V. P. Kochereshko

• Dr. Sci.
• Principal Investigator at Ioffe Institute

Time-resolved and time-integrated circularly polarized photoluminescence of excitons and trions in external magnetic fields up to 10 T has been studied in undoped and n-type doped quantum well structures based on ZnSe. In an undoped structure, a circular polarization of photoluminescence induced by magnetic fields corresponded to the Boltzmann dist...

In this paper, polarized photoluminescence caused by exciton quasi-equilibrium spin orientation on Zeeman sublevels in an ensemble of quantum dots of different sizes is theoretically studied. It is found that: (i) the splitting of the photoluminescence bands in a magnetic field in an ensemble of quantum dots is several orders of magnitude larger th...

Photoluminescence and polarized reflection spectra of quantum well structures with symmetric Cd0.9Zn0.1Te/CdTe/Cd0.9Zn0.1Te and asymmetric Cd0.9Zn0.1Te/CdTe/Cd0.4Mg0.6Te barriers were studied. The Stokes parameters of the reflected light from these structures were measured. In structures with asymmetric barriers, in the region of exciton resonances...

The spectra of photoluminescence and polarized reflection under normal and oblique incidence of light from structures with quantum wells with symmetric Cd 0.9 Zn 0.1 Te/CdTe/Cd 0.9 Zn 0.1 Te and asymmetric Cd 0.9 Zn 0.1 Te/CdTe/Cd 0.4 Mg 0.6 Te barriers have been studied. Due to the mechanical stresses caused by the mismatch of the crystal lattices...

Unusual optical anisotropy was experimentally observed in hexagonal boron nitride thin films produced from bulk boron nitride via ultrasonication. Both the linear and circular polarisation demonstrated a well-defined single axis of anisotropy over a large sample area. To understand this phenomenon, we employed statistical analysis of optical micros...

Photoluminescence and polarized reflection spectra of quantum well structures with symmetric Cd$_{0.9}$Zn$_{0.1}$Te/CdTe/Cd$_{0.9}$Zn$_{0.1}$Te and asymmetric Cd$_{0.9}$Zn$_{0.1}$Te/CdTe/Cd$_{0.4}$Mg$_{0.6}$Te barriers were studied. The Stokes parameters of the reflected light from these structures were measured. In the structures with symmetric ba...

Few-layer flakes of hexagonal boron nitride were prepared by ultrasonication of bulk crystals and agglomerated to form thin films. The transmission and reflection spectra of the thin films were measured. The spectral dependences of the linear and circular polarization revealed a hidden anisotropy of the films over the whole sample area which could...

Polariton photoluminescence from cylindrical microcavities was studied under optical excitation below and above a nonlinear regime threshold. Lines of lateral excitonic polaritons quantization were observed in the spectra. In addition to quantization lines with a small orbital quantum number m, exciton-polariton whispering gallery modes with large...

Heterostructures with atomically thin double quantum wells based on CdTe/ZnTe are investigated by optical spectroscopy (photoluminescence and reflectivity methods) as a function of temperature and density of excitation. Heavy and light exciton luminescence lines are observed with comparable intensities and different temperature behaviors (they cros...

The dependences of the exciton-polariton photoluminescence spectra on the intensity of optical excitation and external magnetic fields are investigated in cylindrical mesas based on high-quality microcavities. It was found that with an increase of the intensity of optical excitation above a certain threshold at sufficiently large magnetic field or...

Bulk inversion asymmetry (BIA) of III-V and II-VI semiconductor quantum wells is demonstrated by reflection experiments in magnetic field oriented in the structure plane. The linear in the magnetic field contribution to the reflection coefficients is measured at oblique incidence of s and p polarized light in the vicinity of exciton resonances. We...

Photoluminescence spectra of an isolated GaAs quantum dot within an AlGaAs quantum wire are studied. The examination of behavior of spectra in a magnetic field provided an opportunity to estimate the exciton binding energy in the quantum dot, which turned out to be 10 times higher than the bulk exciton binding energy in GaAs. It is demonstrated tha...

Reflectance and photoluminescence spectra of the ZnO/Zn0.78Mg0.22O structures with ZnO quantum wells and thick ZnO and Zn0.78Mg0.22O layers have been thoroughly investigated at different temperatures and excitation intensities and wavelengths. All the observed spectral lines have been identified. It has been established that the built-in electric f...

