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International audience; Absolute far-infrared magnetotransmission experiments have been performed in magnetic fields up to 33.5 T on a series of single GaAs quantum wells doped with different electron concentrations. The transmission spectra have been simulated with a multilayer dielectric model. The imaginary part of the optical response function, which reveals singular features related to the electron-phonon interactions, has been extracted. In addition to the expected polaronic effects due to the longitudinal-optical phonon of GaAs, additional interactions with interface phonons are observed. The main interaction is analyzed quantitatively with the Feynman-Hellwarth-Iddings-Platzman model, which is shown to predict correctly the concentration of carriers beyond which the Fröhlich interaction is completely screened.

Intra Landau level thermal activation, from localized states in the tail, to delocalized states above the mobility edge in the same Landau level, explains the $B_c(T)$ (half width of the dissipationless state) phase diagram for a number of different quantum Hall samples with widely ranging carrier density, mobility and disorder. Good agreement is achieved over $2-3$ orders of magnitude in temperature and magnetic field for a wide range of filling factors. The Landau level width is found to be independent of magnetic field. The mobility edge moves, in the case of changing Landau level overlap to maintain a sample dependent critical density of states at that energy. An analysis of filling factor $\nu=2/3$ shows that the composite Fermion Landau levels have exactly the same width as their electron counterparts. An important ingredient of the model is the Lorentzian broadening with long tails which provide localized states deep in the gap which are essential in order to reproduce the robust high temperature $B_c(T)$ phase observed in experiment., Comment: 15 pages, 12 figures

We report a new technique to amplify coherent acoustic phonons in a semiconductor superlattice under intraminiband transport. The interaction between drift electrons and copropagating acoustic phonons leads to an amplification of the sound amplitude by 200 %.

Femtosecond pump-probe experiments demonstrate a strong amplification of optically generated coherent phonons by interaction with an intra-miniband current in a biased semiconductor superlattice. Electron-phonon coupling predominantly occurs via the acoustic deformation potential.

An efficient photon detector based on the ambipolar transport of photogenerated electrons and holes by surface acoustic waves (SAWs) has been investigated. In the experiments, the photogenerated electrons and holes are separated and transported in a GaAs channel and electrically detected by a lateral p–i–n junction. We show that the acoustic transport efficiency improves by using biased metallic guides along the SAW beam to create independent transport channels for electrons and holes. By optimizing the photon absorption efficiency and the amplitude of the SAW field, we demonstrated overall transport efficiencies above 85% for transport lengths exceeding 200 μ m .

We investigate the quantum Hall effect in a highmobility, two-dimensional electron gas on a cylindrical surface. Due to the varying filling factor along the current path, the electron transport is characterized by sequential transmission along incompressible stripes and compressible bulk regions. For the compressible regions, we observe an exponential decay of the longitudinal resistances by rotating the sample around the cylinder axis. We adopt the model of the so-called static skin effect for the conductance in the compressible bulk regions and demonstrate that this behavior can be ascribed to probing the skin channel width by moving the Hall bar terminals along the gradient of the magnetic field along the cylinder periphery. We determine L as the characteristic length for the exponential decay of the skin channel width by moving away from the position where the magnetic field is perpendicular to the surface. The values of L are in the sub-micrometer range, similarly as for low magnetic fields. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

It is now possible to acoustically control the transfer of electrons and holes between a quantum well and a quantum dot by exploiting the moving piezoelectric potential modulation induced by an acoustic phonon. The effect has been used to demonstrate a high-frequency single-photon source with tunable emission energy, by acoustically transferring carriers to selected quantum dots.

