We propose an experimental method for the optical detection of transport along the growth axis of... more We propose an experimental method for the optical detection of transport along the growth axis of CiaAs/Al Ga& "As superlattices (SL) that combines stationary photoluminescence excitation experiments (PLE) and time-resolved picosecond spectroscopy. We show that both the absorption coe+cient of the structure and the effective diffusion length of the photogenerated excitations can be deduced from PLE experiments performed on structures with two enlarged test wells (EW) grown on both sides of the SL. The diffusion coefficients of the excitations involved in the low-temperature diffusion processes are then deduced from time-resolved experiments. A one-dimensional diffusion model is proposed taking into account two kinds of diffusive excitations: free-exciton states and localized-exciton states. We show that the capture process of the SL excitations by the enlarged wells must be taken into account in order to fit the dynamical experiments with capture times in the range 1-S ps. The dependence of the freeexciton states s diffusion coefficient on the miniband dispersion of the structure is clearly demonstrated. Finally, temperature effects are discussed such as the influence of the free-exciton cooling on the diffusion process; the nonthermalization of the EW excitations with the lattice is also revealed.
ABSTRACT Optical and spin properties of individual GaAs droplet dots in AlGaAs barriers are studi... more ABSTRACT Optical and spin properties of individual GaAs droplet dots in AlGaAs barriers are studied in photoluminescence experiments at 4K. First we report strong mixing of heavy hole-light hole states. Using the neutral and charged exciton emission as a monitor we observe the direct consequence of quantum dot symmetry reduction in this strain free system. By fitting the polar diagram of the emission with simple analytical expressions obtained from k•p theory we are able to extract the mixing that arises from the heavy-light hole coupling due to the geometrical asymmetry of the quantum dot. Second we report optical orientation experiments. Circularly polarized optical excitation yields strong circular polarization of the resulting photoluminescence. Optical injection of spin polarized electrons into a GaAs dot gives rise to dynamical nuclear polarization that considerably changes the exciton Zeeman splitting (Overhauser shift). We show that the created nuclear polarization is bistable and present a direct measurement of the build-up time of the nuclear polarization in a single GaAs dot in the order of one second.
Ultrafast Phenomena in Semiconductors and Nanostructure Materials XV, 2011
The energy states in semiconductor quantum dots are discrete as in atoms, and quantum states can ... more The energy states in semiconductor quantum dots are discrete as in atoms, and quantum states can be coherently controlled with resonant laser pulses. Long coherence times allow the observation of Rabi-flopping of a single dipole transition in a solid state device, for which occupancy of the upper state depends sensitively on the dipole moment and the excitation laser power. We
The authors have calculated the band structure of 1.3-mm InGaAsN/GaAs(N)/GaAs compressively strai... more The authors have calculated the band structure of 1.3-mm InGaAsN/GaAs(N)/GaAs compressively strained quantum wells (QW) using the band anticrossing model and an eight-band k.p Hamiltonian. The calculated interband optical transition energies have been compared to the experimental ones deduced from photocurrent. Because of the high compressive strain in the QW, strain-compensated structures may be required in order to grow stable multiple QWs; with this in view, the band structure of InGaAsN/GaAsP/GaAs QWs emitting at 1.3 mm was studied. Dilute nitride structures also offer the possibility of growing tensile-strained QW lasers emitting at 1.55 mm on an InP substrate. In order to evaluate the potentialities of such structures, the band characteristics of InGaAsN/InGaAsP/InP heterostructures were determined with a TM-polarised fundamental transition around 1.55 mm.
... Acknowledgements We are grateful to Professor JP Peyrade and Dr. DJ Dunstan for supplying the... more ... Acknowledgements We are grateful to Professor JP Peyrade and Dr. DJ Dunstan for supplying the samples. References [1] TC Damen, Luis Vina, JE Cunningham, J. Shah and LJ Sham, Phys. Rev. ... References. [1] TC Damen, Luis Vina, JE Cunningham, J. Shah and LJ Sham. ...
