Balance-equation approach to terahertz-field-driven magnetotransport in semiconductors

We investigate the magnetotransport in semiconductors under the influence of a dc or slowly-varying electric field, an intense polarized radiation field of terahertz frequency, and a uniform magnetic field, being in arbitrary directions and having arbitrary strengths. Effective force- and energy-balance equations are derived by using a gauge that the magnetic field and the high-frequency radiation field are described by a vector potential and the dc or slowly-varying field by a scalar potential, and by distinguishing the slowly-varying velocity from the rapidly-oscillating velocity related to the high-frequency field. These equations, which include the elastic photon process and all orders of multiphoton absorption and emission processes, are applied to the examination of the effect of a terahertz radiation on the magnetophonon resonance of the longitudinal resistivity in the transverse configuration in nonpolar and polar semiconductors. We find that the previous zero-photon resonance peaks are suppressed by the irradiation of the terahertz field, while many new peaks, which may be related to multiphoton absorption and emission processes, emerge and can become quite distinct, at moderately strong radiation field. Received 17 May 1999     

Superlattice conductivity sign change induced by intense electromagnetic radiation

The current density in a superlattice exposed to a quantizing electric field and the terahertz field has been calculated. The calculations have been carried out taking into account inelastic scattering of charge carriers by phonons. The possibility of an absolute negative conductivity, i.e., the emergence of electric current opposing the direction of the quantizing electric field, has been demonstrated.     

Effect of the charge dragging in a graphene-based superlattice under a constant electric field

The effect of a constant electric field on the charge dragging by an electromagnetic wave is studied for graphene-based superlattice. An expression is derived for the electric current density in a graphene-based superlattice under a constant electric field in an approximation of a constant time of relaxation. It is demonstrated that the current as a function of wave intensity is of a nonmonotonous character.     

Balance equations for electron transport in an arbitrary energy band driven by an intense terahertz field. Application to superlattice miniband transport

We suggest a balance-equation approach to hot-electron transport in a single arbitrary energy band subject to an intense radiation field of terahertz (THz) frequency, including all the multiphoton emission and absorption processes and taking account of realistic scatterings due to impurities and phonons. This approach, which allows one to calculate THz-driving, time-averaging transport based on a set of time-independent equations, provides a convenient method to study the effect of an intense THz electric field on carrier transport in a nonparabolic energy band. As an example, these fully three-dimensional, acceleration- and energy-balance equations are applied to the discussion of superlattice miniband transport at lattice temperature T=77 and 300 K driven by the THz radiation field of varying strengths. It is shown that the current through a dc biased miniband superlattice is greatly reduced by the irradiation of an intense THz electric field. Received: 23 January 1998 / Revised: 31 March 1998 / Accepted: 20 April 1998     

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.

     

Terahertz radiation of Bloch oscillators excited by an electromagnetic field in lateral semiconductor superlattices

The action of a strong high-frequency electromagnetic field on a lateral semiconductor superlattice is considered based on the quasi-classical electron transport theory in the self-consistent wave formulation. The theory predicts that a lateral superlattice can emit terahertz radiation wave trains, which are associated with periodic excitation of Bloch oscillations in the superlattice arising because of the development of transient processes in it in a variable self-consistent electric field. The conditions necessary for observing Bloch oscillator radiation were found. The spectral composition of radiation transmitted through the superlattice and the energy efficiency of frequency multiplication related to Bloch oscillator excitation were calculated.     

太赫兹场和倾斜磁场对超晶格电子动力学特性调控规律研究

微带超晶格在磁场和太赫兹场调控下表现出丰富而复杂的动力学行为, 研究微带电子在外场作用下的输运性质对于太赫兹器件设计与研制具有重要意义. 本文采用准经典的运动方程描述了超晶格微带电子在沿超晶格生长方向(z方向)的THz场和相对于z轴倾斜的磁场共同作用下的非线性动力学特性. 研究表明, 在太赫兹场和倾斜磁场共同作用下, 超晶格微带电子随时间的演化表现出周期和混沌等新奇的运动状态. 采用庞加莱分支图详细研究了微带电子在磁场和太赫兹场调控下的运动规律, 给出了电子运行于周期和混沌运动状态的参数区间. 在电场和磁场作用下, 微带电子将产生布洛赫振荡和回旋振荡, 形成复杂的协同耦合振荡. 太赫兹场与这些协同振荡模式之间的相互作用是导致电子表现出周期态、混沌态以及倍周期分叉等现象的主要原因.     

