Excitation of surface plasma waves over metallic surfaces by lasersand electron beams

A laser incident on a metal film (deposited on a glass substrate) from the glass side at a specific angle of incidence, excites a surface plasma wave (SPW) at the metal-free space interface. The ratio of the SPW field to the laser field increases with the laser spot size b attaining a value much greater than one at b>exp(2wα/c) where a is the film thickness and ω is the laser frequency. The SPW (ω, k_z,) can also he excited by a relativistic electron beam, propagating parallel to the interface in the free space region, via Cerenkov interaction when beam energy ϵb=(|ϵ|-1)mc² where ϵ is the effective metal permittivity, and mc² is the electron rest mass energy. When the surface has a ripple of wave number k₀, the SPW (ω, k_zz) can be excited at lower beam energy via sideband coupling, ω=(k_zz +k₀)v_b where v_bzˆ is the beam velocity. In both cases, however, the positioning of the beam in the close proximity of the interface is required. The scheme is useful for the generation of wavelengths longer than 1 μm     

Surface plasma waves over bismuth-vacuum interface

A surface plasma wave (SPW) over bismuth-vacuum interface has a signature of mass anisotropy of free electrons. For SPW propagation along the trigonal axis there is no birefringence. The frequency cutoff of SPW lies in the far infrared region and can be accessed using free electron laser. The damping rate of waves at low temperatures is low. The surface plasma wave may be excited by an electron beam of current ∼100 mA propagating parallel to the interface in its close proximity.     

Structure of unsupported bismuth nanoparticles

We present new results of electron diffraction experiments on unsupported nanometer-sized bismuth clusters. The high intensity cluster beam, necessary for electron diffraction, is provided by an inert-gas aggregation source. The cluster beam contains particles with average cluster sizes between 4.5 and 10 nm. When using Helium as a carrier gas we are able to observe a transition from crystalline clusters to a new structure, which we identify with that of amorphous or liquid clusters. Received 28 November 2000     

Terahertz Quantum Cascade Laser at 3.39THz

We demonstrate the growth of terahertz quantum cascade laser （THz QCL） by gas source molecular beam epitaxy. X-ray diffraction and cross-sectional transmission electron microscopic measurements show the high crystalline quality of the THz QCL active region, From the cross-sectional transmission electron microscopy image, sharp interfaces are observed and the deduced cascade period thickness is consistent with the result of x-ray diffraction. The test device is lasing at 3.39THz and operating up to lOOK in pulsed mode. At IOK, the maximum output power is greater than 1 mW with a threshold current density of 738 A/cm^2.     

A Zone Plate as a Tunable Terahertz Filter

A broadband tunable terahertz filter based on a zone plate is demonstrated in our terahertz time domain spec- trometer. The central bandpass frequency covers the whole spectral range of the terahertz wave emitted from a ZnTe emitter, from 0.5 THz to 2.5 THz, and can be tuned continuously by simply moving the zone plate along the terahertz beam path. The peak transmission is about 40% and the bandwidth varies from 0.16 THz to 0.25 THz at different bandpass frequencies when the aperture size is kept constant.     

Tailored terahertz pulses from a laser-modulated electron beam

We present a new method to generate steady and tunable, coherent, broadband terahertz radiation from a relativistic electron beam modulated by a femtosecond laser. We have demonstrated this in the electron storage ring at the Advanced Light Source. Interaction of an electron beam with a femtosecond laser pulse copropagating through a wiggler modulates the electron energies within a short slice of the electron bunch with about the same duration of the laser pulse. The bunch develops a longitudinal density perturbation due to the dispersion of electron trajectories, and the resulting hole emits short pulses of temporally and spatially coherent terahertz pulses synchronized to the laser. We present measurements of the intensity and spectra of these pulses. This technique allows tremendous flexibility in shaping the terahertz pulse by appropriate modulation of the laser pulse.     

