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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

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.

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.

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.

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.

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)

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 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 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.

The highest 19th-order longitudinal optical (LO) phonon-mediated relaxation was observed in photoluminescence excitation spectra of CdTe self-assembled quantum dots grown in ZnTe. Hot excitons photoexcited highly in the ZnTe barrier layer are relaxed into the wetting-layer state by emitting multiple LO phonons of the barrier layer successively. Below the wetting-layer state, the LO phonons involved in the relaxation are transformed to those of interfacial Zn x Cd 1− x Te surrounding CdTe quantum dots. The ZnTe-like and CdTe-like LO phonons of Zn x Cd 1− x Te and lastly acoustic phonons are emitted in the relaxation into the CdTe dots. The observed main relaxation is the fast relaxation directly into CdTe quantum dots and is not the relaxation through either the wetting-layer quantum well or the band bottom of the ZnTe barrier layer. This observation shows very efficient optical phonon-mediated relaxation of hot excitons excited highly in the ZnTe conduction band through not only the ZnTe extended state but also localized state in the CdTe quantum dots reflecting strong exciton–LO phonon interaction of telluride compounds.

We report the photoluminescence and excitation spectra of CdTe self-assembled quantum dots grown by molecular beam epitaxy in ZnTe. In photoluminescence excitation spectra, multiple longitudinal-op...

A method of coherent control of the resonant spontaneous optical phonon assisted secondary emission has been proposed for the study of exciton dynamics in a single semiconductor quantum dot (QD). A theoretical analysis of the time-integrated emission excited by a couple of phase-locked pulses of light resonant to fundamental transition of QD has shown that a dependence of the signal envelope function on the mutual delay between the pulses is directly determined by the dephasing rate of the transition.

Features of the photoluminescence spectra observed for various polarizations and intensities of the pumping radiation and the kinetics of photoluminescence of the CdS and CdSe nanocrystals grown in hollow nanochannels of an Al2O3 matrix are explained in terms of exciton transitions in semiconducting quantum wires with dielectric barriers. The observed exciton transition energies coincide with the values calculated with an allowance for the effects of quantum confinement and the “dielectric enhancement” of excitons. The latter effect is manifested by a significant increase in the Coulomb attraction between electrons and holes (the exciton binding energy exceeds 100 meV) due to a difference between the permittivities of semiconductor and insulator. It is shown that the exciton transition energy remains constant when the quantum wire diameter varies within broad limits. This is related to the fact that a growth in the one-dimensional bandgap width of the quantum wire caused by a decrease in the diameter is compensated by an increase in the exciton binding energy.

The problem of modification of the energy spectrum of electronic and phonon excitations in semiconductor quantum dots (QDs) under conditions of vibrational resonance is considered. Analytical expressions for the energy of polaron-like states in QDs in the form of a sphere or rectangular parallelepiped, taking into account the size dependence of the electron-phonon interaction, are derived. Experimental data on renormalization of the lowest exciton states in QDs based on CuCl in a NaCl matrix have been obtained with the use of two-photon secondary emission resonance spectroscopy. Comparison with the calculated results demonstrates quantitative agreement indicating the adequacy of the employed theoretical model. It is established that the vibrational resonance strongly modifies the energy spectrum of small QDs.

The temperature-dependent homogeneous width of confined excitons in CuCl, CuBr and CdSe quantum dots was investigated at very low temperatures down to 0.6 K by means of accumulated photon echo. The observed nonlinear temperature dependence of the homogeneous width at low temperatures is explained by the interaction with confined acoustic phonons for CdSe quantum dots or very small excitation in the two-level system for CuCl and CuBr quantum dots. At the lowest temperature, the homogeneous width of confined excitons in CuCl, CuBr and CdSe quantum dots becomes 1-2 μeV, 23-59 μeV and 0.16-0.25 meV, respectively. It depends on the surrounding matrix for CuCl quantum dots and the radius of dots for CuBr and CdSe quantum dots. These results suggest surroundings of quantum dots dominate the low temperature dephasing mechanism, i.e. homogeneous width of the optical spectra, of real quantum dots.

