Frequency multiplication using induced dipole domains in a semiconductor superlattice

We theoretically studied the possibility of frequency multiplication using propagating dipole domains which are induced in a semiconductor superlattice by microwave radiation. We have investigated the dynamics of electrons in a superlattice submitted to both a static voltage and a microwave field by performing a simulation based on a drift-diffusion model and incorporating current-limiting boundary conditions. The motion of electrons in the superlattice was governed by an Esaki–Tsu drift velocity field characteristic with a negative differential mobility above a critical electrical field. The simulation delivered, for a static voltage larger than a critical voltage, the periodic formation and annihilation of propagating dipole domains and, as a consequence, a reduction of the direct current through the superlattice. Our simulation showed that an additional microwave field can periodically induce and subsequently quench domains giving rise to a strongly anharmonic current. The anharmonicity of the current is the origin for the generation of higher harmonics of the microwave field. Both the formation and annihilation of a domain can take place within a time of about 1 ps suggesting that the mechanism of domain induction and quenching can be used for generation of radiation up to almost 1 THz.     

Intersubband plasmons of a semiconductor superlattice

Collective intersubband plasmon-like excitations are predicted to exist for a semiconductor superlattice. These modes arise because the single quantum well depolarization shifted intersubband excitation couples via the long-range Coulomb interaction with the corresponding excitations of the other quantum wells of the superlattice. The dispersion relation for these intersubband plasmons is obtained.     

Operation of a semiconductor superlattice oscillator

We report an experimental study of a semiconductor superlattice oscillator and present an analysis of the origin of gain. The oscillator generated microwave radiation (at frequencies around 60 GHz). An analysis of the results suggests that the oscillator operated in a pure charge accumulation mode that can occur in a medium with a negative differential mobility. We relate the negative differential mobility to miniband transport. Additionally, we propose a microwave-terahertz double oscillator that may be suitable to realize a terahertz Bloch oscillator.     

Topological scenarios of the creation and annihilation of polarization singularities in nonstationary optical fields

The dynamics of the polarization structure of the speckle field of the photo-induced scattering in LiNbO₃:Fe (unsteady regime) has been analyzed with the use of the methods of the modified Stokes polarimetry. It has been shown that the creation and annihilation of pairs of circularly polarized points (C points) in varying singular elliptically polarized fields proceed via two topological scenarios. It has been found that the creation of pairs of C points is preceded by the appearance of a region where the average ellipticity is larger than the values in the neighboring regions. A similar structure holds after the annihilation of C points.     

Wave-function reconstruction in a graded semiconductor superlattice

We reconstruct a test wave function in a strongly coupled, graded well-width superlattice by resolving the spatial extension of the interband polarisation and deducing the wave function employing non-linear optical spectroscopy. The graded gap superlattice allows us to precisely control the distance between test and probe wave functions. By spatially tuning one wave function with respect to the other and recording the amplitude and the sign of the modulation of the spectrally resolved four-wave-mixing (FWM) signal with respect to delay, we are able to reconstruct the test wave function. Our numerical simulation of the third-order response of an inhomogeneously broadened system reproduces the experimental data in great detail. The wave function used for the modelling is computed by a one-dimensional transfer matrix model including electron–hole Coulomb interaction. Our experimental scheme inherently allows us to quantitatively distinguish between non-linear mechanisms leading to the FWM signal, namely phase-space filling and excitation-induced dephasing. PACS 78.47.+p; 71.35.Cc; 42.50.Md; 42.65.Re     

Coherent Hall effect in a semiconductor superlattice

The coherent Hall effect denotes the transient Hall response of impulsively excited coherent charge-carrier wave packets in a solid. We report the first experimental study of this phenomenon (i) using a semiconductor superlattice in crossed electric and magnetic fields as a model for three-dimensional materials and (ii) employing a contactless optoelectronic technique to probe the transient currents. Two field regimes with distinctly different oscillatory wave packet dynamics are found, separated from each other by a transition region where all oscillations are suppressed.     

Defects in semiconductors observed by positron annihilation

The origin of the effect of defects on positron annihilation in semiconductors has been studied. The electron-positron momentum densities in elemental semiconductors (Si and Ge), III-V compound semiconductors (GaAs, InP and GaSb), diamond and the proton irradiated Si were investigated by a full-scale use of the two-dimensional angular correlation of positron annihilation radiations (2D-ACAR). The obtained results showed, as a whole, good agreement with the electron momentum distribution of the fully occupied Jones zone with a small exception for the fact that the low density channels are running along the three principal axes. This anisotropy was strong in elemental semiconductors, while it was weakened in compound semiconductors. This anisotropy and its dependence on the material were found to be generally understood by the incorporation of crystal symmetry. The anisotropy will be discussed by group theory in conjunction to the effect of defects on positron annihilation.     

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.     

Plasmons and their damping in a doped semiconductor superlattice

The complex zeroes of dielectric response function of a doped GaAs superlattice are computed to study the frequencies and damping rates of oscillations in coupled electron-hole plasma. The real part of a complex zero describes the plasma frequency, whereas imaginary part of it yields the damping rate. Strong scattering of charge carriers from random impurity potentials in a doped GaAs superlattice gives rise to a large value of damping rate which causes over-damping of plasma oscillations of coupled electron-hole gas below q_c, a critical value of wave vector component (q) along the plane of a layer of electrons (holes). The plasma oscillations which correspond to electrons gas enter into over-damped regime for the case of weak coupling between layers. Whereas, plasma oscillations which belong to hole gas go to over-damped regime of oscillations for both strong as well as weak coupling between layers. The damping rate shows strongq-dependence forq < q_c, whereas it weakly depends onq forq ≥q _c . The damping rate exhibits a sudden change atq =q _c , indicating a transition from non-diffusive regime (where collective excitation can be excited) to diffusive regime (over-damped oscillations).     

Longitudinal stark-cyclotron waves in semiconductor with a superlattice

Semiconductor with a superlattice in static electrical and magnetic fields parallel to the superlattice axis is considered. The existence of Stark-cyclotron longitudinal waves in non-local approximation is predicted.     

Conductivity of a semiconductor superlattice in a transverse magnetic field

The conductivity of a semiconductor superlattice in a magnetic field perpendicular to the superlattice axis is calculated. The dependence of the conductivity on the magnetic field strength is studied. It is found that there is a magnetic-field range where the resistivity is proportional to the magnetic field strength.     

Optic and acoustic plasmon modes of a semiconductor superlattice

The collective plasmon modes of s semiconductor superlattices consisting of alternating layers of electrons and holes or alternating layers of electrons with different densities are investigated. For wells widely separated in space, such that Bloch wavefunction overlap between wells is negligible, we find optical-like and acoustical-like plasmon modes propagating along the superlattice direction. Perpendicular to the superlattice direction, the acoustic mode (ω～q_″) recently observed by Olego and Pinczuk⁸ is found to be split into two acoustic branches.     

Ultrafast band-gap shift induced by a strain pulse in semiconductor heterostructures

The conventional piezospectroscopic effect is extended to picosecond time scales by using ultrashort strain pulses injected into semiconductor heterostructures. The strain pulses with durations of approximately 10 ps are generated in a metal transducer film by intense femtosecond laser pulses. They propagate coherently in the GaAs/(Al,Ga)As heterostructure over a distance of 100 microm and shift the band gaps by several meV as detected optically for quantum well exciton resonances by pump-probe techniques and time-resolved photoluminescence.