低温生长砷化镓光电导天线产生太赫兹波的辐射特性

研究了低温生长砷化镓光电导天线(LT-GaAs PCA)产生太赫兹(THz)波的辐射特性。利用太赫兹时域光谱(TDS)技术测量了光电导发射极在飞秒激光作用下辐射的太赫兹脉冲,得到了时域发射光谱,并通过快速傅里叶变换(FFT)得到相应的频域光谱。结果表明,低温砷化镓光电导天线产生的太赫兹波信号比飞秒激光激发半导体表面产生的太赫兹波信号具有更高的强度和信噪比;太赫兹波信号与光电导天线的偏置电压成线性关系;随着抽运激光功率的增强,太赫兹波信号增大并出现饱和。     

光电导天线太赫兹辐射研究

利用超短激光脉冲触发光电导天线产生太赫兹波是目前研究比较深入,同时得到广泛采用的一种太赫兹波源.电流瞬冲模型能较好地解释光电导天线辐射太赫兹波的机理,并且与实验相吻合,因而被普遍接受.影响光电导天线的因素有材料性质、偏转电压、激光脉冲宽度和单脉冲能量等,其中激光脉冲宽度和激光脉冲能量对太赫兹辐射有很重要的影响.材料中的光生载流子寿命对辐射也有一定的影响.当改变这些参数时,辐射的太赫兹波有不同的特性变化.此外,逐步增大激光脉冲能量,会发现辐射的太赫兹波强度存在一个饱和特性.     

可产生任意偏振方向太赫兹波的光电导太赫兹辐射源

基于太赫兹时域光谱（THz-TDS）系统，使用两个相互垂直的光电导天线构建了1×2 GaAs光电导太赫兹源阵列。通过调控各个阵元的偏置电压，对其辐射太赫兹波的偏振方向进行研究。结果表明：在已实现光电导发射天线阵列的高效合成以及可同时检测脉冲太赫兹波的振幅、相位及偏振态的探测天线的基础上，通过调控各个阵元的偏置电压分别改变了平行和垂直两个阵元辐射太赫兹波的强度；经过1×2 GaAs光电导太赫兹源阵列在远场的同步合成，可产生不同偏振方向的脉冲太赫兹波，实现了以全电控的方式产生任意偏振方向太赫兹波的光电导太赫兹辐射源。     

偏振可调的太赫兹光电导天线设计北大核心

提出了一种偏振可调的太赫兹光电导天线,其结构由四个弧形金属电极、低温砷化镓衬底和氮化硅抗反射涂层构成。通过对光电导天线的工作原理分析,可以发现弧形金属电极结构决定天线的辐射偏振,且衬底的载流子迁移率、载流子寿命和衬底材料对激光的吸收直接影响天线的辐射特性。利用COMSOL软件对该光电导天线进行建模仿真,其结果表明该光电导天线可以在45°方向倍增地辐射线偏振太赫兹波,且辐射强度相较于常规光电导天线提高了30%,辐射带宽高达10 THz。所设计太赫兹光电导天线具有偏振可调、结构简单和易于加工等特点,在太赫兹光谱检测领域具有广阔的应用前景。     

基于整形飞秒激光脉冲泵浦光电导天线的宽带太赫兹辐射研究

飞秒激光脉冲泵浦光电导天线可以产生宽带太赫兹脉冲辐射。采用具有缓慢上升-快速下降整形脉冲包络的飞秒激光脉冲与光电导天线作用产生宽带太赫兹辐射,基于Drude-Lorentz模型计算了太赫兹辐射的规律,并讨论了激光脉冲脉宽和半导体载流子寿命对太赫兹辐射产生的影响。研究结果表明,尽管这种整形飞秒激光脉冲的能量有所损失,但是这种泵浦方式能够比普通飞秒激光脉冲泵浦光电导天线产生更宽的太赫兹辐射。该结果为实验上基于光电导天线产生宽带太赫兹脉冲辐射提供了新的思路     

飞秒激光触发光电导天线产生太赫兹波的研究

研究了光电导天线产生太赫兹波的辐射特性,利用麦克斯韦方程及其边界条件,计算了近远场的电场强度;采用电磁波时域有限差分方法（FDTD）,在Matlab系统软件中,用C语言编写程序计算光电导偶极天线的辐射太赫兹波的空间电磁场分布,并在计算机上以伪彩色图形显示,这种电磁场的可视化结果为天线的设计和改进提供了直观的物理依据。     

