Search

Purcell effect enhancement of spontaneous emission rates is demonstrated for isoelectronic trap single-photon emitters. Two-dimensional photonic crystal slabs with L3 defects were fabricated in nit...

Linear in the wave-vector terms of an electron Hamiltonian play an important role in topological insulators and spintronic devices. Here we demonstrate how an external electric field controls the magnitude of a linear-in-K term in the exciton Hamiltonian in wide GaAs quantum wells. The dependence of this term on the applied field in a high quality sample was studied by means of the differential reflection spectroscopy. An excellent agreement between the experimental data and the results of calculations using semi-classical non-local dielectric response model confirms the validity of the method and paves the way for the realisation of excitonic Datta-and-Das transistors. In full analogy with the spin-orbit transistor proposed by Datta and Das [Appl. Phys. Lett. {\bf 56}, 665 (1990)], the switch between positive and negative interference of exciton polaritons propagating forward and backward in a GaAs film is achieved by application of an electric field with non-zero component in the plane of the quantum well layer., Comment: 30 pages, 6 figures

Highly luminescent Mn doped perovskite nanocrystals (NCs) are promising in solid state lighting. The thermal stability of Mn2+ luminescence in Mn: CsPbCl3 NC/mesoporous silica (MPS) composites was studied at temperature ranging from 80 K to 450 K by steady-state and time-resolved photoluminescence (PL) spectroscopy. The Mn: CsPbCl3 NCs with various Mn doping concentrations were synthesized by changing the Mn/Pb molar ratios at 190 °C. It was found that the Mn doped NC films showed a more significant decrease in PL intensities and lifetimes at high temperature above 320 K than the Mn doped NC/MPS films, indicating that the Mn2+ luminescence in doped NC/MPS films exhibited better thermal stability than that of NC films. Further the broadening and shifts of Mn2+ emission bands in doped NC and NC/MPS films were observed at elevated temperature. The temperature-dependent linewidths of Mn2+ emission bands were discussed by electron-phonon coupling.

Manganese ion (Mn

The ultrafast carrier dynamics and hot electron extraction in tetrapod-shaped CdSe nanocrystals was studied by femtosecond transient absorption (TA) spectroscopy. The carriers relaxation process from the higher electronic states (CB2, CB3(2), and CB4) to the lowest electronic state (CB1) was demonstrated to have a time constant of 1.04 ps, resulting from the spatial electron transfer from arms to a core. The lowest electronic state in the central core exhibited a long decay time of 5.07 ns in agreement with the reported theoretical calculation. The state filling mechanism and Coulomb blockade effect in the CdSe tetrapod were clearly observed in the pump-fluence-dependent transient absorption spectra. Hot electrons were transferred from arm states into the electron acceptor molecules before relaxation into core states.

Photocarrier recombination dynamics in ternary chalcogenide CuInS2 quantum dots (CIS QDs) was studied by means of femtosecond transient-absorption (TA) and nanosecond time-resolved photoluminescence (PL) spectroscopy. Under strong excitation, the TA dynamics in CIS QDs is well described by a simple rate equation including single-carrier trapping, free-to-bound recombination, and trap-assisted Auger recombination. Under weak excitation, on the other hand, the PL decays of the QDs are composed of a short-lived component caused by surface trapping and a long-lived one caused by free-to-bound recombination. It is found that the surface trapping accelerates markedly with decreasing QD size while the free-to-bound radiative recombination hardly depends on the QD size. Besides this, we observed both a decrease in the PL lifetimes and a dynamic spectral redshift, which are attributed to the surface trapping and the coexistent inhomogeneous broadening in CIS QDs. The spectral redshift becomes less pronounced in CIS/ZnS core/shell QDs because of the suppression of the fast nonradiative recombination caused by the passivation of the surface traps. These results give clear evidence that the free-to-bound model is appropriate for interpreting the optical properties of CIS QDs.

We studied the spin rotation of electrons in CdS quantum dots (QDs) and CdS QDs with charge acceptors by means of time-resolved Faraday rotation (TRFR) at room temperature. The electron spin rotation had an oscillatory component in the TRFR signal and the oscillation frequency proportional to the magnetic field gave a g-factor of the electrons of 1.965 ± 0.006. The non-oscillatory component came from the population of excitons and showed an additional decay in CdS QDs with hole acceptors. The electron spin rotation signal was largely enhanced and lasted for a spin coherence time of T2* = 450 ps in CdS QDs tethered to TiO2 electron acceptors, where the spin initialization was triggered by the positive trion transition. These results give clear evidence that the electron spin rotation signal in QDs can be enhanced by transient p-doping.

