NOVEL TERAHERTZ PHOTOCONDUCTIVE ANTENNAS

The photoconductive (PC) antenna is a key device for the recent terahertz (THz) photonics based on laser-pumped generation and detection of THz radiation. In this paper we report on two new types of PC antennas: the Schottky PC antenna and the multi-contacts PC antenna. The former one is able to detect THz radiation intensity without the time-delay scan and useful for applications where spectroscopic information is not important, such as the THz intensity imaging. The latter one is useful for the polarization sensitive THz spectroscopy, such as the THz ellipsometry. The characteristic features of these new types of PC antennas are studied by using a THz time-domain spectroscopy system.     

基于LT-GaAs外延片的THz片上系统

太赫兹（THz）波在物质检测方面发挥着巨大的作用,是一种非常有潜力的生化传感工具。但是传统的太赫兹时域光谱系统（TDS）结构复杂,系统的集成度低,占用空间较大。所以,如何对THz波进行有效引导、实现集成化传输并得到高质量光谱就成为太赫兹光谱系统的研究热点。太赫兹片上系统是将THz的产生、传输以及探测都集成到同一芯片上,然后通过相干探测的方法获得THz时域光谱。它可以实现对多种样品的检测,尤其在对难于取样的微量样品探测方面具有广泛的应用价值。它无需光路准直,操作简便,成品率高。两个研究工作都是基于低温砷化镓(LT-GaAs)外延片开展的。首先将一根直径为200 μm的铜线固定在LT-GaAs外延片的上方,通过真空蒸镀的方法制备出天线电极,同时得到天线间隙,研制出基于LT-GaAs外延片的THz天线。利用波长为800 nm的飞秒激光对其进行测试,得到了质量较高的THz信号,验证了天线的实用性。然后在另一外延片上利用光刻微加工工艺制作出传输线和微电极,得到了集成的THz片上系统。使用波长为1 550 nm的飞秒激光分别激发片上系统的太赫兹产生天线和探测天线,天线产生的太赫兹波在传输线上传播,在探测端同样得到了质量较高的THz时域信号,证实了THz片上系统的可行性。该方法省去了腐蚀牺牲层以及LT-GaAs薄膜的转移、键合等步骤,极大地提高了片上系统的成品率,避免了薄膜转移过程中易破碎及腐蚀液存在毒性的问题。最后,研究了外加电压对从片上系统中获得的THz波性能的影响,结果为电压越高,THz波的信号强度越强;另外,通过在传输线上方垂直放置铜箔的方法验证了THz波沿着传输线传播的事实。该研究中采用的基于LT-GaAs外延片的片上系统的制备方法简单,制作周期短,制作过程安全,应用领域广泛,这为将来与微流控芯片相结合实现对液体样品的探测打下了基础。     

太赫兹波探测光子晶体涂层覆盖目标的可行性

覆盖于高温目标表面的光子晶体红外涂层可实现对目标红外辐射的抑制,而太赫兹波所具有的强穿透特性使其对该类目标的探测成为可能。以相关文献中设计的光子晶体涂层为例,采用特征矩阵理论对0.3~3 THz频率范围内的太赫兹波在该类涂层中的传输特性进行理论计算和分析,重点研究了不同入射角度的太赫兹波在该类涂层中的传输特性。研究发现,上述太赫兹波段处于光子晶体的带隙之外,0.3~0.5 THz频率范围内的太赫兹波对该类红外涂层具有较强的穿透特性,其光谱透过率大于90%;而在2.4~3 THz范围内,其在涂层上具有较强的反射,且整个波段内的吸收率小于0.2%。当入射角小于60°时,其对太赫兹波的传输特性影响较小;进一步增大入射角,其透过率逐渐降低,而反射率逐渐增大。研究结果证明了利用太赫兹波进行涂层覆盖目标探测的可行性,有望利用太赫兹雷达探测弥补红外探测系统的不足。     

光电导天线太赫兹辐射峰值调控研究

针对微结构光电导天线与飞秒激光之间相互作用效应以及辐射太赫兹波调控问题进行了研究。采用德鲁德-洛伦兹理论模型获得微结构光电导天线辐射光电流密度,通过时域有限差分把光电流密度迭代在激励网格上,结合麦克斯韦方程求解时变电磁场,并通过传输线格林函数获得多层介质近场到远场的辐射太赫兹波,建立了辐射光电流与辐射阻抗、电磁共振模式之间的关系模型,模拟仿真分析了微结构S型光电导天线太赫兹波辐射调控机理。研究结果表明:微结构改变了天线等效模型的辐射阻抗;同时得知耦合系数不为零时存在耦合作用,且随着耦合系数增大共振频率峰值发生辐射增强和位移;并通过设计S型光电导天线获得辐射峰值频率调整范围为0.50~0.80 THz之间,对比工形天线辐射峰值频率由原来的0.40 T移动到0.76 T,频率调整度75%,峰值辐射效率约提高70%。该研究工作为后续高功率光导天线太赫兹波辐射的共振中心频点以及结构设计奠定重要基础。     

