Multiband Metamaterial Absorber at Terahertz Frequencies

We propose a multi-band metamaterial absorber operating at terahertz frequencies. The design, characteriza- tion, and theoretical calculation of the high performance metamaterial absorber are reported. The multi-band metamaterial absorber consists of two metallic layers separated by a dielectric spacer. Theoretical and simulated results show that the metamaterial absorber has four distinct absorption points at frequenc/es 0.57 THz, 1.03 THz, 1.44 THz and 1.89 THz, with the absorption rates of 99.9%, 90.3%, 83.0%, 96.1%, respectively. Two single band metamaterial absorbers and a dual band metamaterial absorber on the top layer are designed. Some multi-band absorbers can be designed by virtue of combining some single band absorbers. The multiple-reflection theory is used to explain the absorption mechanism of our investigated structures.     

一种光激发可切换的双频太赫兹超材料吸收器

提出了一种基于方环-金属线结构的光激发动态可切换双频太赫兹超材料吸收器.通过调节嵌入在结构间隙中的光敏硅和锗的绝缘/导通状态,该吸收器可在不改变结构的前提下在三个双频完美吸收态之间自由切换.结果表明:当没有光泵浦时,该吸收器工作在0.987 THz和1.767 THz双频吸收态;当采用800 nm激光泵浦时,该吸收器可...     

Laser-Excited Frequency-Switchable and Polarization-Controlled Amplitude-Tunable Terahertz Metamaterial Absorber

Journal of Russian Laser Research - We achieve a frequency-switchable and amplitude-tunable terahertz metamaterial absorber (MMA) by means of introducing two different controlling degrees of...     

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...     

Terahertz Thermometry: Combining Hyperspectral Imaging and Temperature Mapping at Terahertz Frequencies

The accurate and non‐invasive determination of multiple physical parameters, with well‐defined spatial resolution, is crucial for applications in manufacturing, chemistry, medicine and biology. Specifically, the ability to simultaneously measure both temperature and spectral signatures is still experimentally unavailable. To this end, we propose a mapping technique for biological systems, which exploits a linear correlation between terahertz wave reflectivity and temperature, and allows to spatially and spectrally resolve thermal distributions. This method is applied to a model biological system in two relevant cases where in one example, nanoplasmonic‐induced photothermal effects are imaged gaining new insights into collective heating phenomena. In the second example, we demonstrate a joint thermal‐hyperspectral imaging approach to chemically map the presence of a model drug formulation and simultaneously investigate its thermal stability in our biological system. This concept can be easily extended and widely applied to all materials that demonstrate a measurable change in their dielectric properties.     

一种宽频带超材料吸波体的设计及其特性

为了获得吸收率高、吸波带宽宽的超材料,设计了一种谐振超材料吸波体.该吸波体由多个开口圆环组成,采用商业软件CST Studio Suite 2009频域求解器计算了其在25～35 GHz波段内的S参量,并计算了其吸波率A(ω),在28.4 GHz处吸收率达到86％,带宽达到3.5 GHz.利用不同吸波频段的叠加效应,设计了一种谐振超材料吸波组合体,计算了在25～35 GHz波段的S参量,在29.7 GHz处吸波率达99.9％,吸波带宽达到3.1 GHz,吸收率明显增加.将GHz波段的结构缩小1 000倍,在THz波段同样可以达到高吸收,说明超材料吸波体可以通过对结构尺寸调节改变吸收波段.同时,对其阵列进行仿真计算,发现不同的排列方式仿真结果不同.由于各个谐振环之间的相互作用对吸收效果影响较大,吸收率减小.该吸波材料由金属组成,能灵活地对介电常量和磁导率进行调节,从而实现高吸收.     

一种宽频带超材料吸波体的设计及其特性

为了获得吸收率高、吸波带宽宽的超材料,设计了一种谐振超材料吸波体.该吸波体由多个开口圆环组成,采用商业软件CST Studio Suite 2009频域求解器计算了其在25~35 GHz波段内的S参量,并计算了其吸波率A(ω),在28.4 GHz处吸收率达到86%,带宽达到3.5 GHz.利用不同吸波频段的叠加效应,设计了一种谐振超材料吸波组合体,计算了在25~35 GHz波段的S参量,在29.7 GHz处吸波率达99.9%,吸波带宽达到3.1 GHz,吸收率明显增加.将GHz波段的结构缩小1 000倍,在THz波段同样可以达到高吸收,说明超材料吸波体可以通过对结构尺寸调节改变吸收波段.同时,对其阵列进行仿真计算,发现不同的排列方式仿真结果不同.由于各个谐振环之间的相互作用对吸收效果影响较大,吸收率减小.该吸波材料由金属组成,能灵活地对介电常量和磁导率进行调节,从而实现高吸收.     