The charge separation and recombination processes between CdSe quantum dot (QD) and graphene oxide (GO) composites with linking molecule methylene blue (MB+) were studied by femtosecond transient absorption spectroscopy. Anchoring MB+ molecules on GO results in significant changes in steady-state and transient absorption spectra, where the exciton dissociation time in the CdSe QD-MB+-GO composite was determined to be 1.8 ps. Surprisingly, the ground state bleaching signal increased for MB+-GO complex was found to be 5.2 ps, in relation with electron transfer from QD to GO. On the other hand, the strong electronic coupling between MB center dot-GO radical and GO prolonged charge recombination process (>= 5 ns) in QD-MB+-GO composites. Charge separation and recombination processes at the interface between semiconductor QDs and graphene can thus be modulated by the functionalized dye molecules.

We have experimentally observed the spin polarization process of single electrons in InP/InGaP quantum dots by time-resolved Kerr rotation measurements. It is found that the inversion of the spin polarization direction occurs with the variation of the intensity of the optical pulse. The spin coherence lifetime abruptly changes on the occurrence of the inversion. We have reproduced the inversion in numerical simulations using the density operator of the electron–trion four-level system, assuming a small deviation of the optical pulse from circular polarization. The change of the spin lifetime is attributed to the qualitative change of the four-level system in the electric polarization state.

We studied the energy transfer processes from organic charge transporting materials (CTMs) to ZnCuInS (ZCIS) quantum dots (QDs) with different emission wavelength by steady-state and time-resolved photoluminescence (PL) spectroscopy. The change in the PL excitation intensity of the ZCIS QDs and the PL decay time of the CTMs clearly demonstrated an efficient energy transfer process in the ZCIS/CTM blend films. It was found that the efficiency of Forster resonance energy transfer significantly increases with increasing the particle size and decreasing the Zn content in the QDs, which is well consistent with the estimated Forster radii (R0) varying from 3 to 5 nm. In addition, the PL quenching of the QDs related to the charge separation process was also observed in some of the samples. The energy transfer and charge separation processes in the films were well explained based on the band alignment between the ZCIS QDs and CTMs.

We have investigated the exciton spin dynamics in an ultrathin CdTe/ZnTe single quantum well (QW) and self-assembled quantum dots (QDs) by observing time-resolved Kerr rotation (TRKR). At temperatures low enough to suppress the thermal escape of holes from the quantum-confinement structures, excitons dominate the TRKR signal. It shows characteristic dependence on the magnetic field, which is distinctly different from that of isolated electrons. From the data, we have estimated the electron–hole exchange energy to be 137 µeV in the QW, and larger than 190 µeV in the QDs.

We studied the energy transfer between CdSe core/shell quantum dots (QDs) and 1,3,5-tris(N-phenylbenzimidazol-2,yl) benzene (TPBI) in inorganic/organic blend films using steady-state and time-resolved photoluminescence (PL) spectroscopy. The shortening in PL lifetime of TPBI molecules and the resulting lengthening in PL lifetime of the QDs demonstrated an efficient energy transfer process from donor to acceptor. The slowest PL decays of CdSe core/shell QDs observed in the blend films with low QD concentration were considered to result from the maximum energy transfer process from the surrounding TPBI molecules of a QD to itself. The PL decay curves of the core/shell QDs with a CdS, ZnS, and CdS/ZnCdS/ZnS shells were simulated to obtain the excited state lifetimes of the surrounding TPBI molecules for understanding the effect of the shells on the energy transfer process. It was surprisingly found that the obtained energy transfer rate to a QD with a thick CdS/ZnCdS/ZnS multishell from the surrounding TPBI ...

Spectral diffusion of type-II excitons in InP/InAs/InP core-multishell nanowires (CMNs) was studied at 5 K by means of time- and spectrally resolved photoluminescence. Up to 1.2 ns type-II excitons lose their energy slowly at the initial rate of 8 × 10 6 eV / s and at the later rate of 3 × 10 6 eV / s reflecting two-dimensional spectral diffusion in the quantum well region in CMNs. Acoustic-phonon-assisted migration of excitons to lower-energy-localized states leads to the spectral diffusion in two-dimension. Spectral diffusion of type-II excitons is discussed by extending the theory of spectral diffusion of type-I excitons.

