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An ensemble of InAs quantum dots with ground state transition energies centered at 1.216 eV and density 1011dots/cm2 has been studied by time-resolved photoluminescence (PL). The wavelength of the 100-fs excitation pulse was tuned through the ground (excited) state transitions, resulting in resonant (optical phonon sideband) PL. The decay of the PL was time resolved with a streak camera in the interval 1.5–3 ns to avoid scattered laser light. The intensity of the PL was recorded with its polarization both parallel with and perpendicular to the excitation polarization (along one of the crystal’s cleave axes); the ratio is 2.22 at low temperatures and low excitation. A phenomenological rate equation analysis is made, separating the excitations into two classes, one polarized along the excitation polarization and the other unpolarized (either that way immediately after the excitation pulse or scattered from the first class). Excellent fits to the data lead to the conclusion that both classes decay radiatively with a lifetime of 1 ns, and a transfer from the polarized to the unpolarized species takes place with a distribution time of 12 ns at low temperatures and low excitation, dropping rapidly toward zero for temperatures above 30 K and for intense excitation levels. The polarization of a coherently excited ground state exciton should dephase with a rate equal to the sum of the radiative rate plus the inverse of this distribution time.

We report a systematic study of the photoluminescence properties of ZnO nanostructures. In particular, mesoporous ZnO powders of varying surface-to-mass ratio are investigated and compared to a bulk reference. At low temperatures the emission from higher-energy states is very pronounced and even dominant for samples with high surface-to-mass ratio. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Formation of self-organized Ga(AsSb) quantum-islands during growth is shown to occur in a series of Ga(AsSb)/GaAs/(AlGa)As heterostructures, resulting in an in-plane hole confinement of several hundred meV. The shape of the in-plane confinement potential is nearly parabolic and thus yields almost equidistant hole energy levels. Transmission electronmicroscopy reveals that the quantum islands (QIs) are 100 nm in diameter and exhibit an inplane variation of the Sb concentration of more than 30%, which is the origin of the confinement. Up to seven bound hole states are observed in the photoluminescence spectra. Time-resolved photoluminescence data are shown as function of excitation density. The influence of size and density of such structures as well as the mechanisms of growth are discussed. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

An overview of recent experimental and theoretical results on stationary and time-dependent photoluminescence spectra in disordered semiconductor heterostructures is presented. In particular, temperature-dependent peak position and linewidth of the luminescence spectra, as well as the luminescence intensity are considered along with the time evolution of the luminescence intensity after pulsed excitation. Emphasis is given on the comparison between experimental and theoretical results aiming at a characterization of disorder in the underlying structures.

We present results of experimental and theoretical studies of thermal quenching of the photoluminescence intensity in (GaIn)(NAs)/GaAs quantum wells after various postgrowth treatments. The experimental data are well described in the framework of a phenomenological model, if thermal emission of excitons out of localized states into delocalized states at the mobility edge prior to their non-radiative recombination is taken into account. Comparison between theoretical results and experimental data yields such material parameters as the energy scale of the band tail and the relative concentrations of non-radiative centers and traps. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Exciton photoluminescence (PL) in a GaInNAs/GaNAs quantum well was measured in the temperature range from 15 K to 300 K. Two striking features of the PL were observed: the nonmonotoneous temperature dependence of the Stokes shift and the abrupt increase of the PL linewidth in a rather narrow temperature range. These features are known to be strong indications of the hopping relaxation of excitons via localized states distributed in space and energy. Computer simulations of the hopping relaxation of excitons were carried out. Comparison between the simulation results and the experimental data provides an important and reliable information on the energy shape of the density of states and also on the energy range, in which localized states for excitons are distributed. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ballistic currents of spin-polarized carriers are generated by quantum interference of one and two photon absorption. Polarization and relative phase of the exciting femtosecond laser fields control the magnitude and direction of the currents as well as the electron spin orientation. These spin-polarized currents are experimentally detected by a phase-dependent spatial shift of the circularly analyzed photoluminescence in cubic ZnSe.