In this article we present our results on Thzspectroscopy of intra-excitonic transitions. Especially, properties of different III-V multi quantum well systems are investigated. After an introduction to the methodology of THz Time-Domain Spectroscopy, results for both a GaAs/(AlGa)As and a (GaIn)As/GaAs multi quantum well structure are presented. We find good agreement with previous experiments [1,2] for the GaAs/(AlGa)As structure, while the (GaIn)As/GaAs structure behaves significantly different. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We observe a new type of resistance oscillations in the ballistic transport of two-dimensional electrons on cylindrical surfaces with a tangentially directed magnetic field. The longitudinal resistance oscillates with a periodicity proportional to the square root of the magnetic field √B, which indicates a commensurability of longitudinal distances with a length scale related to the gradient of the perpendicular to the surface component of magnetic field B⊥.We show, that the length of the stripe with energetically free electrons, forming the so called ‘snake’-like trajectories around the position where B⊥ = 0 is the basic length scale for the √B oscillations. We speculate, that ‘zitterbewegung’ of the one-dimensional ‘skin’-channel due to switching of the spin-orbit interaction around the B⊥ = 0 position may cause this new type of resistance oscillations. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical investigation of the transport of electrons and holes in GaAs quantum wires by surface acoustic waves shows that carriers can be efficiently transported over several tens of mm. Despite the efficient charge transport, the transport distances for photoexcited spins were found to be very short. These short spin transport lengths indicate that spin dephasing is not limited by spin motion (the DP mechanism) but rather by spin relaxation during momentum scattering. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Long-range acoustic transport of excitons in GaAs quantum wells (QWs) is demonstrated. The mobile strain field of a surface acoustic wave creates a dynamic lateral type I modulation of the conduction and valence bands in a double-quantum-well (DQW) structure. This mobile potential modulation transports long-living indirect excitons in the DQW over several hundreds of μm.

The structural and optical properties of (In,Ga)As/(Al,Ga)As quantum wells grown on GaAs(1 1 0) by conventional molecular beam epitaxy as well as by migration-enhanced epitaxy are studied by low-temperature photoluminescence (PL), cathodoluminescence (CL), atomic force microscopy (AFM) and transmission electron microscopy (TEM). The quality of GaAs/(Al,Ga)As quantum wells (QWs) allows to detect spin transport mediated by surface acoustic waves over distances up to 65 μm with spin lifetimes of about 22 ns. For (In,Ga)As/GaAs QWs, the accumulated strain due to the increase of the In mole fractions or the well thicknesses degrades the structural perfection more dramatically in structures with (1 1 0)-orientation than in structures with (0 0 1)-orientation. The quality of (In,Ga)As wells is remarkably improved by migration-enhanced epitaxy.

Sound amplification in an electrically biased superlattice (SL) is studied in optical experiments with 100 fs time resolution. Coherent SL phonons with frequencies of 40, 375, and 410 GHz give rise to oscillatory reflectivity changes. With currents from 0.5 to 1.3 A, the Fourier amplitude of the 410 GHz phonon increases by more than a factor of 2 over a 200 ps period. This amplification is due to stimulated Čerenkov phonon emission by electrons undergoing intraminiband transport. The gain coefficient of 8×10^{3} cm^{-1} is reproduced by theoretical calculations and holds potential for novel sub-THz phonon emitters.

We review processes for long-range spin transport and manipulation in GaAs quantum wells using mobile potentials created by the piezoelectric field of a surface acoustic wave. By reducing spin dephasing mechanisms associated with the spin orbit-coupling, these potentials can coherently transport spins over distances on the order of 100 µm. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The geometry of the sample is shown to dictate the importance of strain on the spin-orbit splitting of the conduction band for electrons transported via dynamic quantum dots (DQDs) generated by surface acoustic waves (SAWs). Photoluminescence measurements monitoring the Larmor precession of electron spins in the absence of an applied magnetic field are used to characterize the spin-orbit splitting of the conduction band. We show that, in thick quantum wells (QWs), the Dresselhaus spin-orbit contribution to the precession frequency reduces and may become comparable to the one associated with the strain field induced by the SAW. We attribute deviations in the measured precession frequency to strain effects, which become more pronounced for QWs located closer to the surface because of the larger SAW strain field. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The interaction between surface acoustic waves (SAWs) with (Al,Ga)As microcavity polaritons results in the formation of a dynamic optical superlattice with folded light dispersion and energy stop bands when the lower polariton branch is predominantly of photonic character. For small detunings between the excitonic and optical cavity resonances, the SAW bleaches the polariton resonances through the efficient dissociation of the excitons by its piezoelectric field.

