Publications (114)0 Total impact
-
[show abstract]
[hide abstract]
ABSTRACT: 24th International Conference on the Physics of Semiconductors : Jerusalem, Israel August 2-7, 1998 We present a theory of LO phonon renormalization by coupling with the exciton in semiconductor quantum dots. We find that the one phonon side band of the exciton ground state shows an anticrossing with the excited exciton states. Even away from such a resonant coupling, the phonon frequency shift is significant. For example, in CuCl quantum dots in the weak confinement regime, the phonon frequency in the presence of a single exciton is reduced by about 10% from its value in the ground state. Similar results are predicted for CdS and GaAs quantum dots in the strong confinement regime.
-
[show abstract]
[hide abstract]
ABSTRACT: The size dependence of the longitudinal optical (LO) phonons in the excited state of CuBr quantum dots (QD’s) in glass and NaBr crystals in the intermediate confinement regime was studied by means of persistent spectral hole burning spectroscopy. The phonon-exciton coupled states were clearly observed at a photon energy of about 2.993 eV when the LO phonon energy is close to the energy difference between the ground 1S and excited 1P states of CuBr QD’s in glass. The energies of the LO phonons observed in smaller CuBr QD’s in glass and NaBr crystals were determined to be about 18.6 and 17.6 meV, respectively, which are smaller than that of LO phonons in the bulk CuBr material. The energy softening of the LO phonons was explained in terms of the phonon renormalization.
-
[show abstract]
[hide abstract]
ABSTRACT: 24th International Conference on the Physics of Semiconductors : Jerusalem, Israel August 2-7, 1998 Heterostructures with InP self-assembled quantum dots were studied. Strong Franz-Keldysh oscillations were found in their nonlinear re ection spectra mea- sured by pump-probe method. These oscillations manifest built-in electric field of about 30 kV/cm. We suppose that this field originates from electric charge cap- tured by the structural defects on the dots interface. An estimated areal density of electric charge is about 2×1011cm-2.
-
[show abstract]
[hide abstract]
ABSTRACT: We have studied the oscillator strength of the lowest 1s heavy excitons in GaAs-AlAs quantum wells as a function of well-layer thickness by means of optical absorption. The oscillator strength of the lowest 1s heavy excitons is largely enhanced with the decrease of well-layer thickness. The result is the first full experimental observation of two-dimensional shrinkage of the exciton wave function in quantum wells.
-
[show abstract]
[hide abstract]
ABSTRACT: Microdisk and microring lasers of the organic semiconductor material thiophene/phenylene co-oligomer (TPCO) have been fabricated on Si/SiO2 substrates. Laser emission with the whispering gallery modes (WGM) is observed in 0-1 vibration transitions. Comparing with the bulk film parts, a few times reduction of the lasing threshold has been observed in microdisks. Microring lasing threshold is reduced further when the width of the waveguide is less than 1 μm. WGM with waveguide confinement is supposed to contribute to the decrease of the threshold.
-
[show abstract]
[hide abstract]
ABSTRACT: We observed electron spin precession under magnetic field in single-layer quantum dots (QDs) by highly sensitive time-resolved Kerr rotation measurement. The spin lifetime is longer than that for the quantum well (QW). This is a result of the additional spatial confinement of electrons in QDs. Below 60 K, the spin lifetime is almost constant, and is 7 times shorter than the carrier lifetime. This suggests that the strong electron-hole exchange interaction dominates over the electron spin lifetime in QDs at low temperature.
-
[show abstract]
[hide abstract]
ABSTRACT: We report the observation of inverse exciton series (M-emission series) in the optical decay of an excitonic molecule to the exciton excited states (n=2s, 3s, and 4s). A theory is developed which relates the inverse exciton series to the molecule wave function using the bipolariton model. The components of the exciton excited states n=2, 3, and 4 in the biexciton wave function in CuCl are determined from the relative intensities of M1s, M2s, M3s, and M4s emission lines. This opens a way to reconstruct the internal molecule wave function.
-
[show abstract]
[hide abstract]
ABSTRACT: We observed a light-hole-to-electron transition of strain-induced quantum dots (SIQDs) in a strongly strained GaAs quantum well (QW). Circularly-polarized luminescence excitation spectroscopy showed a pronounced counter-circularly polarized component below the heavy-hole exciton transition in the QW. This special feature can be explained by the existence of the light-hole-to-electron transition at this energy level. On the other hand, a very high co-circular polarization (=0.8) was obtained near the lowest transition of SIQD.
