基于IrMn/Fe/Pt交换偏置结构的无场自旋太赫兹源

与目前商用的太赫兹源相比,自旋太赫兹源具有超宽频谱、固态稳定以及成本低廉等优点,这使其成为下一代太赫兹源的主要研究焦点.但使用自旋太赫兹源时,通常需要外加磁场使铁磁层的磁化强度饱和,才能产生太赫兹波,这制约了其应用前景.基于此,本文制备了一种基于Ir Mn/Fe/Pt交换偏置结构的自旋太赫兹波发生器,通过Ir Mn/Fe中的交换偏置场和Fe/Pt中的超快自旋流注入与逆自旋霍尔效应相结合,在无外加磁场下产生了强度可观的太赫兹波.在Ir Mn和Fe的界面中插入超薄的Cu,可以使Fe在厚度很薄时零场下实现饱和磁化,并且其正向饱和场最高可达–10 m T,从而进一步提升无场下的太赫兹发射效率.零场下出射的太赫兹波的动态范围超过60 d B,达到可实用化的水平.通过旋转样品,发现产生的太赫兹波的偏振方向也会随之旋转,并且始终沿着面内垂直于交换偏置场的方向.此外,在此交换偏置结构的基础上,引入了一层自由的铁磁金属层Fe,设计了一种以Ir Mn/Fe/Pt/Fe为核心结构的自旋阀太赫兹源,发现产生的太赫兹强度在两层铁磁层反平行排列时比平行排列以及不引入自由铁磁金属层时均大约提升了40%.结果表明,基...     

Excitation-wavelength-dependent THz wave modulation via external bias electric field

A theoretical model was proposed to describe the effects of external bias electric field on terahertz(THz) generated in air plasma. The model predicted that for a plasma in a bias electric field, the amplification effect of the THz wave intensity increases with the increase of the excitation laser wavelength. We experimentally observed the relationship between the THz enhancement effect and the electric field strength at different wavelengths. Experimental results showed a good agreement with the model predictions. These results enhance our understanding of the physical mechanism by which femtosecond lasers excite air to generate THz and extend the practical applications of THz generation and modulation.     

Studies on the mechanisms of powerful terahertz radiations from laser plasmas

A survey on the mechanisms of powerful terahertz (THz) radiation from laser plasmas is presented.Firstly,an analytical model is described,showing that a transverse net current formed in a plasma can be converted into THz radiations at the plasma oscillation frequency.This theory is applied to explain THz generation in a gas driven by two-color laser pulses.It is also applied to THz generation in a tenuous plasma driven by a chirped laser pulse,a few-cycle laser pulse,a DC/AC bias electric field.These are well verified by particle-in-cell simulations,demonstrating that THz radiations produced in these approaches are nearly single-cycles and linear polarized.In the chirped laser scheme and the few-cycle laser scheme,THz radiations with the peak field strength of tens of MV/cm and the peak power of gigawatt can be achieved with the incident laser intensity less than 10 17 W/cm 2.     

超薄金属膜在太赫兹波段的光学特性研究

超薄金属膜在太赫兹波段的探测器、反射镜、波导器件以及太赫兹量子级联激光器中得到了广泛应用。超薄金属膜的光学常数不仅是这些器件设计中不可缺少的参数,而且是开发新型光电材料的一个重要依据。文章运用太赫兹差分时域光谱技术对超薄金属铬、镍和钛膜的光学特性研究,获得其在太赫兹波段的折射率和消光系数,并根据菲涅尔公式计算入射介质为高阻GaAs时,GaAs/Metal界面的反射谱,三种金属在0.3～1.5THz的波段范围内的平均反射率均超过80%。研究超薄金属膜在太赫兹波段的反射特性,为设计性能优良的太赫兹辐射源、探测器及太赫兹光学元件奠定基础。     

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.     

Noise temperature distribution of superconducting hot electron bolometer mixers

We report on the investigation of optimal bias region of a wide-band superconducting hot electron bolometer(HEB)mixer in terms of noise temperature performance for multi-pixel heterodyne receiver application in the 5-meter Dome A Terahertz Explorer(DATE5) telescope. By evaluating the double sideband(DSB) receiver noise temperature(Trec) across a wide frequency range from 0.2 THz to 1.34 THz and with a large number of bias points, a broad optimal bias region has been observed, illustrating a good bias applicability for multipixel application since the performance of the HEB mixer is uniquely determined by each bias point. The noise temperature of the HEB mixer has been analyzed by calibrating the noise contribution of all RF components, whose transmissions have been measured by a time-domain spectroscopy. The corrected noise temperature distribution shows a frequency independence relation. The dependence of the optimal bias region on the bath temperature of the HEB mixer has also been investigated, the bath temperature has limited effect on the lowest receiver noise temperature until 7 K, however the optimal bias region deteriorates obviously with increasing bath temperature.     