The ultrafast emission dynamics of a 1.3-/spl mu/m (GaIn)(NAs)/GaAs vertical-cavity surface-emitting laser is studied by femtosecond luminescence upconversion. We obtain a minimum peak delay of 15.5 ps and a minimum pulse width of 10.5 ps. Laser operation with picosecond emission dynamics is demonstrated over a temperature range from 30 to 388 K. The bandgap shift with temperature of (GaIn)(NAs)/GaAs is determined to be about -2.9/spl middot/10/sup -4/ eV/K, which is smaller than for GaAs. Our measurements of the optical gain provide gain spectra similar to those of commercial (GaIn)(PAs)/InP-structures at moderate densities but broaden considerably for elevated carrier densities due to the stronger carrier confinement. We compare our experimental results with gain spectra calculated from a microscopic model and confirm the predictive capability of the model. The theoretical gain spectra are used as the input for a calculation of the temperature dependence of the (GaIn)(NAs)/GaAs surface-emitter emission which results in very good agreement with experiment.

Summary form only given. Over the last two years much effort has been devoted to experimentally explore spin electronics in semiconductors. These experiments also support the hope of implementing quantum information devices based on spins using the mature technology of semiconductors. A basic requirement for quantum information processing with spins is the control of the mutual interaction between single spins. We extend here the investigation of spin coherence to the case of exchange coupled electron hole spins within a single exciton. The sample under investigation is an intrinsic 3 nm GaAs/(AlGa)As multiple quantum well (MQW) with a large Stokes shift of 9 meV.

We measured time and spectrally resolved, resonantly and nonresonantly excited photoluminescence of self-assembled vertically aligned InP/(GaIn)P quantum dots on GaAs substrates with various spacer thicknesses (2, 4, 8, and 16 nm) between the dot layers. The different interdot coupling in the various structures leads to considerable differences in the electron relaxation process. Carriers tunnel in the vertically aligned dot stacks to the lowest energy state when the spacer thickness is small, resulting in a pronounced red shift and slowing down of the photoluminescence decay. We present simple models to explain the results of our measurements.

We demonstrate the mutual influence of the dynamics in the real-space and the spin parts of the exciton wavefunction evoked by the exchange interaction. In the regime of high exciton density, the exciton–exciton interaction in ZnSe/ZnMgSSe quantum wells is found to be strongly affected by the exciton spin orientation and the longitudinal spin relaxation via single-fermion spin flips. The exciton's homogeneous linewidth as deduced from degenerate four-wave mixing with prepulse is significantly enhanced in spin-pure exciton populations by scattering including fermionic exchange. The transverse spin relaxation in the low-density limit is deduced from the decay of spin quantum-beats in photoluminescence experiments. We find an anti-proportionality between the spin relaxation and the optical dephasing times. The temperature dependence of both relaxation times is determined by the exciton–phonon interaction. The latter gradually reduces the electron–hole exchange coupling which is the dominating term in the exciton's transverse spin relaxation.

We present an investigation of the excitonic resonances in wurtzite GaN by ultrafast linear, and nonlinear optical spectroscopy. The exciton lifetime and the cooling of the exciton distribution are investigated by time-resolved photoluminescence (PL) studies of the LO-phonon replicas. We obtain a biexponential decay of the exciton number with time and find that excitons cool mainly by scattering with acoustic phonons via deformation potential interaction. Transient four-wave-mixing (FWM) studies yield the homogeneous broadening of the A-exciton and the strengths of exciton–exciton and exciton–phonon interactions. These values are comparable to other direct gap semiconductors. The biexciton binding-energy is (5.7±0.3) meV. The energy splitting between A- and B-exciton is obtained with very high accuracy from quantum-beat spectroscopy.

A new experimental method is presented to determine the sign of the electron Land' g factor in semiconductors by time-resolved magneto photoluminescence. This technique is used to demonstrate the reversal of the electron g factor for different material compositions. Measurements and theoretical estimates for the spin dynamics in quantum well systems are compared.

We investigate epitaxial layers of GaN on c-plane sapphire by photoluminescence, optical density and x-ray diffraction. Besides the well known luminescence from hexagonal GaN we have identified two emission bands from cubic GaN. We observe the emission of the donor bound exciton in cubic GaN at 3.279 eV. The luminescence at 3.15 - 3.21 eV is explained as the cubic donor - acceptor recombination. The corresponding acceptor binding energy is found to be low as . For layers of 400 nm thickness, the optical density yield values for the average contents of cubic GaN between 10 and 25%. Proper growth conditions minimize the cubic contents in the upper regions of such layers.

