Pump-probe transmission experiments have been performed on PbSe above the fundamental absorption edge near 4 m in the temperature range 30 to 300 K, using the Dutch ps free-electron laser. For temperatures below 200 K and carrier densities above the threshold for stimulated emission, stimulated recombination represents the most efficient recombination mechanism with relatively fast kinetics in the 50-100-ps regime, in good agreement with earlier reports of photoluminescent emission. Above this temperature Auger recombination dominates, and the Auger coefficient C is determined from the pump-probe decay curves. In the lowtemperature regime the Auger coefficient is determined from the decay curves at times beyond 100 ps. The Auger coefficient is approximately constant ͑with a value of about 8ϫ10 Ϫ28 cm 6 s Ϫ1) between 300 and 70 K, and then drops a value of about 1ϫ10 Ϫ28 cm 6 s Ϫ1 at 30 K, in good agreement with the theory for nonparabolic near-mirror bands and nondegenerate statistics. It is found that C for PbSe is between one and two orders of magnitude lower than for Hg 1Ϫx Cd x Te of comparable band gap. ͓S0163-1829͑98͒07243-9͔

Picosecond time-resolved far-infrared measurements are presented of the scattering between conductionband states in a doped quasi quantum dot. These states are created by the application of a magnetic field along the growth direction of an InAs/AlSb quantum well. A clear suppression of the cooling rate is seen, from 10 12 s Ϫ1 when the level spacing is equal to the phonon energy, to 10 10 s Ϫ1 away from this resonance, and thus the results provide unambiguous evidence for the phonon bottleneck. Furthermore, the lifetimes had only weak dependence on temperature between 4 and 80 K. ͓S0163-1829͑99͒50612-7͔Electronic lifetimes of two-dimensional ͑2D͒ systems in magnetic fields are of fundamental interest in part because the quantization effect of the magnetic field mimics the effect of a quantum dot potential with an easily variable degree of confinement. 1 The magnetic field perpendicular to the layers forces the electrons into confined orbits and the density of states becomes a ladder of broadened ␦ functions similar to that of a quantum dot. Recently much work has been carried out on the so-called ''phonon bottleneck'' that has been claimed to inhibit the cooling of carriers in quantum dots when the level separation is not equal to the phonon energy. [2][3][4][5][6] However, partly as a result of different groups using different growth techniques for interband photoluminescence samples and partly on fundamental grounds, this is controversial and is the subject of much debate. 7-12 Indeed several mechanisms have been proposed that may bypass the bottleneck, such as multiphonon scattering, 7 Auger processes, 8 excitonic effects, 9 and defect related processes. 10 In the present work we observe clear phonon suppression in n-type quasi 0D dots ͑i.e., Landau quantized rather than spatially quantized͒ by a time-resolved intraband absorption measurement. This provides unambiguous evidence for the phonon bottleneck effect independently of arguments concerning which processes dominate in the interband photoluminescence measurements in dots 3-6,9-12 and quasi dots 13 such as electron-hole scattering. Further, because of the very clean model system ͑much sharper interfaces and no wetting layer, etc.͒, the interpretation is not complicated by detailed questions about different growth techniques and the quality of different dot sample structures. The results should assist in the understanding of which aspect of these processes is fundamental and which is dependent on sample structure.Resonant absorption of longitudinal optic ͑LO͒ phonons causes a variety of transport properties to oscillate with applied magnetic field, such as the magnetoresistance. 14 Resonant phonon scattering occurs whenwhere ប LO is the LO phonon energy, ប c ϭបeB/m* is the cyclotron energy, and ⌬l is an integer. At these resonances the LO phonon absorption/emission probability is strongly enhanced giving rise to large changes in the electron energy relaxation lifetime. 15,16 In the present work we have used the pump-probe technique, with cyclotron resonan...