利用康普顿散射实现太赫兹皮秒脉冲的分析

 研究了利用微波与电子束团的康普顿散射实现太赫兹的方法、光子产额和辐射功率。推导出了单个电子产生的太赫兹的光子产额和辐射功率表达式,也推导出了电子束团产生的太赫兹的光子总产额和辐射总功率表达式。结果发现：利用微波与电子束团发生康普顿垂直散射,可以产生太赫兹皮秒脉冲;单个电子产生的太赫兹光子产额与微波功率、微波波长成正比,与微波束截面积成反比;单个电子产生的太赫兹辐射功率与微波功率、电子Lorentz因子的平方成正比,与微波束截面积成反比;电子束团产生的太赫兹光子总产额与微波功率的平方、微波波长的平方成正比,与微波束截面积的平方成反比;电子束团产生的太赫兹辐射总功率与微波功率的平方、微波波长以及电子Lorentz因子的平方成正比,与微波束截面积的平方成反比。     

0.14 THz双环超材料慢波结构表面波振荡器数值研究

本文研究了一种太赫兹波段双环超材料慢波结构, 并具有同轴引出结构的相对论过模表面波振荡器. 设计了超材料同轴过模慢波结构; 通过色散特性, 进行了模式选择和过模结构电子束电参数和几何参数的设计; 根据超材料同轴慢波结构的特点, 设计了具有同轴引出结构的末端同轴输出段. 粒子模拟结果表明, 在电子束电压为600 kV和电流为1.0 kA, 引导磁场为2.0 T 时, 同轴超材料慢波结构过模表面波振荡器输出稳定单频的0.141 THz电磁波, 峰值功率为316.8 MW.     

Features of the Development of Electron-Optical Systems for Pulsed Terahertz Traveling-Wave Tubes (Review)

We present an analysis of the current state and development of pulsed amplifying traveling-wave tubes for terahertz radiation operation at frequencies of at least 200 GHz, as well as the development prospects of the principles of creating electron-optical and magnetic systems. The possibility of using field emission cathodes based on carbon nanotubes for constructing an electron-optical system with the compression of a sheet beam is discussed. A numerical simulation of a field emission electron gun forming a sheet electron beam for traveling-wave tubes of the terahertz range is carried out.

     

基于相对论电子束的太赫兹源

太赫兹辐射在基础科学和产业应用中具有重要的应用前景,但传统的电子学和光学方法难以在1～10 THz产生相干的高功率、窄带且连续可调的太赫兹辐射。基于相对论性超短电子束和预调制电子束序列的加速器太赫兹源将能在上述范围内产生可调的高能谱强度窄带太赫兹辐射。综述了清华大学加速器实验室近年来在基于相对论电子束的加速器太赫兹源方面的理论和实验进展,以及与加速器太赫兹源一起发展起来的太赫兹辐射测量、束流诊断和先进加速技术。     

Directional terahertz radiation through groove-assisted hole array

The authors theoretically demonstrate the terahertz beam shaping with metallic subwavelength holes array surrounded by concentric periodic grooves. High transmission and directional radiation can be obtained simultaneously for the resonant excitation of the surface wave in the combined structure. Finite-difference time-domain simulation results shows that the transmission mainly depends on the lattice constant of hole array and the features of incident surface around it, while the far-field angle distribution is dominated by the details of the output surface. This compact beam shaping structure is hoped to serve as a basic device for future terahertz systems.     

Optically controlled terahertz beam steering and imaging

We propose a spatial modulator for terahertz waves based on light induced electron plasma in photo-active semiconductors. A two-dimensional array of computer controlled light is used to create free carries in bulk silicon, which results in a spatial modulation of the transmission at terahertz frequencies. This method not only exhibits a remarkable modulation depth over a broad frequency range but also allows for an optically controlled beam steering of terahertz waves by inducing virtual grating structures. In addition, we analyze the possibility of all-optically controlled terahertz imaging.     

多晶碲化锌薄膜载能子超快动力学实验研究

利用双波长飞秒激光抽运-探测实验方法测量了掺氮多晶ZnTe薄膜在飞秒激光加热情况下载能子超快动力学过程. 采用包含电子弛豫过程和晶格加热过程的理论模型拟合实验数据, 二者符合得很好. 拟合得到10 ps以内影响掺氮多晶ZnTe薄膜表面超快反射率变化的三个弛豫过程的时间常数均为亚皮秒量级. 其中, 正振幅电子弛豫过程是由电子-光子相互作用引起的载流子扩散和带间载流子冷却过程, 负振幅电子弛豫过程是由缺陷造成的光激载能子的俘获效应引起的, 晶格加热过程主要通过电子-声子耦合过程进行的.     

