太赫兹液晶材料与器件研究进展

液晶是一种性能优异的可调控光电功能材料,基于液晶的太赫兹器件有着广泛的应用前景,但高性能太赫兹功能器件的研发仍处于初级阶段.本文综述了太赫兹领域液晶材料与器件的研究现状,探讨了液晶技术与太赫兹技术相结合的发展趋势.     

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.     

SU-8 based planar metamaterials with fourfold symmetry as selective terahertz absorbers

We report on the absorption properties of polarization-insensitive transmissive and reflective metamaterial absorbers based on two planar aluminium periodic structures and SU-8 epoxy resist. These absorbers were investigated using numerical simulation and experimental methods in the terahertz range (below 2 THz). SU-8 is a very promising organic material for dielectric layers in planar metamaterials, because its application simplifies the process of fabricating these structures and significantly reduces the fabrication time. The experimental absorption of the metamaterial absorbers has narrowband characteristics that were consistent with the numerical simulations. Power flow analysis in the transmissive metamaterial unit cell shows that the absorption in the terahertz range occurs primarily in the SU-8 layer of the absorber.     

基于石墨烯和二氧化钒的太赫兹宽带可调谐超材料吸收器

太赫兹超材料吸收器作为一种重要的太赫兹功能器件,被广泛应用于生物医学传感、电磁隐身、军用雷达等多个领域.但这种传统的超材料吸收器结构具有可调谐性差、功能单一、性能指标不足等缺点,已经无法满足复杂多变的电磁环境的要求,因此可调谐超材料吸收器逐渐成为了太赫兹功能器件领域的研究热点.为实现超材料吸收器吸收特性的调谐,通常从调...     

Thermal tunability in terahertz metamaterials fabricated on strontium titanate single-crystal substrates

We report an experimental demonstration of thermal tuning of resonance frequency in a planar terahertz metamaterial consisting of a gold split-ring resonator array fabricated on a bulk single-crystal strontium titanate (SrTiO?) substrate. Cooling the metamaterial starting from 409 K down to 150 K causes about a 43% shift in resonance frequency, and there is very little variation in resonance strength. The resonance shift is due to the temperature-dependent dielectric constant of the strontium titanate. The experiment opens up avenues for designing tunable terahertz devices by exploiting the temperature-sensitive characteristic of high dielectric constant substrates and complex metal oxide materials.     

A terahertz polarization insensitive dual band metamaterial absorber

Metamaterial absorbers have attracted considerable attention for applications in the terahertz range. In this Letter, we report the design, fabrication, and characterization of a terahertz dual band metamaterial absorber that shows two distinct absorption peaks with high absorption. By manipulating the periodic patterned structures as well as the dielectric layer thickness of the metal-dielectric-metal structure, significantly high absorption can be obtained at specific resonance frequencies. Finite-difference time-domain modeling is used to design the structure of the absorber. The fabricated devices have been characterized using a Fourier transform IR spectrometer. The experimental results show two distinct absorption peaks at 2.7 and 5.2?THz, which are in good agreement with the simulation. The absorption magnitudes at 2.7 and 5.2?THz are 0.68 and 0.74, respectively.     

基于光导微探针的近场/远场可扫描太赫兹光谱技术

太赫兹技术已经成为涉及公共安全、军事国防和国民经济等国家核心利益的前沿研究领域.以往太赫兹测量技术中通常以远场测量为主,如常用的太赫兹时域光谱仪.近年来太赫兹近场技术得到了迅猛的发展,特别是基于光导天线的探针技术的发展,为可扫描的太赫兹近场测量提供了可能.本文详细报道了我们近期在可扫描太赫兹近场光谱仪研究中的进展.采用光纤耦合的光导微探针实现了方便灵活的太赫兹近场/远场三维扫描,并同时获得振幅和相位信息.该系统将有可能广泛应用于人工微结构、石墨烯、表面等离子激元、波导传输、近场成像、生物样品检测、芯片检测等研究领域.     