We present far-/near-infrared double resonance measurements of self-assembled InAs/GaAs quantum dots. The far-infrared resonance is unambiguously associated with a bound-bound intraband transition in the neutral dots. The results show that the interband photoluminescence ͑PL͒ lines originate from conduction levels with successively increasing in-plane quantum numbers. We determine the confinement energies for both electrons and holes in the same dots. Furthermore, we show that the inhomogeneous broadening of the PL cannot be attributed solely to size and composition fluctuation.Crystal growth and lithographic techniques now allow the fabrication of semiconductor microstructures such as quantum box or dot systems.1 The study of these dots is motivated in part by interest in confinement of charges on very small length scales and interactions between them. They also have potential advantages for optoelectronic emitters, quantum computation etc., due to the singular density of states of individual dots. Some of these advantages have yet to be fully realized due to the lack of complete understanding of the nature of the excited excitonic states and the way charges relax down the ladder of quantized levels. Furthermore, dot ensembles can show considerable inhomogeneity, and even the best photoluminescence ͑PL͒ linewidths are typically 15 to 20 meV.There have been many measurements of intraband spectroscopy in semiconductor quantum dots 2-8 as a means to probe the excited states, but we describe here an inter/ intraband double resonance investigation of quantum dots. The interest of this technique is that unlike far-infrared ͑FIR͒ absorption or even photoinduced FIR absorption it has allowed us to make an unequivocal assignment of the resonant electronic bound-bound intraband absorption simultaneously with the interband excitonic transition in neutral dots ͑i.e., giving the electron and hole splittings within the same dots, and without the need for n-type and p-type samples͒. These transitions may be strongly dependent on charge due to the Coulomb energy, 8 and it is important to study the transitions in neutral systems since these are of primary technological interest. The technique has allowed us to investigate the relative importance of the causes of the inhomogeneous broadening.The samples used were InAs/GaAs self-assembled quantum dots grown at a low rate, 9 which are capped with GaAs. The low temperature PL from these dots under high laser power shows a series of very well resolved lines ͓Fig. 1͑a͔͒.For sample A the lowest energy transition, E1-H1, is at 1046 meV, and higher peaks are observed at 1114 meV, 1181 meV, which are assigned to transitions involving higher bound states ͑E2-H2 and E3-H3͒ in the dots. The peaks are spaced roughly 68 meV apart and exhibit a full width at half maximum of about 23 meV. Sample B has lowest PL peaks at 1239 and 1278 meV, i.e., separated by 39 meV, and full width 28 meV ͑data not shown͒. The PL from the confining layer 9 and the GaAs matrix are at 1.43 and 1.5 eV, respectively....

No abstract

We have developed an inter-/intra-band double-resonance technique that has allowed us to make an unequivocal assignment afthe resonant bound-bound intersublevel absorption in self-organired InAs/GaAs quantum dots We have applied the technique to dots grown at a IOW growth-rate, which show well resolved photoluminescence (PL) lines The doublc resonance (far infrared modulated photoluminescence. FIR-MPL) and its polarisation selection mles aUows us to identify not only the levels involved in the PL transition but the relative importance of the causes of its inhomugeneous broadening The interband excitation was with a 4 m W c w. He Ne laser, focussed $0 that the peak illtensity was ahaut I photon per dot every I h s , incident on the top surfaee o f the sample The PL was collected and focussed into a 0 5 m grating monochrometer, and detected by a liquid nitrogen cooled Ge p-i-n diode The FIR t o m the Dutch free-eledron laser (FELIX), which is continuously tiinmhle from 5 ,unto 250 pm, was incident normal to the substrate side of the sample 'The PI. and the FlR-WI. were collected simultaneously to ensure the conerpondence o f features in each by monitanng both the dc and ac outputs of the detector Spectra were obtained by scanning either the monachrometer at fixed FJR frequency or ac'uming FELIX at a fixed PL frequency The FJR beam was utfoocussed in order to avoid heating the ample, and the intensity was estiniated to bephotuns/micropulsei~m*, i e ofthe order of IO6 photons per dotThe results unequivocally confirm that the intehand PL lines arise from Swcessively inneasing in-gl8ne quantum numbers, with the lowest electroric states separated by 55 inrV and corresponding hole states by 13meV Most iinpartantly we believe that our method can distinguish between the three PI, inhomogeneous broadening mechanisms expected to be significant in our self-assembled system o f dots a) in-plane dot size vanation, b) out-of-plane. or pofcntial broadeniy, and c) compositional broadening associated with fluctuations o f In within the dot. In case (a) for fixed FIR frequency (i I: centred on a largdsmall dot size) one expects to see a sharp modulated PI. line shifled to lowhigh frequency side o f the PL spectrum, in =se (b) for fixed PL frequency one similarly expects a sharp FIR line. for case (c) one would expect both a broad FIR line for Bxed PI. irequcncy and a broad modulated PL line for fixed FIR frequency Case (e) is in fact what we observe, so we speculate that the PL inhomogeneous line broadening is dominated by composition fluchiations o f In within the dot Preliminaly supponive r e~u l t~ on annealed specimens, where the composition fluctuations of In arc reduced. do indeed show sharper modulated PL lines. these are presently under iwcstigatiuri 13.45 QThE2 Carder caphlrc and rrcombinatioo dynamics in single pyramidal quailtutii dots Uepartment oiPhysius, Swiss kcderal lnstituk of'leclmology-EPFL. CIi-1015 Lausanne Photic: +41 21 693 5492, Fax: +41 21 693 5480, c-mail . yairn.ducommun~~~epfl.ch W o g self-ardcrcd p w l ...