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The influence of an intra-cavity anti-reflection coating on the switch-on dynamics and the power input-output characteristic of a vertical-external-cavity surface-emitting laser (VECSEL) system are investigated. The experiments show a significantly higher threshold and slower switch-on dynamics for anti-reflection coated systems. These differences originate in the changes in the mode strength at the active medium due to the more or less pronounced sub-cavity. The presented simulations based on microscopic calculations are in good qualitatively agreement with the experiments. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Signatures of exciton and two-exciton states in semiconductor systems are investigated experimentally and theoretically. A set of quantum well samples with different well widths is studied using coherent excitation spectroscopy. The experimental results are analyzed using a microscopic model demonstrating the importance of biexciton correlations and disorder effects. Additional numerical investigations of absorption changes in small nanorings are employed to analyze optical transitions from two-exciton to three-exciton resonances.

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 study semiconductor quantum-well samples with different disorder strengths by coherent excitation spectroscopy with particular emphasis on disorder-mediated couplings of exciton subensembles. Our experiments show that there is no coherent coupling of excitons in different quantum islands and in a strongly inhomogeneously broadened sample. Latest results with polarization-resolved coherent excitation spectroscopy indicate a disorder-induced dephasing of biexcitons in the inhomogeneously broadened sample.

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.

Microscopic calculations including energetic disorder and Coulomb correlations up to third order in the laser field are performed. The resulting four-wave-mixing signals show polarization-dependent dephasing induced by diagonal disorder. The correct modeling of this disorder-induced dephasing requires the proper inclusion of Coulomb correlations. The theoretical results are in good qualitative agreement with measurements performed on a variety of quantum-well samples.

Lasing is demonstrated for a Ga(NAsP) multi quantumwell structure grown pseudomorphically on GaP using metal-organic vapour-phase epitaxy. For a series of temperatures ranging from 8 K to 290 K, the sample emission characteristics are determined. With increasing pump power, a spectral narrowing of the emitted light along with a blue-shift is observed as the lasing threshold is approached. Above the threshold, the emission splits up spectrally into a pattern of clearly distinguishable modes. In addition, the potential of this material concerning roomtemperature lasing is investigated by means of a variable stripe-length experiment, where a modal gain of 10 cm–1 is observed. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A new spectroscopic technique is demonstrated on three characteristically different semiconductor quantum-well samples. Coherent excitation spectroscopy reveals coherent coupling and effectively suppresses the signatures of inhomogeneous broadening and incoherent transfer between various excitonic states in quantum wells. Coherently coupled homogeneous subsystems are analyzed in an inhomogeneously broadened excitonic ensemble.

Pump-probe measurements were performed with a subwavelength spatial resolution of 355 nm and a temporal resolution of 130 fs in a multiple quantum well sample at T= 8 K. A solid immersion lens was used to increase the spatial resolution to 0.41 lambda, demonstrating that the limit of conventional microscopy was surpassed.

GaAs-based vertical cavity surface emitting laser structures (VCSEL) with strain-compensated (GaIn)As/Ga(PAs) multiple quantum well heterostructures (MQWH) active regions have been deposited using the less hazardous, liquid group-V-sources tertiarybutyl arsine (TBAs) and tertiary butyl phosphine (TBP) in metal organic vapour phase epitaxy (MOVPE). We report on growth optimization, characterization of the VCSEL structures, and ultrafast emission dynamics after femtosecond optical excitation. The improved decomposition characteristics of the alternative compounds yield a small variation in the cavity thickness of only ±0.35% across the 2″ wafer. The high quality of the VCSEL structures having a 2 λ cavity with 4 stacks of 3 (GaIn)As/Ga(PAs)-MQWH and AlAs/GaAs-Bragg mirrors is revealed by the large normal mode splitting of 10.6 meV. Excellent pulse response with a pulse width of 3.2 ps and a peak delay of 4.8 ps at 30 K is obtained after femtosecond optical excitation. The high crystalline perfection with respect to layer homogeneity, optical properties as well as ultrafast emission dynamics demonstrate the advantages of the strain-compensated material system for VCSEL structures grown by MOVPE using TBAs and TBP.

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.

A new method using the broad spectrum of a 10 fs Ti:sapphire laser is demonstrated for measuring the gain spectra of semiconductor lasers with high and quantitative accuracy. Results are shown for an edge-emitting ridge-waveguide In0.05Ga0.95As single quantum well (SQW) laser. The device is studied from the absorption regime up to the strong gain regime recording both, TE and TM polarizations. The experiments are compared to the predictions of a microscopic model based on the semiconductor Bloch equations including microscopic scattering and dephasing terms. A very good quantitative agreement is obtained.

