Generation of direct current in a semiconductor superlattice under the action of a bichromatic field as a parametric effect

Generation of direct current in a semiconductor superlattice under the action of an ac bichromatic field is considered in the most general case of an arbitrary ratio of the frequencies of the fields being mixed. It is shown that this effect is of parametric origin associated with oscillations of the electron effective mass in the miniband of the superlattice.     

Subterahertz superlattice parametric oscillator

We report a superlattice parametric oscillator (SPO), with a GaAs/AlAs superlattice as the active element. The SPO was pumped by a microwave field (power 4 mW) and produced third harmonic radiation at subterahertz frequencies (near 300 GHz; 0.1 mW). We attribute the parametric gain to the nonlinearity of the miniband transport.     

Microscopic Three-Dimensional Theory of Space-Charge-Wave Modes in GaAs-Based Planar Superlattices

We report a systematic, fully three-dimensional thkoreticd examination of convective space-charge wave modes of a planar superlattice subject to negative differential miniband conductance (NDC) in a dc bias electric field. Our analysis is based on a set of hydrodynamic balance equations for system having an arbitrary energy band, with an accurate microscopic treatment of phonon and impurity scatterings. When applied to the longitudinal transport of an unconfined planar superlattice, these equations, which take full account of the role of carrier transverse motion, give rise to bulk NDC in the dc steady state conduction and provide a unified formulation to analyze the small-signal ac response in the space homogeneous case and the drift-relaxational modes under spatidy inhomogeneous condition.     

Intraminiband plasmons in a superlattice within a parabolic well

A well defined collective plasma resonance corresponding to electrons tunneling back and forth through superlattice barriers is observed in a series of modulation doped AlGaAs heterostructures containing a superlattice within a parabolic potential. Due to occupation of a large fraction of the miniband, the resonance frequency decreases strongly with increasing superlattice barrier thickness.     

Dynamic localization and self-induced transparency in a two-dimensional superlattice with a nonadditive dispersion law

Electron dynamics and the nonlinear effects of electric conduction in a two-dimensional semiconductor superlattice with nonadditive miniband dispersion relations have been studied in the case of strong high-frequency electric fields applied to the superlattice. The conditions for dynamic electron localization and electromagnetic transparency in such superlattices have been revealed.     

Transport in superlattices in the stark ladder regime

The current-voltage characteristic of a semiconductor superlattice in the Stark ladder regime has been calculated. It is shown that the negative differential conductivity onset electric field is defined by the scattering time, which depends only very slightly on the superlattice miniband width.     

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.     

Amplification of terahertz radiation in carbon nanotubes

We investigate theoretically the feasibility of amplification of terahertz radiation in aligned achiral carbon nanotubes, a zigzag (12,0) and an armchair (10,10) in comparison with a superlattice using a combination of a constant direct current (dc) and a high-frequency alternate current (ac) electric fields. The electric current density expression is derived using the semiclassical Boltzmann transport equation with a constant relaxation time. The electric field is applied along the nanotube axis. Analysis of the current density versus electric field characteristics reveals a negative differential conductivity behavior at high frequency, as well as photon assisted peaks. The photon assisted peaks are about an order of magnitude higher in the carbon nanotubes compared to the superlattice. These strong phenomena in carbon nanotubes can be used to obtain domainless amplification of terahertz radiation at room temperature.     

Double-resonance Semiconductor Superlattice Parametric Oscillator for Generation of Subterahertz Radiation

We report on a subterahertz superlattice parametric oscillator that operated simultaneously at two different harmonic frequencies of a microwave pump field. A pump field (frequency near 100 GHz) was coupled to a GaAs/AlAs superlattice in a resonator for the third and the fifth harmonic. The pump field produced a third harmonic field and this together with the pump field created a fifth harmonic field. A theoretical analysis indicates that the nonlinearity, which is based on the dynamics of miniband electrons, should allow for the upconversion of pump radiation of higher frequency into the terahertz frequency range.      

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     

A theoretical study of operation conditions for a terahertz superlattice parametric oscillator

We report a theoretical study of operation conditions for a terahertz superlattice parametric oscillator (SPO). The SPO converts radiation of frequency ω to radiation at frequency 3ω. The parametric process is based on the nonlinearity of the motion of a miniband electron in a high-frequency field consisting of a strong fundamental-frequency field and a higher harmonic field. In our study we made use of a semiclassical theory. Our analysis indicates that parametric frequency upconversion should allow for production of radiation up to several terahertz.     

Wave packet dynamics in a semiconductor superlattice

We examine the time development of a wave packet in a superlattice miniband in a constant electric field, explicitly determining the time dependence of the probability density. Our analytic results permit the identification of breathing motion of the wave packet as well as Bloch oscillatory motion of its center-of-mass. The dynamical development of a Gaussian wave packet is exhibited, showing the appearance of images of the initial wave packet in nearby superlattice cells, exemplifying their participation in Bloch oscillatory and breathing motions.     