Bulk inversion asymmetry (BIA) of III-V and II-VI semiconductor quantum wells is demonstrated by reflection experiments in magnetic field oriented in the structure plane. The linear in the magnetic field contribution to the reflection coefficients is measured at oblique incidence of $s$ and $p$ polarized light in vicinity of exciton resonances. We...

Polarization conversion of light reflected from quantum wells governed by both magnetic field and light propagation direction is observed. We demonstrate that the polarization conversion is caused by the magnetospatial dispersion in quantum wells which manifests itself in the reflection coefficient contribution bilinear in the in-plane components o...

The reflection and photoluminescence excitation spectra under circular polarization of the incident light are measured in wide quantum wells whose width is much larger than the exciton Bohr radius. It is found that, in the absence of a magnetic field, the relation between the intensities of even and odd absorption peaks is reversed upon reversal of...

Polarization conversion of light reflected from quantum wells governed by both magnetic field and light propagation direction is observed. We demonstrate that the polarization conversion is caused by the magneto-spatial dispersion in quantum wells which manifests itself in the reflection coefficient contribution bilinear in the in-plane components...

The optical activity of crystals leads to the occurrence of ellipticity at the reflection of light which is linearly polarized in the interface plane or normally to it. The effect experimentally discovered in the investigated structures with quantum wells is due to a spin-orbit interaction and birefringence. The qualitative dependence of the effect...

The photoluminescence spectra of individual quantum dots incorporated into a quantum wire are studied. From the behavior of the spectra in a magnetic field, it is possible to estimate the exciton binding energy in a quantum dot incorporated into a quantum wire. It is found that the exciton photoluminescence signal emitted from a quantum dot along t...

The photoluminescence spectra of exciton polaritons in microcavities under conditions of three-dimensional quantization are studied as a factor of the density of the optical excitation and magnetic field. The behavior of the degree of circular polarization of the exciton luminescence in a magnetic field shows that, when the concentration of exciton...

We report on the observation of optical activity of quantum wells resulting in the conversion of the light polarization state controlled by the light propagation direction. The polarization conversion is detected in reflection measurements. We show that a pure $s$-polarized light incident on a quantum well is reflected as an elliptically polarized...

We report on the observation of optical activity of quantum wells resulting in the conversion of the light polarization state controlled by the light propagation direction. The polarization conversion is detected in reflection measurements. We show that a pure $s$-polarized light incident on a quantum well is reflected as an elliptically polarized...

Light amplification by stimulated emission of radiation, well-known for revolutionising photonic science, has been realised primarily in fermionic systems including widely applied diode lasers. The prerequisite for fermionic lasing is the inversion of electronic population, which governs the lasing threshold. More recently, bosonic lasers have also...

A new magnetooptical phenomenon referred to as the parity effect is predicted and observed. This effect takes the form of a redistribution of the exciton oscillator strength between spatially even and odd states of the exciton’s center of mass quantization in a wide quantum well.

In this paper we predicted and observed a new magneto-optical effect – the “effect of parity”. The effect is manifest as a redistribution of the oscillator strengths between spatially even and odd center of mass quantized states of the exciton in the quantum well. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on a study of the photoluminescence spectra taken from quasi one-dimensional and quasi zero-dimensional semiconductor heterostructures. The structures were grown by molecular-beam epitaxy in (111) direction and were cylindrical nanowires based on AlGaAs, of 20 - 50 nm in diameter and 0.5 - 1 μm in length. Inside the nanowires contain one...

The influence of free carriers on the ground state of the exciton at zero magnetic field in a quasi-two-dimensional quantum well doped with electrons is considered in the framework of the random phase approximation. The effects of the exciton-charge-density interaction and the inelastic scattering processes due to the Hartree-Fock electron-electron...

The dependence of the spectra of the polarized photoluminescence of excitons in microcavities under conditions of three-dimensional quantization on the optical-excitation intensity is investigated. The cascade relaxation of polaritons between quantized states of a polariton Bose condensate is observed.

The magnetic field effect on phase transitions in electrically neutral bosonic systems is much less studied than those in fermionic systems, such as superconducting or ferromagnetic phase transitions. Nevertheless, composite bosons are strongly sensitive to magnetic fields: both their internal structure and motion as whole particles may be affected...