We investigate the influence of carrier relaxation on the emission dynamics of a semiconductor microcavity laser. The structure is optically excited with energies of 1.477 down to 1.346 eV (resonant excitation). The stimulated emission dynamics clearly becomes faster for decreasing excitation energy and the influence of the light hole on the emission dynamics is demonstrated. Theoretical calculations reproduce the results only if the nonequilibrium carrier dynamics is treated on the basis of a microscopic model.

We study the dependence of the binding energies of excitonic molecules on the confinement in semiconductor quantum wells. A set of symmetrically strained (GaIn)As/Ga(PAs) quantum wells with different well depths and equal well widths is investigated with transient degenerate four-wave mixing. The ratio of biexcitonic to excitonic binding energy increases with stronger confinement. This experimental result is discussed in detail in the framework of recent theoretical predictions and experimental results.

Circularly polarized optical excitation of a semiconductor microcavity in a transverse magnetic field leads to pulsed laser emission with alternating circular polarization synchronized to the electron Larmor precession. The synchronization originates from the transfer of electron-spin coherence to the dynamics of the optical field. A microscopic theory is developed to analyze the coupled carrier--light-field dynamics.

Spin quantum-beat spectroscopy is presented as a powerful tool for the investigation of the electron Lande g factor and the spin dynamics in bulk and low-dimensional semiconductors. The technique has several advantages including high sensitivity, broad applicability, and ease of implementation and, therefore, proves to be superior to other techniques as, e.g., the Hanle-effect, spin-flip Raman scattering, and conduction band spin-resonance. We demonstrate the utility of this technique by the study of the temperature dependence of the electron Lande g factor g* in GaAs up to room temperature. We present as well results on the temperature dependence of g* in InP, the dependence of g* on quantum film thickness in GaAs-AlGaAs heterostructures, and the anisotropy of g* in quantum films and wires. A modification of this technique yields the direction of the spin rotation, i.e., the sign of g*. Finally, recent results on spin quantum beats in microcavities will be presented.

The aim of the present study was to determine the correlation of cotinine levels in amniotic fluid and in fetal and corresponding maternal blood samples. Amniotic fluid samples (N = 130) were taken during second trimester amniocentesis, umbilical artery blood samples (N = 75) at birth, both together with corresponding maternal blood samples. Self-reported smokers showed maternal serum cotinine levels > 15 ng/ml in 93%, self-reported nonsmokers levels < 15 ng/ml in 89%. Correlation of corresponding values for cotinine was 0.81 - 0.92. Cotinine values were increased in fetal blood and amniotic fluid in comparison to maternal serum levels. Despite the fact that pathophysiology is not fully understood, an accumulation of nicotine and its metabolites both in the fetus and in the amniotic fluid appears to be evident

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.

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...

Optical gain spectra of (GaIn)(NAs)/GaAs quantum-well lasers operating in the 1.3-μm-emission-wavelength regime are measured and compared to those of a commercial (GaIn)(AsP)/InP structure. Good agreement of the experimental results with computed spectra of a microscopic many-body theory is obtained. Due to the contributions of a second confined subband, a spectrally broad gain region is expected for (GaIn)(NAs)/GaAs at elevated carrier densities.

Spatially resolved photocurrent-spectroscopy and spatially resolved current-voltage characteristics are introduced as new methods to characterize solar cells. A combination of these two methods is shown to localize and characterize deficiencies and structural damages in processed solar cells with high spatial resolution. The local external and internal quantum efficiencies as well as the local characteristic parameters of the p-n junction like the short circuit current, the saturation current, the ideality factor, and the optically induced shunt resistance can be determined quantitatively. Both, a slab of a damaged and an undamaged (GaIn)(NAs) concentrator solar cell, are used as test structures. Upon these test structures domains with a high concentration of impurities in the crystal structure and structural imperfections in the upper contact region are identified and analyzed. Additional numerical simulations prove the reliability and show limits of the methods.

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.

New techniques for a solar cell characterization with high spatial resolution are introduced and evaluated both by experiments on test structures and numerical simulations. The reliability is demonstrated and technical limits are assessed.

Characterization techniques based on measuring the photocurrent of a solar cell under local illumination are introduced. Experiments on test structures and numerical simulations proof the applicability and reliability under realistic conditions and show technical limits.