Excitation power density dependence of InP self-assembled quantum dots (SADs) was investigated under band-to-band excitation in a Ga0.5In0.5P matrix by means of macro- (conventional) and micro-spectroscopy. State filling of confined excitonic states was studied in detail by micro-photoluminescence (μ-PL) spectra and images. The efficiency of the state filling differs from dot to dot, which depends on the inflow and outflow rates of the excitons. Sharp μ-PL lines from a single confined exciton and biexciton were clearly observed. The observed biexciton binding energy of an InP SAD is a few times larger than that of the bulk InP, suggesting that the multi-exciton states are stabilized because of the confinement.

A polyvinyl alcohol film containing ZnS:Mn 2+ nanocrystals is prepared by a chemical method, and the temperature dependence of the Mn 2+ luminescence intensity is investigated in the 9–305 K range. Under the interband excitation of the ZnS host crystals, the temperature quenching of the Mn 2+ luminescence in the nanocrystals is found to be remarkably weak in comparison with the commercially available bulk powder. Probable causes of this difference are discussed in terms of the thermally induced dissociation of the exciton, the exciton-phonon interaction, and the rate of the energy transfer from the exciton to the Mn 2+ ion.

We prepare a polyvinyl alcohol film containing undoped and Mn 2+ -doped CdS nanocrystals by a chemical method, and investigate the photoluminescence properties. The luminescence of Mn 2+ of this sample shows long decay times of about 1.1 and 0.2 ms, unlike the previous report on ZnS : Mn 2+ nanocrystals. Under the interband excitation, the temperature quenching of the Mn 2+ luminescence is found to be remarkably weak in comparison with the bulk crystals of CdS: Mn 2+ . Probable causes of this difference are discussed in terms of the electron-hole pair (or exciton)-phonon interaction and the thermally induced dissociation of the electron-hole pair. Further, we measure site-selective luminescence spectra and discuss the origin of the green luminescence of the undoped CdS nanocrystals.

Ultranarrow homogeneous linewidth of a ${Z}_{3}$ exciton in CuCl quantum dots was studied by using accumulated photon echo. CuCl quantum dots embedded in two kinds of matrices, namely, an alminoborosilicate glass and a NaCl crystal, were studied. The narrowest homogeneous linewidth at low temperature was 1 $\ensuremath{\mu}\mathrm{eV}.$ The temperature dependence of the homogeneous linewidth was similar, above 5 K for both kinds of samples, but in the lower-temperature region, a remarkable difference was observed between them. It indicates that the matrix definitely affects the dephasing process of a confined exciton in quantum dots at low temperatures. The observed temperature dependence was explained by the contribution of the confined phonon in quantum dots and the two-level system in the matrix.

We investigated exciton dynamics in ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ multiple quantum wells at 2 K and room temperature by means of time-resolved photoluminescence and pump-and-probe measurements. Under low excitation density, the temporal change of the spontaneous emission indicated slow dynamical features of the two-dimensional exciton localization, while, above a stimulation threshold density, the decay time of the emission was shortened to be less than $\ensuremath{\sim}30 \mathrm{ps}$ due to the stimulated emission process. Further, the time-resolved pump-and-probe measurement revealed a fast relaxation of photoexcited delocalized electron-hole $(e\ensuremath{-}h)$ pairs into localized states. Above stimulation threshold, localized states were saturated, and $e\ensuremath{-}h$ pairs at delocalized states were observed. Two-dimensional $e\ensuremath{-}h$ pairs at delocalized states relaxed into localized states and excitons were formed, from which the optical gain was formed in terms of the stimulated emission process.