A New Theoretical Modeling of Surface Resistance in Normal Metals at Terahertz Frequencies

A number of researchers have reported discrepancies between surface resistance measurements and classical theoretical predictions for simple mm-wave and sub-mm-wave structures. We have developed a rigorous phenomenological model for analyzing surface resistance of normal metals. The model is based on quantum mechanical analysis for spatial dispersion within the metal. We have used the model to predict the surface resistance in normal metals at terahertz frequencies. Our analysis shows that the conductivity is not only frequency but also wave vector dependent. Moreover, we have compared the results of this model with the results of the classical skin-effect model and classical relaxation- effect model. We demonstrate that our model has good quantitative agreement with the published experimental data for the room temperature surface resistance of normal metals at terahertz frequencies range from the literature.     

Design of Tunable Devices at Terahertz Frequencies Based on Quasi-Photonic Crystals Incorporated with Graphene

In this study, we present a new theoretical model including Thue-Morse and double-period sequences as quasi-photonic crystals are incorporation with graphene and investigate the transmission properties of the THz waves in both structures using a straightforward computational method. We also consider properties of nonlinear conductivity in addition to surface linear conductivity for graphene. We observe the sharp peaks and proper forbidden bands are created in the range of $0.3$ THz to $30$ THz. In addition, we find that by considering the nonlinear term of graphene and engineering the structural parameters such as the chemical potential of graphene, number of layers and the incidence wave angle, transmission levels of peaks enhance scientifically and quality factor improve considerably. These results show that it would be possible to design of high-Q tunable filters with multi-stop bands in the THz regime which can reduce the noise associated with THz frequency peaks and increase the number of sharp frequency peaks.     

Growth and Characterization of a Kind of Nitrogen-Rich Niobium Nitride for Bolometer Applications at Terahertz Frequencies

Niobium is sputtered onto a single crystalline silicon substrate in N2：Ar=4：1 gas mixture at the total pressure of 2 Pa. The temperature coefficient of resistance of the sample is about 0.5% at 30OK, and up to 7% at 77K, indicating the possibility of using it to make room-temperature bolometers with performances better than those based on Pt, Bi, or Nb. For a 60-nm-thick sample, the rms surface roughness is 0.45nm over an area of 2 μm × 2 μm. Analyses based on x-ray diffraction and x-ray photoelectronic spectroscopy indicate that the samples are Nb5N6 thin films in which there is a combination of Nb^3＋ and Nb^5＋, or Nb^4＋.     

Simultaneous Amplification of Terahertz Difference Frequencies by an Injection-Seeded Semiconductor Laser Amplifier at 850 nm

Two-frequency operation of an 850 nm semiconductor optical amplifier was achieved by simultaneously injection seeding it with two diode lasers. The two frequencies could be independently amplified without strong interference when they were separated by more than 10 GHz, and the spectral purity was preserved by the amplification process. At frequency differences below 10 GHz, unbalanced two-frequency output was observed, which can be explained by a two-mode interaction driven by the refractive index modulation at the beat frequency. The laser system is suitable for the difference-frequency generation of coherent terahertz radiation in ultra-fast photoconductors or nonlinear optical media.     

Tunable and Switchable Narrow Bandwidth Semiconductor-Saturable Absorber Mirror Mode-Locked Yb-Doped Fiber Laser Delivering Different Pulse Widths

The wavelength-tunable and switchable narrow bandwidth mode-locking operation is demonstrated in an all fiber laser based on semiconductor-saturable absorber mirror(SESAM).Two narrow-band fiber Bragg gratings centered at 1029.9 nm and 1032 nm respectively with a polarization controller inserted between them are used to realize the wavelength switchable between 1029.9nm and 1032 nm.The laser delivers different pulse widths of 7.5ps for 1030 nm and 20 ps for 1032 nm.The maximum output power for both could reach ~6.5mW at single pulse operation.The output wavelength could be tuned to about 0.9nm intervals ranging from 1030.2nm to 1031.1 nm and from 1032.15 nm to 1033.7nm with the temperature change of the fiber Bragg grating,respectively.     

Temperature Evolution of the Interaction of Relaxation Processes with Local Dynamics at Terahertz Frequencies in Polymers with Hydrogen Bonds

Physics of the Solid State - At terahertz frequencies, the torsional vibration motion is associated with dielectric relaxation in disordered solids with hydrogen bonds. The interaction between...     

Photoexcited processes at metal and alloy surfaces: electronic structure and adsorption site

Photoexcited processes of NO and CO at photon energies ranging from 2.3 to 6.4 eV are investigated on Pt(111), Ni(111) and Pt(111)---Ge surface alloys by reflection-absorption infrared spectroscopy and resonance-enhanced multiphoton ionization. The branching between three competitive processes of desorption, recapture and dissociation upon laser irradiation is dramatically changed on the three surfaces. On Pt(111), NO is either photodesorbed or photodissociated depending on the coverage, while NO is exclusively photodissociated on Ni(111). UV-photon irradiation of NO on Pt(111)---Ge, on the other hand, induces only desorption of NO. Desorption of CO bound at the on-top site of Pt(111) is induced by laser irradiation. The electronic mechanism for photodesorption and competitive branching is discussed in terms of the electronic structure of the substrate and the adsorbate.     