-
[show abstract]
[hide abstract]
ABSTRACT: An intense anti-Stokes photoluminescence was observed in a structure with InP quantum dots (QD’s) in the presence of both a direct electric current and optical pumping below the lowest electron-hole transition in quantum dots. The discovered phenomenon provides clear evidence of deep energy levels in the vicinity of the QD’s. A simple model was proposed which allowed us to estimate the energies of the deep states and the lower limit of the product of the electron and hole relaxation rates from the QD’s to the deep states.
-
[show abstract]
[hide abstract]
ABSTRACT: We have studied persistent spectral hole-burning (PSHB) phenomena of CuCl nanocrystals embedded in a NaCl crystal. We found correlations between the PSHB phenomena and the photoluminescence spectral changes of excitons in CuCl nanocrystals and of Cu+ dimers in a NaCl crystal. The spectral changes can be triggered by photoinduced Cu+-ion displacements in CuCl nanocrystals and in a NaCl crystal.
-
[show abstract]
[hide abstract]
ABSTRACT: Physics of semiconductors 2002 : proceedings of the 26th International Conference on the Physics of Semiconductors held in Edinburgh, UK, 29 July-2 August 2002 Kinetics of polarized photoluminescence (PL) of the InP quantum dots with variable number of resident electrons is studied. A long-lived component of the degree of circular polarisation of the PL is found to be present for charged quantum dots and absent for neutral ones. Behaviour of the long-lived polarisation in longitudinal and transverse magnetic field is studied. Experimental data obtained allowed us to conclude that the long-lived polarisation is related to spin orientation in the quantum dots containing odd number of carriers. We found that the irreversible spin relaxation time in these dots substantially exceeds 1 ns.
-
[show abstract]
[hide abstract]
ABSTRACT: The free induction decay signal from excitons in a ZnP2 single crystal shows an unexpected temporal evolution, polariton beat, and coherent dip. We show that the effects come from a nonzero thickness of the sample and nonzero time width of the excitation pulses. A systematic numerical calculation provides good qualitative agreement with the data. These effects are very important in understanding the experimental results which deal with the coherent interaction between excitons and optical fields.
-
[show abstract]
[hide abstract]
ABSTRACT: The interlayer Γ-X scattering rate of photoexcited electrons was measured in staggered-alignment Al0.34Ga0.66As-AlAs ternary alloy multiple-quantum-well structures. The scattering process was directly probed by femtosecond pump-and-probe spectroscopy. The mean Γ-X scattering time of electrons across the interface between Al0.34Ga0.66As layers and AlAs layers at 4.2 K is determined to be 1.2 ps, which is longer than that observed recently in GaAs-AlAs short-period superlattices and is 20 times longer than that observed in bulk GaAs at 295 K. The slowing mechanism is ascribed to the small penetration of the evanescent Γ electrons into the AlAs barrier layers.
-
[show abstract]
[hide abstract]
ABSTRACT: 24th International Conference on the Physics of Semiconductors : Jerusalem, Israel August 2-7, 1998 Homogeneous linewidth of the excitons confined in CuCl quantum dots embedded in a NaCl crystal was studied by heterodyne-detected accumulated photon echo technique. At 2K, photon echo signal shows an exponential decay with a decay time of 160ps except for a slow rise. The homogeneous linewidth calculated from the dephasing time was found to be much smaller than that reported previously by the other methods. At 1.5K, we observed the linewidth of 1.6meV. As far as we know, this is the narrowest exciton linewidth in quantum dots.