ERATO Single Quantum Dot Project, Japan Science and Technology Corporation, 5-9-9 Tokodai, Tsukuba 300-2635, Japanand Institute of Physics, University of Tsukuba, Tsukuba 305-8571, Japan~Received 29 December 1998!We report the observation of inverse exciton series (M-emission series! in the optical decay of an excitonicmolecule to the exciton excited states (n52s,3s, and 4s). A theory is developed which relates the inverseexciton series to the molecule wave function using the bipolariton model. The components of the excitonexcited states n52, 3, and 4 in the biexciton wave function in CuCl are determined from the relative intensitiesof M

Dynamical dephasing processes of an exciton and a charged exciton in single InP quantum dots were studied by using interferometric spectroscopy. Interferometric spectroscopy enabled us to observe with high sensitivity the dephasing of exciton or other exciton complexes in single quantum dots. In order to observe the dephasing of the exciton or exciton complexes, emitted single-photons generated from single InP quantum dots were detected through the Michelson interferometer. The contrast of the interferometric signal due to the exciton and the charged exciton shows non-exponential decay under band-to-band excitation for the GaInP matrix. The band-to-band excitation generates carriers trapped in the matrix and the trapped carriers modulate the energy of the quantum dots because of the quantum-confined Stark effect. Therefore the non-exponential decays are caused by energy fluctuation due to the trap carriers in the long timescale.

Strong optical anisotropy is observed in the photoluminescence (PL) bands of both the InP self-assembled quantum dots and the ${\mathrm{Ga}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}$ matrix. From the linearly polarized PL spectra measured under weak excitation, we found that large size quantum dots show strong anisotropy. The luminescence from a single quantum dot observed by the micro-PL technique revealed a doublet fine structure of the exciton levels that obey the linear polarization selection rule. The observed fine structure is shown to arise from an interplay of the electron-hole exchange interaction and the asymmetric crystal structure of the ${\mathrm{I}\mathrm{n}\mathrm{P}/\mathrm{G}\mathrm{a}}_{0.5}{\mathrm{In}}_{0.5}\mathrm{P}$ system.

A single-quantum dot spectroscopy enabled us to observe exciton, biexciton and triexciton in the s-shell, p-shell and d-shell structures in InP quantum dots. Polarized luminescence spectroscopy of the single-quantum dots showed optical anisotropy of excitons and biexcitons. Exciton and biexciton luminescence are co-linearly polarized but polarization is oriented towards different directions dot by dot depending on the local order of the surrounding In 0.5 Ga 0.5 P matrix. Anisotropic exchange energy splitting was seen in the polarized luminescence of single-quantum dots.

Optical properties of InP self-assembled quantum dots (SADs) were investigated in the presence of a magnetic field up to 10 T. By using a μm–sized InP/GaInP mesa, we succeeded in observing sharp photoluminescence (PL) lines from a single InP quantum dot. With the increase of the magnetic field, the diamagnetic shift and the Zeeman splitting were clearly observed in the Faraday configuration. The diamagnetic coefficient and the effective g-value of the confined excitons show fluctuations reflecting dispersion in the size and the shape of the quantum dots. The estimated g-value of the InP quantum dot is about half of that of the bulk InP because of the heavy-hole light-hole band mixing.

Sharp photoluminescence (PL) lines were observed in a single InAs quantum dot embedded in a micro-sized mesa of GaAs. These lines are in groups which conform to the conventional PL spectra. Power and temperature dependence of PL were measured to study the energy level structure. Clear state filling was observed in each sharp PL line by increasing the excitation power density. With the increase of the temperature, the PL intensity of these lines became gradually very weak successively from the higher energy side. These results suggest that most of the PL lines observed in our experiments come from the excited states of excitons confined in InAs quantum dots. Level filling and the relaxation process of excitons are discussed.

