低浓度吡虫啉的超材料太赫兹光谱检测研究

农产品质量安全是社会广泛关注的重大民生问题。近年来,农产品生产过程中农药的广泛使用和滥用会导致农药残留,对人类健康和环境造成潜在危害。吡虫啉是一种硝基亚甲基类新烟碱内吸杀虫剂,因其具有广谱、高效和低毒的特性已广泛用于农业生产中,但其过量残留也给人类的健康带来了威胁。首先对超材料结构的透射谱进行了分析,对共振频率的形成原因进行了解释;其次分别在超材料结构和二氧化硅基底上涂覆了500 mg·L-1的吡虫啉溶液并进行了测量,排除了二氧化硅基底的影响;接着制备了3个梯度15个浓度的吡虫啉溶液,分别为：100~500 mg·L-1（梯度为100 mg·L-1）、10~50 mg·L-1（梯度为10 mg·L-1）、1~5 mg·L-1（梯度为1 mg·L-1）;测量了喷涂在超材料结构上的吡虫啉薄膜的太赫兹时域光谱,根据太赫兹透射谱峰值频率红移量的不同实现了对不同溶液浓度的鉴别,建立了峰值频率红移量和吡虫啉浓度的函数关系。实验结果表明,借助超材料的调制特性,太赫兹光谱法可以检测到浓度低至1 mg·L-1的吡虫啉薄膜。将实验测得的不同浓度吡虫啉溶液的折射率,代入CST软件进行仿真验证,结果说明不同浓度的吡虫啉的透射曲线具有不同程度的红移,且红移量随着浓度的增大而增加。实验和仿真结果表明,超材料对太赫兹光谱透射峰值频率的调制可用于低浓度吡虫啉含量的太赫兹时域光谱检测。此研究为食品基质中农药残留的检测提供了一种新方法。     

Metamaterials mimicking dynamic spacetime,D-brane and noncommutativity in string theory

We propose a scheme to mimic the expanding cosmos in 1+2$1+2$ dimensions in laboratory. Furthermore, we develop a general procedure to use nonlinear metamaterials to mimic D-brane and noncommutativity in string theory.     

Two-dimensional Maxwell's equations with sign-changing coefficients

We consider the theoretical study of time harmonic Maxwell's equations in presence of sign-changing coefficients, in a two-dimensional configuration. Classically, the problems for both the Transverse Magnetic and the Transverse Electric polarizations reduce to an equivalent scalar Helmholtz type equation. For this scalar equation, we have already studied consequences of the presence of sign-changing coefficients in previous papers, and we summarize here the main results. Then we focus on the alternative approach which relies on the two-dimensional vectorial formulations of the TM or TE problems, and we exhibit some unexpected effects of the sign-change of the coefficients. In the process, we provide new results on the scalar equations.     

Scattering from a coated nihility circular cylinder placed in chiral metamaterial

The scattering of electromagnetic plane wave from a coated nihility circular cylinder placed in chiral metamaterial is investigated. The nihility cylinder is coated with chiral or chiral-nihility metamaterial and the medium hosting the coated cylinder is also considered as chiral or chiral-nihility metamaterial. The vector wave equation is used to find out the solution for the fields scattered from the concentric chiral ciruclar cylinders immersed in chiral metamaterial. Inner chiral cylinder is reduced to nihility cylinder taking permittivity and permeability approaching to zero with chirality parameter equal to zero. Coated perfect electric conductor (PEC)/perfect magnetic conductor (PMC) are obtained by taking very large value of permittivity/permeability of the inner cylinder. Numerical results for fields reflected from coated nihility cylinder are compared with those obtained for coated PEC/PMC cylinder. Our results for the special case of dielectric coated nihility/PEC cylinders in free space are shown to be in agreement with already published results thus validating the scheme of analysis.     

纳米粒子形貌与表面等离子体激元关系

通过调控纳米粒子表面形貌,研究了纳米粒子形貌与表面等离子体激元之间的关系.采用水相化学合成法制备出粗糙表面“花朵”形银纳米粒子.通过自组装形成单层阵列,并进一步组装成复合结构超材料.测试了其光学行为,并将实验结果与树枝形纳米粒子、光滑表面纳米粒子进行对比分析.结果表明:光滑表面纳米粒子不能出现超材料效应,当粗糙程度增加,纳米粒子呈类“花朵”形时,样品出现透射峰和平板聚焦行为,但强度不高|当粗糙程度继续增加,纳米粒子呈树枝状时,出现了较强的透射峰与平板聚焦行为.研究证实通过改变纳米粒子表面形貌,可以调控表面等离子体激元与入射光的相互作用,从而实现对光传播的操控.     