Coherent spin precession of electrons and excitons is observed in charge tunable InP quantum dots under the transverse magnetic field by means of time-resolved Kerr rotation. In a quantum dot doped by one electron, spin precession of the doped electron in the quantum dot starts out of phase with spin precession of the doped electrons in a GaAs substrate just after a trion is formed and persists for more than 2 ns even after the trion recombines. Simultaneously spin precession of a trion (hole) starts. Observation of spin precession of both a doped electron and a trion (hole) confirms creating coherent superposition of an electron and a trion as the initialization process of spin of doped electrons in quantum dots. In a neutral quantum dot, the exciton spin precession starts out of phase with spin precession of the doped electrons in a GaAs substrate and the precession frequency does not converge to 0 at the zero field limit. It contains the electron–hole exchange interaction and corresponds to the splitting between bright and dark excitons under the transverse magnetic field.

We studied the diffusion of Mn ions in MnS/ZnS core/shell nanocrystals (NCs) from the MnS core to the ZnS shell by steady-state and time-resolved photoluminescence (PL) spectroscopy. The colloidal MnS/ZnS core/shell NCs with different thicknesses of the ZnS shell synthesized in octadecene with nucleation doping strategy were annealed at different temperatures. It was found that the PL intensity and lifetime of the MnS/ZnS NCs with a thin ZnS shell significantly decreased with increasing annealing time in the heat treatment temperature range of 220−300 °C, resulting from the diffusion of Mn ions in the MnS/ZnS core/shell NCs to the surface of the ZnS shell, while the PL intensity and lifetime of the NCs with a thick ZnS shell remained almost constant in the temperature range. Furthermore, it was noted that the MnS/ZnS NCs with a small MnS core of about 2.0 nm in diameter exhibited high PL quantum yield of greater than 40%. The experimental results indicated that highly efficient luminescent Mn-doped ZnS NC...

Microdisk lasers of the organic semiconductor material thiophene/phenylene co-oligomer (TPCO) have been fabricated on Si/SiO 2 substrates. Quality of the organic films is improved by using the high temperature deposition method, and microcavities without grain boundaries and cracks are obtained. As a result, microdisks with 1 μm diameter are fabricated successfully. Comparing with the bulk film part, the lasing threshold is reduced to 2% of the annealed bulk film parts. A part of the reduction comes from the improved quality of crystal films and another part of the reduction from the small disk size. To our knowledge, this is the smallest microdisk cavity made from organic crystals ever reported. The TPCO crystal films fabricated by using the high temperature deposition method also show the good performance of p-channel field effect transistor operation.

The peculiarities of Hanle effect (i.e., the depolarization of edge photoluminescence in a transverse magnetic field) in semiconductors, brought about by spin-dependent recombination of free electrons with deep paramagnetic centers, have been investigated in GaAs0.979N0.021 alloy at room temperature. The measured Hanle curve consists of narrow and wide parts with the widths at the half-height being ∼100 and ∼120000 G. The difference between the widths by three orders of magnitude results from strongly differing spin lifetimes of bound and free electrons, Tsc and Ts. Using g-factor values +2 and +1 for bound and free electrons, respectively, we have found that Tsc≈700 ps and Ts≈2 ps. Thus obtained values of Tsc and Ts allow us to describe theoretically the experimental Hanle curve as well as its dependence on the pump intensity.

Spin polarization in charge-tunable InP quantum dots (QDs) was investigated as a function of the longitudinal magnetic field under the linearly polarized excitation. In neutral QDs, photoluminescence (PL) polarizes resonantly at two magnetic fields where bright and dark excitons anticross each other. The resonant spin-orientation structure turns from up to down with the change of the observation photon energy, reflecting the size-variable g-factor of holes in InP/InGaP QDs. In one-electron doped QDs, on the other hand, PL polarization linearly increases with the increase of magnetic field. In two-electron doped QDs, small differential structure coming from the electron–hole anisotropic exchange interaction is superposed on the linear increase of PL polarization versus magnetic field. The linear slope observed for trions and tetraons is smaller than that expected for holes uncorrelated with the electron spins and thermalized in Zeeman sublevels and is reduced by the electron–hole exchange interaction at low temperatures.