We have studied exciton localization and its stimulated emission in InGaN multiple quantum wells at 2 K. Mobility edge was determined by the use of site-selectively excited photoluminescence (PL) measurement. A time-resolved PL spectroscopy was performed to study exciton relaxation processes. The PL decay time increased with the decrease of the detection photon energy, indicating the dynamical features of exciton localization. Under the high-density excitation, we observed the localized exciton luminescence turned into the stimulated emission just below the mobility edge. In addition, a nonlinear luminescence measurement was performed to study the detailed optical property of the stimulated emission.

application/pdf, We have revealed sharp structures in the absorption spectrum of CuCl nanocrystals using persistent hole-burning spectroscopy and observed sidebands due to phonon-assisted absorption. The LO phonon frequency in the excited state of the nanocrystal is reduced by about 10% from the free value due to mixing with the exciton. When the mixing becomes resonant, the phonon sideband anticrosses with the excited exciton state. We present a theory of the phonon renormalization in nanocrystals and obtain results in excellent agreement with the experiment.

Sharp line width of excitons in CuCl quantum dots was observed by means of persistent spectral hole burning. At 2 K, the hole spectrum consists of a zero-phonon line (line width = 0.14 meV) and its acoustic phonon wing, and they are well separated from each other. With increasing the temperature T , the zero-phonon hole and its phonon side band are merged into each other and finally the hole spectrum shows broadening. Temperature-dependent hole spectra were compared with a theoretical simulation of the absorption spectra of single-sized dots. Contrary to the usual exciton broadening of the linear temperature dependence, the broadening is proportional to 1/[ exp ( δ / kT ) − 1] at low temperature, where δ is the confined acoustic phonon energy in quantum dots. This observation demonstrates unique characteristics of the exciton-phonon interaction in quantum dots

Exciton localization in ${\mathrm{In}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ was studied. At 2 K, the time-integrated photoluminescence (PL) spectrum showed a Stokes shift from the absorption shoulder and broadening at the lower photon energy side. Site-selectively excited PL measurements determined the mobility edge. The exciton relaxation processes were studied by use of time-resolved PL spectroscopy. The PL decay time increased with the decrease of the detection-photon energy, indicating the dynamical features of exciton localization. In addition, we observed localized exciton luminescence turned into stimulated emission just below the mobility edge.

Electron-hole plasma was observed together with biexciton at 77 K in the luminescence spectra of CuBr under femtosecond band-to-band excitation. Under the strong excitation, temporal change of the electron-hole plasma luminescence showed unique pulsation structures superposed on the monotonous decay, while that of the biexciton luminescence showed monotonous decay. Time span between pulsed luminescence is inversely proportional to the group velocity of photon (polariton) near the exciton resonance. Pulsation of the electron-hole plasma luminescence is ascribed to the stimulated emission coming from the large optical gain of electron-hole plasma.

A coherent emission of excitons in semiconductor multiple quantum wells was investigated by using transient Brewster reflection spectroscopy. The method reduced nonresonant background reflection greatly and allowed us to observe the coherent dynamics of the excitons clearly. The coherent signal showed not only the quantum beats of the heavy- and light-hole excitons but also the oscillatory modulation structures that resulted from the radiative coupling of exciton polaritons. In the linear regime, these observations were compared with results calculated from the transfer-matrix method involving multiple-reflection effects, and good agreement was obtained. As the excitation density of the incident pulses was raised, the decay rate of the coherent signal increased due to the exciton-exciton scattering.

application/pdf, Biexciton and triexciton states in CuCl quantum dots were studied by means of time-resolved size-selective pump-and-probe technique. A clear induced absorption band is observed on the high-energy side of the excitation photon energy. The new induced absorption is assigned to the transition from the exciton ground state to one of the weakly correlated exciton pair states which were theoretically predicted to exist and to play an important role in nonlinear optical processes. Its pump energy dependence and temporal evolution strongly support this assignment. Under high-density or two-color excitation condition, a triexciton state in quantum dots is observed for the first time.