We have studied the influence of radio-frequency plasma cell operating conditions on the plasma-a... more We have studied the influence of radio-frequency plasma cell operating conditions on the plasma-assisted molecularbeam epitaxy growth of bulk GaAsN and GaInAsN quantum wells. For low N 2 flow rates, the species that mostly incorporate are atoms and the dissociation fraction of N 2 for the lowest flow rate and the highest RF power has been found to be up to 0.7. For high N 2 flow rates, the species that mostly incorporate are vibrationally excited molecules in the N 2 ðX ; vÞ ground state. The vibrational temperature of the N 2 ðX ; vÞ ground state has been found to be higher than 10 4 K. The ion damage has been decreased by increasing the N 2 pressure which induces a decrease of the electron temperature and consequently a decrease of the plasma ionization. Oxygen has been evidenced to incorporate proportionally to nitrogen and its emission line has been identified in the plasma emission spectrum. Finally, quantum-well structures emitting at around 1.3 lm at 300 K have been fabricated and characterized by photoluminescence and photocurrent spectroscopy.
Using picosecond time resolved spectroscopy, we study the extinction kinetics of the optical tran... more Using picosecond time resolved spectroscopy, we study the extinction kinetics of the optical transmission of CdSe platelets induced by strong optical pumping. We investigate particularly the all-optical gate capabilities (resulting from the bandgap shrinkage due to the Mott transition) for laser beams the wavelength of which ranges from 676 to 678 nm. The electronhole plasma density necessary to enable the optical switching is determined (p ~ 2 x I017cm-a at 20K). The switch-off time (i.e. the transparency recovery time) is also studied at different wavelengths by picosecond spectroscopy.
We have investigated the electron spin dynamics on the discrete energy levels of self-organized I... more We have investigated the electron spin dynamics on the discrete energy levels of self-organized InAs quantum dots (QDs) covered by the thin InGaAs layer and embedded in the GaAs matrix. The InGaAs layer forms the external quantum well (QW). In particular, we report on the electron mean spin redistribution over the QD and QW energy spectrum resulting in a drastic increase of electron spin polarization of the QD excited levels and the QW ground state. The electron polarization is determined by measuring the degree of circular polarization of the QD and QW emissions after excitation by 1.5 ps circular polarized pulses in the GaAs barrier.
The exciton spin alignment is measured in an ensemble of self-organized cubic GaN/AlN quantum dot... more The exciton spin alignment is measured in an ensemble of self-organized cubic GaN/AlN quantum dots. By picosecond time-resolved photoluminescence experiments, we show that the exciton linear polarization does not decay with time from 20 to 300 K.
We propose an experimental method for the optical detection of transport along the growth axis of... more We propose an experimental method for the optical detection of transport along the growth axis of CiaAs/Al Ga& "As superlattices (SL) that combines stationary photoluminescence excitation experiments (PLE) and time-resolved picosecond spectroscopy. We show that both the absorption coe+cient of the structure and the effective diffusion length of the photogenerated excitations can be deduced from PLE experiments performed on structures with two enlarged test wells (EW) grown on both sides of the SL. The diffusion coefficients of the excitations involved in the low-temperature diffusion processes are then deduced from time-resolved experiments. A one-dimensional diffusion model is proposed taking into account two kinds of diffusive excitations: free-exciton states and localized-exciton states. We show that the capture process of the SL excitations by the enlarged wells must be taken into account in order to fit the dynamical experiments with capture times in the range 1-S ps. The dependence of the freeexciton states s diffusion coefficient on the miniband dispersion of the structure is clearly demonstrated. Finally, temperature effects are discussed such as the influence of the free-exciton cooling on the diffusion process; the nonthermalization of the EW excitations with the lattice is also revealed.
ABSTRACT Optical and spin properties of individual GaAs droplet dots in AlGaAs barriers are studi... more ABSTRACT Optical and spin properties of individual GaAs droplet dots in AlGaAs barriers are studied in photoluminescence experiments at 4K. First we report strong mixing of heavy hole-light hole states. Using the neutral and charged exciton emission as a monitor we observe the direct consequence of quantum dot symmetry reduction in this strain free system. By fitting the polar diagram of the emission with simple analytical expressions obtained from k•p theory we are able to extract the mixing that arises from the heavy-light hole coupling due to the geometrical asymmetry of the quantum dot. Second we report optical orientation experiments. Circularly polarized optical excitation yields strong circular polarization of the resulting photoluminescence. Optical injection of spin polarized electrons into a GaAs dot gives rise to dynamical nuclear polarization that considerably changes the exciton Zeeman splitting (Overhauser shift). We show that the created nuclear polarization is bistable and present a direct measurement of the build-up time of the nuclear polarization in a single GaAs dot in the order of one second.