We find laser emission of a (GaIn)(NAs)/GaAs VCSEL with picosecond dynamics from 30 up to 388 K. The measured band-gap shift of (GaIn)(NAs) is comparatively small. Theoretically predicted gain spectra at high densities are broad. We conclude that the combination of both effects leads to the VCSEL operation in such a broad temperature regime.

We measure the normal-mode linewidths in a semiconductor microcavity with various exciton–photon interaction strengths. Variation of the normal mode coupling and thus of the exciton–photon interaction is obtained reducing the cavity quality by stepwise removing of top mirror pairs. Excellent agreement of the measured linewidths with results of a linear dispersion theory is obtained.

Ga(NAsP) multi quantum well heterostructures were grown pseudomorphically on exactly oriented (001) Silicon substrates without the formation of misfit dislocations. Optical pumped lasing operation was observed at temperatures up to 125 K.

Nowadays Silicon Photonics gains more and more interest especially for the future architecture of Si based optoelectronic integrated circuits (OEI C). OEIC building blocks such a s modulators, detectors as well as light wave guides have been demonstrated in a down-scalable process technology. However, a commercial solution for t he monolithic integration of long-term stable laser diodes has not been achieved yet, which is, however, the key device component to full y profit from Silicon Photonics.

We report lasing of optically pumped Ga(NAsP)/(BGa)(AsP) heterostructures grown lattice-matched on Si. Modal gain of up to 80cm−1 is determined at 300K and a distinct threshold behaviour and mode spectrum is observed up to 100K.

In 50 cases with an end-diastolic zero flow or reverse flow all antenatal and perinatal abnormalities have been recorded. The fetal outcome was registered. The percentage of highly dystrophic newborns (percentile less than 5) was 88%. The perinatal mortality counted up to 16% and the percentage of congenital malformations (including chromosomal anomalies) was 12%. A reverse flow was registered in 4 cases. The perinatal mortality of those cases with reverse flow was 100%. In approx. one-quarter of those pregnancies and in 50% of the perinatally deceased newborns, there were no pathological or suspicious changes in the antepartal and/or subpartal CTG-recordings. The Duplex sonographical diagnosis of an end-diastolic zero flow/reverse flow, has a highly positive predictive value, whereas its sensitivity is low. It can be regarded as a very helpful parameter in clinical diagnosis, particularly as it is independent of borderline values. The correct choice of the high-pass wall filter (50-100 Hz) is important.

We demonstrate a new concept to identify the population of 1s excitons in semiconductors in the time between optical excitation and recombination. The experiment determines the overlap of electrons and holes by tracing the electron–hole exchange energy. A dominant population of excitons is only present under stringent experimental conditions.

The variation of the normal-mode coupling in a semiconductor microcavity is demonstrated by reducing the cavity quality through stepwise removing top mirror pairs. The dependence of the measured normal-mode coupling linewidths on the cavity quality is well reproduced by calculations on the basis of a linear dispersion theory with broadened excitons in a microcavity.

(GaIn)(NAs) vertical-cavity surface-emitting lasers for room-temperature emission at 1.3 μm wavelength are designed and grown by metal-organic vapor-phase epitaxy using dimethylhydrazine and tertiarybutylarsine. Room-temperature operation at wavelengths up to 1.285 μm is achieved with low optical pumping thresholds between 1.6 and 2.0 kW/cm2. Stimulated emission dynamics after femtosecond optical pumping are measured and compare favorably with results on (GaIn)As/Ga(PAs)-based structures.

Pump–probe measurements with a subwavelength spatial resolution of 355 nm and a temporal resolution of 130 fs have been performed in a multiple quantum well sample at T=8 K. A solid immersion lens was used to increase the spatial resolution to 0.41⋅λ and the effective numerical aperture to 1.23, demonstrating that the limit of conventional microscopy has been surpassed. The lateral carrier transport was investigated on subpicosecond timescales.

We present a vertical-cavity surface-emitting laser structure optimized for fast intrinsic emission dynamics, using the strain-compensated (GaIn)As/Ga(PAs) material system with a 2λ sin-type cavity. The high quality of the epitaxial growth is revealed by the large normal mode splitting of 10.5 meV found in reflectivity measurements. The fast dynamical response of our structure after femtosecond optical excitation at 30 K yields a pulse width of 3.2 ps and a peak delay of only 4.8 ps. A structure designed for laser emission at higher temperatures exhibits picosecond dynamics at room temperature.

