掺杂石墨烯系统电场调控的非线性太赫兹光学特性研究

石墨烯是单原子厚的二维狄拉克相对论费米子系统,其优秀的光电学性质得到了广泛的关注和研究. 本论文利用量子理论研究掺杂石墨烯系统外电场和光场共同作用下的非平衡载流子的非线性太赫兹光学性质. 研究发现,掺杂石墨烯带内光吸收表现出强的非线性太赫兹光学特性. 随着外加偏压电场的增大,石墨烯非线性光学响应增强;随着外界太赫兹光频率的减小,非线特性增强. 研究表明通过改变电场强度,可以有效调节石墨烯系统太赫兹非线性光学特性. 研究结果为探索和发展以石墨烯为基础的新型纳米太赫兹光电器件的研究和实际应用提供了理论依据. 关键词： 石墨烯 太赫兹 非线性 光电流     

碳纳米管薄膜周期结构的太赫兹表面等离子波特性研究

在获得太赫兹波段碳纳米管薄膜的介电特性基础上,利用数值THz时域光谱技术研究了碳纳米管薄膜栅周期结构的表面等离子激元的传播特性和局域化现象. 研究结果表明,在栅周期为168 μm时,频率在0.5-2.5 THz之间出现两个等离子模式的共振峰值,分别位于0.99 THz和1.95 THz,这与理论计算结果相符合. 数值计算的表面等离子激元传播距离与理论预测值相一致,达到了146 μm. 此外,分析了栅厚度与栅宽度变化对表面等离子波特性的影响. 关键词： 太赫兹 碳纳米管 表面等离子波     

一种基于不平坦基底结构的混合等离子体波导

提出了一种工作于太赫兹波段的、基于半圆柱形"不平坦"InSb基底结构的混合等离子体波导.分析了半导体材料InSb的相对介电常量随工作频率的变化特性,重点研究了该混合波导以及两种不同形变结构的有效模场面积、传播长度、品质因数、能量分布等随波导工作频率、波导尺寸的变化情况.结果表明,本文提出的"不平坦"基底结构波导可以达到与传统结构几乎相同的传播长度,有效模场面积较传统结构减小了一个数量级以上,具有非常强的模场约束性,适合于太赫兹频段高密度集成电路中的应用.     

亚波长分形结构太赫兹透射增强的机理研究

利用太赫兹时域光谱(terahertz time domain spectroscopy,简称THz-TDS),研究了亚波长金属分形结构在THz波段的透射增强特性.分别从实验和理论两个方面,研究了铜箔上各级分形结构THz透射增强现象的产生机理.结果表明,在低频区的透射增强主要是由低级分形线中电子运动的共振引起的,而高频区的透射增强则主要由高级分形线中电子运动的共振引起的.从而将这种透射增强效应归结为分形结构中电子的共振辐射,即分形结构的局域共振效应. 关键词： 分形 太赫兹 透射 共振峰     

基于人工复合材料的太赫兹波双波段滤波

本文分别从实验和理论模拟两方面详细的讨论了金属双环和反双环两种亚波长阵列结构在太赫兹波段的双频率响应特性. 这两种双环结构都可以实现双频率调制,金属双环结构表现为两个明显的吸收峰,反双环结构则表现出明显的透过峰. 这两种结构不仅在物理结构上互补,在太赫兹波段的响应也表现为吸收和透过互补的特性. 其中高频响应来源于内环电子振荡,低频影响则来源于外环电子振荡,并且这种结构在太赫兹波段的响应与周期无关. 关键词： 太赫兹 滤波器 亚波长阵列 FDTD     

基于四波混频效应的窄带太赫兹波源

采用频率差在太赫兹范围的双波长激光器进行泵浦,利用光纤的四波混频效应,得到结构紧凑、频率可调的窄带太赫兹波源。为减小光纤材料对太赫兹波的吸收,采用了表面发射机制。从耦合波理论出发,详细分析了保偏光纤中的四波混频过程,得到了太赫兹波输出功率的解析表达式,并讨论了实现相位匹配的条件。结果表明,太赫兹波功率与泵浦光功率和光纤长度成正比,与太赫兹波长的3次方成反比。当泵浦光峰值功率为1 kW,在6 THz处得到的太赫兹波峰值功率达350 mW,功率转换效率约为0.01%。通过合理设置泵浦波长,可以实现太赫兹辐射在3~8 THz范围内连续调谐。该方案提供了一种新型的高功率、紧凑型的窄带太赫兹辐射源。     