雪崩倍增GaAs光电导太赫兹辐射源研究进展

在飞秒激光激励下用GaAs光电导开关作为太赫兹(THz)辐射天线, 已经广泛用于太赫兹时域光谱系统, 但目前国际上都是使用GaAs光电导开关的线性工作模式, 而GaAs光电导开关的雪崩倍增工作模式所输出的超快电脉冲功率容量远大于其线性工作模式, 迄今为止, 还没有人提出用雪崩倍增机理的GaAs 光电导开关作为辐射源产生THz电磁辐射. 本文探讨了用 雪崩倍增工作模式的GaAs光电导开关作为光电导天线产生THz电磁波的可能性及研究进展. 通过理论分析及实验研究, 在实验上实现了: 1) 利用nJ量级飞秒激光触发GaAs光电导天线, 可以进入雪崩倍增工作模式; 2) 利用光激发电荷畴的猝灭模式, 可以使GaAs光电导天线载流子雪崩倍增模式的延续时间(lock-on 时间)变短. 这为利用具有雪崩倍增机理的GaAs光电导天线产生强THz辐射奠定了基础.     

GaAs光电导天线辐射太赫兹波功率的计算

在Larmor公式的基础上建立了适合计算光电导天线辐射太赫兹波功率的数学模型,利用此数学模型通过蒙特卡罗方法分别计算了不同实验条件下GaAs光电导天线辐射太赫兹电磁波功率.计算结果表明,增加光电导天线的偏置电场或触发光能量,都能够提高天线辐射太赫兹波功率,大孔径光电导天线能够承载更多的光生载流子,因而可以产生比小孔径光电导天线功率更高的太赫兹波. 关键词： 光电导天线 Larmor公式 太赫兹波功率     

Analytical framework of small-gap photoconductive dipole antenna using equivalent circuit model

A compact planar antenna sources with on-chip fabrication and high directivity in order to achieve large depth-of-field for better image resolution is the prospective demand for THz imaging application. Therefore, the small-gap photoconductive dipole antennas have been explored to fulfil such applications demand. However, there are certain modalities for improving the photoconductive dipole antenna performance which need to identify to accomplish high THz average radiated power and improved total efficiency. The unit-cell small-gap photoconductive dipole antenna radiation power enhancement methods need to optimize the design parameters with photoconductive material selection from theoretical simulation. Further, the potential improvement of coupling efficiency of THz wave with air as well as femto-second laser incident efficiency is also important parameters to enhance the radiation power of small-gap photoconductive dipole antenna. In this paper, we have presented an analytical procedure employing explicit mathematical expression leading to the physical behaviour of small-gap photoconductive dipole antenna. The effects of biased lines on the antenna performance parameters are discussed with the help of proposed equivalent circuit model. We have explored the effect of gap-size on the THz radiated power and on total radiation efficiency from the proposed photoconductive dipole antennas.     

基于纳观域碳纳米管的太赫兹波天线研究

基于碳纳米管独特的结构特点建立了以其为基础的Pocklington积分方程,并设计了一种全新的碳纳米管太赫兹(THz)波天线.数值仿真和理论计算结果表明,碳纳米管能够产生高频THz电磁辐射,半波长为60μm、半径为2.712nm的单壁碳纳米管偶极天线在-10dB反射系数以下可以实现2.5THz与7.6THz的双频带工作,带宽分别为8.4%与2.7%,由其构成的纳米管天线阵可以获得10.3dB的高增益特性.所得结果有助于在纳观域开展高频THz波辐射源及天线的研究与设计. 关键词： 太赫兹波 碳纳米管 天线 辐射源     

Terahertz-wave generation in a conventional optical fiber

We theoretically propose surface-emitted and collinear phase-matched terahertz (THz)-wave generation in a conventional optical fiber. The third-order nonlinear effect, four-wave mixing (FWM), is used to generate THz waves in an optical fiber. Surface-emitted THz-wave generation via FWM is realized using a single-mode fiber. Perfect phase matching is obtained at ~800 nm and 1.5 microm pumping, and it follows that third-order polarization in an optical fiber has the same phase at any point. In this situation, the optical fiber acts like a phased array antenna of the THz wave. Collinear phase-matching THz waves are obtained under the same conditions as for surface-emitted THz waves, and the THz wave is propagated in the silica cladding of the optical fiber. This is a promising method for realizing a reasonable THz-wave source.     