We study the optical properties of a periodic array of InP/InAs/InP core-multishell nanowires by means of time- and spectrally resolved PL and PL excitation spectroscopy. Inhomogeneous broadening is present in our sample due to short range monolayer fluctuation and As–P intermixing at the interface, resulting in exciton localization. Acoustic phonon assisted migration of exciton to lower energy localized states leads to an energy-dependent PL decay time. A time-dependent redshift of the PL spectra (spectral diffusion) resulted from this is observed and a localization energy of ∼ 4 meV is estimated.

We have studied the mechanism of photoluminescence (PL) from MnS/ZnS core/shell quantum dots (QDs) synthesized via hot solution phase chemistry using a nucleation−doping strategy. Efficient PL of the Mn2+ ions with a quantum yield (QY) of over 35% is demonstrated in the resulting QDs coated with a thick ZnS shell on the MnS core. The MnS/ZnS core/shell QDs with a thick shell exhibit PL enhancement with increasing temperature in the range between 140 and 300 K, resulting from the thermal activation of charge carriers localized at the interface between the MnS core and the ZnS shell. The PL decays of the Mn2+ ions in the core/shell QDs consist of three exponential components with time constants on the scales of 1−2 ms, hundreds of μs, and tens of μs. Surprisingly, the PL lifetimes of Mn2+ ions show a very weak dependence on the shell thickness, which is clearly different from that of the PL QY of the QDs. The experimental results indicate that the mechanism for improving the PL QY in MnS/ZnS QDs can be unde...

The photoluminescence (PL) spectra of CdSe-core CdS/CdZnS/ZnS-multishell quantum dots (QDs) were studied to understand the radiative and nonradiative relaxation processes in the temperature range from 80 to 360 K. The mechanism of temperature-dependent nonradiative relaxation processes in the CdSe QDs with changing the shell structures was found to evolve from thermal activation of carrier trapping by surface defects/traps in CdSe core QDs to the multiple longitudinal-optical (LO) phonon-assisted thermal escape of carriers in the core/shell QDs. An increase in PL intensity with increasing temperature was clearly observed in the core/shell QDs with a thick CdS monoshell and a CdS/ZnCdS/ZnS multishell. The PL enhancement was considered to come from delocalization of charge carriers localized at the CdSe/CdS interface with the potential depth of ∼30 meV. The experimental results indicated that the improvement of PL quantum efficiency in CdSe-core CdS/CdZnS/ZnS-multishell QDs could be understood in terms of t...

Luminescence from individual nitrogen pairs (NN) were studied in nitrogen δ-doped gallium phosphide. Density of NN pairs (NN3,4,5,6) were directly measured by counting the spot density in μ-photoluminescence images in samples with different nitrogen densities. It was found that all the NN densities depend on the square of nitrogen sheet density suggesting that NN3∼6 definitely consists of two nitrogen atoms, although quantitative discrepancy was found for NN5 and NN6. In high resolution polarized spectrum of single NN4, two different polarization patterns due to different N-N orientation were observed. Single photon generation from a single NN4 was demonstrated under pulsed optical excitation at 30 K. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on the photoluminescence (PL) and time-resolved PL (TR-PL) spectroscopy of wurtzite InP/InAs/InP core-multishell nanowires. Multiple peaks appeared in the PL spectra due to monolayer variation of InAs layers and each PL peak showed a blueshift with a cube-root dependence on the excitation power. This blueshift results from the band-bending in the type-II band configuration. Fast decay caused by the relaxation of the band-bending was observed at higher energy sides of the PL peaks in the TR-PL measurement. The blueshift calculated by a triangular quantum well approximation in type-II configuration was in good agreement with that observed in the experiment. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Spectral narrowing of exciton luminescence at the elevated temperature was observed in a semiconductor quantum dot (QD). The Fourier spectroscopy was used for the study of decoherence processes of excitons in the single QD. High resolution spectra of the excitons were obtained from the Fourier transform of the decay profile of the exciton decoherence. We found that the exciton linewidth in the QD at low temperature was predominated by spectral diffusion due to carriers trapped at localized sites around the QD and the narrowing of the exciton line was induced by thermally activated escape processes of the carriers from the trap sites. The narrowing of the exciton line also indicates that the exciton decoherence in the semiconductor quantum dots is predominated by environmental fluctuation rather than the exciton-phonon interaction at low temperature. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