-
[show abstract]
[hide abstract]
ABSTRACT: Picosecond kinetics of polarized resonant photoluminescence (PL) of HH excitons in the GaAs/AlGaAs superlattices is studied in an external magnetic field. The measurements were made in real time using a streak camera. For the magnetic field aligned along the heterostructure growth direction, the resonant PL exhibits oscillations in the degree of linear polarization. The oscillations are ascribed to quantum beats between sublevels of the optically active excitonic doublet split by the magnetic field. For the magnetic field aligned along the plane of the layers, the resonant PL is found to exhibit oscillations in circular polarization. These oscillations are related to beats between states of the optically active and optically inactive excitonic doublets. Experimental dependence of the oscillation frequency on the magnetic field strength and orientation has allowed us to determine the hole and electron g factors and the electron-hole exchange energy. Dynamics of the degree of circular polarization of the PL in magnetic field is used to measure the energy relaxation rate of the exciton spin states. In the magnetic field exceeding 1 T, the energy relaxation rate is shown to be magnetic-field independent and equal (1±0.2)×1010 s-1. This value is found to be much smaller than the spin phase relaxation rate determined from decay of the quantum beats in linear polarization. Unlike the energy relaxation rate, the latter grows linearly with the field strength and equals (5±0.5)×1010 s-1 at 5 T. It is concluded that the main mechanism responsible for loss of macroscopic spin coherence is the reversible dephasing within inhomogeneously broadened system.
-
[show abstract]
[hide abstract]
ABSTRACT: The paper reports on quantum beats observed in the photoluminescence kinetics of a single layer of the InP self-assembled quantum dots in a magnetic field. It is found that the beats arise only after removal of excess charges from the quantum dots by an external electrical bias. The quantum beats are shown to be related to the interference of the excitonic fine-structure states split by the magnetic-field. The dependences of the beat characteristics on the magnetic-field strength and orientation are studied. Theoretical analysis based on a model spin Hamiltonian has allowed us to describe adequately the shape of the oscillating component of the signal. We have determined the values of the electron g-factor components and estimated the spread and the mean value of the hole g factor, as well as of the electron-hole exchange splitting parameters.
-
[show abstract]
[hide abstract]
ABSTRACT: Quantum beats of the strain-induced GaAs quantum dots were observed in the time-resolved photoluminescence in the magnetic field parallel and perpendicular to the growth direction. Quantum beats observed under the longitudinal magnetic field are caused by quantum interference of bright excitons showing Zeeman splitting. The oscillation period depends on the angle between the growth direction of the crystal and the magnetic field. Analysis based on the spin Hamiltonian for excitons explains the observed data and gives g factors 0.51, 0.17, and 0.34 to the exciton, electron, and heavy hole, respectively. Quantum beats coming from electron Larmor precession were observed under the transverse magnetic field. 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.
-
[show abstract]
[hide abstract]
ABSTRACT: We conducted time-resolved luminescence spectroscopy on self-assembled InP dots in a Ga0.5In0.5P matrix at different photon excitation energies. In comparison with results on Ga0.5In0.5P without dots, the influence of the Ga0.5In0.5P matrix on luminescence-decay profiles of InP dots was clarified. By excitation at the Ga0.5In0.5P matrix, the long-decay component (∼50 ns), as well as the 400 ps radiative-decay component, was observed in luminescence of InP dots. This long-decay component reflects carrier lifetime in the Ga0.5In0.5P matrix. Carrier injection time from the Ga0.5In0.5P matrix to InP dots is estimated to be ∼100 ps, and is determined by carrier diffusion in the matrix.
-
[show abstract]
[hide abstract]
ABSTRACT: Physics of semiconductors 2002 : proceedings of the 26th International Conference on the Physics of Semiconductors held in Edinburgh, UK, 29 July-2 August 2002 The authors studied confined acoustic phonons in semiconductor quantum dots by using coherent phonon spectroscopy. Time-resolved pump-and-probe method was applied to CuBr QDs embedded in glass matrices with the second harmonic pulses from a femtosecond Ti:S laser oscillator. A clear damped oscillation signal with frequency of sub-THz was observed at room temperature. The frequency of the oscillation increases with decreasing the mean radius of dots, but the size-dependence does not show a simple 1/R characteristics of confined acoustic phonon.
-
[show abstract]
[hide abstract]
ABSTRACT: Indium Phosphide and Related Materials, 2000. Conference Proceedings. 2000 International Conference on 14-18 May 2000 Coupled quantum dot (QD)-pairs were fabricated by stressing the near-surface InGaAs/GaAs coupled quantum wells (QWs) with self-assembled InP islands. The coupling strength in the dot-pair was studied by varying the barrier layer width separating the two dots and the indium composition in the lower dot. Strong coupling was observed at a barrier less than 4 nm. Anti-bonding of the two ground states as well as all the bonding states were observed by state filling in the photoluminescence spectra. By tuning the indium composition in the lower QW to adjust the lower QD state energies before coupling relative to those in the upper QD, crossing of the bonding and anti-bonding states is achieved