相变材料与超表面复合结构太赫兹移相器

利用相变材料嵌入超表面组成复合结构实现太赫兹移相器,该器件自上而下依次为二氧化钒嵌入金属层、液晶、二氧化钒嵌入金属层、二氧化硅层.通过二氧化钒的相变特性和液晶的双折率特性同时作用实现对器件相位调控.随着外加温度变化二氧化钒电导率发生改变,器件的相位随之产生移动,同样的对液晶层施加不同的电压导致液晶折射率发生变化,器件相位也会有影响.经过这两种介质共同作用,最终实现对太赫兹波相位有效调控.仿真结果验证了该相移器在频率f=0.736 THz时,太赫兹移相器的最大相移量达到355.37°,在0.731—0.752 THz(带宽为22 GHz)频率范围相移量超过350°.这种基于相变材料与超表面复合结构为灵活调控太赫兹波提供了一种新思路,将在太赫兹成像、通信等领域有着广泛的应用前景.     

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.     

A simple design of ultra-broadband and polarization insensitive terahertz metamaterial absorber

Ultra-broadband metamaterial absorbers have attracted considerable attention due to their great prospect for practical applications. These absorbers are usually stacked by many (no. <20) different shaped or sized subunits in a unit cell, making it quite troublesome to be fabricated. Simple design for ultra-broadband absorber is urgently necessary. Herein, we propose a simple design of ultra-broadband and polarization insensitive terahertz metamaterial absorber based on a double-layered composite structure on a metallic board, and each layer consists of two sets of different sized square metallic plates. Greater than 90 % absorption is obtained across a frequency range of 0.85 THz with the central frequency around 1.60 THz. The relative absorption bandwidth of the device is greatly improved to 53.3 %, which is much larger than previous results. The mechanism of the ultra-broadband absorber is attributed to the overlapping of four closely resonance frequencies. The proposed metamaterial absorber has potential applications in detection, imaging and stealth technology.     

Highly confined hybrid spoof surface plasmons in ultrathin metal-dielectric heterostructures

Highly confined "spoof" surface plasmons (SSPs) are theoretically predicted to exist in a perforated metal film coated with a thin dielectric layer. Strong modes confinement results from the additional waveguiding by the layer. Spectral characteristics, field distribution, and lifetime of these SSPs are tunable by the holes' size and shape. SSPs exist both above and below the light line, offering two classes of applications: "perfect" far-field absorption and efficient emission into guided modes. It is experimentally shown that these plasmonlike modes can turn thin, weakly absorbing semiconductor films into perfect absorbers.     

Nanosphere dispersed liquid crystals for tunable negative-zero-positive index of refraction in the optical and terahertz regimes

An analysis of aligned nematic liquid crystal cells containing core-shell nanospheres shows that it is possible to devise a new type of metamaterial whose index of refraction is tunable from negative, through zero, to positive values. The design parameters for the constituents can be scaled for application in the optical as well as very long wavelength (e.g., terahertz and microwave) regions.     

Tunable multi-band terahertz absorber based on graphene nano-ribbon metamaterial

The tunable multi-band selective absorption effect with graphene nano-ribbon metamaterial is investigated. It is achieved by depositing a set of graphene nano-ribbons on a dielectric spacer backed with a metallic mirror. A dual-band and tri-band absorbers are designed to illustrate such multi-band selective absorption behavior. The designed graphene absorbers exhibit near-unity absorption at multiple resonance frequencies, which is attributed to the plasmonic resonance of graphene nano-ribbons with different widths. Moreover, the multiple resonance wavelengths can be tuned in a wide frequency range by changing the Fermi energy and the absorbers possess large angle insensitivity. Last, a Fabry-Perot model is employed to give a physical understanding of such multi-band selective absorption effect. It is believed that the conclusions may be useful for designing of next-generation graphene-based optoelectronic devices.     

Tunable terahertz metamaterial based on resonant dielectric inclusions with disturbed Mie resonance

Tunable metamaterial operating in terahertz (THz) frequency range based on dielectric cubic particles with deposited conducting resonant strip was investigated. The frequency of the first magnetic type Mie resonance depends on the electric length of the strip. It can be changed under photoexcitation or applied voltage. This method of control was used for a design of tunable double negative metamaterial based on dielectric resonant inclusions and wire medium.     