典型大气窗口太赫兹波传输特性和信道分析

在已有大气传输模型的基础上,发展了新的太赫兹波大气传输衰减与色散模型,对宽频太赫兹波在真实大气中传输的衰减和色散特性进行了数值模拟研究.改进太赫兹时域光谱技术,对0.3—2.0 THz频段太赫兹波的大气传输特性进行了透射光谱测量,并得到了一组连续吸收参数.比对发现实验窗口区强度和吸收峰的位置都与计算结果符合得很好.据此选取了三个可行的信道:340,410和667 GHz窗口区,利用线性色散理论和无线通信原理分别从物理上精确地计算了这些信道的群速色散参数和信道容量,并分析了影响最大传输数据率的因素-天线增益.研究结果表明:太赫兹波大气传输1 km时,这三个信道群速色散很小,信号不易被展宽;最大传输速率达十几Gbps,高于单模光纤,但需要更高的天线增益.     

THz wave polarization-controlled spectroscopic imaging for anisotropic materials

We present a polarization-controlled terahertz (THz) wave spectroscopic imaging modality to investigate the anisotropy of the detected materials. The polarization of the emitted THz wave is controlled by changing the relative phase between the fundamental and second-harmonic waves in the two-color laser-induced air plasma THz generation configuration. The THz wave polarization direction is extracted by measuring the two electric field amplitudes when the polarization of the incident wave is controlled to be horizontal and vertical. The anisotropy of the industrial Sprayed-On-Foam-Insulation (SOFI) is characterized by measuring its azimuthal angle dependent THz polarization response. This work demonstrates that THz wave polarization-controlled imaging technique can be used for highly sensitive industrial nondestructive inspection and biological related characterization.     

平面天线在场效应晶体管太赫兹探测器中的应用

为了提高场效应晶体管太赫兹探测器的响应度并降低噪声等效功率,需要对探测器集成平面天线的结构进行合理设计与优化,本文对集成平面天线结构的场效应晶体管太赫兹探测器的研究进行了深入调研。首先,对场效应晶体管太赫兹探测器的工作原理进行了分析,介绍了集成平面天线如何解决耦合太赫兹波效率低的问题。然后,介绍了一些常用的平面天线结构,包括偶极子天线、贴片天线、缝隙天线、grating-gate和其他类型的结构,比较了各种天线的性能以及引入后对太赫兹探测器响应度的影响。通过对比不同天线结构的探测器响应度和噪声等效功率等参数指标,发现:采用平面天线结构之后,场效应晶体管太赫兹探测器的响应度有了大幅度的提升,各种类型的天线对探测器响应度都有不同程度的提升。本文着重介绍了几种集成于场效应晶体管的平面天线结构,包括各种天线的性能和研究进展,最后分析了场效应晶体管太赫兹探测器存在的问题和发展趋势。     

纳米网络中的太赫兹波大气传输和信道分析（英）

利用太赫兹大气传输衰减模型,比对太赫兹时域光谱系统的实验结果,结合最新的HITRAN数据库,发展了一个适用于纳米尺度的太赫兹信道分析模型。提出了一个0.1~5 THz宽的信道,分析了此信道在纳米尺度的传输损耗和最大传输数据率。研究结果表明,在纳米尺度0.1~5 THz宽的信道的传输数据率达几百Gbit/s,随着天线增益等硬件性能的不断提升,信道的最大传输数据率将达Tbit/s,此研究对于纳米器件之间的快速、大数据量的信息共享具有重要的参考价值。     

Design of terahertz wave polarizing beam splitter

In this paper, we design a THz wave polarizer based on a periodic symmetrical thin film structure, which operates over a broadband THz frequency region (1.0–2.0 THz) and over an effective wide range of incident THz wave (72–84°). The spectral performance of this structure is characterized by transfer matrix method calculations. Results of simulations show that the polarizer is highly transmittance for TE polarized THz wave as well as highly reflecting for TM polarized THz wave.     