Magnetic circular dichroism and time-resolved photoluminescence spectra were measured for cubic GaN epitaxial layers grown by gas source molecular beam epitaxy using NH3 plasma. Pronounced spectral features were observed in the magnetic circular dichroism spectra at the same energy position as the highest energy signals in reflection and photoluminescence spectra of the epilayers. The time-resolved photoluminescence measurement showed that the corresponding transition has a lifetime of around 15 ps. Considering these results, the exciton bandgap of cubic GaN is estimated to be 3.27 ± 0.01 eV at 4.2 K.

We present a study of tunnelling processes between CdTe CdMnTe quantum wells by means of steady-state and time-resolved spectroscopy. The energy difference of the involved states and the barrier height can be controlled by an external magnetic field and resonances of the heavy-hole and exciton levels can be achieved. The heavy-hole (hh) resonance is essential for the type and direction of the transfer process. An efficient transfer of excitons is possible even without emission of LO phonons.

Steady-state and picosecond photoluminescence experiments on CdTe/CdZnTe and CdTe/CdMnTe asymmetric double quantum wells are presented. An external magnetic field is used to tune the coupling between the two QWs by allowing resonances to occur between excitonic states. Very efficient tunnelling is found when an exciton from the high-energy QW can transfer either with the emission of an LO phonon or via the resonance with the 2s state of the low-energy QW.

We report on the resonantly ground-state excited, time-resolved photoluminescence of self assembled InP dots embedded in In 0.48 Ga 0.52 P grown by molecular beam epitaxy. Measurements dependent on excitation power and temperature show that the resonant luminescence is sensitive neither to exciton-exciton scattering nor phonon scattering, demonstrating the quantum dot nature of the InP islands. The measured decay times yield the intrinsic radiative lifetimes of excitons, typically 400 ps and one order of magnitude larger than that found in quantum wells.

We report on the intrinsic radiative lifetime of excitons in InP quantum dots embedded in an In0.48Ga0.52 P matrix. This lifetime is about 400 ps as measured by time-resolved photoluminescence with resonant excitation of the ground states. It is independent of excitation density and temperature demonstrating the quantum dot nature of the InP islands and the strong localization of the excitons. Similar experiments on excitons localized by well width fluctuations in a GaAs quantum well reveal distinct differences: weakly localized excitons disappear with increasing density or temperature.

The origins and structure of molecular and solid-state fullerenes are reviewed. Comparison of the optical properties of solution and solid state indicates strongly that the molecular nature is preserved in the solid state. Picosecond time resolved photoluminescence, photoconductivity and resonant Raman measurements are performed to investigate the influence of high-intensity illumination on the properties of Fullerene single crystals. A highly non-linear dependence of the luminescence emission efficiency and lifetime is observed on increasing the intensity. This non-linear increase is associated with a dramatic shift to the red of the emission maximum. Under similar conditions, the photoconductive response of the fullerenes is also seen to increase non-linearly with input intensity. Temperature-dependent measurements indicate that the non-linear processes are associated with an insulator-metal phase transition in the material. The transition is reversible and the observed photophysical changes coincide with a reversible shifting of the characteristic fullerene Raman lines to lower energies. At room temperature, in many samples, the shifting becomes irreversible, and a high molecular weight, insoluble material is formed. The photochemical process is proposed to be a polymerisation-like reaction of the fullerene molecules in the triplet excited state. This is supported by the observation that the rate of the reaction is reduced greatly in the presence of oxygen, an efficient triplet quencher. In conclusion, the response of Fullerene crystals to light is divided into three categories. At low intensities the photophysical processes are characteristic of those of a molecular insulator, the electronic wavefunctions being molecularly localised. At higher intensities, the material undergoes an optically-induced Mott-like transition to a semiconductor/metal, in which the electrons become delocalised in three dimensions. Thirdly, the material is found to be photochemically unstable under some conditions but analysis of the temperature and intensity dependence of Raman spectroscopy shows that the photodegradation process can be predicted and therefore controlled.

Spin quantum beats of free-electron Larmor precession are measured by time-resolved photoluminescence spectroscopy and yield an extreme anisotropy of the electron Lande g-factor in quantum wires. The g-factor changes from 0.56 along the wires to approximately 0 perpendicular to the wires. The experimental results are in good agreement with an estimate according to k p theory.