The photoluminescence (PL) spectra of semiconductor structures, namely, Al0.3Ga0.7As-based quasi-one-dimensional cylindrical nanowires (nanowhiskers), are measured. The diameter of a typical nanowire is 20–50 nm, and its length was 0.5–1.0 μm. Samples containing one or several GaAs-based quantum dots at the center of the quantum wire are studied. T...

The reflection and photoluminescence spectra of modulation-doped CdTe/CdMgTe quantum-well structures have been studied. It was found that the magnitude and sign of the Zeeman splitting of the trion reflection line depend on the electron concentration in the quantum well, whereas the magnitude and sign of the splitting of the exciton line are absolu...

The results of a study into the photoluminescence spectra of a set of quantum dots based on GaAs enclosed in AlGaAs nanowires are presented. The steady state and time resolved spectra of photoluminescence under optical excitation both from an array of quantum wires/dots and a single quantum wire/dot have been measured. In the photoluminescence spec...

Technology for the fabrication of a homogeneous array of pores in an A3B5 semiconductor matrix has been developed. Characterization with Scanning Electron Microscope and optical characterization of these samples have been carried out. Refractive indexes for the light polarized along and perpendicular to the pores have been measured. We have found t...

Existence of intrinsic carrier interface states in heterostructures with no common atom at the interface (such as ZnSe/BeTe) is evidenced experimentally by ellipsometry and photoluminescence spectroscopy. These states are located on interfaces and lie inside the effective band gap of the structure; they are characterized by a high density and high...

The effects of charge-density fluctuations on negative trion states in a quantum well are considered for the case of intermediate doping by electrons at zero magnetic field. The quasi-two-dimensionality of the system and the trion-plasmon interaction are taken into account in the framework of the random-phase approximation. It is shown that the tri...

The magnetic properties of heavy-hole excitons in wide quantum wells of ZnTe with ZnxMg1-xTe barriers have been studied with photoluminescence and reflectivity measurements. The exciton magnetic moments (as characterized by the g values) and the diamagnetic shifts of the exciton transitions are found to depend strongly on the wave-vector component...

We report the photoluminescence characterization of GaAs quantum dots embedded in AlGaAs nanowires. Time-integrated and time-resolved photoluminescence was measured for both arrays and single quantum dot/nanowires. The optical spectroscopy data show the influence of growth temperature on the distribution of diameters and the presence of different c...

In this paper we demonstrate that the magnetic moment of the exciton increases due to its motion along the magnetic field, and the diamagnetic shift decreases. These effects were observed for a set of quantum well structures with different well widths and based on a variety of semiconductor compounds. The reason for these effects is mixing of the e...

We have shown recently that the magnetic properties of excitons change significantly as the excitons acquire kinetic energy. In particular, the exciton magnetic moments are enhanced considerably, whilst the diamagnetism decreases. The behaviour can be investigated through spectroscopic studies of excitons confined in quantum wells of large width (g...

Exciton-polariton reflectivity of wide CdTe/CdZnTe quantum-wells (with well widths exceeding by far the exciton Bohr radius) was measured in the Voigt geometry in a magnetic field. In a magnetic field, forbidden exciton lines were observed to appear for each exciton quantization level. The exciton diamagnetic shift constant was observed to decrease...

The effects of charge‐density fluctuations on trion states and spin‐density fluctuation on the exciton states in quantum well are considered in the limit of low doping by electrons. In both cases the quasi‐two dimensionality of the systems is taken into account. The trion‐chargedensity interaction is considered in the random phase approximation. Th...

Reflectivity and photoluminescence spectra from CdTe/CdMgTe modulation doped quantum well structures were studied. We have found that the value and the sign of the trion reflectivity line's Zeeman splitting depends on the electron concentration in the quantum well, whereas the value and sign of the exciton line splitting are absolutely equal for al...

Reflectivity and photoluminescence spectra from CdTe/CdMgTe modulation-doped quantum well structures were studied. We have found that in reflectivity spectra the value and the sign of the Zeeman splitting of the trion lines depend on the electron concentration in the quantum well, whereas, the value and sign of the exciton line splitting are consta...

Recent studies of excitons in wide quantum wells have shown that the magnetic properties are strongly affected as the excitons acquire kinetic energy. In the center of mass approximation, these motion-induced changes are ascribed to mixing between the 1S exciton ground state and the higher lying nP states. The origin of the mixing is due to the dis...

