Spin polarization of semiconductor carriers by reflection off a ferromagnet

We present a theory of generation or alteration of the electron spin coherence and population in an n-doped semiconductor by reflection at the interface with a ferromagnet. The dependence of the spin reflection on the Schottky barrier height and the doping concentration in the semiconductor was computed for a generic model. The theory provides an explanation for the spontaneous electron spin coherence and nuclear polarization in the semiconductor interfaced with a ferromagnet and associated phenomena recently observed by time-resolved Faraday rotation experiments. The study also points to an alternative approach to spintronics different from spin injection.     

Parametric generation of twin photons in vertical triple microcavities

We report the realization of a monolithic vertical-cavity, surface emitting micro-optical parametric conversion nanostructure, triply resonant with the parametric frequencies, allowing parametric oscillation with ultra-low pump power threshold. The photonic phase-space naturally provides triple resonance for the parametric frequencies, together with built-in cavity phase-matching for the pump wave at normal incidence. Parametric oscillation is observed in both the strong and weak exciton–photon coupling regime, allowing a high operating temperature. Signal and idler beams can be collected at 0° or at finite angles. The OPO threshold is low enough to envisage the realization of an all-semiconductor electrically-pumped micro-parametric oscillator. To cite this article: C. Diederichs et al., C. R. Physique 8 (2007).     

Cavitation activity stimulation by low frequency field pulses

The influence of a short-time action of a low-frequency ultrasound on the sonoluminescence generation by a high frequency pulsed field has been studied. This action remarkably lowers the cavitation thresholds and increases the sonoluminescence intensity. The stimulating effect of the low-frequency field action depends on its duration and on the intensities of both fields. Possible mechanisms of this effect are discussed.     

Enhancement of high-frequency acoustic cavitation effects by a low-frequency stimulation

The cavitation effects given by a high-frequency pulsed ultrasound field are studied with and without the stimulation of a low-frequency field. Sonoluminescence intensity and subharmonic one-half intensity of the high-frequency field are measured. The stimulation gives a sharp rise of both subharmonic and sonoluminescence intensities.     

Coherent control of polariton parametric scattering in semiconductor microcavities

In a pump-probe experiment, we have been able to control, with phase-locked probe pulses, the ultrafast nonlinear optical emission of a semiconductor microcavity, arising from polariton parametric amplification. This evidences the coherence of the polariton population near k=0, even for delays much longer than the pulse width. The control of a large population at k=0 is possible although the probe pulses are much weaker than the large polarization they control. With rising pump power the dynamics of the scattering get faster. Just above threshold the parametric scattering process shows unexpected long coherence times, whereas when pump power is risen the contrast decays due to a significant pump reservoir depletion. The weak pulses at normal incidence control the whole angular emission pattern of the microcavity.     

Light modulation by a real stationary ultrasonic field

The temporal modulation of a light beam diffracted by a real stationary ultrasonic field in a liquid is studied. The stationary fields, experimentally obtained, are really ‘pseudo-stationary’, due to volume absorption and reflection losses.The modulation waveforms are analysed in a ‘thick-screen model’. The pseudo-stationary field has been considered as built up both by two antiparallel progressive beams of different amplitude, and by a stationary field plus a residual progressive beam. Different modulation formulae are obtained for beams either spatially superposed or placed at a finite distance.     

Optical visualization of non-linear acoustic propagation in cavitating liquids

Non-linearity effects on sound propagation induced by cavitation bubbles are investigated. The convergence of an acoustic wave due to the interaction with the microbubbles produced in the cavitation zone is shown experimentally. In these conditions the theoretical analysis shows that the self-focusing primarily depends on the effective microbubble volume fraction. This fraction turns out to be about 10^?6 with a corresponding self-focusing distance of about 9 cm in the Fraunhofer region of a plane circular transducer.     

Quantum vacuum radiation spectra from a semiconductor microcavity with a time-modulated vacuum Rabi frequency

We develop a general theory of the quantum vacuum radiation generated by an arbitrary time modulation of the vacuum Rabi frequency of an intersubband transition in a doped quantum well system embedded in a planar microcavity. Both nonradiative and radiative losses are included within an input-output quantum Langevin framework. The intensity and the spectral signatures of the extra-cavity emission are characterized versus the modulation properties. For realistic parameters, the photon pair emission is predicted to largely exceed the blackbody radiation in the mid and far infrared. For strong and resonant modulation a parametric oscillation regime is achievable.     

Enhancement of sonoluminescence emission from a multibubble cavitation zone

Investigations have been performed on various methods of increasing cavitation activity measured by the intensity of sonoluminescence. It is shown that the effect of the combined action of (a) pulsed modulation of an acoustic field, (b) liquid degassing and cooling and (c) increasing the static pressure considerably exceeds the sum of the effects achieved by each of these methods individually. A more than 250-fold increase of the sonoluminescence intensity has been attained compared with continuous irradiation under normal conditions (room temperature, atmospheric pressure, gas-saturated liquid). An interpretation of the results obtained is proposed.     

Electrically injected cavity polaritons

We have realized an electroluminescent device operating in the light-matter strong-coupling regime based on a GaAs/AlGaAs quantum cascade structure embedded in a planar microcavity. At zero bias, reflectivity measurements show a polariton anticrossing between the intersubband transition and the cavity mode. Under electrical injection the spectral features of the emitted light change drastically, as electrons are resonantly injected in a reduced part of the polariton branches. Our experiments demonstrate that electrons can be selectively injected into polariton states up to room temperature.