We report on time-resolved luminescence in GaAs quantum wells in high magnetic fields up to 21 T. Results on resonant tunneling and exciton relaxation including spin flip are presented

Nonlinearities photoluminescence, photoconductivity and resonant Raman measurements of fullerenes at high excitation densities are described and the observations are discussed in terms of an optically induced insulator-metal transition in fullerene crystals. The intensity dependence of the transient photoconductive response is reported to show similar nonlinear behaviour. At low intensities, the response decays exponentially within a time of 15nsec. At increased intensities, a second, delayed component emerges and evolves nonlinearly into a long lived component of lifetime 100-200nsec. The system is fitted with a simple three level model in accordance with a Mott-like transition.

Photoluminescence, photoconductivity and resonant Raman measurements are performed to investigate the influence of high intensity illumination on the properties of Fullerenes. A highly nonlinear dependence of the luminescence emission efficiency and lifetime is observed on increasing the intensity. This nonlinear increase is associated with a dramatic shift to the red of the emission maximum. The photoconductive response of the fullerenes is also seen to increase nonlinearly with input intensity. Temperature dependent measurements indicate that the nonlinear processes are associated with an insulator-metal phase transition in the material. The transition is reversible and the observed photophysical changes coincide with a reversible shifting of the characteristic fullerene Raman lines to lower energies

The characteristics of photo and electroluminescent emission from C60 crystals are reported. The photoluminescence emission is seen to increase nonlinearly with the third power of the input intensity above a threshold intensity. Associated with this nonlinear increase is the emergence of a long lifetime emission component and a redshifting of the emission spectrum. Furthermore, above an intensity which coincides with the onset of the nonlinear emission, the photoconductive response increases with the cube of the input power. At high excitation densities, the photoconductive response becomes relatively temperature independent compared to the thermally activated behaviour observed at low intensities. The characteristics of the temperature dependence are associated with a high electron mobility phase in the highly excited state and therefore an optically driven insulator to metal transition is proposed as a description of the observed phenomena. Using gold and aluminium electrodes as contacts, a bro...

We have studied the energy band structure and the Γ − X carrier transfer mechanism for type II tunneling bi-quantum wells consisting of GaAs wells, AlGaAs barriers of different thicknesses, and AlAs layers by cw and time-resolved photoluminescence measurements. The cw photoluminescence spectra of the indirect recombination of X electrons in the 7.1 nm thick AlAs layers with Γ holes in the 2.8 nm thick GaAs wells show weak zero-phonon lines indicating that the AlAs confined states at X xy are lower than those at Y z . Time-resolved photoluminescence reveals that the carrier transfer time depends stronger on temperature for thicker AlGaAs barriers. Two scattering mechanisms, temperature-dependent phonon scattering and the temperature-independent interface scattering, are probably involved in the carrier transfer, the latter becoming smaller with increasing AlGaAs barrier thickness. Our results are compared with those obtained for similar type II GaAs/AlAs superlattices.

We investigate the dependence of carrier lifetime in compressed semi-insulating GaAs on defect concentration, excitation intensity, and temperature by means of time-resolved photoluminescence spectroscopy. The carrier lifetime decreases from 240 ps over 50 ps down to 10 ps as the arsenic antisite defect concentration increases from 0.410 16 cm -3 over 2.010 16 cm -3 to 10.010 16 cm -3 .

The highly excited state of fullerenes is characterised by a luminescence output which is dependent on the cube of the input intensity. This nonlinear emission is red shifted from the low-level emission and has a long, intensity dependent lifetime. Under similar irradiation conditions, the photoconductive response is seen to increase with the cube of the input intensity and the photocurrent in the highly excited state is observed to be largely independent of temperature, contrasting sharply with the thermally activated behaviour at low excitation densities. The degree of nonlinearity of the observed phenomena exclude an interpretation in terms of intra-molecular processes and the temperature dependence of the photoconductive response is suggestive of a Mott-like transition. The nonlinear behaviour is compared to that of indirect band-gap semiconductors in which the origin of similar nonlinear phenomena in the highly excited state luminescence and photoconductivity are explained in terms of electron-hole droplet formation. The similarities of the behaviours leads to a consideration of exchange and correlation energies in fullerenes, which are calculated according to a phenomenological model. Estimates of the contributions are consistent with a Mott-like transition at high excitation densities and an excess exchange/correlation energy in the highly excited state of ∼150 meV.