Mobile quantum dots (denoted as dynamic quantum dots) formed by the interference of orthogonal beams of surface acoustic waves (SAWs) in an undoped GaAs quantum well coherently transport electron spins over distances exceeding 70 μm. We determine the spin-splitting parameter for a GaAs quantum well to be 7 = 17 ± 2 eV A 3 as well as investigate the effects of the SAW strain field on the spin dynamics.

Quantum wells consisting of (In,Ga)As/GaAs and InAs/GaAs were grown on GaAs(110) by conventional molecular beam epitaxy as well as by migration enhanced epitaxy and studied by low-temperature photoluminescence, atomic force microscopy and transmission electron microscopy. Increasing the In mole fraction or the well thickness degrades the structural perfection more dramatically in (110)-oriented structures than in (001)-oriented ones while post-growth annealing improves the optical properties. The quality of (In,Ga)As wells is remarkably improved by migration enhanced epitaxy. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electron spins in an undoped quantum well are transported using dynamic quantum dots (DQDs) formed by the piezoelectric potential of surface acoustic waves. The temperature dependence of the coherence length of electron spins during transport is investigated using spatially resolved photoluminescence. For temperatures between 4.2 and 20 K, the spin coherence length, which is proportional to the spin coherence lifetime, remains constant at approximately 100 μ m. This results from the ability of the DQDs to confine the spins within a small area (1 μ m × 1 μ m to 2 μ m × 2 μ m) during transport and reduce the temperature dependence associated with D’yakonov-Perel’ (DP) spin scattering. At 30 K, the electron spin coherence length rapidly decreases as the DP mechanism becomes dominant because of the higher thermal velocity on the Fermi surface.

Mobile potential dots (dynamic dots, DDs) formed by surface acoustic waves (SAWs) are used to transport photogenerated electrons and holes in GaAs quantum wells (QWs). We investigate the interaction between the transported carriers and microscopic trap centers in the QW plane using spatially and time-resolved photoluminescence (PL) spectroscopy. The carriers recombine at the trap site emitting short (width ≤ 0.6 ns) light pulses at a repetition rate corresponding to the SAW frequency. The dependence of the PL intensity from the traps on the number of carriers transported per DD n exhibits a well-defined, distinct plateau for n in the range from 5-20, which is attributed to the emission of a well-defined number of photons.

GaAs∕Al0.33Ga0.67As quantum-cascade lasers with plasmon-assisted waveguides exhibit a decreasing threshold current density jth with increasing wave number ν0 of the laser line, which changes as a function of the injector doping density. We have developed an analytical approach based on the effective dielectric tensor component for the p-polarized light emitted from a quantum-cascade laser, which explains the observed dependence of jth(ν0) in terms of losses due to free-carrier absorption predominantly in the doped waveguides αWG(ν0). A contribution to the losses by free-carrier absorption in the quantum-cascade structure itself and subsequently to jth can be neglected except for very high injector doping densities. The calculated values for αWG(ν0) are in good agreement with the experimental data. Our approach quantitatively predicts the observed decrease of jth from 17to7kAcm−2 with increasing ν0 between 900 and 1100cm−1. In addition to achieving a direct physical insight into the influence of free-carri...

We investigate the cyclotron resonance (CR) on a single GaAs quantum well containing a two-dimensional electron gas (7.0×10 11 cm −2 ) , in magnetic fields up to 28 T in order to study the resonant polaron coupling. When the normal to the surface of the sample is in the same direction as that of light propagation, we do not observe an anomaly of the CR absorption in the optical phonon range of energy. When the normal of the sample is tilted with respect to the light propagation direction (and with the magnetic field direction) we observe a pinning of the CR peak at an energy lower than that of the longitudinal optical (LO) phonon energy. This mode is identified as the coupled intersubband plasmon–LO phonon of the quantum well.