In the longitudinal magnetic field, photoluminescence (PL) polarization in InP quantum dots (QDs) dramatically changed depending on the number of doped electrons. Under linearly polarized excitation, PL polarizes resonantly at two magnetic fields where bright and dark excitons anticross each other in neutral QDs. On the other hand, PL polarization monotonously increases with the increase of longitudinal magnetic field in one- and two-electron doped QDs, where thermal spin orientation of a photoexcited hole determines the PL polarization of a trion and a tetraon. At low temperatures, however, the electron-hole exchange interaction reduces the thermal spin orientation of holes. In two-electron doped QDs, the anisotropic electron-hole exchange interaction gives small structure in the magnetic-field-dependent circular polarization at the low field region. Time-resolved circular polarization clearly shows the dynamical spin polarization process in charged QDs in the longitudinal magnetic field. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Nitrogen (N) δ-doping in GaP was studied using low-pressure metalorganic chemical vapor deposition at 650 °C. Secondary ion mass spectroscopy measurements revealed that the profile of atomic N concentrations is abrupt and symmetrical, indicating a good-quality δ-doping. For the δ-doped sample of a dilute N sheet density, spatially dependent emission spectra from N-related isoelectronic traps were observed at 30 K with micro-photoluminescence (μ-PL). Besides the well-known NNi pairs, as well as the so-called A line, a new peak was observed at 2.2847 eV. We successfully mapped the spatial distribution of NN4 pairs by using 0th order diffraction of a spectrometer with a narrow-band pass filter. By applying this technique to several samples with different N sheet densities, it was found that the densities of NNi and the new peak have square and cubic dependence on the sheet density of N atoms, respectively. After identifying a single NN4 pair using the μ-PL, strong photon antibunching under continuous optical pumping was demonstrated, which is good evidence for clear single-photon emission from an individual N–N pair.

Different types of quantum beats were experimentally observed in the photoluminescence kinetics of semiconductor nanostructures with InP quantum dots characterized by strong inhomogeneous broadening of the optical transitions. Specific types of beats were selected by varying the magnitude and orientation of the magnetic field, the applied bias voltage, and the frequencies of the exciting and detected light. Parameters of the fine structure of electrons and holes in the system under study and characteristic relaxation times of the spin coherence are determined from the experimental data.

Kinetics of polarized photoluminescence of the negatively charged InP quantum dots in weak magnetic field is studied experimentally. Effect of both the nuclear spin fluctuations and the dynamical nuclear polarization on the electron spin orientation is observed.

Effect of external magnetic field on the optical orientation of electron spins in negatively charged InP quantum dots is studied experimentally using the PL pump–probe method.

The authors have studied the excitons and excitonic complexes in two kinds of self-assembled quantum dots (QDs) using photon echo measurements. In GaAs strain-induced quantum dots (SIQDs), a pronounced biexcitonic beat with a period of 1 ps is observed. The biexciton binding energy in SIQDs is obtained from the beat period, and its magnetic field dependence is investigated. It is found that the biexciton binding energy is remarkably increased by the lateral confinement and they are almost independent of the applied magnetic field up to 8 T. A theoretical calculation of the biexciton binding energy in SIQDs is presented to explain the observed magnetic field dependence. In charge-tunable InP QDs, the photon echo signal shows dramatic changes depending on the electric bias. The decay profile of the echo intensity is not a single exponential but Gaussian-like function, which indicates non-Markovian nature of the dephasing process in this system. Theoretical calculation is done assuming tunneling induced dephasing mechanism, and it reproduces the experimental results quite well.

We construct the highly sensitive Kerr rotation system to observe electron spin precession in single-layer quantum dots. The system consists of a photoelastic modulator, a balanced detector and tandem double lock-in amplifiers. The angle resolution achieves 5×10−6 degree, and is highest among the reported so far. To evaluate the possibility of measuring spin relaxation in single-layer quantum dots, we studied the strain-induced GaAs quantum dots. We observe successfully the electron spin precession in the single-layer quantum dots under resonant excitation.

A highly sensitive heterodyne-detected photon echo enabled us to observe the signal from one layer of neutral self-assembled InP quantum dots under an electric field. The photon echo signal dramatically increased when charged InP quantum dots were neutralized with the application of negative electric bias. This fact suggests the hypothesis that charged quantum dots do not produce the photon echo. With the further increase of negative electric bias, photoluminescence was quenched and the photon echo decayed faster. The photon echo showed tunneling-induced dephasing and decayed nonexponentially, reflecting the non-Markovian nature of the tunneling process.