0.34 THz无线通信收发前端

描述了一种基于肖特基二极管技术的0.34 THz无线通信收发前端。该前端采用超外差结构,由0.34 THz谐波混频器、0.17 THz本振8倍频链和偏置电路组成。0.34 THz谐波混频器基于反向并联肖特基二极管,可以实现信号的上变频发射和下变频低噪声检测。0.17 THz本振8倍频链由三级二倍频及驱动放大链路组成,可将20~22.5 GHz信号倍频至0.16~0.18 THz,为混频器提供5~10 dBm左右的本振信号。实验测试结果表明:该前端用于信号发射时,在0.34 THz频点的饱和输出功率为-14.58 dBm;用于信号检测时,最低单边带(SSB)变频损耗为10.0 dB,3 dB中频带宽约30 GHz。限于测试条件,未能测试前端接收噪声温度,仿真得到的双边带噪声温度数值低于1000 K。在该前端基础上,完成了首次基于16QAM 数字调制体制的0.34 THz无线通信实验,传输速率达3 Gb/s。     

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     

Terahertz conductivity and possible BLOCH gain in semiconductor superlattices

We have investigated terahertz emission due to dynamical electron transport in wide-miniband GaAs/Al(0.3)Ga0.7As superlattices. By noting that the time-domain THz emission spectroscopy inherently measures the step-response of the electron system to the bias electric field, the obtained THz spectra were compared with the high-frequency conductivities predicted for miniband transport. Excellent agreement between theory and experiment strongly supports that the THz gain due to Bloch oscillating electrons persists at least up to 1.7 THz. It was also found that Zener tunneling into the second miniband sets the high-frequency limit to the THz gain for the samples studied here.     

光电导开关产生太赫兹电磁波双极特性分析

利用光电导体产生太赫兹电磁波(THz波)，THz远场辐射波形与光电导体材料的载流子寿命、偏置电场以及触发光有直接关系.用不同方法对低温GaAs(LT-GaAs)和半绝缘GaAs(SI-GaAs)光电导开关辐射的THz电磁波所呈现的双极特性进行了模拟计算.结果表明，LT-GaAs光电导开关辐射THz波呈现双极性的主要原因是光生载流子寿命小于一个THz波产生时间；而光生载流子寿命大于100ps的SI-GaAs光电导开关，在不同的实验条件（不同偏置电场、不同光脉冲能量）下，产生的THz波呈现双极特性的主要原因分别是载流子发生了谷间散射和空间电荷电场屏蔽. 关键词： 光电导开关 THz电磁波 载流子寿命 空间电荷屏蔽     

n-Type GaAs/AlGaAs heterostructure detector with a 3.2 THz threshold frequency

Terahertz detection using the free-carrier absorption requires a small internal work function of the order of a few millielectron volts. A threshold frequency of 3.2 THz (93 microm or approximately 13 meV work function) is demonstrated by using a 1 x 10(18) cm(-3) Si-doped GaAs emitter and an undoped Al(0.04)Ga(0.96)As barrier structure. The peak responsivity of 6.5 A/W, detectivity of 5.5 x 10(8) Jones, and quantum efficiency of 19% were obtained at 7.1 THz under a bias field of 0.7 kV/cm at 6 K, while the detector spectral response range spans from 3.2 to 30 THz.     

超低温高电场下GaAs的电子太赫兹功耗谱的研究

利用自由空间太赫兹电光取样方法,测量了在高电场下,GaAs中受飞秒激光脉冲激发的电子所辐射出的太赫兹电磁波,发现从样品中辐射出的和电子加速度成正比的太赫兹电磁波电场强度E_THz（t）,表现出双极特性.通过分析GaAs中辐射出的太赫兹电磁波的傅里叶变换谱,首次实验上得到在阶跃电场下的GaAs的电子太赫兹功耗谱.研究发现,当电场小于50 kV/cm时,由电子谷间散射引起的负功耗（即增益）的截止频率ν_c,随着电场的增大而增大;当电场大于50 kV/cm时,负功耗的截止频率ν_c开始在750 GHz（10 K）附近饱和. 关键词： 太赫兹 非平衡载流子 功耗谱 谷间散射     

All-semiconductor room-temperature terahertz time domain spectrometer

We report what we believe to be the first demonstration of an all-semiconductor room-temperature terahertz time domain spectrometer. An optical Stark mode-locked vertical-external-cavity surface-emitting laser with 480 fs pulses at 1044 nm was used to illuminate low-temperature-grown photoconductive antennae with 5 mum-gap bow-tie-shaped electrodes. The coherently detected spectrum has a bandwidth close to 1 THz, in which water absorption lines at 0.555 and 0.751 THz can be resolved.     

