can support an arbitrarily large number of species. Further, we show analytically that as the degree of asymmetry in competition increases, the probability of a given number of species co-existing also increases, but note that even in the most favourable conditions, large numbers of species co-existing along a single trade-off is highly unlikely. On this basis, we suggest it is unlikely that single trade-offs are able to support high levels of bio-diversity without interacting other processes.     

Use of Whole Cells of Pseudomonas aeruginosa for Synthesis of the Antioxidant Hydroxytyrosol via Conversion of Tyrosol

For the first time, a soil bacterium, designated Pseudomonas aeruginosa, was isolated based on its ability to grow on tyrosol as a sole source of carbon and energy. During growth on tyrosol, this strain was capable of promoting the formation of a significant amount of hydroxytyrosol and trace quantities of parahydroxyphenyl acetic acid and 3,4-dihydroxyphenyl acetic acid. The products were confirmed by high-performance liquid chromatography and gas chromatography-mass spectrometry analyses. Using an optimized tyrosol concentration of 2 g liter⁻¹, the maximal hydroxytyrosol yield (80%) was achieved after a 7-h reaction in a growth experiment. To enhance the formation of hydroxytyrosol and prevent its degradation, a resting-cell method using P. aeruginosa was performed. The growth state of the culture utilized for biomass production, the carbon source on which the biomass was grown, the concentration of the biomass, and the amount of tyrosol that was treated were optimized. The optimal yield of hydroxytyrosol (96%) was obtained after a 7-h reaction using 4 g of tyrosol liter⁻¹ and 5 g of cells liter⁻¹ pregrown on tyrosol and harvested at the end of the exponential phase. This proposed procedure is an alternative approach to obtain hydroxytyrosol in an environmentally friendly way. In addition, the reaction is easy to perform and can be adapted to a bioreactor for industrial purposes.     

Matching Pursuit with Asymmetric Functions for Signal Decomposition and Parameterization

The method of adaptive approximations by Matching Pursuit makes it possible to decompose signals into basic components (called atoms). The approach relies on fitting, in an iterative way, functions from a large predefined set (called dictionary) to an analyzed signal. Usually, symmetric functions coming from the Gabor family (sine modulated Gaussian) are used. However Gabor functions may not be optimal in describing waveforms present in physiological and medical signals. Many biomedical signals contain asymmetric components, usually with a steep rise and slower decay. For the decomposition of this kind of signal we introduce a dictionary of functions of various degrees of asymmetry – from symmetric Gabor atoms to highly asymmetric waveforms. The application of this enriched dictionary to Otoacoustic Emissions and Steady-State Visually Evoked Potentials demonstrated the advantages of the proposed method. The approach provides more sparse representation, allows for correct determination of the latencies of the components and removes the "energy leakage" effect generated by symmetric waveforms that do not sufficiently match the structures of the analyzed signal. Additionally, we introduced a time-frequency-amplitude distribution that is more adequate for representation of asymmetric atoms than the conventional time-frequency-energy distribution.     

How Backward Poynting Flows Arise for Surface Plasmon Waves with Lossy Metals

We revisit the surface plasmon resonances established along a planar interface lying between a lossless dielectric and a lossy metal. By examining the orbital and spin parts of the Poynting vector, the mechanisms behind forward or backward flows are clearly illustrated. Consequently, we were able to construct more intuitive pictures of two-dimensional energy flows induced by the metallic losses. In addition, we recognized the importance of both asymmetry and symmetry hidden behind the familiar transverse-magnetic waves. Our numerical results are close to reality, since experimentally observed optical data of gold is employed for a lossy metal.