0.14THz高功率太赫兹脉冲的频率测量

提出了采用截止波导法与谐波混频法相结合的方式,进行0.14THz高功率短脉冲的频率测量.首先将两个截止频率分别为0.125和0.15THz的非标准矩形波导作为接收端,通过截止波导滤波法获得了太赫兹辐射源的频率范围.然后根据已知的频率范围,将本振频率选择为15—20GHz,则谐波混频的谐波次数确定为8.随后的Ka波段的脉冲测试和0.14THz连续波测试表明,该8次谐波混频器可用于0.14THz脉冲的混频测量.最后,0.14THz脉冲频率测量实验给出了太赫兹辐射源的准确频率为0.1465THz.该方法大大降低了对本振信号的频率要求,且结果准确可信,为长波段太赫兹脉冲的频率测量提供了一种新的思路.     

太赫兹技术及其在国防与安全领域的应用

<正>太赫兹波是指频率介于0.1~10THz之间的电磁波,其波长范围为0.03~3 mm。太赫兹波在电磁波谱中的位置位于微波和红外辐射之间(如图1所示)。由于太赫兹波直接以其频率范围命名,实际上在低频波段与微波重合,在高频波段与红外重合,与之相应,其研究手段也由电子学理论逐渐过渡为光子学理论。所以太赫兹波是宏观电子学与微观光子学的交叉融合区域。     

太赫兹表面极化激元

作为束缚于表面或界面的电磁波与极性元激发的耦合模量子,表面极化激元是克服衍射极限的核心物理.在紫外、可见以及近红外波段,表面等离子极化激元展现出了亚波长特性,具有高分辨成像等应用,并发展成为"表面等离子极化激元亚波长光学"学科;在中红外波段,表面声子极化激元发挥着同样的作用.太赫兹波段曾是人类认识的空白区域,近三十年来得以高速发展,其战略意义重大.具有克服衍射极限能力的太赫兹表面极化激元同样是小型化与集成化太赫兹器件,以及太赫兹超高分辨成像的重要物理基础.近几年来,对以石墨烯为代表的二维材料的研究突飞猛进,诞生了"石墨烯表面等离子极化激元亚波长光学"这门学科,并贡献于太赫兹领域.本文对可在太赫兹波段工作的人工超构材料、掺杂半导体、二维电子气、二维材料、拓扑绝缘体等结构材料的表面极化激元进行了较为全面的总结与介绍,为研制克服衍射极限的太赫兹集成光子学器件提供可资借鉴的物理基础.     

石墨烯超材料等离诱导透明特性和光电开关设计

基于单层图案化石墨烯超材料在太赫兹波段实现了等离子诱导透明效应，利用耦合模式理论（CMT）分析了等离子诱导透明产生的机理，得到的理论结果与时域有限差分方法计算的结果高度一致。通过调节石墨烯费米能级对等离子诱导透明特性进行了动态调控，并实现了多模同步异步开关的设计，在2.16、3.01、3.84 THz三个频率处的振幅调制度分别为95.77%、83.42%、95.58%，消光比最高可达13.73 dB。对慢光效应的研究结果表明群折射率可达180。本研究为设计光电子器件提供方案和指导。     

The performance of Schottky diodes as far-infrared modulators

We analyze the performance as a terahertz-frequency modulator of a small-area Schottky diode mounted in a corner-cube antenna. The analysis includes the effects of carrier inertia and dielectric relaxation as modeled by Champlin and Eisenstein (1978). It also includes the effect of the vanishing of the depletion region above the flat-band potential, as modeled by Crowe and Mattauch (1986). Our baseline calculation refers to a 1.4 m diameter diode (Univ. of Virginia batch no. 1E12) operated at a carrier frequency of 2.52 THz and a modulation frequency of 8 GHz, as was used in the experiments of Watson, Grossman, and Phillips (1988). The effects on reflectivity modulation, and therefore on sideband-generation efficiency, of varying the diode parameters are investigated. Our conclusions are: A) For all realistic diode parameters, the phase modulation completely dominates the amplitude modulation. B) Performance is degraded well below the plasma frequency in the undepleted epilayer due to the presence of a second resonance, caused by the interaction of the barrier capacitance and the effective inductance due to carrier inertia, C) The effects of varying the antenna impedance, temperature, diode substrate size, and Schottky barrier height over realistic ranges are small. D) An improvement in single-sideband conversion efficiency of approximately 20 db may be obtained by increasing the epilayer doping and simultaneously reducing the diode radius.     