Confined acoustic phonons in CuBr quantum dots (QDs) are studied for the first time using coherent phonon measurements and persistent spectral hole burning (PSHB) spectroscopy. A size-dependent dam...

The spin-dependent recombination (SDR) has been observed in GaAs_{1-x}N_{x} (x = 2.1, 2.7, 3.4%) at room temperature. It reveals itself in a decrease of the edge photoluminescence (PL) intensity by more than a factor of 3 when either the polarization of the exciting light is changed from circular to linear or the transverse magnetic field of ~300 gauss (G) is applied. The interband absorption of the circularly polarized light results in a spin polarization of conduction electrons, which reaches 35% with increasing the pump intensity. The effects observed are explained by dynamical polarization of deep paramagnetic centers and spin-dependent capture of conduction electrons by these centers. The PL depolarization in a transverse magnetic field (Hanle effect) allows us to estimate the electron spin relaxation time in the range of 1 ns. Theoretically, it has been concluded that, due to the SDR, this long time is controlled by slow spin relaxation of bound electrons. In all the three alloy samples, a positive sign of the bound-electron g-factor is determined experimentally from the direction of their mean spin rotation in the magnetic field.

Misfit dislocation was investigated by observing crosshatch over surfaces of In x Ga 1-x As-In 0.52 Al 0.48 As strained multiple quantum wells (MQWs) and In x Ga 1-x As bulk-like thin epifilms grown on InP substrates. It was found that no crosshatch was observed on surfaces of non-doped In x Ga 1-x As-In 0.52 Al 0.48 As strained MQW over an In composition range of 0.32

We observed the tunneling process of photo-excited holes in neutral InP quantum dots and Pauli blocking of charged InP quantum dots. A highly sensitive heterodyne-detected photon echo method enabled us to observe the signal from one layer of self-assembled InP quantum dots under the electric field. The electric field could control the charging or neutralization of the InP quantum dots and hence the photon echo signal decreased considerably with the increase of electron doping. The photon echo of neutral InP quantum dots under the electric field showed tunneling-induced dephasing, which decays non-exponentially reflecting the non-Markovian nature of the tunneling process.

The work is an experimental study of optical spin polarization in InAs/GaAs quantum dots (QDs) with 2 resident electrons or holes. A capture of a photo-generated electron-hole pair into such a QD creates a negative or positive tetron (double-charged exciton). Spin polarization was registered by the circular polarization of the QD photoluminescence (PL). The spin state was found to differ radically in the dots with opposite in sign charge. Particularly, under excitation in a GaAs barrier, the ground state PL polarization is negative (relative to the polarization of an exciting light) in the negatively charged QDs and positive in the positively charged QDs. With increasing excitation intensity, the negative polarization rises from zero up to a saturation level, while the positive polarization decreases. The negative polarization increases in weak magnetic fields applied in Faraday geometry, but strong fields suppress it. The positive polarization always increases as a function of magnetic field. We propose a theoretical model that qualitatively explains the experimental results.

Four-wave mixing in strain-induced GaAs quantum dots formed in a GaAs quantum well is studied using a highly sensitive heterodyne technique. A clear oscillation with a period of 1 ps is observed, recognized as a biexcitonic quantum beat based on its polarization dependence. The period of the beat is about 1.5 times shorter than that from the quantum well in the same sample. The beat represents direct evidence of an increase in the biexciton binding energy in the dots due to lateral confinement potential originating from the stressors.

We studied the optical absorption, polarized reflectance (PR) and photoluminescence (PL) of β-FeSi 2 single crystals grown from solution. In low-absorption measurements, we found a phonon emission and absorption structure, which suggests an indirect transition. The exciton energy gap of 0.814 eV was determined from the absorption spectrum at 3.5 K. We also found a direct transition with the gap energy of 0.939 eV. PR measurements for Ea, Eb and Ec revealed the anisotropy of reflectivity of β-FeSi 2 . We observed the PL with a peak wavelength of about 1.56 μm at 20 K.

Excitonic lasing from ZnO single nanobelts was observed at room temperature, which was due to exciton–exciton scattering processes appearing under intense light excitation. Morphologies of the nanobelts are rectangular shapes, so that crystalline facets of the nanobelts acted as laser-cavity mirrors. Therefore, mode spacings corresponding to cavity lengths of the respective nanobelts were observed in luminescence spectra. Lasing from ZnO single nanobelts at room temperature and their lasing properties are reported.