An Infrared Metamaterial Broadband Absorber Based on a Simple Titanium Disk with High Absorption and a Tunable Spectral Absorption Band

A metamaterial absorber is proposed that functions in the medium- (3–5 µm) and long-wavelength (8–12 µm) infrared (medium-wavelength infrared, MWIR, and long-wavelength infrared, LWIR, respectively) regions. The proposed design, which consists of periodic cells, can be tuned to achieve single-band or dual-band light absorption by changing the periodicity of the structure. Each cell forming the metamaterial absorber consists of a bottom metal plate (Al), a top metal disk (Ti), and an intermediate dielectric medium (Si or ZnS) in which a metal disk (Ti) is embedded. For a period of 0.85 µm, the absorber achieves broadband absorption in the LWIR region, with an average absorption of 92.1%. Further, the absorber shows acceptable tolerance to irradiation at oblique incidence. For a period of 2 µm, a peak absorption of 99.05% is achieved in the MWIR region, thereby providing dual-band absorption. Tuning the periodicity of the structure enhances the localized surface plasmon resonance, with the absorption mechanism explained by establishing an equivalent parallel LC circuit. The absorption properties demonstrated by the proposed metamaterial absorber are promising for thermal imaging and infrared spectroscopy.     

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 VO₂ 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 VO₂ is at metal phase,and convert x-polarized wave to circularly polarized wave when ...     

L-阿拉伯糖指纹区太赫兹光谱和拉曼光谱的研究

为了解生物体内L-阿拉伯糖在代谢过程中的合成与降解机制,采用太赫兹和拉曼光谱系统,对其指纹区的振动进行检测。结果表明,L-阿拉伯糖太赫兹图谱在频率49.5和72.2 cm-1分别检测出了振动吸收,其中72.2 cm-1的振动为首次检出。该振动频率与其折射率图谱反常色散的频率基本一致,故这两个振动吸收可以作为L-阿拉伯糖的特征吸收。最为重要的是,在该频域内,检测得到图谱的波型与三种异构体理论值简单叠加后波型极为相似,故可以初步判定样品含有三种构象异构体(α-型、β-型和l-型结构),非单一组分,而是混合组分;对于拉曼图谱而言,其特点简洁而明晰,一般将指纹区的振动,从高到低分为四个区域：吡喃环结构的伸缩振动、亚甲基的摇摆振动、环上羟基的扭曲振动及环骨架扭曲和畸变振动。同时也根据密度泛函理论B3LYP/6-311G**基组,分别对L-阿拉伯糖的三种构象异构体的振动进行模拟计算,利用势能分布对这些振动进行归属和指认。与理论值相比,振动频率检测值有不同程度的红移,即振动频率向低频发生了偏移,其原因是样品内不同分子间相互影响所致。     

低损耗高双折射太赫兹Topas光子带隙光纤

以太赫兹低损耗聚合物材料Topas环烯烃共聚物为基材,设计了一种带隙型光子晶体光纤.光纤由三角形排列的圆角正六边形空气孔构成包层,缺失四个近邻空气孔构成近菱形的二重对称空气芯.采用有限元法分析了该光纤在太赫兹波段的传输特性.结果表明:在1.5THz附近约0.3THz的宽频范围内存在光子带隙,光纤可以基于光子带隙效应将太赫兹波束缚在空气芯中传输.在1.4~1.6THz范围内具有10-3数量级的高双折射;x偏振基模和y偏振基模的损耗都小于0.1cm-1,分别在1.53THz和1.5THz处达到最小值0.029 1cm-1和0.028 7cm-1.所设计的太赫兹Topas光子带隙光纤具备结构简单、易制备、直径小而易弯曲的特点.     

Generation and Radiation of Waves at Combined Frequencies in a Warm Inhomogeneous Plasma Layer

We investigate generation and radiation of waves at combined frequencies from an arbitrary inhomogeneous, isotropic plasma layer, when electromagnetic waves are obliquely incident on it to interact with a surface wave at the plasma boundary. This interaction has been described for both S- and P-polarized waves. We consider a warm plasma layer with thickness very small as compared to the wavelengths of oscillations. It is shown that generated waves are strongly amplified, compared to cold plasma, when phase velocities of generated waves approaches the electron thermal velocity. Waves are not emitted when P-polarized waves are incident perpendicular onto the plasma layer.     

Conditions for Separation Rays by Their Arrival Time at Large Distances and Low Frequencies

Acoustical Physics - Abstract—The results of experimental studies on the propagation of low-frequency pulsed signals in deep- and shallow-water areas of the World Ocean are presented....