Resonantly excited secondary emission of light (RSE) in GaAs/AlGaAs superlattices gives a direct proof for the stability of coherent Bloch dynamics beyond the optical coherent regime. The polarization analysis of the RSE signal confirms its nonlinear origin resulting from coherent intraband density dynamics. lt is shown that the intraband coherence of Bloch oscillations (BO) persists after momentum scattering of the photoexcited states. A variation of emission angle and ex-citation density allows to distinguish two momentum scattering processes contributing to the RSE. A static disorder mediated contribution is restricted to directions near the reflection direction. A further contribution of carrier induced momentum scattering is resolved in directions well separated from diffraction directions. The dependence of the RSE as a function of an external electric field demonstrates an enhancement of momentum scattering with increasing field.

We have investigated the transport and population properties of an undoped GaAs/(Al,Ga)As quantum-cascade structure (QCS) below and above the designed threshold field strength F c r i t using current-voltage characteristics and photoluminescence (PL) spectroscopy. The device exhibits a current instability at F c r i t , which is correlated with an abrupt change in the field-dependent PL spectra. Below F c r i t , the PL spectra are dominated by the emission from the lowest subband of the QCS exhibiting the usual Stark shift. Above F c r i t , a significant emission from the lower laser level as well as, though much weaker, from the upper laser level is observed. This current instability may be due to the occupation of the laser levels and/or of additional barrier states, which open transport channels for the electrons within the (Al,Ga)As barriers.

Dynamic dots (DDs) consisting of confined and mobile potentials are realized by the interference of orthogonal surface acoustic wave (SAW) beams in GaAs quantum wells. Photoluminescence spectroscopy reveals that the DDs are characterized by a peculiar distribution of strain and piezoelectric fields dictated by the lattice symmetry, which is quite different from the one induced by a single SAW. We demonstrate the unique ability of DDs to control the flow of photogenerated electron–hole pairs and of photons by realizing an electronic switch based on SAWs.

We have investigated the transport and population properties of undoped GaAs/(Al,Ga)As quantum-cascade structures (QCS) below and above the designed threshold field strength Fcrit using current–voltage characteristics and photoluminescence (PL) spectroscopy. The devices exhibit a negative differential conductance (NDC) at Fcrit. The NDC is correlated with an abrupt change of intensity and energy positions in the field-dependent PL spectra. Below Fcrit, the PL spectra show only emission from the lowest interband transition in the QCS exhibiting the usual Stark shift. Above Fcrit, a significant population of the laser levels is observed. The switching behaviour is tentatively attributed to an abrupt spatial redistribution of the carriers at Fcrit mediated by additional states within the (Al,Ga)As barriers.

We have investigated the current density–electric field (j-F) characteristics and photoluminescence (PL) spectra of several GaAs/(Al,Ga)As quantum-cascade structures (QCS’s) up to the threshold field strength to obtain information on the carrier distribution and field inhomogeneities. Due to the complex structure of each single period of the cascaded device, the dark j-F characteristics of undoped QCS’s exhibit distinct current maxima, resulting in regions of negative differential conductivity, while a plateaulike feature appears under illumination. For doped QCS’s, a plateau with sawtoothlike structures appears, which are well known from doped, weakly coupled superlattices under electric-field-domain formation. However, in contrast to the splitting of the PL line that is observed for doped, weakly coupled superlattices, no splitting has been observed in the doped QCS’s. The j-F characteristic of an undoped QCS with thicker barriers than the original QCS exhibits a much more pronounced current maximum in ...

We utilize imprint lithography and the epitaxial lift-off technique to fabricate devices in which the photoluminescence (PL) is modulated under irradiation of surface acoustic waves (SAWs). Imprint lithography provides short-wavelength SAW transducers on LiNbO3. A high-quality GaAs–(Al,Ga)As quantum well is bonded in front of the transducers using the epitaxial lift-off technique. The large electromechanical coupling in LiNbO3 enables us to manipulate the carrier dynamics in the quantum well through the SAW-induced piezoelectric fields far more effectively than with transducers prepared directly on GaAs. The influences of the scattering of the SAWs from imperfections in the bonded film are investigated by measuring the quenching of the quantum-well emission by micro-PL.