This paper reports on a study of the exciton polariton region in reflection spectra of wide CdTe/CdZnTe quantum wells (with well width exceeding by far the exciton Bohr radius) measured in Voigt geometry in a magnetic field. In a magnetic field, forbidden exciton lines were observed to appear for each exciton quantization level. The exciton diamagn...

Anisotropy of the relaxation of electron spin in AlGaAs quantum wells with an asymmetric profile in the growth-axis direction is studied by the technique of optical orientation. It is demonstrated that the difference between the Hanle curves for [110] and [1$ \bar 1 $ \bar 1 0] orientations does originate from the spin-relaxation anisotropy rather...

Photoluminescence spectra taken from modulationdoped CdTe/(Cd0.7Mg0.3)Te quantum well structures with a two-dimensional electron gas of moderate to high density (from ne ∼ 1010 up to ne ∼ 1012 cm–2) in magnetic fields from 0 T to 45 T were studied. The observed spectra were analyzed in terms of combined excitonelectron processes. (© 2009 WILEY-VCH...

We report remarkable changes in the magnetic properties of excitons as they acquire kinetic energy in wide (001) quantum wells of CdTe, ZnSe, ZnTe and GaAs. In particular, the magnetic moments (g-values) when magnetic fields are applied in the growth direction are shown to. have a strong dependence on the translational wavevector K-z. The dependenc...

This paper overviews our recent results on the magneto-optics of excitons in wide quantum wells, when the well width is much larger than the exciton Bohr radius. We have demonstrated that the magnetic moment of the exciton increases by an order of magnitude due to its motion along the magnetic field. The effect was observed for a set of quantum wel...

Very large changes in the Zeeman splittings and in the diamagnetism of excitons as they acquire kinetic energy in wide quantum wells of CdTe are reported. The changes are found to be functions of the translational wave vector Kz of the exciton in the growth direction of the well, irrespective of the width of the well, and are also found to be stron...

Modulation-doped CdTe/Cd0.7Mg0.3Te quantum well structures with a two dimensional electron gas of low and moderate density (from ne = 3 × 1010 cm–2 up to ne ≈ 1012 cm–2) have been studied. Photoluminescence spectra taken from the quantum well in magnetic fields from 0 T to 45 T at different electron densities were studied. The observed spectra were...

We have observed remarkable changes in the magnetic properties of excitons as they acquire kinetic energy. In particular, the Zeeman splittings and diamagnetic shifts of excitonic transitions when magnetic fields are applied along the growth direction of (001) wide quantum wells of CdTe, ZnSe, ZnTe and GaAs are found to to have a strong dependence...

Photoluminescence spectra of strained structures Zn1 − x Mn x Te/Zn1 − y Mg y Te with magnetic quantum wells and nonmagnetic barriers are studied. The Zeeman splitting of the heavy exciton is found to follow an unusual behavior: both spin components shift down in energy. The heavy-exciton photoluminescence Zeeman components are observed to be inver...

The temperature-induced variations in the photoluminescence spectra measured in a magnetic field of trions and excitons in CdTe/CdMgTe quantum wells with modulated doping are studied. It is found that the temperature-induced redistribution of the intensity between the exciton and trion emission lines in a magnetic field is opposite to that expected...

The phenomenon in which giant enhancement of exciton magnetic moments occurs due to translational motion was found for light hole excitons in ZnTe/ZnMgTe quantum well structures. Decreasing diamagnetic shifts as the number of the exciton quantized state increases were found for the first time.

The paper is an overview of recent studies of the optical anisotropy of heterostructures with quantum wells, quantum wires, and quantum dots based on cubic semiconductors. This subject area is essentially an extension of the research conducted by professor A.A. Kaplyanskii in the optical anisotropy of cubic crystals.

A new physical phenomenon has been found, namely a giant enhancement of the exciton magnetic moment due to its motion. This phenomenon was observed in GaAs, CdTe and ZnSe based quantum wells with QW widths much larger than the exciton Bohr radius. Consequently, it is relevant to any crystal with zinc blende structure.