This paper reports observation of four kinds of luminescence quantum beats (QBs) in semiconductor quantum dots (QDs). QBs of Zeeman-split bright heavy-hole excitons under a longitudinal magnetic field and QBs coming from the electron spin Larmor precession under a transverse magnetic field were observed for strain-induced GaAs QDs. Another type of QB between bright and dark heavy-hole excitons was observed in neutralized self-assembled InP QDs in a tilted magnetic field. Trionic QBs were observed in negatively charged InP QDs possessing only one excess electron in the absence of the magnetic field. Analysis of QBs gives us fine energy splittings of exchange energy and Zeeman splittings of exciton, electron and hole with an accuracy of 10 μeV despite the broad inhomogeneous linewidth of 20–50 meV.

MBE growth of self-organized dots of CdTe and (Cd,Mn)Te on the lattice-mismatched ZnTe surface and their optical properties are reported. The formation of nano-scale dots was confirmed when CdTe or Cd1−xMnxTe with the Mn composition x≤0.1 was deposited on the ZnTe (0 0 1) surface beyond the critical thickness of 1.5–2 monolayers (MLs). In the PL measurement on CdTe dots, the zero-dimensional (0D) excitonic luminescence from the dots was observed. Though the emission from the wetting layer was not observed in the PL spectra, the existence of the wetting layer state was confirmed in the PLE measurement. The PL spectra from (Cd,Mn)Te dots were split into two lines in low temperatures less than 20 K. The lower-energy line showed anomalous behaviors in the dependences on temperature and magnetic field. This is considered due to the magnetic polaron effect enhanced by the 0D confinement.

We report an observation of the optical polarization of hole spins in the negatively charged self-assembled InAs/GaAs quantum dots when the dot is occupied by 2 or 5 electrons on average. It was found that the spin relaxation time of holes is comparable with their lifetime. It was demonstrated that the hole spin polarization can be destroyed by both the transverse magnetic field (the Hanle effect) and the electric bias.

We report the observation of two kinds of quantum beats in the time-resolved photoluminescence of the strain-induced GaAs quantum dots in the magnetic field parallel and perpendicular to the growth direction. They come from the Zeeman splitting of bright excitons in the Faraday configuration and electron Larmor precession in the Voigt configuration. Analysis of the oscillation period depending on the magnetic field strength, its orientation and the polarizations of excitation and luminescent light gives g-factors of the exciton, the electron and the heavy hole depending on the crystal orientation. The electron g-factor obtained by the analysis of the quantum beat observed in the Faraday configuration agrees with that obtained directly from the electron Larmor precession in the Voigt configuration. The isotropic electron g-factor is observed in contrast to the anisotropic electron g-factor for the corresponding quantum well and is ascribed to the strain-induced opposite energy shift of heavy and light hole bands.

Bimolecular interlayer scattering of electrons was observed in wurtzite InP/InAs/InP core–multishell nanowires by means of femtosecond up-conversion luminescence spectroscopy. Two-body Auger scattering took place in hexagonal InAs nanotubes constituting type-II quantum wires and wells.

The role of longitudinal surface collective excitations of doped elements of heterostructure in electronic dynamics of quantum dots was studied theoretically. A new mechanism of intraband quantum dot carrier relaxation is proposed where an interaction between the quantum dot carriers and electric potential induced by charge density fluctuations in the element with frequencies of the surface plasmon–LO–phonon modes is responsible for the process. It has been found that two branches of the surface plasmon–LO–phonon modes of the doped element are effectively involved in the relaxation even in the case when a distance between the element and the quantum dots is relatively long (50÷150 nm ). The calculations show that relaxation rates about 10 12 s −1 are expected at the distance of 100 nm rising by an order of magnitude at a distance of 30 nm.

Temperature dependence of the homogeneous linewidth of CuBr quantum dots in glass and NaBr matrices and CdSe quantum dots in glass matrix has been investigated using heterodyne-detected accumulated photon echo. We can reproduce the observed nonlinear temperature dependence considering two characteristic mechanisms: the small energy excitation in two-level system and the two-phonon Raman process of confined acoustic phonons. The interaction with the two-level system is dominant below 15 K and 10 K for CuBr and CdSe quantum dots, respectively. Above these temperatures the interaction with confined phonons is dominant. These results are similar to those of a previous study of CuCl quantum dots in glass, showing the universal influence of confined phonons and surrounding matrices on the low-temperature dephasing process of very small semiconductor quantum dots.