Dual and broadband power dividers at microwave frequencies based on composite right/left handed (CRLH) lattice networks

This paper proposes a dual-band power divider operating at GHz frequencies and implemented by means of impedance transformers (also called inverters) based on lattice networks and transmission line sections. The dual-band functionality of the proposed device is achieved thanks to the composite right/left handed (CRLH) behavior of the impedance transformers, able to provide −90° and +90° phase shift at the first and second design frequencies, respectively, of the divider. By using such combination of transmission line sections and lattice networks, the characteristic impedance of the impedance transformers is roughly constant over wide bandwidths, with the results of broad operating bands. To demonstrate the possibilities of the approach, a prototype device is designed, fabricated and characterized.     

Efficient Design of Directive Patch Antennas in Mobile Communications Using Metamaterials

An efficient approach for designing directive patch antennas using metamaterials is presented. Based on the left-handed-material (LHM) transmission line model, circuit elements L and C of the LHM equivalent circuit model are computed in terms of the prescribed desirable characteristics of directive patch antennas. Formulae to calculate the LHM design parameters from the circuit elements L and C are developed. Design examples for the GSM900 and GSM1800 frequency bands in mobile communications are performed to illustrate validation and efficiency of this design approach.     

Recent advancements in surface plasmon polaritons-plasmonics in subwavelength structures in microwave and terahertz regimes

This paper presents a review of recent investigational studies on exciting Surface Plasmon Polaritons (SPPs) in MicroWave (MW) and TeraHertZ (THz) regimes by using subwavelength corrugated patterns on conductive or metal surfaces. This article also describes SPP Microstrip (MS) structures at microwave and terahertz frequencies, and compares their significance with that of conventional MS Transmission Lines (TL), in order to tackle the key challenges of high gain, bandwidth size, compactness, TL losses, and signal integrity in high-end electronic devices. Because they have subwavelength properties, surface plasmon polaritons are gaining attention for their improved performance and ability for miniaturization in high-speed dense circuits. They possess comparably minuscule wavelength compared to incident light (photons). Consequently, they can demonstrate stronger spatial confinement and higher local field intensity at optical frequencies. In addition to engineering spoof SPP waveguides, which are created by engraving grooves and slits on metal surfaces to allow operation on at low frequencies (microwave and terahertz), semiconductors with smaller permittivity values and thus lower free charge carrier concentration have been demonstrated as a potential candidate in plasmonic devices. If necessary, further tuning of semiconductor-based SPP structures is aided by controlling the charge carrier concentration through doping, or by external stimuli such as optical illumination or thermal excitation of charge carriers from valence to conduction bands. This article conclusively covers previously elucidated perspectives on manipulating SSPPs in the MW and THz ranges, and emphasizes how these could steer next-generation plasmonic devices.     

Design of metaporous supercells by genetic algorithm for absorption optimization on a wide frequency band

The optimization of acoustic absorption by metaporous materials made of complex unit cells with 2D resonant inclusions is realized using genetic algorithm. A nearly total absorption over a wide frequency band can be obtained for thin structures, even for frequencies below the quarter wavelength resonances i.e., in a sub-wavelength regime. The high absorption performances of this material are due to the interplay of usual visco-thermal losses, local resonances and trapped modes. The density of resonant and trapped modes in this dissipative porous layer, is a key parameter for broadband absorption. The best configurations and critical coupling conditions are found by genetic algorithm optimization. Several types of resonators are included gradually in the studied configurations (split-rings, Helmholtz resonators, back cavities) with increasing complexity. The optimization leads to a metaporous structure with a 2-cm sub-wavelength layer thickness, exhibiting a nearly total absorption between 1800 Hz and 7000 Hz. The influence of the incidence angle on the absorption properties is also shown.     

Electromagnetic scattering from a perfect electromagnetic conductor circular cylinder coated with a metamaterial having negative permittivity and/or permeability

Electromagnetic scattering of an incident uniform plane wave from a perfect electromagnetic conductor (PEMC) circular cylinder, coated with a metamaterial is investigated theoretically. In the analysis the coating layer may be double-positive (DPS), double-negative (DNG), epsilon-negative (ENG) or mu-negative (MNG). It is assumed that both PEMC cylinder and the coating layer are infinite along the cylinder axis. Both parallel and perpendicular polarization cases are considered for the analysis. Comparison between the monostatic and bistatic echo widths, of a PEMC cylinder coated with metamaterial and coated with ordinary dielectric material (DPS), is presented. The numerical results are compared with the published literature, and comparison is found to be in good agreement.