THz技术在农产品/食品品质检测中的应用

农产品/食品的质量和品质问题越来越受人们关注。探索实际可行的农产品/食品的无损检测与品质评估技术正在成为研究热点。太赫兹(THz)辐射是位于中红外和微波波段之间的一段电磁波,具有非常重要的科学研究和应用价值。长期以来由于缺乏可行的THz波产生方法与探测手段,该波段相关领域的研究滞缓。THz光谱传感和成像技术是THz波的两个主要应用技术。THz光谱检测技术作为一种新型检测技术能够获得传统检测无法获得的信息。近十几年来,THz波用于来研究固、液、气相等各种物质的光电特性、分子内部振动和组成信息,在生物分析、医疗诊断、安全检测、环境控制等领域,THz技术显示出广阔的应用前景。文章介绍了THz波的主要性质、THz波检测技术的特点,论述了THz技术在农产品、食品质量与品质检测中的最新进展及其应用的潜力。     

Terahertz (THz) Frequency Sources and Antennas - A Brief Review

In this paper we review THz radiation properties, generation methods, and antenna configurations. This paper suggests some new class of antennas that can be used at THz frequency, like optical antennas or Carbon nanotube antennas. THz technology has become attractive due to the low energy content and nonionizing nature of the signal. This property makes them suitable for imaging and sensing applications. But at the same time detection and generation of THz signals has been technologically challenging. This paper presents a comparative study of the generation techniques for THz frequency signals giving emphasis to the some new techniques like Quantum Cascade lasers which has created significant research interest. The main aim for this study is to find out the materials suitable for fabricating THz devices and antennas, a suitable method for generation of high power at THz frequency and an antenna that will make THz communication possible.     

Generation of THz radiation using bulk, periodically and aperiodically poled lithium niobate – Part 1: Theory

Optical rectification of femtosecond pulses in nonlinear materials is an efficient method to generate ultra short terahertz (THz) pulses over a wide frequency range extending from 100 GHz to well above 10 THz. Lithium niobate is particularly well suited for such purposes and can be used both in bulk and periodically poled forms. Different optical techniques for the generation of THz pulses are presented and compared theoretically. The whole discussion is performed for the interaction of gaussian beams using the radiating antenna approach that takes into account the diffraction of the THz wave, and therefore may predict the THz emission in a direction that differs from the optical pulse propagation. PACS 42.65.Ky; 42.70.Mp; 42.72.Ai     

A Novel Woodpile Three-Dimensional Terahertz Photonic Crystal

A novel woodpile lattice structure is proposed. Based on plane wave expansion （PWE） method, the complete photonic band gaps （PBGs） of the novel woodpile three~dimensional （3D） terahertz （THz） photonic crystal （PC） with a decreasing symmetry relative to a face-centred-tetragonal （fct） symmetry are optimized by varying some structural parameters and the highest band gap ratio can reach 27.61%. Compared to the traditional woodpile lattice, the novel woodpile lattice has a wider range of the filling ratios to gain high quality PBGs, which provides greater convenience for the manufacturing process. The novel woodpile 3D PC will be very promising for materials of THz Junctional components.     

Antenna gain enhancement by using metamaterial radome at THz band with reconfigurable characteristics based on graphene load

The reconfigurable concept of a graphene loaded patch antenna with increasing gain is proposed in this paper. We have developed the patch antenna with inset feed for THz band application for which graphene load is used to obtain the reconfigurable characteristic. It has been shown that by increasing the graphene chemical potential, the antenna resonant frequency shifted and the gain increased drastically up to 4 dBi. Additionally, we have shown that antenna efficiency is improved up to 78% which shows more than 100% of enhancement in comparison to basic antenna by increasing the graphene chemical potential. Finally, considering the antenna gain improvement, we have implemented the metamaterial layer over the antenna. In this case, the gain is increased more than 5–6 dBi. In addition, when we put the metamaterial layer over the antenna, the graphene layer shows more linear characteristics. By using parametric studies, we have defined the best point for metamaterial layer around 0.58λ. The final antenna gain is more than 11 dBi, which is useful for THz communication and THz medical imaging systems.     

周期极化非线性晶体中太赫兹波辐射研究

 从理论上详细研究了飞秒激光在周期极化非线性晶体中由光整流效应产生的太赫兹（THz）波辐射。利用天线辐射原理和光栅衍射理论，着重研究THz波辐射的频域场和时域场的分布。讨论和分析了THz波辐射的中心频率、频谱宽度和电场随辐射角的变化。研究表明，THz波的带宽反比于晶体的长度或光栅数，电场随辐射角呈准谐波变化。