电磁超材料中负磁导率对负介电常数的影响

为深入了解电磁超材料中物质间相互作用关系,理论分析了金属导体线阵列宏结构嵌入单负磁导率媒质中时其等效介电常数的变化特性。数值计算和电磁仿真方法相结合,讨论了单负磁导率媒质和单负介电常数媒质的相互作用关系,提出了减小其相互作用的解决方法。仿真结果显示：将金属线阵列直接嵌入到单负磁导率媒质中时,电磁超材料传输特性在整个频段内为传输禁带;将金属线裹附一层绝缘材料后,传输禁带变为传输通带,这表明金属线阵列和单负磁导率媒质之间必须加入一种绝缘材料才能合成双负的电磁超材料。     

Active terahertz metamaterials

In this paper we present an overview of research in our group in terahertz (THz) metamaterials and their applications. We have developed a series of planar metamaterials operating at THz frequencies, all of which exhibit a strong resonant response. By incorporating natural materials, e.g., semiconductors, as the substrates or as critical regions of metamaterial elements, we are able to effectively control the metamaterial resonance by the application of external stimuli, e.g., photoexcitation and electrical bias. Such actively controllable metamaterials provide novel functionalities for solid-state device applications with unprecedented performance, such as THz spectroscopy, imaging, and many others.     

太赫兹人工电磁媒质研究进展

近年来, 随着太赫兹科学技术的发展, 越来越多的科学家向太赫兹间隙这一传统空白领域发起挑战. 其中, 人工电磁媒质因为能够设计太赫兹波段中紧缺的功能器件而受到广泛关注. 近年来, 对人工电磁媒质尤其是太赫兹方面的研究进展突飞猛进. 人工电磁媒质的性质不仅仅由其构成材料决定, 更与其结构单元的形状和空间排布密切相关. 本文介绍了人工电磁媒质在太赫兹波段的发展、原理、设计和应用, 并着重介绍完美吸波器和人工表面等离激元, 为太赫兹波段功能器件的研究提供了参考, 并对可能的发展方向予以展望.     

基于谐振结构的左右手传输线的奇异传输性质

通过对一种具有谐振结构的左右手传输线进行了数值仿真和分析，从有效磁导率和介电常数及色散关系来说明其奇异的传输特性.把这种左右手传输线用作漏波天线并测量了在不同频率点的方向图，从而实验上验证了这种特殊结构在不同的频率范围内可以实现一个左手性通带，也可以实现两个左手性通带.对这种结构传输特性的研究，有助于其在漏波天线方面的应用.     

Z-shaped meta-atom for negative permittivity metamaterials

A printed Z-shaped electric meta-atom is presented as an alternative design to the conventional electric-LC (ELC) resonator. We propose an easy way to redesign the ELC resonator pattern to get a compact and a low cost electric resonator exhibiting a strong electric response. Our approach involves, in the effective medium regime, redressing the resonator shape to accommodate higher inductance and lead to a lower resonance frequency without being limited by fabrication tolerances. The electromagnetic behaviour of the meta-atom has been investigated through both simulations and experiments in the microwave regime. Our results show that the Z meta-atom exhibits an electric response to normally incident radiation and can be used very effectively in producing materials with negative permittivity. The proposed planar meta-atom can find various applications in high frequency passive circuits which are designed in planar technology. Moreover, the proposed structure can be scaled to much higher frequencies via appropriate lithographic scaling.     

可调太赫兹与光学超材料

本文对可调太赫兹与光学超材料的研究进展进行了综述,并对其发展趋势和应用前景进行了展望。可以预见,可调超材料将继续成为超材料研究中的热点课题,并将成为引领光学器件和光学系统变革的潜在技术途径,对光学和太赫兹技术的发展将产生深远的影响。     

Manipulation of terahertz radiation using metamaterials

During the past decade electromagnetic metamaterials have realized many exotic phenomena that are difficult or impossible using naturally occurring materials. It is their resonantly enhanced interaction with electromagnetic waves that underpins their attractive qualities, which are increasingly important in the terahertz frequency range. Passive and active terahertz metamaterials and devices have enabled novel functionality and unprecedented terahertz device performance. These demonstrations prove their potential to address the so‐called terahertz gap, a technology vacuum associated with the deficiency of natural materials with a desirable terahertz response.     