The Mott diode as a heterodyne receiver element

This paper considers a novel doping profile for Schottky barrier mixer diodes called the Mott barrier. The structure consists of a metal-semiconductor junction in which the semiconductor's epitaxial layer is very lightly doped and thin enough so that it remains depleted even under substantial forward bias. It has been proposed that Mott barrier diodes will generate less noise and have lower series resistance-junction capacitance products than standard Schottky diodes, thus increasing the sensitivity and cut-off frequency of heterodyne receivers. In this paper, the band structure and electron transport properties of the Mott diode are evaluated. This analysis shows that the Mott diode actually will have a large series resistance-junction capacitance product and excessive hot electron noise, making it a poor candidate for high-frequency applications. Experimental results are presented which substantiate these conclusions.     

Design and fabrication of 0.5 micron GaAs Schottky barrier diodes for low-noise terahertz receiver applications

Recent technological advances have made possible the development of heterodyne receivers with high sensitivity and high spectral resolution for frequencies in the range 1,000–3,000 GHz (1–3 THz). These receivers rely on GaAs Schottky barrier mixer diodes to translate the high-frequency signal to a lower frequency where amplification and signal processing are possible. At these frequencies, the diode quality is a major limitation to the performance of the receiver. The design, fabrication and DC evaluation of a diode for this frequency range is presented. A figure-of-merit cut-off frequency of over 10 THz is achieved with a record low zero biased capacitance of 0.5 fF. Results from RF tests are also given.This work has been supported in part by the National Science Foundation under contract ECS-8720850 and the US Army.     

Silicon diodes in avalanche pulse-sharpening applications

Silicon diodes operated in an avalanche breakdown mode can he used to reduce, or sharpen, the rise times of driving pulses. Proper operation of a diode in this manner requires the application of a driving pulse with sufficient time rate of change of voltage dV/dt. The rapidly changing reverse bias produces an electron-hole plasma of sufficient density that the electric field strength in the n region of a p⁺-n-n⁺ structure is significantly reduced and the plasma is essentially trapped. In effect, the plasma generation causes the device to transition from a high-impedance state to a low-impedance state in a short period of time, and thus acts as a fast closing switch. This paper provides an overview of this mode of operation. A simplified theory of operation is presented. A comparison is made among the results of numerical modeling, the theory of operation of the silicon avalanche shaper (SAS) diode, and the theory of operation of the trapped-plasma avalanche-triggered transit (TRAPATT) mode of operation of a diode. Based on the results of numerical modeling, conclusions are drawn on what factors most greatly affect the performance of avalanche shaper diodes, and one optimized design is provided     

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.     

激光等离子体太赫兹辐射源的频率控制

实验研究了双色超快强激光场作用于氮气分子束所产生的宽带太赫兹(THz)辐射光谱随等离子体介质的密度和长度的依赖关系, 发现THz辐射的中心频率随等离子体密度提高和长度减小而增大(0.8-1.4 THz), 且谱宽也随之增加(0.78-1.53 THz). 分析和计算表明, 太赫兹光谱的变化由等离子体振荡频率和谱宽决定. 该发现为等离子体宽带太赫兹辐射源的光谱操控提供了新思路.     

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。     

Design consideration of monolithic millimeter-wave barrier varactor diode frequency multiplier arrays

A new device structure for highly efficient frequency tripling in the millimeter and submillimeter wavelength regions is presented. The Barrier-Intrinsic-N⁺ (BIN) diode structure [1,2] is modified for highly efficient millimeter- and submillimeter-wave frequency tripling device to be employed into the monolithic back-to-back diode frequency tripler array. The modified BIN diode structures have series resistances of a few ohms and cut-off frequencies in the terahertz range. The modified BIN diode structures have weaker C-V nonlinearities than the BIN diode structure does, however, the modified BIN structures have much higher intrinsic cut-off frequencies than does the BIN counterpart. The C-V nonlinearity, capacitance ratio, breakdown voltage, series resistance and cut-off frequency of this new device structure will be discussed in this paper. In addition, the calculated high-frequency performance of this device using a large-signal nonlinear circuit model will be presented in this paper. The operation and performance of the monolithic diode-grid frequency tripler arrays employing large numbers of the modified BIN diodes will also be discussed in this paper.     

Properties and bias dependence of metal-insulator-metal point-contact diodes as harmonic mixers in the mid-infrared

Tungsten whisker-nickel point-contact (W-Ni) diodes have been used to investigate fourth-, fifth-, sixth-, and seventh-order harmonic mixing processes in the frequency range of 30–120 THz. The beat notes obtained by these processes have been measured as a function of the applied bias voltages of the diode. The results were compared with the corresponding derivative evaluated by a polynomial fit of the dc current-voltage (I–V) characteristic of the diode. It was found that the beat signals obtained by the various processes can in most cases be described qualitatively by the dc I–V characteristic of the diode.