Broadband THz emission from gas plasmas induced by femtosecond optical pulses: From fundamentals to applications

We review the generation of broadband THz radiation from femtosecond photo‐induced gas plasmas, with an emphasis on the highly efficient “AC‐bias” case where the plasma is generated and driven by a superposition of fundamental and second‐harmonic optical fields. The dependence on experimental parameters such as pulse energy, air pressure, polarization and focusing are presented, and compared to the predictions from semi‐quantitative models for the THz generation process, namely (i) a microscopic photocurrent model and (ii) a four‐wave mixing model. We also employ these models to the case of few‐cycle pulses, where the observed THz emission is related directly to the carrier‐envelope phase of the pulses, and hence provides a mechanism with which to measure this phase.}     

Circumventing the Manley-Rowe quantum efficiency limit in an optically pumped terahertz quantum-cascade amplifier

Using a microscopic theory based on the Maxwell-semiconductor Bloch equations, we investigate the feasibility of an optically pumped electrically driven terahertz (THz) quantum-cascade laser as a pathway to room-temperature THz generation. In optical conversion schemes the power conversion efficiency is limited by the Manley-Rowe relation. We circumvent this constraint by incorporating an electrical bias in a four level intersubband scheme, thereby allowing coherent recovery of the optical pump energy. The observed THz radiation is generated through both stimulated emission and automatically phase-matched quantum coherence contributions--making the proposed approach both a promising source for THz radiation and a model system for quantum coherence effects such as lasing without inversion and electromagnetically induced transparency.     

Si doped GaAs/AlGaAs terahertz detector and phonon effect on the responsivity

Terahertz detection capability of an n-type heterojunction interfacial work function internal photoemission (HEIWIP) detector is demonstrated. Threshold frequency, f₀, of 3.2 THz (93 μm) was obtained by using n-type GaAs emitter doped to 1 × 10¹⁸ cm⁻³ and Al_0.04Ga_0.96As single barrier structure. The detector shows a broad spectral response from 30 to 3.2 THz (10–93 μm) with peak responsivity of 6.5 A/W at 7.1 THz under a forward bias field of 0.7 kV/cm at 6 K. The peak quantum efficiency and peak detectivity are 19% and 5.5 × 10⁸ Jones, respectively under a bias field of 0.7 kV/cm at 6 K. In addition, the detector can be operated up to 25 K.     

Dispersive terahertz gain of a nonclassical oscillator: BLOCH oscillation in semiconductor superlattices

We have directly determined the spectral shape of the complex conductivities of Bloch oscillating electrons by using the time-domain terahertz (THz) electro-optic sampling technique, and presented experimental evidence for a dispersive Bloch gain in superlattices. This unique dispersive gain without population inversion arises from a nonclassical nature of Bloch oscillations; that is, the phase of the Bloch oscillation is shifted by pi/2 from that of the semiclassical charged harmonic oscillation when driven by the same ac field. By increasing the bias electric field, the gain bandwidth reached approximately 3 THz in our particular sample.     

基于钛酸锶钡介电非线性效应的可调谐太赫兹人工电磁媒质

数值仿真研究了内嵌钛酸锶钡薄膜夹层的金属网格谐振单元人工电磁媒质的谐振行为. 垂直电磁波激励下, 电磁响应频率随着内嵌夹层钛酸锶钡薄膜介电常数的变大呈现红移, 调谐率为22.6%. 本文提出的人工电磁媒质调谐方法只需要以结构单元的上下两层金属图形本身作为电极对内嵌夹层钛酸锶钡薄膜施加电压, 极大地简化了可调谐人工电磁媒质的制备及应用, 在太赫兹人工电磁媒质调制器等方面具有潜在的应用. 关键词： 人工电磁媒质 钛酸锶钡薄膜 介电非线性 调谐     

Terahertz electrically controlled nematic liquid crystal lens

We report some preliminary results on the realization of terahertz (THz) nematic liquid crystal (LC) lens with electric tunable focal length. A LC cell with a single circular-hole electrode is driven by an AC voltage to yield the desired spatial distribution of the refractive index. The two-dimensional function of the phase delay introduced into THz wave depends on the AC voltage and the electrodes contacting the LC layer. This lens is operated at room temperature and its convergent optical characteristic is demonstrated at 118 μm. We realize the lens with a clear aperture of 1 mm. The lens matched with a bolometer array is capable of obtaining real-time THz images of a sample without any mechanical scanning.     

Slow light performance enhancement of graphene-based photonic crystal waveguide

We propose a graphene-based photonic crystal (PC) slow light waveguide, which is realized by creating periodical air holes in a silicon layer to achieve spatially varying chemical potentials of graphene. The structure is optimized around 30 THz, and a large group index of 166.6 is obtained, with a very low propagation loss of ?2.1 dB/um. The corresponding normalized delay-bandwidth product reaches as high as 4.00. Furthermore, the slow light performance can be dynamically tuned by changing a bias voltage. The center frequency of the slow light waveguide can be tuned between 19.1 THz and 27.4 THz. Our results suggest that graphene-based PC structures are very promising for slow light devices.