Reflectivity and transmission spectra from wide GaAs and CdTe quantum wells (L=50–1000 nm) were studied in magnetic fields in Faraday geometry. The exciton Zeeman splitting was found to increase with the index N of the exciton quantization level. Since N is related to the magnitude of the exciton wave vector K, the observed effect means there is a...

Modifications of photoluminescence (PL) spectra taken from modulation doped CdTe/CdMgTe quantum well (QW) structures have been studied in magnetic fields up to 45T as a function of 2D electron concentration.

Spin relaxation times of two-dimensional electrons in asymmetrical (001) AlGaAs quantum wells are measured by means of Hanle effect. Three different spin relaxation times for spins oriented along [110], [110] and [001] crystallographic directions are extracted demonstrating anisotropy of the D'yakonov-Perel' spin relaxation mechanism. The relative...

We study the temperature-dependent modifications of trion and exciton photoluminescence (PL) spectra in modulation-doped CdTe/CdMgTe quantum wells in high magnetic field. We find that, in magnetic field, the temperature-dependent redistribution of exciton and trion PL intensities is opposite to that expected from a simple Boltzmann distribution mod...

Photoluminescence (PL) spectra of modulation-doped CdTe/CdMgTe quantum-well structures containing two dimensional electron gases with electron concentrations around 1010 cm–2 have been recorded under a high magnetic field up to 45 T. Radiative recombination lines of dark and bright triplet trion states were found in the spectra at magnetic field st...

The observation of a new physical phenomenon, a giant increase in the longitudinal magnetic moment of an exciton in motion, is reported. The effect is observed in the wide GaAs-, CdTe-, and ZnSe-based quantum wells, with a width much larger than the exciton Bohr radius, and hence relates to any crystals with a zinc-blend structure.

We report remarkable changes in the magnetic properties of excitons as they acquire kinetic energy in wide (001) quantum wells of CdTe, ZnSe, ZnTe and GaAs. In particular, the magnetic moments (g-values) when magnetic fields are applied in the growth direction are shown to. have a strong dependence on the translational wavevector K-z. The dependenc...

We report a remarkable enhancement of the magnetic moments of excitons as a result of their motion. This surprising result, which we have observed in magneto-optical studies of three distinct zinc-blende semiconductors, GaAs, CdTe, and ZnSe, becomes significant as the kinetic energy of the exciton becomes comparable with its Rydberg energy and is a...

The optical reflection spectra of semiconductor GaAs/AlGaAs structures with wide quantum wells are studied experimentally. A theoretical analysis of the spectra is performed in terms of the exciton-polariton model in the approximation of quantum confinement of the exciton center of mass with regard to the contributions of both heavy and light excit...

Modifications of photoluminescence (PL) spectra taken from modulation doped CdTe/CdMgTe quantum well (QW) structures have been studied in magnetic fields up to 45T as a function of 2D electron concentration.

We present an experimental data, which demonstrate a basically new mechanism of carrier radiative recombination in semiconductor heterostructures-recombination via Tamm-like interface states. Bright line was observed in photoluminescence spectra of periodical ZnSe/BeTe heterostructures at the energies, which correspond to the optical transitions be...

Spin-relaxation times of two-dimensional electrons in asymmetrical (001) AlGaAs quantum wells are measured by means of Hanle effect. Three different spin-relaxation times for spins oriented along [110], [11¯0] , and [001] crystallographic directions are extracted demonstrating anisotropy of the D'yakonov-Perel' spin relaxation mechanism. The relati...

Spin relaxation of two-dimensional electrons in asymmetrical (001) AlGaAs quantum wells are measured by means of Hanle effect. Three different spin relaxation times for spins oriented along [110], [1-10] and [001] crystallographic directions are extracted demonstrating anisotropy of D'yakonov-Perel' spin relaxation mechanism. The relative strengths...

Reflectivity spectra from GaAs and CdTe based wide quantum wells (widths L = 50–1000 nm) have been studied in magnetic fields in Faraday geometry. The magnitude of the exciton Zeeman splitting has been found to grow with increase of the quantization index N for the exciton energy levels. Since N is related to the magnitude of the exciton wave vecto...

The spectral response of the lateral optical anisotropy of periodic undoped type-II ZnSe/BeTe heterostructures with nonequivalent interfaces was studied by ellipsometry. The spectra revealed two types of features corresponding to optical transitions with energies lying in the bandgap. The position of features of the first type does not depend on th...

