Influence of the polarization and optical axis on the transmission behavior in a one dimensional photonic crystal containing uniaxial metamaterial

In this letter, we introduce a one-dimensional photonic crystal (1DPC) structure with a uniaxial metamaterial defect layer. It is proposed to control the transmitted wave of the defect mode by adjusting the orientation of the optical axis and incidence angle for both polarization states. The 4 × 4 transfer matrix method was employed to calculate the transmittance spectrum of the proposed structure. It is shown that the photonic band gaps, the intensity and the peak wavelength of the defect mode depends on the polarization, the orientation of the optical axis and the incidence angle of the wave, due to the strong anisotropy of the metamaterial. The transmittance spectrum curves at different optical axes of the uniaxial metamaterial and the distinct incidence angles are illustrated graphically. It is shown that the defect mode appears as a peak in the transmission spectrum. Pronounced contrasts in the intensity, wavelength positions of the defect mode and photonic band gap were demonstrated depending on the incidence angle and the orientation of the optical axis of the uniaxial metamaterial defect layer for both polarizations. Our structure offers a great variety of possibilities for designing and controlling the transmitted intensity of the defect mode.     

Negative-index metamaterial at 780 nm wavelength

We further miniaturize a recently established silver-based negative-index metamaterial design. By comparing transmittance, reflectance, and phase-sensitive time-of-flight experiments with theory, we infer a real part of the refractive index of -0.6 at a 780 nm wavelength--which is visible in the laboratory.     

Photonic band gap in a one-dimensional periodic structure with semiconductor metamaterial in the near infrared

We construct a one-dimensional photonic crystal (1DPC) by use of semiconductor metamaterial in the near infrared which is composed of Al-doped ZnO (AZO) and ZnO and dielectric material. The expressions of transmittance and field distribution are deduced by transfer matrix method, and the dispersion curves of anisotropic permittivity of semiconductor metamaterial are analyzed at different ratio of AZO and ZnO. At last simulation results are given to analyze the influence of thickness of dielectric material and incident angle on the transmittance of the proposed PC.     

An ultra-broadband and highly-efficient tunable terahertz polarization converter based on composite metamaterial

A near perfect, ultra-broadband and highly-efficient terahertz reflective polarization converter based on multilayer metamaterial is proposed in this paper. The hybrid metamaterial unit structure consists of a split-ring metal pattern and a metal sheet separated by a dielectric resonator spacer. The polarization conversion ratio (PCR) is above 90% from 2.06 to 4.26 THz, with an optimal range between 2.98 and 4.16 THz where the efficiency is above 98% for normal incidence. It also shows excellent performance for oblique incidences. Moreover, the working band and the optimized frequency range for the polarization conversion can be manipulated by changing the open angle between the metallic arcs and the substrate thickness, respectively. Based on this design, two types of hybrid metamaterial converters are also investigated to swith off polarization conversion altogether or modify the polarization conversion bandwidth. A polarization converter with temperature controlled PCR is realized by adding a vanadium oxide (VO₂) mask on the designed structure. In addition, by integrating photoconductive silicon islands between the split-ring and metal bar a dual-band polarization converter can be realized. Combining with a polarizer, this window can act as an active THz filter.     

Optimized Modular Design of Acoustic Metamaterials: Targeted Noise Attenuation

A modular design optimization method for acoustic metamaterial cells is proposed, which can ensure satisfying the targeted noise attenuation by using only the objective function of an arbitrary noise source and the constraints of the application at hand. The core algorithm is a fusion of the generalized particle swarm algorithm and wave finite element method specialized for structural and targeted optimization of metamaterials. The output is the optimized metamaterial cell and its sound barrier structure. The desired noise reduction performance is verified via acoustic testing of a 3D-printed prototype. The simulation and experimental results of test cases show that the designed acoustic metamaterial can perfectly block the target noise band and have significant advantages in terms of design efficiency, light weight, and noise attenuation.     

Low-loss negative-index metamaterial at telecommunication wavelengths

We fabricate and characterize a low-loss silver-based negative-index metamaterial based on the design of a recent theoretical proposal. Comparing the measured transmittance and reflectance spectra with theory reveals good agreement. We retrieve a real part of the refractive index of Re(n)= -2 around 1.5 microm wavelength. The maximum of the ratio of the real to the imaginary part of the refractive index is about three at a spectral position where Re(n)= -1. To the best of our knowledge, this is the best figure of merit reported for any negative-index photonic metamaterial to date.     