Resonant hyper-Raman-scattering spectroscopy was applied for studies of the exciton\char21{}LO-phonon interaction in spherical CuCl nanocrystals of different sizes embedded in a glass matrix. Both the LO and 2LO phonon bands have shown a prominent resonance with the lowest energy $1S$ confined exciton state. The ratio of the integral intensities of the LO and 2LO bands \ensuremath{\rho} was found to increase with increase of the incident photon energy, or decrease of the nanocrystal radius. The Huang-Rhys factor $S$ has been calculated as a function of the nanocrystal size from the experimentally measured values of \ensuremath{\rho} on the offset harmonic-oscillator model of electron-vibrational coupling. It has been found that $S$ increases monotonically from 0.22 to 0.7, with the nanocrystal radius decreasing from 3.6 to 1.6 nm.

A persistent spectral-hole-burning (PSHB) phenomenon was successfully applied to the precise site-selective spectroscopy of CuCl quantum dots embedded in NaCl crystals. In the PSHB spectra of CuCl quantum dots, a resonantly burned hole and lower-energy satellite holes were observed. These satellite holes are supposed to originate from hole burning of the ground states, which results from site-selective excitation of the corresponding excited states of excitons confined in CuCl quantum dots. Energy relation between the resonantly burned hole and each satellite hole is well explained by the simple concept of a particle in a quantum cube with an infinitely high potential barrier. However, actual quantum dots are considered to be a little deviated from cubes, resulting in the violation of the optical selection rule in quantum cubes. A cubic-shaped quantum-dot model is almost consistent with oscillatory fine structures observed in the ${Z}_{3}$ exciton absorption band. Its spectral decomposition into the ground state and the first excited state of excitons was made, and showed that the first excited state is in majority at the higher-energy region of the ${Z}_{3}$ exciton absorption band. This result was supported by the photoluminescence spectrum of the ${Z}_{3}$ exciton.

We analyze luminescence spectra resulting from a resonant size-selective two-photon excitation of the low-energy confined ${\mathrm{Z}}_{3}$-exciton states in CuCl spherical nanocrystals. Excitation spectra of the two-photon-excited luminescence (TPL) bands and size dependences of band energies show that the TPL bands arise due to annihilation of the exciton in the lowest-energy state excited both directly and through the higher-energy confined states. It is shown that the oserved features of the TPL fine structure can be unambiguously interpreted in terms of longitudinal (L) and transverse (T) confined exciton states and resonant enhancement of phonon-assisted luminescence when the energy gap between 1S and 1P states of the T exciton is close to the LO-phonon energy.

application/pdf, Luminescence hole-burning phenomena were observed in CuCl quantum dots embedded in a NaCl crystal. As a result of the selective excitation of the Z3 exciton band of CuCl quantum dots, a resonantly burned hole and moreover its sidebands appear in the luminescence spectrum. The Stokes shift of the sidebands increases with the increase in the burning photon energy, and its dependence is explained by the quantum size effect of the negatively charged exciton X- and the positively charged exciton X2+. New exciton complexes, X- and X2+, confined in quantum dots were first observed by the luminescence hole burning.

The persistent spectral hole burning (PSHB) phenomenon was found to occur in many kinds of nanocrystalline semiconductors, such as CdSe, CdS, CuCl, CuBr and CuI, embedded in crystals, glass or polymers. In inhomogeneously broadened exciton absorption spectra of these nanocrystals, the spectral hole and its associated structure were created by the narrow-band laser excitation and were conserved for more than several hours at 2 K. Hole depth grew in proportion to the logarithm of the burning fluence. Thermally-annealing and light-induced hole-filling phenomena were observed. The hole burning takes place by the tunneling process through potential barriers with broadly distributed barrier height and thickness. Unusual luminescence behaviors related to the PSHB phenomena were also observed. They are luminescence elongation with increase of the light exposure and hole burning in the luminescence spectrum. The observed PSHB phenomena are explained by the exciton localization and the succeeding ionization of nanocrystals. The energy of the photoionized nanocrystal is released from the original energy and the new energies depend on the spatial arrangement of the trapped carriers. Quantum confinement of carriers and resulting strong Coulomb interaction between confined carriers and trapped carriers are essential for the energy change. Possible applications of the PSHB phenomenon is discussed.