The dispersion of the Rayleigh wave in a strain-unrelaxed GaAs/AlAs/GaAs heterostructure is evaluated. We determine the wavelengths at which the velocity of surface acoustic waves (SAWs) is maximum or comparable to that in GaAs. The information is utilized to design interdigital transducers to investigate the SAW transmission properties when GaAs micro-beams are realized in the SAW propagation path by selectively etching the buried AlAs layer. A frequency shift as well as an attenuation of the SAW transmission peak is observed for certain SAW wavelengths within the range of the enhanced SAW velocity in the unpatterned heterostructure.

We have investigated the electric-field-dependent subband population and carrier dynamics in double-quantum-well superlattices (DQW-SLs) as well as quantum cascade structures (QCSs) of the GaAs/(Al,Ga)As material system by (interband) photoluminescence spectroscopy and current–voltage characteristics. In several DQW-SLs, absolute values for the subband population of electrons and the transfer coefficients of holes have been determined. In typical QCSs, the characteristic times for the hole relaxation into the widest well have been estimated to be of the order of several picoseconds. Although the characteristic times become very short, the electric-field-dependent population of the ground states in the active region can be qualitatively characterized.

The conductance in high-mobility and high-density, modulation-doped GaAs single quantum wells on GaAs (0 0 1) substrates with thin spacer layers is strongly anisotropic with a 2–3 times higher mobility in the [ 1 1 0] than that in the [1 1 0] direction. We show that the anisotropic scattering potential is strongly influenced by additional X-like electrons in the barriers formed by short-period superlattices. The X-electrons are able to considerably smooth the fluctuations of the potential; thus, increasing the correlation length of the fluctuations. We investigate the correlation length of the potential fluctuations by oscillations in the low-field magnetoresistance.

We have investigated the dynamics of photogenerated carriers in GaAs quantum wells under the influence of high-frequency fields produced by metal gratings and by surface acoustic waves (SAW's) using spatially and time-resolved photoluminescence (PL). The frequency and phase of the PL oscillations induced by the high-frequency field yield information about the spatial distribution of the carriers and, in the case of SAW's, about the band-gap modulation induced by the SAW strain.

Femtosecond midinfrared four-wave mixing is used to study the ultrafast dynamics of coherent intersubband polarizations in n-type modulation-doped GaAs/AlGaAs quantum wells. Time-integrated four-wave mixing yields a dephasing time of T 2 =320 fs , independent of excitation density. Time-resolved four-wave mixing unambiguously shows that the intersubband transition is purely homogeneously broadened by intraband Coulomb scattering within the low-density (n=5×10 10 cm −2 ) electron plasma. The nonlinear propagation of tailored midinfrared pulses is studied by electro-optic sampling, which allows direct time-resolved measurements of the electric field transients. Excitation pulses of only 1 pJ energy are needed to observe partial Rabi flops of up to 60° in the Bloch sphere.

The temperature dependence of the emission properties in a composite quantum well structure consisting of a wide and a narrow GaAs quantum well (QW) separated by and embedded in a GaAs/AlAs short-period superlattice (SPS) has been studied by steady-state and time-resolved photoluminescence (PL) measurements. At low temperatures (20K), distinct PL peaks originating from the QWs and SPS are observed. When the temperature is increased to 60K, the PL intensity of the wide QW with strongly confined states significantly increases, while the ones of the narrow QW and the SPS gradually decrease. Above 100K, however, the PL intensity of the wide QW decreases and the others increase. The temperature dependence of the PL dynamics indicates that the PL properties of the composite QWs are connected to Bloch-type transport of photoexcited carriers in the SPS between the wide and narrow QW.

The ultrafast dynamics of coherent intersubband polarizations in n-type modulation-doped GaAs/AlGaAs quantum wells is studied by femtosecond time-integrated and time-resolved four-wave-mixing. At a low electron density of 5×10 10 cm −2 , intraband Coulomb scattering results in irreversible dephasing with a decay time of T 2 =320 fs which is independent of excitation density and fully accounts for the 3.7 meV linewidth of the intersubband transition. Nonlinear propagation of tailored mid-infrared field transients is studied using amplitude and phase-resolved detection. A coherent response corresponding to partial Rabi flops of up to 60° in the Bloch sphere is observed with excitation pulses of only 1 pJ energy.