超材料谐振子间的电耦合谐振理论与实验研究

通过将两个金属开口环谐振器口对口地放置, 实现了超材料谐振子间的电耦合谐振. 对电耦合谐振的微波等效电路进行了理论分析和数值计算, 结果表明耦合后的超材料谐振子能产生两个谐振频率, 其中一个随耦合强度的增加逐渐向低频方向移动, 而另一个固定在单谐振子的谐振频率处不变. 微波透射谱的实验测试和电磁仿真结果表明, 两个谐振峰随耦合强度的增加分别向低频和高频方向移动. 分析表明: 低频谐振峰的位置主要是由超材料谐振子间的电耦合强度决定的; 高频谐振偏离单谐振子的谐振频率主要是由不可避免的磁耦合引起的, 而且在耦合间距越小时磁耦合影响越大. 提出的基于超材料谐振子间的电磁耦合实现的双频谐振及其可调性极大地增加了超材料的设计与应用空间.     

Switching between the Functions of Half-Wave Plate and Quarter-Wave Plate Simply by Using a Vanadium Dioxide Film in a Terahertz Metamaterial

We propose a switchable THz metamaterial that can be switched between two functions of half-wave plate and quarter-wave plate.The two switchable functions can be simply achieved by inserting a VO_2 film in the metamaterial design.Finite-difference time-domain(FDTD) simulation results show that the proposed metamaterial can convert x-polarized incident wave to y-polarized reflected wave when VO_2 is at metal phase,and convert x-polarized wave to circularly polarized wave when VO_2 is at insulator phase.The metamaterial performs well in the two functions,i.e.,the same broad working frequency band and near perfect polarization conversion.The switching effect originates from the switchable Fabry-Perot cavity length induced by the phase change of VO_2.We believe that our findings provide a reference in designing switchable metamaterials.     

基于电磁谐振的极化无关透射吸收超材料吸波体

仿真并实验验证了基于电磁谐振的极化无关透射吸收超材料吸波体, 该吸波体可以实现低频透射和高频吸收.实验测试结果表明, 该吸波体在6.77 GHz 吸收率峰值为83.6%, 半功率带宽为4.3%, 实现窄带强吸收.为进一步拓展该谐振型超材料吸波体的吸收带宽, 利用其低频透射特性, 将两个工作于不同频段的吸波体叠加在一起, 测试结果表明, 复合后超材料吸波体的半功率带宽可以增大到10.9%, 吸收率也略有增强. 该超材料吸波体设计简单, 具有较强的实用性和应用前景. 关键词： 极化无关 透射吸收 超材料吸波体     

Optimization of locally resonant acoustic metamaterials on underwater sound absorption characteristics

The study of acoustic metamaterials, also known as locally resonant sonic materials, has recently focused on the topic of underwater sound absorption. The high absorption occurs only within a narrow frequency band around the locally resonant frequency. Nevertheless, this problem can be addressed through a combination of several acoustic metamaterial layers that have different resonant frequencies. In this paper, an optimization scheme, a genetic and a general nonlinear constrained algorithm, is utilized to enhance the low-frequency underwater sound absorption of an acoustic metamaterial slab with several layers. Both the physical and structural parameters of the acoustic metamaterial slab are optimized to enlarge the absorption band. In addition, the sound absorption mechanism of the acoustic metamaterial slab is also analyzed. The result shows that each layer is found to oscillate as a nearly independent unit at its corresponding resonant frequency. The theoretical and experimental results both demonstrate that the optimized metamaterial slab can achieve a broadband (800–2500 Hz) absorption of underwater sound, which is a helpful guidance on the design of anechoic coatings.     

Circular polarization converters based on bi-layered asymmetrical split ring metamaterials

In this paper, a kind of bi-layered asymmetrical split ring metamaterial was proposed as a circular polarization converter. Simulations and experiments at the microwave regime showed that the proposed structures can achieve the conversions from right-handed circularly polarized electromagnetic waves to left-handed ones and the reversed conversions in the opposite propagating direction. The linear to circular polarization transmission coefficients and the surface currents were investigated to understand the mechanism of the circular polarization conversions. Moreover, we optimized the proposed metamaterials by increasing the distance between the two metal layers. The proposed circular polarization converters have applications in microwave wave plates and metamaterial antennas.     

Perfect metamaterial absorber

We present the design for an absorbing metamaterial (MM) with near unity absorbance A(omega). Our structure consists of two MM resonators that couple separately to electric and magnetic fields so as to absorb all incident radiation within a single unit cell layer. We fabricate, characterize, and analyze a MM absorber with a slightly lower predicted A(omega) of 96%. Unlike conventional absorbers, our MM consists solely of metallic elements. The substrate can therefore be optimized for other parameters of interest. We experimentally demonstrate a peak A(omega) greater than 88% at 11.5 GHz.     