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.

The focus of the present study is the optical characterisation of the novel direct band gap material Ga(NAsP) monolithically grown on Si substrate. Ga(NAsP)/(BGa)(AsP) multi-quantum-well heterostructures (MQWHs) were deposited by low temperature metal organic vapour phase epitaxy (MOVPE) in a commercial reactor system on nominally exact (001) Si substrates. In order to realise III/V laser diode the MQWH layer-stack was embedded in 1mum thick (BGa)P separate confinement structure (SCH) layers. Structure investigation proved that a precise strain-management allows for the deposition of thick III/V devices without the formation of cracks or misfit dislocations. First gain measurements at room temperature using the stripe-length method show a modal gain of more than 20cm-1 of the active Ga(NAsP) material monolithically integrated on Si substrates, which was discussed in detail.

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.

The nonlinear propagation of picosecond pulses in CdTe is investigated. First, the band‐gap dynamics is studied in the presence of a dense electron‐hole plasma using time‐resolved luminescence in the picosecond regime. Good agreement is found between experimental results for band‐gap shrinkage and theory for densities of 5×1016–4×1017 cm−3. Second, the nonlinear transmission of optical pulses through CdTe/CdZn0.04Te0.96 heterostructures is investigated for photon energies close to the unexcited band gap. We demonstrate that, depending on the energy of the incident pulse, induced transmission or induced absorption takes place: Self‐induced transmission is caused by band filling at high intensity E of the incident pulses (E≂100–200 μJ/cm2). Self‐induced absorption is observed at relatively low incident intensity (E≂15–20 μJ/cm2). This effect is related to the band‐gap shrinkage of the CdTe epilayer in the presence of a nondegenerate electron‐hole plasma. A transmission theory of picosecond pulses propagatin...

A simple new pump geometry for optical excitation of microcavities and vertical-cavity surface-emitting lasers is presented. The technique circumvents the high reflectivity of the cavity stop band by excitation through the substrate at a large angle of incidence. Under these conditions, the reflectivity of the bottom Bragg reflector is small, and optical pumping at any desired photon energy becomes possible. Experimental results for optical excitation with this new geometry are compared with resonant optical pumping through the cavity mode.

The ultrafast dynamics of optical excitations in semiconductors and semiconductor nanostructures can be computed on a microscopic theoretical basis using the semiconductor Bloch equations. This set of equations allows one to nonperturbatively evaluate light-field-induced intraband and interband excitations and is also well suited for the analysis of many-body effects such as excitonic resonances and carrier-carrier and carrier-phonon scattering processes. In this article we start with a description of an experimental observation of coherent spin photocurrents, then we briefly introduce the theoretical approach and review some results of our theory concerning the microscopic description of the ultrafast coherent optical generation and the temporal decay of charge and spin currents in semiconductor nanostructures. The computed transients show an enhanced damping of the spin current relative to the charge current as a consequence of Coulomb scattering between carriers with opposite spin. The influence of quantum kinetic memory effects on the dynamics of the photocurrents is analyzed by evaluating the scattering terms beyond the Markov approximation.

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.

Spin electronics in semiconductors is a fascinating new field of research with high potential towards new kinds of semiconductor devices. We pick spin optoelectronics as examplary subfield and discuss from this point of view potentials, recent advances, and problems. We show spin optoelectronic devices, critically analyse spin injection and spin transport, show results concerning GaMnAs, and present data about spin dephasing in semiconductors.

Transient gain measurements are performed for (Galn)As quantum well structures. Gain up to 2000 cm-1 on a timescale of several hundred ps is observed. A microscopic model quantitatively provides theoretical support without introducing fit parameters.

The gain spectrum of a (GaIn)As/(AlGa)As single-quantum-well laser diode is precisely measured at various currents in order to quantitatively check the predictions of a microscopic model. The theory includes carrier—carrier and carrier—LO-phonon collisions which lead to optical dephasing and screening of the Coulomb interaction. The measurements are based on a transmission technique using the broad spectrum of a 10 fs Ti:sapphire laser to obtain sufficient signal to noise ratio over a wide spectral range. We obtain excellent agreement between theoretical and experimental gain spectra and thus can clearly demonstrate the predictive capability of our microscopic model.

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

We significantly improve the dynamics of a vertical-cavity surface-emitting laser by using chirped rather than bandwidth-limited optical excitation. The optimal excitation pulse is found by a genetic algorithm in a learning loop.