Rectification of terahertz radiation in semiconductor superlattices in the absence of domains

We study theoretically the dynamical rectification of a terahertz AC?electric field, i.e.?the DC?current and voltage response to the incident radiation, in strongly coupled semiconductor superlattices. We address the problem of stability against electric field domains: a spontaneous DC?voltage is known to appear exactly for parameters for which a spatially homogeneous electron distribution is unstable. We show that by applying a weak direct current bias the rectifier can be switched from a state with zero DC?voltage to one with a finite voltage in full absence of domains. The switching occurs near the conditions of dynamical symmetry breaking of an unbiased semiconductor superlattice. Therefore our scheme allows for the generation of DC?voltages that would otherwise be unreachable due to domain instabilities. Furthermore, for realistic, highly doped wide miniband superlattices at room temperature, the generated DC?field can be nearly quantized, that is, be approximately proportional to an integer multiple of ?ω/ea where a is the superlattice period and ω is the AC?field frequency.     

半导体超晶格声子激光器的研究进展

太赫兹频率的相干声子在纳米尺度器件的探测和操控领域具有重要的应用价值。半导体超晶格声子激光器是实现太赫兹频率相干声子源稳定输出的重要途径。本文首先回顾了GHz到THz频率范围声学放大的多种方法,然后详细阐述了超晶格声子放大、超晶格声学布拉格镜的工作原理与设计方法以及声子激光器的阈值条件,同时总结了电抽运和光抽运结构器件的研究现状,最后简要讨论了亚太赫兹声子激光器在声-电子领域的应用。分析表明,这种能够产生强相干太赫兹声子的半导体超晶格声子激光器在纳米尺度器件的探测与成像等方面具有广阔的发展前景。     

Numerical study on the field emission properties of aligned carbon nanotubes using the hybrid field enhancement scheme

The magnitude of the local electric field and the electron emission current density for an array of aligned carbon nanotubes is estimated. For describing in detail the properties of the local electric field in the vicinity of the nanotube tips, a hybrid method allowing for the local determination of the field enhancement factor is introduced. The field factor consists of two parts: an internal factor which describes the structure of the carbon nanotubes and an external factor which represents the field screening effect due to neighboring nanotubes. The current density is obtained using the Fowler–Nordheim equation with the hybrid field enhancement scheme. As a result, the emission properties for an array of nanotubes with a given length are described satisfactorily, and an optimum value for the nanotube spacing is determined. PACS 85.45.Fd; 85.45.Db     

碳纳米管辐射太赫兹波的理论分析与数值验证

基于简单紧束缚方法理论,利用一维碳纳米管的量子化周期边界条件得到了碳纳米管的电子能级结构关系.并结合已报道的碳纳米管中电子与声子相互作用的实验结果,得到了碳纳米管能够辐射太赫兹波的结论.通过数值结果验证了碳纳米管在外场作用下能够产生太赫兹波,并对数值计算结果中太赫兹波的振荡现象给予合理的解释.其结果为进行碳纳米管产生太赫兹辐射的实验研究提供深入的理论基础. 关键词： 太赫兹波 碳纳米管     

Intraband Dynamics and Terahertz Emission in Biased GaAs/In0.53Ga0.47As Semiconductor Superlattices

Using an excitonic basis, we investigate the intraband polarization, opticalabsorption spectra, and terahertz emission of semiconductor superlattice withthe density matrix theory. The excitonic Bloch oscillation is driven by the dcand ac electric fields. The slow variation in the intraband polarizationdepends on the ac electric field frequency. The intraband polarizationincreases when the ac electric field frequency is below the Bloch frequency.When the ac electric field frequency is above the Bloch frequency, theintraband polarization downwards and its intensity decreases. The satellitestructures in the optical absorption spectra are presented. Due to excitonicdynamic localization, the emission lines of terahertz shift in different acelectric field and dc electric field.     

Forced vibration of a carbon nanotube with emission currents in an electromagnetic field

The occurrence of vibrations in a single carbon nanotubes placed in an electromagnetic field through which constant field-emission current passes has been analyzed. It has been shown experimentally that the emission current, along with the constant component, has a variable one that resonates at a certain frequency. Calculations show a relationship between the resonance frequency and the parameters of the whole system and nanotube itself. The conditions under which resonance may occur in the terahertz range of vibration frequencies have been analyzed.     

不同电场下碳纳米管场致发射电流密度研究

本文运用密度泛函理论和金属电子论, 深入研究了碳纳米管场致发射电流的变化规律. 结果显示其发射电流密度取决于体系的态密度、赝能隙、管长和局域电场, 在不同范围电场下的变化规律不同. 在较低电场下, 发射电流密度随电场增强而近似线性增大(对应的宏观电场须小于18 V· μm^-1); 但在较高电场下, 发射电流密度随外电场增加呈现非周期性振荡增长趋势, 碳纳米管表现为电离发射. 本文进一步研究了金属性碳纳米管电导率在不同电场下的变化规律.     