Double-slit diffraction of terahertz wave generated by tilted-pulse-front pumping

We propose a spatial diffraction diagnostic method via inserting a millimeter-gap double slit into the collimated terahertz beam to monitor the minute variation of the terahertz beam in strong-field terahertz sources, which is difficult to be resolved in conventional terahertz imaging systems. To verify the method, we intentionally fabricate tiny variations of the terahertz beam through tuning the iris for the infrared pumping beam before the tilted-pulse-front pumping setups.The phenomena can be well explained by the theory based on the tilted-pulse-front technique and terahertz diffraction.     

束流发射度对太赫兹微电真空折叠波导行波管性能的影响

采用理论分析与数值模拟相结合的方法,分别对140,220和345 GHz折叠波导行波管中的束流发射度的影响因素及其对直流导通率的影响进行了分析,总结了发射度随频率、结构参数和电子束参数的变化规律。研究发现,在太赫兹频段束流发射度直接决定着聚焦磁场的选取设计, 是表征太赫兹频段束流品质的一个重要参量。     

Tunable few-cycle and multicycle coherent terahertz radiation from relativistic electrons

We report the generation of tunable, narrow-band, few-cycle and multicycle coherent terahertz (THz) pulses from a temporally modulated relativistic electron beam. We demonstrate that the frequency of the THz radiation and the number of the oscillation cycles of the THz electric field can be tuned by changing the modulation period of the electron beam through a temporally shaped photocathode drive laser. The central frequency of the THz spectrum is tunable from ～0.26 to 2.6 THz with a bandwidth of ～0.16 THz.     

9MeV,小于0.1mm微焦点X射线源验证实验

密度高、成像分辨率高、成像速度快的X射线数字成像检测需要高能微焦点大剂量X射线源,高品质电子源是实现这一X射线源的关键手段。基于中国工程物理研究院太赫兹自由电子激光的主加速器,验证了低发射度、低能散度的高亮度电子束实现高能微焦点的可行性,得到电子束半高全宽尺寸小于70μm的9 MeV微焦点,并初步开展成像实验,双丝像质计焦斑清晰分辨9D号丝,丝直径0.13 mm。     

光纤表面等离子体波传感器的温度特性的研究

本文从粒子振动的角度讨论了光纤表面等离子体波传感器对于温度的敏感特性。对于金属 /介质的表面等离子体波传感器 ,在界面处产生的等离子体振动实质是大量电子的集体振荡。在某一特定的波长的P光激励下 ,电子吸收了光子的能量而改变了原来的运动方式 ,当激励的光信号与电子群的固有振荡频率一致时 ,大量的电子振动变为一种谐振。由于界面处的电子密度与温度是密切相关的 ,不同的温度时密度不同———温度越高 ,电子的密度越大 ,而电子群的集体振荡又与电子的密度密切相关。所以温度的变化将对等离子体的共振频率产生非常重要的影响。利用了这一效应来补偿环境介质的温度变化 ,可以克服环境温度对测试所带来的影响。此外 ,本文也讨论了采用光纤SPW传感器可以进行多参数的测量。     

双共焦波导结构二次谐波太赫兹回旋管谐振腔设计

准光共焦波导具有功率容量大、模式密度低的特点,能够有效地减少模式竞争对回旋管互作用的影响,有利于高次谐波太赫兹回旋管的设计.为提高太赫兹准光回旋管的互作用效率,在共焦柱面波导的基础上,研究了一种新型高频互作用结构——双共焦波导结构,设计了一种330 GHz二次谐波双共焦结构回旋管谐振腔并对其进行了理论分析和粒子模拟.研究结果表明,双共焦谐振腔中的高阶模式能够与高次电子回旋谐波发生稳定的相互作用,并且没有模式竞争现象,具备工作在太赫兹波段的潜力.相比普通共焦波导谐振腔,双共焦谐振腔能够增强准光回旋管的注波互作用强度,提高回旋管的输出功率和工作效率.此外,结果还表明双共焦波导中的电磁波模式是一种由两个独立的共焦波导模式叠加而成的混合模式.利用这种混合模式有望实现太赫兹回旋管的单注双频工作,为新型太赫兹辐射源的研究提供了新的途径.     

Coherent diffraction radiation as a source of radiation in far-infrared and terahertz range

The results of measurements of the coherent diffraction radiation yield generated by femtosecond electron beam are presented. It is shown that the simulated and experimental data agreed quite well. The coherent diffraction radiation can be considered as a real candidate to develop a source of radiation in far-infrared and terahertz range.