Photoluminescence (PL) and reflectivity spectra from modulation-doped CdTe∕CdxMg1−xTe quantum well structures containing a two-dimensional electron gas have been studied in magnetic fields up to 45 T. In high fields, the recombination from the dark triplet trion state was found to be one of the most intense PL lines. A “bright” triplet line is reve...

Spectral response of lateral optical anisotropy of periodic undoped type-II ZnSe/BeTe heterostructures with nonequivalent interfaces has been studied by spectroscopic ellipsometry. The spectra revealed two types of features corresponding to optical transitions with energies lying in the bandgap. The position of features of the first type does not d...

Photoluminescence (PL) spectra of modulation‐doped CdTe/CdMgTe quantum well structures containing two dimensional electron gases different electron concentrations have been studied in magnetic fields up to 45T. Recombination line of triplet trion state was found in the spectra. A model calculation of PL spectra in magnetic fields, which takes into...

Modification of photoluminescence spectra taken from modulation doped CdTe/CdMgTe QW structures have been studied in magnetic fields up to 45 T at high concentrations of 2D electrons. The following peculiarities were found in relatively low magnetic fields when filling factors ν was higher than 2: (i) linear energy shift of the photoluminescence (P...

Photoluminescence spectra of modulation-doped CdTe/CdMgTe quantum well structures containing two-dimensional electron gases of low, moderate and high electron concentrations were studied in high magnetic fields up to 45 T. The recombination line of triplet trion state was found in the spectra. A model calculation of photoluminescence spectra in mag...

The in-plane anisotropy of the refractive index and absorption coefficient in the blue-green spectral region in ZnSe/BeTe type-II heterostructures with no common atoms at the interface has been studied by ellipsometry in reflection. It was established that the relative difference (anisotropy) in the refractive index and absorption coefficient remai...

Time-resolved photoluminescence (PL) spectra of self-assembled CdSe/ZnSe quantum dots (QDs) are measured with a view to identifying the QD-size dependence of carrier capture and recombination in a single QD. The PL is excited by optical absorption in the ZnSe barrier layers under weak and strong irradiation with femto-and nanosecond laser pulses, r...

Time-resolved and time-integrated circularly-polarized photoluminescence of excitons and trions have been studied in external magnetic fields up to 10 T. ZnSe-based quantum well structures of n-type with carrier densities varied from 5×109 to 1011cm-2 were used in this study. Absence of the chemical equilibrium in the exciton–trion system has been...

Combined exciton–electron processes were found and studied in reflectivity and photoluminescence spectra taken from modulation-doped quantum well structures with different 2DEG densities. At low magnetic fields a Stokes shift between reflectivity and PL exciton and trion peaks due to the renormalization of the exciton and trion self-energy have bee...

In modulation-doped quantum well structures with electron concentration of about 10(11) cm(-2), a new resonant optical transition involving four particles (one hole and three electrons) was found in external magnetic fields in photoluminescence excitation and reflectivity spectra. This transition reveals as a narrow line between exciton and trion l...

Excitons and charged excitons (trions) are investigated in ZnSe-based quantum well structures with (Zn,Be,Mg)Se and (Zn,Mg)(S,Se) barriers by means of magneto-optical spectroscopy. Binding energies of negatively (X−) and positively (X+) charged excitons are measured as functions of quantum well width, free carrier density and in external magnetic f...

The formation of two-dimensional negatively charged excitons (negative trions) out of excitons and free electrons is found to be determined by a dynamical equilibrium. This dynamical equilibrium consists of a chemical equilibrium, relating the trion, exciton, and electron populations, modified by finite formation and recombination times of (charged...

Circularly and linearly polarized radiation due to spatially indirect optical transitions is studied in semimagnetic (Zn,Mn)Se/BeTe and nonmagnetic ZnSe/BeTe quantum-well structures with a type-II band alignment. Because of the giant in-plane anisotropy of the optical matrix elements related to a particular interface, complete spin orientation of p...

Time-resolved inhomogeneously broadened spectra of CdSe/ZnSe self-assembled QDs excited by weak picosecond and powerful nanosecond laser pulses have been measured. The peculiarities of the kinetics of the CdSe/ZnSe self-assembled QDs' spectra and their changes at high excitation have been explained by different recombination times of QDs (it shorte...