THz-field induced nonlinear transport and dc voltage generation in a semiconductor superlattice due to Bloch oscillations

We report on a theoretical analysis of terahertz (THz-) field induced nonlinear dynamics of electrons in a semiconductor superlattice that are capable to perform Bloch oscillations. Our results suggest that for a strong THz-field a dc voltage should be generated. We have analyzed the real-time dynamics using a balance equation approach to describe the electron transport in a superlattice miniband. Taking account of both Bloch oscillations of electrons in a superlattice miniband and dissipation, we studied the influence of a strong THz-field on currently available superlattices at room temperature. We found that a THz-field can lead to a negative conductance resulting in turn in a THz-field induced dc voltage, and that the voltage per superlattice period should show, for varying amplitue of the THz-field, a form of wisted plateaus with the middle points being with high precision equal to the photon energy divided by the electron charge. We show voltage to the finite voltage state, and that in the finite voltage state dynamic localization of the electrons in a miniband occurs.     

2D chaotic quantum states in superlattices

The semiclassical motion of an electron along the axis of a superlattice may be calculated from the miniband dispersion curve. Under a weak electric field the electron executes Bloch oscillations which confines the motion along the superlattice axis. When a magnetic field, tilted with respect to the superlattice axis, is applied the electron orbits become chaotic. The onset of chaos is characterised by a complex mixed stable-chaotic phase space and an extension of the orbital trajectories along the superlattice axis. This delocalisation of the orbits is also reflected in the quantum eigenstates of the system some of which show well-defined patterns of high probability density whose shapes resemble certain semiclassical orbits. This suggests that the onset of chaos will be manifest in electron transport through a finite superlattice. We also propose that these phenomena may be observable in the motion of trapped ultra-cold atoms in an optically induced superlattice potential and magnetic quadrupole potential whose axis is tilted relative to the superlattice axis.     

Franz–Keldysh oscillations at the above-barrier miniband in a GaAs/AlxGa1 − xAs superlattice

We report on the observation of the Franz–Keldysh oscillations associated with the second-electron and heavy-hole minibands, in which the second-electron miniband lies above the barrier potential, in a GaAs(7.0 nm)/Al_0.1Ga_0.9As (3.5 nm) superlattice by using electroreflectance spectroscopy. The reduced mass estimated from the Franz–Keldysh oscillations is consistent with that estimated from the miniband–dispersion relation based on the effective mass approximation around a zero-electric field, and becomes heavier with an increase in the electric field. The electric-field dependence of the reduced mass correlates with the localization of the electron envelope function. We discuss these results from an envelope-function forms calculated by a transfer-matrix method.     

Superlattice vertical transport with high-lying minibands

We examine the role of high-lying minibands in superlattice vertical transport using a nonparabolic balance-equation approach. We find that the inclusion of high-lying minibands results in a decrease of the electron temperature, a reduction of the peak drift velocity and a slow-down of the velocity-drop rate in the negative differential mobility (NDM) regime, in comparison with those predicted by a single miniband model. These effects become significant when the strength of the electric field gets close to or falls in the NDM regime.     

Current oscillation and chaotic dynamics in superlattices driven by crossed electric and magnetic fields

We have theoretically studied current oscillation and chaotic dynamics in doped GaAsAlAs superlattices driven by crossed electric and magnetic fields. When the superlattice system is driven by a dc voltage, a stationary or dynamic electric-field domain can be obtained. We carefully studied the electric-field-domain dynamics and current self-oscillation which both display different modes with the change of magnetic field. When an ac electric field is also applied to the superlattice, a typical nonlinear dynamic system is constructed with the ac amplitude, ac frequency, and magnetic field as the control parameters. Different nonlinear behaviors show up when we tune the control parameters.     

A theoretical study of harmonic generation in a short period AlGaN/GaN superlattice induced by a terahertz field

Based on an improved energy dispersion relation, the terahertz field induced nonlinear transport of miniband electrons in a short period AlGaN/GaN superlattice is theoretically studied in this paper with a semiclassical theory. To a short period superlattice, it is not precise enough to calculate the energy dispersion relation by just using the nearest wells in tight binding method: the next to nearest wells should be considered. The results show that the electron drift velocity is 30% lower under a dc field but 10% higher under an ac field than the traditional simple cosine model obtained from the tight binding method. The influence of the terahertz field strength and frequency on the harmonic amplitude, phase and power efficiency is calculated. The relative power efficiency of the third harmonic reaches the peak value when the dc field strength equals about three times the critical field strength and the ac field strength equals about four times the critical field strength. These results show that the AlGaN/GaN superlattice is a promising candidate to convert radiation of frequency ω to radiation of frequency 3ω or even higher.     

Pendulum limit,chaos and phase-locking in the dynamics of ac-driven semiconductor superlattices

We describe a limiting case when nonlinear dynamics of an ac-driven semiconductor superlattice in the miniband transport regime is governed by a periodically forced and damped pendulum equations. We find analytically the conditions for a transition to chaos. With increasing temperature the chaos disappears. We also discuss fractional dc voltage states in a superlattice originating from phase-locked states of the pendulum.     

Millimetre-wave negative differential conductance in GaInAs/AlInAs semiconductor superlattices

We report microwave and temporal measurements on nonlinear transport in GaInAs/AlInAs superlattice n⁺n⁻n⁺diodes. The superlattices have wide minibands, wider than 80 meV. The microwav experiments are performed for 0–65 GHz. The diodes exhibit reflection gain up to 65 GHz over a bias-dependent Hakki resonance. The temporal experiments use electro-optic sampling to push the measurement range beyond 200 GHz. Harmonics of the fundamental Hakki resonance are observed up to order three at 150 GHz. A simple admittance model for negative differential conductance devices accounts for the results in both experimental approaches. They are in agreement with miniband transport and demonstrate superlattice negative differential conductance far in the millimetre-wave domain.