Pascal Desfonds

Hole–Nuclear Spin Interaction in Quantum Dots

We have measured the carrier spin dynamics in p-doped InAs/GaAs quantum dots by pump-probe and time-resolved photoluminescence experiments. We obtained experimental evidence of the hyperfine interaction between hole and nuclear spins. In the absence of an external magnetic field, our calculations based on dipole-dipole coupling between the hole and the quantum dot nuclei lead to a hole-spin dephasing time for an ensemble of dots of 14 ns, in close agreement with experiments.

Non-blinking single-photon generation with anisotropic colloidal nanocrystals: towards room-temperature, efficient, colloidal quantum sources.

Blinking and single-photon emission can be tailored in CdSe/CdS core/shell colloidal dot-in-rods. By increasing the shell thickness it is possible to obtain almost non-blinking nanocrystals, while the shell length can be used to control single-photon emission probability.

Determination of the phosphorus content in diamond using cathodoluminescence spectroscopy

In n-type diamond doped with phosphorus, exciton properties have been investigated by cathodoluminescence as a function of the phosphorus concentration and the temperature. The homoepitaxial diamond layers were grown by microwave plasma-assisted chemical vapor deposition and doped using a liquid organic precursor of phosphorus (tertiarybutylphosphine). The phosphorus concentration ranges from 5.2×1016 to 3.3×1018 cm−3 as measured by secondary ion mass spectrometry. It is shown that the ratio between the luminescence intensities of the neutral phosphorus-bound exciton and the free exciton follows the dopant concentration. Calibration graphs are presented to determine the phosphorus contents in diamond using cathodoluminescence spectroscopy.

Magnetic and transport properties of the room-temperature ferrimagnetic semiconductor Fe1.5Ti0.5O3±δ: Influence of oxygen stoichiometry

Fe1.5Ti0.5O3±δ epitaxial thin films have been grown on α‐Al2O3 (0001) substrates by pulsed laser deposition technique. The samples are both ferrimagnetic and semiconducting beyond room temperature. Oxygen pressure (PO2) during the deposition appears to be a critical parameter in promoting high temperature long range magnetic order and semiconducting properties. For all oxygen pressures, Fe1.5Ti0.5O3±δ thin films are single phase with twin epitaxy. High dc conductivity and low magnetization are obtained at low PO2, whereas high saturation magnetization and low conductivity stand for high PO2. Oxygen vacancies and∕or change of iron valence state are pointed out to be responsible of these properties. Superexchange mechanism via oxygen bonds seems rather to dominate the magnetic properties especially for high PO2, whereas for low PO2 a double exchange mechanism might occur. Fe1.5Ti0.5O3±δ appears thus to be an interesting material for high temperature spintronics applications.

Electron and hole spin cooling efficiency in InAs quantum dots: The role of nuclear field

The spin dynamics of a resident carrier, hole or electron, in singly charged InAs/GaAs quantum dots has been measured by pump-probe experiments. The relative strength of the hole to the electron hyperfine couplings with nuclei is obtained by studying the magnetic-field dependence of the resident-carrier spin polarization. We find, in good agreement with recent theoretical studies, that the hole hyperfine coupling is ten times smaller than the electron one.

Hole and trion spin dynamics in quantum dots under excitation by a train of circularly polarized pulses

We have performed pump-probe experiments in p -doped InAs/GaAs quantum dots leading to the all-optical initialization and readout of hole spins. In order to describe these experiments, we have modelized the interconnected dynamics of the photoelectron spin and the resident hole spin, triggered through the optical excitation by a train of short pulses. A complete description of this spin dynamics is obtained by including the hyperfine coupling as the common decoherence mechanism for the electron and hole spins. Periodic excitation conditions for arbitrary values of the pump power and the external magnetic field are also included in the model. In particular, a good agreement concerning the temporal behavior of the photoinduced circular dichroism is obtained for zero or low magnetic fields. When the applied magnetic field screens the hole-hyperfine interaction, we show that the agreement between the experimental and calculated time-dependent curves requires an additional relaxation mechanism for holes with a characteristic time in the microsecond range.

Hole spin initialization in quantum dots by a periodic train of short pulses

We model pump-probe experiments leading to the all-optical initialization of the hole spin of a p-doped InAs/GaAs quantum dots ensemble. We consider selection rules of mixed hole states and include periodic excitation conditions. Hyperfine interaction is taken into account as the common decoherence mechanism for the spins of electrons and holes. We show that the degree of hole spin polarization can be maximized by quenching the action of the hole hyperfine interaction with a small applied magnetic field. However additional hole spin relaxation mechanisms, in the microsecond time range, determine the absolute value of this maximum.

Engineering of radiative and non-radiative channels in colloidal nanocrystals: Towards room-temperature efficient colloidal quantum sources

Blinking effect and multi-excitonic emission can be independently addressed by tuning both core and shell dimension. By confocal techniques measurement, we show dot-in-rods as blinking-free sources of single photon on demand at room temperature.