低温生长砷化镓光电导天线产生太赫兹波

宽频太赫兹(THz)技术在国防、科研等领域有着广阔的应用前景,光电导天线是产生宽频THz波的重要手段.分析了低温生长和高温退火对光电导天线材料载流子寿命和电阻率的影响.在生长温度为230℃和250℃,退火温度为475℃的低温生长砷化镓(LTG-GaAs)上制备了领结(BowTie)和偶极子(Dipole)两种电极结构的小孔径光电导天线.实验给出,在250℃生长的LTG-GaAs上制备的光电导天线产生的太赫兹波辐射强度和频谱宽度较好,谱宽达到了3.6 THz,BowTie天线的辐射强度优于Dipole天线.两种形状的光电导天线皆可在10 V的偏置电压下产生太赫兹波辐射.     

改进光电导太赫兹天线输出功率与方向的研究

光电导天线被广泛用于太赫兹波的产生,但是产生的太赫兹功率较低,方向性差。为了改进光电导太赫兹天线输出功率与方向,研究了不同基底厚度对光电导天线辐射功率的影响,其仿真结果表明:辐射功率受基底厚度影响明显,大部分辐射功率进入基底的一侧而不是自由空间,厚度薄的基底更能辐射出较大的功率。为了提高光电导天线的方向性,设计出一个加载在光电导天线输出端的锥形喇叭天线结构,仿真得到该喇叭天线的方向性为20.7dBi,这表明可以通过加载锥形喇叭,在期望的方向上获得较高的太赫兹波功率。     

光电导太赫兹天线及其研究进展

太赫兹科学技术是一门新学科,其关键技术是太赫兹波的产生。光电导天线是目前产生宽频带太赫兹波最重要的方法之一。光电导天线的几何结构对提高天线在太赫兹频段的效率、方向性、带宽等都非常重要。因此有必要对光电导太赫兹天线的各种几何结构和国内外研究进展做较为全面的评述,对其中存在的问题和解决思路进行分析,对未来研究发展的方向进行探讨。     

小孔径蝴蝶型光电导天线太赫兹辐射源的研究

研究了5种小孔径光电导天线的太赫兹发射特性,并且对它们所发射的太赫兹波进行了对比,为研制高效率的太赫兹波发射源提供了参考依据。利用太赫兹时域光谱技术测量了光电导天线发射的太赫兹（THz）脉冲,得到了时域发射光谱,并通过快速傅里叶变换得到相应的频域光谱。结果表明,太赫兹信号强度随偏置电压的增大而增强;随着泵浦激光功率的增大而增强并出现饱和现象。偏置电压与泵浦激光功率相同时,我们对比5种光电导天线产生的太赫兹信号,从中找到了一种发射效率较高的小孔径光电导天线,并且研究了电极形状、电极间距对光电导天线发射效率的     

Terahertz radiation of a butterfly-shaped photoconductive antenna (invited)

The terahertz(THz) far-field radiation properties of a butterfly-shaped photoconductive antenna (PCA) were experimentally studied using a home-built THz time-domain spectroscopy(THz-TDS) setup. To distinguish the contribution of in-gap photocurrent and antenna structure to far-field radiation, polarization-dependent THz field was measured and quantified as the illuminating laser beam moved along the bias field within the gap region of electrodes. The result suggests that, although the far-field THz radiation originates from the in-gap photocurrent, the antenna structure of butterfly-shaped PCA dominates the overall THz radiation. In addition, to explore the impact of photoconductive material, radiation properties of butterfly-shaped PCAs fabricated on both low-temperature-grown GaAs(LT-GaAs) and semi-insulating GaAs(Si-GaAs) were characterized and compared. Consistent with previous experiments, it is observed that while Si-GaAs-based PCA can emit higher THz field than LT-GaAs-based PCA at low laser power, it would saturate more severely as laser power increased and eventually be surpassed by LT-GaAs-based PCA. Beyond that, it is found the severe saturation effect of Si-GaAs was due to the longer carrier lifetime and higher carrier mobility, which was confirmed by the numerical simulation.     