Polarization-independent broadband terahertz chiral metamaterials on flexible substrate

A THz band-reject filter is designed based on chiral four-fold rotational symmetry metamaterial. This filter was fabricated by laser micro-lens array lithography and characterized by terahertz time-domain spectroscopy. The resonant frequencies at different twist angles are almost the same, which demonstrates the polarization independence of the structure. The electric field distribution is simulated to explain the physics mechanism behind the polarization independence. By stacking multiple metamaterial layers together, a THz broadband reject filter at a bandwidth of 0.461 THz is experimentally achieved.     

Fabrication of terahertz metamaterials by laser printing

A laser printing technique was used to fabricate split-ring resonators (SRRs) on Si substrates for terahertz (THz) metamaterials and their resonance behavior evaluated by THz time-domain spectroscopy. The laser-printed Ag SRRs exhibited sharp edge definition and excellent thickness uniformity, which resulted in an electromagnetic response similar to that from identical Au SRR structures prepared by conventional photolithography. These results demonstrate that laser printing is a practical alternative to conventional photolithography for fabricating metamaterial structures at terahertz frequencies, since it allows their design to be easily modified and optimized.     

半导体超常媒质矩形谐振腔

设计并研究了两个有不同各向异性参数的半导体超常媒质矩形谐振腔,其中填充了部分右手半导体超常媒质和部分左手半导体超常媒质。通过作图法研究了充满非色散的右手半导体超常媒质和色散的左手半导体超常媒质的谐振腔中谐振模式的解。结果显示,谐振腔的谐振模式取决于半导体超常媒质的色散关系。讨论了每种谐振腔中包含的六种情况,分别表示两个区域的传播常数为实数和虚数的不同的组合,并且揭示了构建具有无相差特性的谐振腔的几何参数要求。     

半导体超常媒质矩形谐振腔(英文)

设计并研究了两个有不同各向异性参数的半导体超常媒质矩形谐振腔,其中填充了部分右手半导体超常媒质和部分左手半导体超常媒质。通过作图法研究了充满非色散的右手半导体超常媒质和色散的左手半导体超常媒质的谐振腔中谐振模式的解。结果显示,谐振腔的谐振模式取决于半导体超常媒质的色散关系。讨论了每种谐振腔中包含的六种情况,分别表示两个区域的传播常数为实数和虚数的不同的组合,并且揭示了构建具有无相差特性的谐振腔的几何参数要求。     

Programmable terahertz metamaterials through V-beam electrothermal devices

We report on the application of various optimization methods as a very promising simulation approach for the design of true integrated optical devices by means of inverse design. We show that these techniques provide a global optimum toward one or various functional objectives at a reasonable computational cost. The results obtained by these methods are far better than intuitive design procedures and clearly outperform trial-and-error based models. We illustrate their performance by using a series of inverse-designed practical photonic devices.     

Mechanically tunable metamaterials terahertz dual-band bandstop filter

We experimentally demonstrate a mechanically tunable metamaterials terahertz(THz) dual-band bandstop filter. The unit cell of the filter contains an inner aluminum circle and an outside aluminum Ohm-ring on high resistance silicon substrate. The performance of the filter is simulated by finite-integration-time-domain(FITD) method. The sample is fabricated using a surface micromachining process and experimentally demonstrated using a THz time-domain-spectroscopy(TDS) system. The results show that, when the incident THz wave is polarized in y-axis, the filter has two intensive absorption peaks locating at 0.71 THz and 1.13 THz, respectively. The position of the high-frequency absorption peak and the amplitude of the low-frequency absorption peak can be simultaneously tuned by rotating the sample along its normal axis.The tunability of the high-frequency absorption peak is due to the shift of resonance frequency of two electrical dipoles,and that of the low-frequency absorption peak results from the effect of rotationally induced transparent. This tunable filter is very useful for switch, manipulation, and frequency selective detection of THz beam.     

Features of electrodynamics of fine-structured resonant metamaterials

We give a brief review of the state of the art of theoretical and experimental studies in the field of electrodynamics of resonant composite materials (metamaterials) whose dielectric permittivity or magnetic permeability or both can be negative. Principles and methods of realization of such metamaterials in the microwave range, their main distinctive electrodynamic properties, prospects for creating new systems and devices (e.g., more perfect lenses), and ways for moving towards the higher frequencies from microwaves to optics are discussed.     