Exciton resonances are observed in the room-temperature absorption and photoluminescence (PL) spectra of CdTe/${\mathrm{Cd}}_{0.85}$${\mathrm{Mn}}_{0.15}$Te multiple-quantum-well structures. Temperature-dependent absorption linewidth and PL decay measurements are reported. Above 45 K, exciton-LO-phonon scattering determines the homogeneous linewidth (${\mathrm{\ensuremath{\Gamma}}}_{\mathit{H}}$). High excitation intensities were used in the PL decay measurements so that delocalized excitons were created. The PL decay time is found to be a measure of the exciton radiative decay time between 45 and 145 K and it increases linearly with ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{H}}$/r(T), where r(T) is the fraction of excitons with transition frequencies in ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{H}}$. From 170 K nonradiative processes dominate the decay so that the PL decay time decreases with temperature.

Under high intensity illumination, the optical and electronic properties of fullerenes are seen to undergo dramatic, nonlinear changes. The photoluminescence emission is seen to increase with approximately the third power of the input intensity above an apparent threshold intensity. Associated with this nonlinear increase is the emergence of a long lifetime emission component and a redshifting of the emission spectrum. Investigations of the photo-transport properties associate the observed behaviour with a phase transition in the highly excited state. Above an intensity which coincides with the onset of the nonlinear emission, the photoconductive response increases with approximately the cube of the input power. In the highly excited state, the photoconductive response becomes relatively temperature independent compared to the thermally activated behaviour observed at low intensities. The characteristics of the temperature dependence are associated with a high electron mobility phase in the highly excited state and therefore an optically driven insulator to metal transition is proposed as a description of the observed phenomena.

Carrier tunneling through (Cd,Zn)Te barriers of different thicknesses is investigated in CdTe/Cd,Zn)Te asymmetric double quantum wells by use of time-resolved photoluminescence and steady-state photoluminescence excitation experiments. The strong dependence of the observed tunneling times on the barrier thickness indicates a resonant tunneling behavior. The analysis of our results allows us to identify the importance of excitons rather than free-carrier states for the tunneling mechanism.

Time-resolved photoluminescence studies in films, powders and single crystals of C60 are reported. The observed spectra in all forms are consistent with those reported for steady state conditions and may be associated with an intramolecular process. The spectrum is largely temperature independent and, at low temperatures, the observed luminescence decays with a lifetime of 1.2 ns. No long-lived decay from the triplet is observed at these intensities.In the case of the films, both the emission spectrum and lifetime are independent of excitation intensity. In the powder and single crystallite, a dramatic increase in the lifetime as well as a red-shift and broadening of the spectrum is observed at higher intensities. The luminescence intensity is seen to increase with the cube of the input intensity. The behaviour is interpreted as an abrupt onset of emission from the highly populated triplet state resulting from a banding of the intramolecular triplet states, a process which is dependent on a critical excited state density.

The synthesis of macroscopic amounts of the fullerenes [1] has stimulated a variety of experimental and theoretical investigations of this new allotropic form of carbon. The photophysical properties in solution have been well characterised. The lowest energy optical transitions are strongly forbidden by symmetry selection rules and no luminescence is observable [2–7]. In the case of the solid state, there is still no conclusive experimental evidence for electronic banding [8], posing questions as to the degree of inter- versus intramolecular interactions. However, in thin films luminescence has been observed, indicating a departure from the behaviour of the isolated molecules [6]. Tn this study, time resolved luminescence of films, powders and crystals of C60 are carried out in an effort to characterise this departure. A strong dependance of the luminescence on excitation intensity is observed in powders and crystals, but not in films. The results are discussed in terms of a transition between intramolecular and banded intermolecular behaviour at high excited state densities, which is dependent on microcrystallite size.

Time-resolved photoluminescence studies in powders and single crystals of C60 are reported. The observed emission spectra, resulting from excitation above the HOMO-LUMO transition energy, are consistent with those reported for steady state conditions which may be associated with an intramolecular process and the observed luminescence decay times are of the order of 1 ns. No long lived decay from the triplet is observed at low intensities. A dramatic increase in the lifetime as well as a redshift and broadening of the spectrum is observed at high excitation densities. The luminescence intensity is seen to increase with the cube of the input intensity. The phenomenon is interpreted as an abrupt onset of emission from the highly populated excited state resulting from an interaction of the intramolecular states, a process which is dependent on a critical excited state density. Furthermore, the phenomenon may be photoexcited at photon energies which lie below the HOMO-LUMO transition energy, under which conditions an intensity dependence of the output luminescence on input to the sixth power is observed. Under “off-resonant” conditions the process is interpreted as two-photon assisted nonlinear emission.