Analysis and design of a photoconductive integrated photomixer/antenna for terahertz applications

A new terahertz (THz) photoconductive photomixer/antenna device is presented. A continuous-wave THz signal is generated in a dc-biased photoconductive film by employing optical heterodyne scheme, and at the same time, the size of the film on the grounded dielectric substrate is designed to have an efficient broadside radiation. Incorporating the photoconductive film as the photomixing media and the radiation element not only eliminates source to antenna coupling problem but also makes the proposed device attractive for THz array source configurations. Analytical expressions for the photocurrent and the radiation power are derived under high dc bias condition, and the effects of the device configuration on its performance is studied. Two possible photomixer configurations, namely longitudinal and transversal, are introduced and their photomixing efficiencies and radiation power are compared. The typical nW output power is achievable by mW laser pump power for frequencies up to 10 THz.     

改变天线泵浦光斑尺寸对太赫兹辐射影响的研究

从实验上研究了光电导天线电极之间激光光斑大小对太赫兹波产生的影响。另外,理论模拟了激光功率密度与太赫兹波辐射强度之间的关系,与实验结果非常吻合。泵浦光激光功率密度的增加能够显著的提高太赫兹波辐射强度,但是当超过一定值后会趋于饱和,此时若继续减小光斑尺寸,激光功率密度的增加不会使太赫兹产生继续增强。但是,太赫兹波却会随着光斑尺寸的减小而变弱。     

34μm孔径GaAs偶极子光电导天线辐射特性仿真研究

光电导天线的辐射特性仿真是设计和制造光电导天线的重要依据。将蒙特卡洛经验公式修正的GaAs迁移率随电场强度的变化关系引入德鲁德-洛伦兹模型,分析了光电导天线载流子输运情况。又将德鲁德-洛伦兹模型计算的光生载流子密度与时域有限差分方法相结合,得到34 m孔径GaAs偶极子光电导天线的时域、频域及三维辐射特性。辐射峰值为0.8 THz,频谱宽度大于1 THz,与太赫兹时域光谱系统实际测量得到的数据吻合。结果表明:仿真方法正确可行,能够为设计和制造高增益、宽频带的太赫兹光电导天线提供参考依据。     

Terahertz Radiation-Band Engineering Through Spatial Beam-Shaping

We propose a terahertz (THz)-band engineering technique based on generation of specifically designed time-domain radiation. The desired time-domain radiation is formed by combining the time-delayed outputs of an array of photoconductive antennas. We have employed this technique to generate tunable narrowband THz radiation. By using an array of identical photoconductive antennas with identical spacings, we have demonstrated a 12.5-mW output peak-power over 0.05-THz bandwidth and a tuning range of 0.65–1.4 THz. Considering the 2.5-THz bandwidth of each individual photoconductive antenna, 50 times higher power efficiency is achieved as a result of radiation-band shaping.      

基于FDTD的太赫兹光导天线三维辐射特性计算方法

光导天线是目前产生和探测太赫兹波最常用的辐射源之一,在太赫兹成像、光谱检测等诸多领域有着广泛的应用前景。本文介绍了基于时域有限差分(FDTD)的太赫兹光导天线三维辐射特性计算方法,在分析光导天线辐射原理的基础上阐述了半导体漂移电流、扩散电流对电场及磁场的作用,并根据实际运用情况对漂移方程、连续性方程进行简化,最终求得电流密度、电场、磁场的迭代方程,给出了光导天线辐射特性计算流程。     

Terahertz-radiation generation and detection in low-temperature-grown GaAs epitaxial films on GaAs (100) and (111)A substrates

The efficiency of the generation and detection of terahertz radiation in the range up to 3 THz by LT-GaAs films containing equidistant Si doping δ layers and grown by molecular beam epitaxy on GaAs (100) and (111)Ga substrates is studied by terahertz spectroscopy. Microstrip photoconductive antennas are fabricated on the film surface. Terahertz radiation is generated by exposure of the antenna gap to femtosecond optical laser pulses. It is shown that the intensity of terahertz radiation from the photoconductive antenna on LT-GaAs/GaAs (111)Ga is twice as large as the intensity of a similar antenna on LT-GaAs/GaAs(100) and the sensitivity of the antenna on LT-GaAs/GaAs (111)Ga as a terahertz-radiation detector exceeds that of the antenna on LT-GaAs/GaAs(100) by a factor of 1.4.