HfO_2-based ferroelectric modulator of terahertz waves with graphene metamaterial

Tunable modulations of terahertz waves in a graphene/ferroelectric-layer/silicon hybrid structure are demonstrated at low bias voltages. The modulation is due to the creation/elimination of an extra barrier in Si layer in response to the polarization in the ferroelectric Si:HfO_2 layer. Considering the good compatibility of HfO_2 with the Si-based semiconductor process, the highly tunable characteristics of the graphene metamaterial device under ferroelectric effect open up new avenues for graphene-based high performance integrated active photonic devices compatible with the silicon technology.     

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.     

Thermal broadband tunable Terahertz metamaterials

A thermally tunable metamaterial comprising a periodic array of metallic split-ring resonators with an embedded semiconductor has been proposed. The resonance frequencies of the metamaterial are demonstrated to be continuously tuned in the terahertz regime by increasing the temperature. The tunability is attributed to the temperature-dependent permittivity of the embedded semiconductor and well analyzed by an equivalent capacitor-inductor model. The proposed designs ensure broadband thermally tunable terahertz devices.     

光栅-液晶复合结构太赫兹移相器

随着太赫兹技术及其应用的快速发展,各类太赫兹控制器件需求也随之增加,作为太赫兹系统重要器件之一,太赫兹波移相器成为当前研究热点。已有移相器存在着尺寸较大、结构复杂、相移量较小等问题,为克服上述缺陷,提出一种光栅-液晶复合结构太赫兹移相器,该器件结构为石英、石墨烯、液晶盒、光栅结构、石墨烯和石英组成。通过改变石墨烯电极上电压,使液晶折射率发生改变,相移器的相位因折射率改变而发生变化,通过控制外加电压可以实现对太赫兹波相移量有效调控。计算结果表明,该移相器在0.39~0.46 THz频率范围内实现了400°相移量,回波损耗小于-11 dB,在频率0.43 THz处,最大相移量达到422°。太赫兹波入射角在0°~30°范围内变化,移相器的相移量保持不变,而且该器件对入射太赫兹波偏振态不敏感。所设计的太赫兹移相器具有相移量大、结构尺寸小等优点,在未来的太赫兹通信、安检、医疗、传感、成像等领域中具有广阔的应用前景。     

石墨烯太赫兹波动态调制的研究进展

石墨烯是一种有着独特电学和光学性质的二维材料,近年来在太赫兹波动态调制的研究中有着广泛的应用。本文主要对基于石墨烯的太赫兹波动态调制器件进行了综述,分析了电调制、光调制和光电混合调制3种调制方法的原理和优缺点,介绍了近几年来将石墨烯应用于太赫兹波动态调制所取得的一系列科研成果,着重对不同器件的调制性能进行了对比,分析了优势和不足。石墨烯可调超材料为实现更快速、高效的太赫兹调制器件提供了新的思路。     

混合谐振模式宽带长波红外超表面吸收器研究

为了满足红外探测器件集成化和对红外宽光谱范围吸收的需求,设计了一种工作在长波红外波段(8~14μm)的超宽带、高吸收、极化不敏感的超材料吸收器。通过在金属-介质-金属三层异质的超材料吸收器结构的顶部金属周围镶嵌一层介质形成超表面,以增加谐振强度和吸收带宽。在8~13.6μm的带宽范围内,该结构有超过90%的平均吸收率,覆盖了大部分长波红外大气窗口波段,对红外探测领域有着重要意义。研究结果表明:镶嵌的金属-介质组成的介质波导模式和谐振腔模式的结合以及传播型表面等离激元模式的激发是形成宽带高吸收的主要原因,并且谐振模式的谐振波长可以通过相关参数来进行调控。本文的研究结果为可调谐宽带长波红外吸收材料的设计提供参考,该设计方法可推广到中波红外波段、甚至长波红外或其它波段。