We present time-resolved photoluminescence measurements performed on an ensemble of InAs quantum dots with density of 10 11 dots/cm2 and ground state transition energies centered at 1.216 eV. The wavelength of the 100fs excitation pulse was tuned through the ground (excited) state transitions, resulting in resonant (optical phonon assisted) photoluminescence (PL). The PL was detected with its polarization both parallel with and perpendicular to the excitation polarization (along one of the crystal's cleave axes). The decay of the PL was time-resolved with a streak camera in the interval 1.5 - 3ns to avoid scattered laser light. A strong polarization dependence was observed. Considerable amount of the resonant fluorescence signal and even of the non-resonant PL signatures remained linearly polarized on a nanosecond time scale. A phenomenological rate equation analysis is made.

The occupation of higher subbands in superlattices is investigated with photoluminescence spectroscopy. The carriers are injected into the higher subbands by resonant tunnelling when an electric field is applied perpendicular to the layers of the superlattice. The occupation of the second subband is larger for a subband spacing below the longitudinal optical phonon energy h(cross) omega LO than for a subband spacing larger then h(cross) omega LO. However, the intensity of the observed transition is strongly reduced for a subband spacing below h(cross) omega LO due to the strong mixing of the heavy- and light-hole subbands.

The optical emission and gain properties of Ga(AsSb) quantum-islands are investigate. These islands form during growth in a self-organized process in a series of Ga(AsSb)/GaAs/(AlGa)As heterostructures, resulting in an additional in-plane hole confinement of several hundreds of meV. The shape of the in-plane confinement potential is nearly parabolic and thus yields almost equidistant hole energy levels. Transmission electron microscopy reveals that the quantum islands are 100nm in diameter and exhibit an in-plane variation of the Sb concentration of more than 30 %. Up to seven bound hole states are observed in the photoluminescence spectra. Time-resolved photoluminescence data are shown as function of excitation density, lattice temperature, and excitation photon energy and reveal fast carrier capture into and relaxation within the quantum islands. Furthermore, the optical gain is measured using the variable stripe-length method and the advantages of such structures as active laser material are discussed.

The intervalley coupling between \ensuremath{\Gamma} and X minima of the conduction band is investigated in highly excited ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As. The efficiency of indirect electron-hole recombination as a function of hydrostatic pressure directly yields the relative strength of alloy-disorder and phonon-assisted intervalley scattering for both GaAs-like and AlAs-like LO phonons. Intervalley transfer assisted by alloy disorder is proved to be very efficient, with a strength of about 25% of the overall deformation-potential scattering. The zero-phonon processes dominate the carrier dynamics close to the direct-to-indirect crossover in the ternary compounds. They are always negligible in GaAs with an indirect gap under high hydrostatic pressure, which proves the attribution of the zero-phonon intervalley coupling in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As to alloy disorder. The properties of direct recombination in indirect-gap ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As further support the existence of efficient disorder-induced \ensuremath{\Gamma}-X transfer.

Electron and hole tunneling transfer processes in asymmetric double quantum well structures are investigated by time-resolved picosecond photoluminescence. Change from nonresonant to resonant tunneling is achieved with a perpendicular electric field. Electron transfer times become considerably faster in the resonant regime, but the transfer times still decrease strongly with barrier thickness. The resonances shift to higher bias values for increasing excitation densities. Nonresonant hole tunneling with fast transfer times occurs without the contributions of optical phonons. Resonances in the hole tunneling transfer times seem to result from mixing effects between heavy and light holes in adjacent wells.

The cooling of a hot photoexcited exciton distribution in wurtzite GaN is studied by time-resolved photoluminescence measurements on the longitudinal optical phonon replicas of the free exciton. The temperature of the excitons decreases for sample temperatures below 60 K within 150 ps down to the lattice temperature. The transient temperature is theoretically described in terms of exciton–phonon scattering via an acoustic deformation potential of 13 eV.

The injection of spin-polarized electrons is presently one of the major challenges in semiconductor spin electronics. We propose and demonstrate a most efficient spin injection using diluted magnetic semiconductors as spin aligners. Time-resolved photoluminescence with a Cd0.98Mn0.02Te/CdTe structure proves the feasibility of the spin-alignment mechanism.