The quenching of photoluminescence of GaAs by surface acoustic waves (SAWs) is investigated in the presence of a magnetic field. The response of the luminescence to the SAWs is weakened in high magnetic fields. The suppression of the quenching is attributed to electrons and holes trapped around disorder potentials. These electrons and holes are generated from the dissociation of excitons by the SAWs. When a two-dimensional electron gas (2DEG) is placed near the GaAs surface, the electron–hole recombination associated with the background carbon acceptors becomes totally undisturbed by the SAWs and the magnetic-field-induced transition of the quenching behaviour for the near-band-edge exciton recombination becomes abrupt. These observations demonstrate the modification of the boundary conditions imposed on the piezoelectric potential by means of changing the 2DEG conductivity.

The intrinsic photoluminescence of high mobility and dense two dimensional electron gas in symmetric narrow GaAs quantum wells has been investigated in magnetic fields up to 22 T and temperatures from 1.7 to 60 K . The observation of a new emission line seen below the ground state recombination and showing a saw-tooth like behaviour with the magnetic field is reported. This transition is tentatively attributed to red shifted replica of a recombination process involving electrons from the Fermi level.

We investigated the cyclotron resonance (CR) on a series of samples containing high-density 2DEG (7.4– 12×10 11 cm −2 ) in GaAs/AlAs single quantum wells (in high magnetic fields up to 28 T ) in order to study the resonant polaron coupling. We did not observe any anomalies of the CR peak's evolution around the longitudinal optical (LO) phonon energy of GaAs (ℏω LO =36.3 meV ) . Instead, a pinning of the CR peak occurs near the transverse optical (TO) phonon energy and is accompanied by an increase of the width and of the intensity of the peak. We have simulated the transmission of the whole structure using a multilayer dielectric model, which allows us to determine the correct values of the relaxation frequency and its variation with magnetic field. We discuss the different possibilities of the coupling observed around the TO energy.

The control of spin dephasing is an essential requirement for quantum information processing using electron spins in IIIV semiconductors. GaAs quantum wells grown along the non-conventional [111] crystallographic direction are particularly interesting for spintronics due to the long spin lifetimes, which can be electrically controlled. Here, we show electron spin dynamics in (111) quantum wells by combining spatially-resolved with time-resolved photoluminescence measurements. The latter allows us to experimentally demonstrate the field induced enhancement of the spin lifetime as well as the transport of spin over several micrometers along the quantum well plane.

In this paper, photonic devices driven by surface acoustic waves and operating in the GHz frequency range are presented. The devices were designed and fabricated in (Al,Ga)As technology. In contrast to previously realized modulators, where part of the light transmission is lost due to destructive interference, in the present devices light only switches paths, avoiding losses. One of the devices presents two output channels with 180◦-dephasing synchronization. Odd multiples of the fundamental driving frequency are enabled by adjusting the applied acoustic power. A second and more complex photonic integrated device, based on the acoustic modulation of tunable Arrayed Waveguide Gratings, is also proposed.

We investigate the impact of two-step molecular beam epitaxy on the performance of GRINSCH lasers with In0.22Ga0.78As and GaAs quantum wells (QWs) in the active region. The buried regrown interface lies in the Al-containing optical waveguide section. The interface has been cleaned in situ prior to regrowth by hydrogen-assisted oxide desorption and re-evaporation of a sacrificial GaAs cap layer. Two-step epitaxy leaves the internal quantum efficiencies of the laser devices unchanged but increases the internal losses. The threshold current density increases for the lasers with GaAs QWs, whereas for those with In0.22Ga0.78As QWs it decreases by up to 35%. We attribute this to a thermal annealing during re-evaporation and find a similar behaviour for structures that have been treated with post-growth rapid thermal annealing.