Ultrafast Phenomena in Semiconductors and Nanostructure Materials XV, 2011
The energy states in semiconductor quantum dots are discrete as in atoms, and quantum states can ... more The energy states in semiconductor quantum dots are discrete as in atoms, and quantum states can be coherently controlled with resonant laser pulses. Long coherence times allow the observation of Rabi-flopping of a single dipole transition in a solid state device, for which occupancy of the upper state depends sensitively on the dipole moment and the excitation laser power. We
The authors have calculated the band structure of 1.3-mm InGaAsN/GaAs(N)/GaAs compressively strai... more The authors have calculated the band structure of 1.3-mm InGaAsN/GaAs(N)/GaAs compressively strained quantum wells (QW) using the band anticrossing model and an eight-band k.p Hamiltonian. The calculated interband optical transition energies have been compared to the experimental ones deduced from photocurrent. Because of the high compressive strain in the QW, strain-compensated structures may be required in order to grow stable multiple QWs; with this in view, the band structure of InGaAsN/GaAsP/GaAs QWs emitting at 1.3 mm was studied. Dilute nitride structures also offer the possibility of growing tensile-strained QW lasers emitting at 1.55 mm on an InP substrate. In order to evaluate the potentialities of such structures, the band characteristics of InGaAsN/InGaAsP/InP heterostructures were determined with a TM-polarised fundamental transition around 1.55 mm.
... Acknowledgements We are grateful to Professor JP Peyrade and Dr. DJ Dunstan for supplying the... more ... Acknowledgements We are grateful to Professor JP Peyrade and Dr. DJ Dunstan for supplying the samples. References [1] TC Damen, Luis Vina, JE Cunningham, J. Shah and LJ Sham, Phys. Rev. ... References. [1] TC Damen, Luis Vina, JE Cunningham, J. Shah and LJ Sham. ...
We have studied the influence of radio-frequency plasma cell operating conditions on the plasma-a... more We have studied the influence of radio-frequency plasma cell operating conditions on the plasma-assisted molecularbeam epitaxy growth of bulk GaAsN and GaInAsN quantum wells. For low N 2 flow rates, the species that mostly incorporate are atoms and the dissociation fraction of N 2 for the lowest flow rate and the highest RF power has been found to be up to 0.7. For high N 2 flow rates, the species that mostly incorporate are vibrationally excited molecules in the N 2 ðX ; vÞ ground state. The vibrational temperature of the N 2 ðX ; vÞ ground state has been found to be higher than 10 4 K. The ion damage has been decreased by increasing the N 2 pressure which induces a decrease of the electron temperature and consequently a decrease of the plasma ionization. Oxygen has been evidenced to incorporate proportionally to nitrogen and its emission line has been identified in the plasma emission spectrum. Finally, quantum-well structures emitting at around 1.3 lm at 300 K have been fabricated and characterized by photoluminescence and photocurrent spectroscopy.
Using picosecond time resolved spectroscopy, we study the extinction kinetics of the optical tran... more Using picosecond time resolved spectroscopy, we study the extinction kinetics of the optical transmission of CdSe platelets induced by strong optical pumping. We investigate particularly the all-optical gate capabilities (resulting from the bandgap shrinkage due to the Mott transition) for laser beams the wavelength of which ranges from 676 to 678 nm. The electronhole plasma density necessary to enable the optical switching is determined (p ~ 2 x I017cm-a at 20K). The switch-off time (i.e. the transparency recovery time) is also studied at different wavelengths by picosecond spectroscopy.
We have investigated the electron spin dynamics on the discrete energy levels of self-organized I... more We have investigated the electron spin dynamics on the discrete energy levels of self-organized InAs quantum dots (QDs) covered by the thin InGaAs layer and embedded in the GaAs matrix. The InGaAs layer forms the external quantum well (QW). In particular, we report on the electron mean spin redistribution over the QD and QW energy spectrum resulting in a drastic increase of electron spin polarization of the QD excited levels and the QW ground state. The electron polarization is determined by measuring the degree of circular polarization of the QD and QW emissions after excitation by 1.5 ps circular polarized pulses in the GaAs barrier.
The exciton spin alignment is measured in an ensemble of self-organized cubic GaN/AlN quantum dot... more The exciton spin alignment is measured in an ensemble of self-organized cubic GaN/AlN quantum dots. By picosecond time-resolved photoluminescence experiments, we show that the exciton linear polarization does not decay with time from 20 to 300 K.
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