A phase separation into a liquid and an exciton gas is found for the optically generated electron-hole system in indirect-gap ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As. The formation of electron-hole droplets occurs on a time scale of a few hundred picoseconds, i.e., much more rapidly than in Si or Ge. This rapid droplet formation is found in ${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As regardless of whether it is an indirect-gap semiconductor due to its alloy composition or due to application of hydrostatic pressure. A subnanosecond phase separation is, however, not observed in GaAs, which is an indirect-gap semiconductor under hydrostatic pressure. This gives strong evidence that the accelerated nucleation dynamics is caused by alloy disorder. We determine the equilibrium density and critical temperature of the liquid phase from a preliminary phase diagram to be 4.5\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$ and 34 K, respectively. The electron-hole plasma with density and temperature above the critical parameters is still bound with respect to the free exciton and shows self-confinement.

We have investigated the time-dependent photoluminescence (PL) of GaAs/${\mathrm{Al}}_{\mathit{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$As quantum-well wires with GaAs cross sections of 70\ifmmode\times\else\texttimes\fi{}14 nm prepared by deep mesa etching into a quantum-well system. The decay times of the two lowest one-dimensional heavy-hole excitonic states, ${\mathrm{hh}}_{11}$ and ${\mathrm{hh}}_{12}$, are both about 300 ps at 5 K, which is surprisingly long and comparable to the PL lifetime of 450 ps of the heavy-hole exciton in the reference quantum-well sample. In the early-time regime, a population transfer from the ${\mathrm{hh}}_{12}$ excitonic state to the ${\mathrm{hh}}_{11}$ state occurs with a transfer time constant of 100 ps, which is explained by a cooling process due to acoustical-phonon scattering. Within this time, the PL is anisotropic for the different polarizations of the exciting radiation with respect to the wire direction. For larger delay times, the excitons lose their information on the polarization in the excitation beam due to the scattering processes.

Electron tunneling through the barrier in asymmetric double-quantum-well structures is investigated by time-resolved picosecond luminescence spectroscopy. Change from nonresonant to resonant tunneling is acheived with a perpendicular electric field. Energetic alignment of electron subbands in the two wells strongly enhances tunneling transfer rates. The resonant transfer times decrease strongly with barrier thickness. The wells are coupled at resonance by energy-conserving scattering processes between states localized in a single well. The buildup of delocalized coherent states at resonance would lead to much shorter transfer times.

Non-resonant electron tunneling through a 6 nm thick Alo.35Ga0.65As barrier is investigated by time-resolved photoluminescence. Application of hydrostatic pressure at 5 K reveals that tunneling via virtual X-states is at least 800 times less efficient than via virtual Γ-states. Above 24.5 kbar an extremely fast scattering of electrons out of the n=1 quantized level of the narrower quantum well into real X-states in the barriers is observed.

We have studied the relaxation of photo-generated electron-hole pairs in CdTe using a time resolved luminescence technique. Temperature kinetics are deduced from the line shape analysis using first a free particle model and second including the carrier damping in the frame of the plasmon pole approximation. The latter model leads to significantly lower plasma temperatures which are compared to plasma cooling theory including hot phonon effects. The agreement is satisfactory for the densities ranging from 5 X 10 16 to 4 X 10 17 cm -3 .

The transition from the nonperturbative reversible emission regime of normal-mode coupling all the way to the perturbative regime of irreversible lasing emission is studied. The microcavity samples contain one or two InGaAs/GaAs quantum wells with very narrow absorption linewidths (1 meV) resulting in record normal-mode-coupling splitting-to-linewidth ratios. For zero exciton-cavity detuning, the transmission peaks and reflectance dips vanish with increased carrier density with little change in splitting. This new nonlinear behavior is observed because exciton broadening with little reduction in oscillator strength is the dominant nonlinearity at low densities for such narrow-linewidth excitons. A microscopic theory, where the effects of carrier and polarization scattering are included at the microscopic level, explains these experiments. The photoluminescence emitted perpendicular to the layers also shows curious density-dependent behavior. When the cavity mode is tuned energetically above the exciton resonance, the upper-polariton peak is weaker (stronger) than the lower-polariton peak at low (high) carrier densities. This crossover occurs at a carrier density less than a factor of two below that for lasing threshold. This behavior is in good agreement with a full quantum mechanic description for both light and carrier, where the mutual Coulomb interaction of carriers is included.

