A. Miard

Polariton Laser Using Single Micropillar GaAs − GaAlAs Semiconductor Cavities

Polariton lasing is demonstrated on the zero-dimensional states of single GaAs/GaAlAs micropillar cavities. Under nonresonant excitation, the measured polariton ground-state occupancy is found as large as 10(4). Changing the spatial excitation conditions, competition between several polariton lasing modes is observed, ruling out Bose-Einstein condensation. When the polariton state occupancy increases, the emission blueshift is the signature of self-interaction within the half-light half-matter polariton lasing mode.

Enhancement of the spin accumulation at the interface between a spin-polarized tunnel junction and a semiconductor.

We report on spin injection experiments at a Co/Al2O3/GaAs interface with electrical detection. The application of a transverse magnetic field induces a large voltage drop DeltaV at the interface as high as 1.2 mV for a current density of 0.34 nA.microm(-2). This represents a dramatic increase of the spin accumulation signal, well above the theoretical predictions for spin injection through a ferromagnet/semiconductor interface. Such an enhancement is consistent with a sequential tunneling process via localized states located in the vicinity of the Al2O3/GaAs interface. For spin-polarized carriers these states act as an accumulation layer where the spin lifetime is large. A model taking into account the spin lifetime and the escape tunneling time for carriers traveling back into the ferromagnetic contact reproduces accurately the experimental results.

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.

Optically Probing the Fine Structure of a Single Mn Atom in an InAs Quantum Dot

We report on the optical spectroscopy of a single InAs/GaAs quantum dot doped with a single Mn atom in a longitudinal magnetic field of a few Tesla. Our findings show that the Mn impurity is a neutral acceptor state A0 whose effective spin J=1 is significantly perturbed by the quantum dot potential and its associated strain field. The spin interaction with photocarriers injected in the quantum dot is shown to be ferromagnetic for holes, with an effective coupling constant of a few hundreds of mueV, but vanishingly small for electrons.

Spontaneous formation of a polariton condensate in a planar GaAs microcavity

We report on polariton condensation in a planar GaAs microcavity under nonresonant optical excitation. Angularly resolved photoluminescence measurements demonstrate polariton condensation for temperature up to 40 K. Numerical simulations using Boltzmann equations give an overall description of the observed condensation for various detunings and temperatures. This model highlights the importance of the polariton relaxation rate as compared to the polariton decay for condensation to occur on the lowest energy polariton states.

Coherent generation of acoustic phonons in an optical microcavity.

Ultrafast coherent generation of acoustic phonons is studied in a semiconductor optical microcavity. The confinement of the light pulse amplifies both the generation and the detection of phonons. In addition, the standing wave character of the photon field modifies the generation and detection phonon bandwidth. Coherent generation experiments in an acoustic nanocavity embedded in an optical microcavity are reported as a function of laser energy and incidence angle to evidence the separate role of the optical and exciton resonances. Amplified signals and phonon spectra modified by the optical confinement are demonstrated.

Scalable implementation of strongly coupled cavity-quantum dot devices

Using low temperature in situ optical lithography, we fabricate pillar microcavities with quality factors around 2×104. Each pillar embeds a spatially and spectrally resonant single InGaAs quantum dot (QD). Light-matter strong coupling regime is reached for 100% of the fabricated pillars for which the resonance can be tuned through temperature. This is a demonstration of scalable and deterministic implementation of strongly coupled cavity-QD devices.

Strain control of the magnetic anisotropy in (Ga,Mn) (As,P) ferromagnetic semiconductor layers

A small fraction of phosphorus (up to 10%) was incorporated in ferromagnetic (Ga,Mn)As epilayers grown on a GaAs substrate. P incorporation allows reducing the epitaxial strain or even change its sign, resulting in strong modifications of the magnetic anisotropy. In particular a reorientation of the easy axis toward the growth direction is observed for high P concentration. It offers an interesting alternative to the metamorphic approach, in particular for magnetization reversal experiments where epitaxial defects strongly affect the domain wall propagation.

Exciton polaritons in two-dimensional photonic crystals

The strong coupling regime between light and matter is characterized by a reversible and coherent exchange of energy between a single material oscillator and a single mode of the electromagnetic field. A particular case is when excitons confined in a semiconductor quantum well QW are spectrally and spatially resonant with the mode of a vertical semiconductor microcavity, e.g., in structures similar to the vertical cavity surface emitting laser. 1 If the coherent lightmatter coupling overcomes excitonic and photonic dissipation rates, the strong coupling regime can be achieved in these structures. 2,3 As a result, exciton-photon hybrid quasi

Transient chirp in high-speed photonic-crystal quantum-dot lasers with controlled spontaneous emission

We report on a series of experiments on the dynamics of spontaneous emission controlled nanolasers. The laser cavity is a photonic-crystal slab cavity, embedding self-assembled quantum dots as gain material. The implementation of cavity electrodynamics effects increases the large signal modulation bandwidth significantly, with measured modulation speeds of the order of 10 GHz while keeping an extinction ratio of 19 dB. A linear transient wavelength shift is reported, corresponding to a chirp of less than 100 pm for a 35 ps laser pulse. We observe that the chirp characteristics are independent of the repetition rate of the laser up to 10 GHz.