Process-controllable modulation of plasmon-induced transparency in terahertz metamaterials

Recently reported plasmon-induced transparency(PIT)in metamaterials endows the optical structures in classical systems with quantum optical effects.In particular,the nonreconfigurable nature in metamaterials makes multifunctional applications of PIT effects in terahertz communications and optical networks remain a great challenge.Here,we present an ultrafast process-selectable modulation of the PIT effect.By incorporating silicon islands into diatomic metamaterials,the PIT effect is modulated reversely,depending on the vertical and horizontal configurations,with giant modulation depths as high as 129%and 109%.Accompanied by the enormous switching of the transparent window,remarkable slow light effect occurs.     

Nonlinear reshaping of terahertz pulses with graphene metamaterials

We study the propagation of electromagnetic waves through a slab of graphene metamaterial composed of the layers of graphene separated by dielectric slabs. Starting from the kinetic expression for two-dimensional electric current in graphene, we derive a novel equation describing the nonlinear propagation of terahertz electromagnetic pulses through the layered graphene-dielectric structure in the presence of losses and non-linearities. We demonstrate strong nonlinearity-induced reshaping of transmitted and reflected terahertz pulses through the interaction with the thin multilayer graphene metamaterial structure.     

Origin of strain-induced resonances in flexible terahertz metamaterials

Two types of flexible terahertz metamaterials were fabricated on polyethylene naphthalate(PEN) substrates. The unit cell of one type consists of two identical split-ring resonators(SRRs) that are arranged face-to-face(i.e., Flex Meta F); the unit cell of the other type has nothing different but is arranged back-to-back(i.e., Flex Meta B). Flex Meta F and Flex Meta B illustrate the similar transmission dips under zero strain because the excitation of fundamental inductive–capacitive(LC)resonance is mainly dependent on the geometric structure of individual SRR. However, if a gradually variant strain is applied to bend Flex Meta F and Flex Meta B, the new resonant peaks appear: in the case of Flex Meta F, the peaks are located at the lower frequencies; in the case of Flex Meta B, the peaks appear at the frequencies adjacent to the LC resonance. The origin and evolution of strain-induced resonances are studied. The origin is ascribed to the detuning effect and the different responses to strain from Flex Meta F and Flex Meta B are associated with the coupling effect. These findings may improve the understanding on flexible terahertz metamaterials and benefit their applications in flexible or curved devices.     

Coupled resonant modes in twisted acoustic metamaterials

Acoustic metamaterials constructed by resonant microelements in subwavelength scale were generally characterized by the effective medium approximation theory, which neglects the interaction between adjacent elements. In this paper, we show that twisting the orientation of resonators in acoustic metamaterials produces secondary coupled resonant modes by introducing internal vibration interaction. Metamaterials composed of a single-slit Helmholtz resonator arranged in two-dimensional square lattice are investigated. We rotate a portion of the resonator so that the adjacent resonators in a ??X direction have a twist angle of ??. For the system with ??=180^°, the coupling interaction produces the symmetric coupled mode in in-phase oscillation and the antisymmetric coupled mode in out-of-phase oscillation. This acoustic analog of ??hybridization effect?? leads to a sharp transparency window in the extended locally-resonant forbidden gap, which is analogous to the phenomenon of electromagnetically induced transparency. Such coupled resonant modes may have potential applications in sound wave manipulations such as acoustic filtering and imaging.     

A reflecting-type highly efficient terahertz cross-polarization converter based on metamaterials

We propose and experimentally demonstrate a wideband linear polarization converter in a reflection mode operating from 2.4 to 4.2 THz with conversion efficiency of more than 80%. Our device can expand the applications to a higher frequency band. A numerical simulation is performed for this metamaterial converter, which shows a good agreement with experimental results. Importantly, a concise and intuitive calculating model is proposed for the Fabry–Pérot cavity. The theoretical results indicate that the underlying reason for the enhanced polarization conversion is the additional phase difference induced by the resonance of the meta-structure and multiple reflections within the Fabry–Pérot cavity.     

Reflection-type electromagnetically induced transparency analogue in terahertz metamaterials

A reflection-type electromagnetically induced transparency（EIT） metamaterial is proposed, which is composed of a dielectric spacer sandwiched with metallic patterns and metallic plane. Experimental results of THz time domain spectrum（THz-TDS） exhibit a typical reflection of EIT at 0.865 THz, which are in excellent agreement with the full-wave simulations. A multi-reflection theory is adopted to analyze the physical mechanism of the reflection-type EIT, showing that the reflection-type EIT is a superposition of multiple reflection of the transmission EIT. Such a reflection-type EIT provides many applications based on the EIT effect, such as slow light devices and nonlinear elements.