Field electron emission from carbon nanotubes in the presence of a weak high-frequency electric field

Series of narrow peaks in the frequency range of f ≈ 50–1200 MHz have been revealed in the frequency responses of the emission current from carbon nanotubes in the presence of a weak high-frequency electric field. The analysis makes it possible to attribute these peaks to resonance of the first and second harmonics of forced mechanical vibrations of carbon nanotubes in a high-frequency electric field. The determined Q factor of nanotubes is in the range of 100–300.     

Effect of a constant electric field on mutual rectification in a graphene superlattice

The effect of a transverse dc electric field on two-wave mutual rectification in a graphene superlattice (GSL) is investigated. Two field orientations are considered: (i) the polarization plane is parallel to the GSL axis and (ii) the polarization plane is perpendicular to the GSL axis. In both cases, the constant field is perpendicular to the polarization plane. The current density is calculated within a one-miniband model using the Boltzmann equation in the approximation of constant relaxation time.     

Steady-state phonon-free current in superlattices in a strong electric field induced by terahertz radiation

The effect of strong terahertz radiation on the current-voltage characteristics in one-dimensional superlattices is studied using the method of two-time nonequilibrium Green’s functions. It is shown that the broadening of discrete spectrum lines due to elastic scattering plays a fundamental role in this problem. A new phonon-free type of photon-induced charge transfer in the hopping mode is predicted. Under certain conditions, the constant components of current and electric field may have opposite directions in this regime. The phonon-free current cannot be described in principle in the formalism of the one-time density matrix and reflects the two-time nature of correlation functions.     

Loss and gain in Bloch oscillating super-superlattices: THz Stark ladder spectroscopy

Bloch oscillation in electrically biased semiconductor superlattices offer broadband terahertz gain from DC up to the Bloch frequency or Stark splitting. Useful gain up to 2–3 THz can provide a basis for solid-state electronic oscillators operating at 10 times the frequency of existing devices.A major stumbling block is the inherent instability of the electrically biased doped superlattices to the formation of static or dynamic electric field domains. To circumvent this, we have fabricated super-superlattices in which a large superlattice is punctuated with heavily doped regions. The short superlattice sections have subcritical “nL” products.Room temperature, terahertz photon-assisted transport in short InGaAs/InAlAs superlattice cells allows us to determine the Stark ladder splitting as the superlattice is electrically biased and confirms the absence of electric field domains in short structures.Absorption of radiation from 1.5 to 2.5 THz by electrically biased InAs/AlSb super-superlattices exhibit a crossover from loss to gain as the Stark ladder is opened. Measurements are carried out at room temperature in a novel planar terahertz waveguide defined by photonic band gap sidewalls and loaded with an array of electrically biased super-superlattices. The frequency-dependent crossover voltage indicates 80% participation of the super-superlattice.     

AN EFFECTIVE TWO-LEVEL CHANNEL FOR AN IMPURITY IN A SUPERLATTICE

The eigenstates and eigenspectrum of a charged particle in a one-dimensional semiconductor superlattice with an impurity under the action of a dc electric field are investigated employing the single-band tight-binding model. We find that the system undergoes a series of avoided crossings, at which resonant oscillations between the impurity and its nearest neighbour occur if appropriate conditions are met, suggesting an effective two-level approximation. This phenomenon shows that introducing an impurity in a perfect lattice provides a promising structure for the observation of terahertz radiation.     

一种平行栅碳纳米管阵列阴极的场发射特性研究

建立一种平行栅碳纳米管阵列阴极,利用悬浮球模型和镜像电荷法进行计算,给出碳纳米管顶端表面电场与电场增强因子的解析式. 在此基础上,进一步分析器件各类参数以及接触电阻对阴极电子发射性能的影响. 分析表明,碳纳米管间距大约为2倍碳纳米管高度时阵列阴极的分布密度最佳,靠边缘部位的碳纳米管发射电子能力比其中心部位的大;除碳纳米管的长径比之外,栅极宽度和栅极间距也对电场增强因子有一定作用;接触电阻的存在大幅度降低碳纳米管顶端表面电场与发射电流,而接触电阻高于800 kΩ时,器件对阳极驱动电压的要求更高. 关键词： 平行栅碳纳米管阵列 悬浮球 场增强因子 接触电阻