An (Al,Ga)As heterostructure consisting of a 10 nm wide GaAs single quantum well and an optimized AlAs/GaAs type-II-superlattice barrier is fused onto a new LiNbO3 substrate by epitaxial lift-off and subsequent wafer bonding. X-electrons formed in the superlattice barrier effectively screen the high mobility electrons in the single quantum well from electronic defects arising at the new hybrid interface. Thus, the electron density as well as its high electron mobility can be preserved in the hybrid system.

The results of the study of cyclotron resonance on a series of high electronic density 2DEG in GaAs/AlAs single quantum well are presented. The samples contain different amount of acceptors in the well. We observe clear oscillations of the CR line width with the filling factor. This effect is accompanied by a periodical modulation of the CR peak energy. We simulated the transmission of the whole structure using a multi-layer dielectric model. This model allows us to determine the correct values of the relaxation frequency and its variation with magnetic field. The oscillations of the CR line width and of the CR peak energy are explained within the theory of the memory function.

The influence of metastable centres on the phonon emission of highly planar-doped GaAs:Si is studied by time of flight measurements. A general decrease of the flux of ballistically propagating phonons is observed in the whole investigated range of electric fields. It is assumed to be mainly caused by a redistribution of emitted phonons due to the creation of vibronic states. An additional channel of the emission of quasi-diffusively propagating phonons appears on account of the formation of metastable centres and their decay according to a dynamic balance.

In order to distinguish between conflicting interpretations regarding the effect of intralayer insertion at semiconductor junctions, we have carried out synchrotron-radiation photoemission studies of GaAs/AlAs(100) heterojunctions and GaAs/GaAs(110) homojunctions, with and without a Si or a Be intralayer, at room temperature and at low temperature. The synchrotron light induces photovoltage effects at low temperature, which are found to be consistent with the room-temperature band profiles we have previously proposed for these heterojunctions [M. Moreno et al., Phys. Rev. B 58, 13 767 (1998)], assuming a doping role for the intralayer atoms. Band discontinuities play an important role in determining the type of photovoltage effects induced. Our experimental observations can be fully understood in terms of intralayer-induced changes of the bandbending profile, and the occurrence of photovoltage effects at low temperature, calling into question the previous interpretation of room-temperature photoemission results from GaAs/AlAs heterojunctions in terms of intralayer-induced "band-offset" changes.

Photoluminescence (PL) spectroscopy is used to study the population properties of a multiple, asymmetric GaAs/(Al,Ga)As double quantum well superlattice, which represents a simplified version of the active region of a quantum cascade laser (QCL). In the case of anti-Stokes PL of the narrower well, which is observed for excitation between the excitonic states of the two quantum wells and sufficiently high electric field strengths, the photocarriers are only excited in one of the two wells. The anti-Stokes signal gives direct evidence for transport of electrons and holes through the (Al,Ga)As barriers. An analysis of the electric-field dependence of the conventional PL as well as the anti-Stokes PL signal demonstrates that the population of the subbands can be determined in the limit of weak excitation intensity.

We report on the femtosecond dynamics of coherent Bloch oscillations in GaAs/Al x Ga 1− x As superlattices. In a superlattice with a miniband width equal to the optical phonon energy of GaAs the Bloch frequency can be tuned into resonance with the LO phonon by an applied electric field. We observe a strong coupling between Bloch oscillations and phonons leading to the coherent excitation of optical phonons when both are brought into resonance. This phenomenon is analyzed theoretically on the base of a microscopic Hamiltonian for the electron–phonon interaction in the superlattice. We show that coupled modes are described by a nonlinear pendulum coupled linearly to an oscillator representing the phonons. Under resonance condition the amplitudes of both modes increase and sidebands appear. The theoretical results are compared to the experimental observations.

First, we study coupling of excitons in different quantum-well islands by coherent excitation spectroscopy and find that excitons in different islands are not coherently coupled. Second, we investigate exciton and biexciton binding energies in inhomogeneously broadened (GaIn)As/Ga(PAs) multi quantum wells. A strong increase of the 1s–2s-splitting with increasing exciton localization is found, whereas the biexciton binding energy shows a very weak decrease with increasing localization.