A phenomenological model is suggested to describe nonradiative recombination of optical excitations in disordered semiconductor heterostructures. The general property of disordered materials is a strong decay of the photoluminescence intensity with rising temperature. We show that this temperature dependence is a consequence of the interplay between radiative and nonradiative recombination and hopping dynamics of excitations in the manifold of localized states created by disorder potential. The dynamics is studied by kinetic Monte Carlo simulations. Experimental data on the thermal quenching of the photoluminescence intensity in GaInNAs / GaAs and GaNAsP / GaP quantum wells are presented, which are in good agreement with the theoretical results. For all disordered materials, it has been well-established that the quantum efficiency of the photoluminescence PL drastically decreases with increasing temperature. Classical theoretical approaches to description of nonradiative processes in perfect crystalline semiconductors are not applicable to disordered materials, since the behavior of photoexcited electrons and holes in the latter systems is essentially determined by disorder potential that causes spatial localization of electron states. We suggest in this Brief Report a model of nonradiative recombination in disordered materials, which takes into account not only the interplay between radiative and nonradiative processes, but also the dynamic exchange of charge carriers between localized states. We show that these dynamic effects determine the thermal quenching of the PL at low temperatures. Furthermore, we show how the temperature dependence of the PL intensity IT is related to important parameters of disordered materials, such as the localization energy scale and the relative concentration of nonradiative centers and traps. Although our model is general and it can be applied to any disordered system, we test it here on a particular case of semiconductor heterostructures containing nitrogen, which are currently intensively studied for applications in optical communications and photovoltaics. Materials containing nitrogen suffer from disorder effects, which become more pronounced with increasing of the nitrogen content. At room temperature, the majority of generated electrons and holes recombine nonradiatively. This leads to unfavorably high threshold current densities in lasers and to an unfavorably short minority carrier diffusion length in solar cells. We studied experimentally the thermal quenching of the PL in Ga0.7In0.3N0.006As0.994/ GaAs and in GaN0.04As0.80P0.16/ GaP quantum wells QWs. The PL measurements were performed under direct excitation of QWs with a Ti:sapphire laser with the wavelength of 800 nm in a continuous wave cw regime for GaNAsP / GaP QWs and using a short pulse excitation with the wavelength of 900 nm for GaInNAs / GaAs QWs. The excitation density was 14 W cm 2 in the short pulse regime. The sample temperature in the cold-finger cryostat was varied from 10 K to room temperature. For cw measurements, the PL signal was dispersed in a 1 m grating monochromator THR 1000 from Jobin-Yvon and collected by a cooled germanium detector applying the standard lock-in technique. For the short pulse measurements, the PL signal was dispersed by a 32 cm spectrometer with a spectral resolution of 2 nm and detected using a Hamamatsu S1 streak camera with a time resolution of 20 ps. Time-integrated PL intensities IT are discussed below. Experimentally obtained IT dependences are shown in Fig. 1. The prominent features of the temperature-induced quenching of the PL are the relatively weak nonexponential temperature dependence at low temperatures, succeeded by a steep drop by several orders of magnitudes and subsequent saturation of the PL. This shape of the thermal quenching of

A set of ${(\mathrm{Ga}}_{0.77}{\mathrm{In}}_{0.23})\mathrm{As}∕\mathrm{Ga}({\mathrm{N}}_{x}{\mathrm{As}}_{1\ensuremath{-}x})$ heterostructures is studied by time-resolved photoluminescence. Four samples with nitrogen concentrations from $x=0.48%$ up to $x=2.2%$ are investigated at different temperatures and with different excitation densities. The experiments suggest that the heterostructure band offset is type I for $x=0.48%$ and type II for $x=2.2%$. The situation is more complex for $x=0.72%$ and $x=1.25%$, since these samples are close to the transition from type I to type II. The experimental findings are analyzed using a detailed microscopic theory. Numerical calculations describe the measured data well. In particular, the interpretation of the experimental results concerning the band alignment is confirmed by the theoretical analysis.

Quantum interference of one- and two-photon excitation of unbiased semiconductors yields ballistic currents of carriers. The magnitudes and directions of the currents and the spin orientations of the carriers are controlled by the polarization and relative phase of the exciting femtosecond laser fields. We provide direct experimental evidence for the spin polarization of the optically injected spin currents by detecting a phase-dependent spatial shift of the circularly polarized photoluminescence in cubic ZnSe.