Optical properties of localized excitons have been studied in a highly strained CdSe quantum well with 1-ML thickness by employing time-resolved photoluminescence (PL) and nonlinear PL spectroscopy under various excitation conditions. At 20 K, the time-integrated PL from the well layer was peaked at 2.7276 eV with a linewidth of 21 meV under low excitation intensity (0.11 \ensuremath{\mu}J/${\mathrm{cm}}^{2}$). The lifetime ranged from 200 ps to 50 ps as the monitored photon energy was changed from the low-energy tail to the high-energy one. The behavior could be well understood as a result of exciton localization, which is induced by terraces and islands with units of ML thickness fluctuation lying at the interface between CdSe and ZnSe. An emission with the fast decay component was observed at the low-energy side of the peak (2.7168 eV) under higher excitation condition. The emission could be well resolved as a positive component by the nonlinear PL measurement, and originates from the many-body effect of localized excitons, which is probably attributed to localized biexcitons. \textcopyright{} 1996 The American Physical Society.

Dynamics of excitons and excitonic molecules in CuCl nanocrystals were investigated by means of both picosecond time-resolved luminescence and femtosecond transient absorption. Stochastic as well as rate-equation treatments are made to explain the temporal changes of both luminescence and absorption. In the stochastic treatment, a Monte Carlo simulation is made for ${10}^{5}$ nanocrystals with the initial condition that the exciton number distribution follows the Poisson distribution. The stochastic treatment satisfactorily explains all the experimental results, while the rate-equation treatment does not. Success of the stochastic treatment shows the unique property of the dynamics of excitons and excitonic molecules in quantum dots arising from discreteness of the exciton number. \textcopyright{} 1996 The American Physical Society.

Time-resolved nonlinear luminescence of excitons and bi-excitons in ZnSe-based single quantum wells has been studied by means of a sub-picosecond excitation correlation technique. The enhancement in luminescence efficiency of bi-excitons was clearly observed in a time-correlation trace of nonlinear bi-exciton luminescence. The origin of the enhancement was attributed to the higher-order nonlinearity with respect to the density of bi-excitons, and was explained by considering the stimulation of bi-exciton luminescence.

Effects of high excitation on localized excitons have been studied in cubic Zn 0.42 Cd 0.58 S lattice-matched to GaAs. The time-integrated photoluminescence (PL) spectrum at 2 K under weak excitation conditions (0.025 μJ/cm 2 ) was composed of a single emission peaking at 2.863 eV and its LO-phonon replica. The linewidth of the main peak was 18.5 meV. With increasing excitation energy density ( I ex ), the spectra broadened, especially on the lower energy side and a small shoulder appeared at about 2.85 eV for I ex ≳ 2 μ J/cm 2 . In order to study the detailed mechanism, the nonlinear luminescence component was studied by the population mixing technique. The positive signals were detected on the lower energy side (2.85 eV) at an I ex value in the range between 1.8 and 2.9 μJ/cm 2 . This nonlinear component was attributed to localized excitonic molecules.

Dynamics of excitons and excitonic molecules in CuCl quantum dots are investigated by means of both time-resolved luminescence and transient absorption with use of sub-picosecond laser pulses. To explain the experimental results, a stochastic treatment by means of the Monte Carlo simulation and a rate-equation treatment are made. The former nicely explains the experimental results, while the latter does not. It is found that the exciton dynamics in CuCl quantum dots are characterized by its stochastic nature due to discreteness of the exciton number in a quantum dot.