Realization of a three-functional-layer negative-index photonic metamaterial

We modify a recently theoretically suggested multilayer negative-index photonic metamaterial design and fabricate corresponding structures with up to three functional layers (seven actual layers) for the first time to our knowledge. Measured transmittance and reflectance spectra agree well with theory.     

二维磁流变包层声学超材料*

该文构建了以钨为内核、环氧树脂为基体、磁流变弹性体作为包层的二维局域共振型声学超材料,采用有限元方法分析了声波的能带结构、透射率、振动模。研究结果表明:利用外磁场可以调控磁流变弹性体包层的弹性模量,从而调节声学超材料带隙的中心位置和宽度,还可以通过改变磁流变弹性体包层的厚度来调节带隙的中心位置和宽度。这些方法对可调型声学超材料的设计有重要的参考意义。     

A broadband,polarisation-insensitive and wide-angle coplanar terahertz metamaterial absorber

Broadband terahertz metamaterial absorbers have attracted considerable attention due totheir significant potential for practical applications. These absorbers are usuallystacked in several differently shaped or sized subunits to form a unit cell, making theirfabrication quite troublesome. A simple design for broadband metamaterial absorbers istherefore urgently needed. Herein, we propose a coplanar broadband andpolarisation-insensitive perfect absorber formed by two patterned square metallic ringswith a dielectric layer on top of a metallic ground plane. The full width at half maximum(FWHM) of the device can be up to 42% (with respect to the central frequency), which is 2times greater than that of a single-layered structure. This property is retained well fora very wide range of incident angles. The two patterned square rings resonating atdifferent but similar frequencies leads to the broadband absorption. Moreover, ahybridised resonance model is proposed to analyse the origin of the resonance bandwidth.The results of this metamaterial absorber design appear to be very promising for solarcell, detection and imaging applications.     

Optically implemented broadband blueshift switch in the terahertz regime

We experimentally demonstrate, for the first time, an optically implemented blueshift tunable metamaterial in the terahertz (THz) regime. The design implies two potential resonance states, and the photoconductive semiconductor (silicon) settled in the critical region plays the role of intermediary for switching the resonator from mode 1 to mode 2. The observed tuning range of the fabricated device is as high as 26% (from 0.76 THz to 0.96 THz) through optical control to silicon. The realization of broadband blueshift tunable metamaterial offers opportunities for achieving switchable metamaterials with simultaneous redshift and blueshift tunability and cascade tunable devices. Our experimental approach is compatible with semiconductor technologies and can be used for other applications in the THz regime.     

Metamaterial coatings for broadband asymmetric mirrors

We report the design and fabrication of nanostructured metal-dielectric mirrors with high reflectance asymmetries in the visible spectral range. Applying dispersion engineering principles to model a broadband and large reflectance asymmetry, we obtain a dielectric function for this metamaterial, closely resembling the effective permittivity of disordered metal-dielectric nanocomposites. Coatings realized by using disordered nanocrystalline silver films on glass substrates confirm the theoretical predictions, exhibiting symmetric transmittance, accompanied by large broadband reflectance asymmetries.     

Investigation of optical properties of Tamm states in higher bands of multilayer stacks including right and left handed materials

We study the electromagnetic surface wave localizing, the so-called surface Tamm states, at an interface separating a left-handed metamaterial (LHM) and a semi-infinite one-dimensional photonic crystal made of alternative left-handed metamaterial and right-handed materials. We show that the existence of metamaterial causes the Tamm states with backward energy flow and allows flexible control of dispersion properties of the surface modes. We study the effect of the physical parameters of the photonic crystal on the dispersion properties and the group velocity of the Tamm states. We also study dispersion properties of the Tamm states in higher order photonic band gaps of the photonic crystal and compare our results with the case when the LHM medium is replaced by a right-handed material (RHM).     

极化无关双向吸收超材料吸波体的仿真与实验验证

设计并制作出极化无关双向吸收超材料吸波体.仿真结果表明,该吸波体对斜入射横电极化和横磁极化电磁波具有较好的吸收效果,其强损耗主要来自介质基板的介电损耗.实验测试结果表明,该吸波体在11.1 GHz对垂直入射电磁波实现双向强吸收,吸收率分别为95.9％和90.8％.该吸波体厚度较薄,工作波长约为1/34.该超材料吸波体具有吸收率高、厚度薄、设计简单、易于加工等优点.