A one-dimensional optomechanical crystal with a complete phononic band gap.

Recent years have witnessed the boom of cavity optomechanics, which exploits the confinement and coupling of optical and mechanical waves at the nanoscale. Among their physical implementations, optomechanical (OM) crystals built on semiconductor slabs enable the integration and manipulation of multiple OM elements in a single chip and provide gigahertz phonons suitable for coherent phonon manipulation. Different demonstrations of coupling of infrared photons and gigahertz phonons in cavities created by inserting defects on OM crystals have been performed. However, the considered structures do not show a complete phononic bandgap, which should enable longer lifetimes, as acoustic leakage is minimized. Here we demonstrate the excitation of acoustic modes in a one-dimensional OM crystal properly designed to display a full phononic bandgap for acoustic modes at 4 GHz. The modes inside the complete bandgap are designed to have high-mechanical Q-factors, limit clamping losses and be invariant to fabrication imperfections.

Optomechanical interactions in two-dimensional Si and GaAs phoXonic cavities.

We investigate theoretically the optomechanical interactions inside cavities created in two-dimensional infinite phoXonic crystals constituted by a square array of holes in silicon (Si) and gallium arsenide (GaAs) matrices. The cavity is simply obtained by removing one hole in the perfect crystal. Our calculations take into account two mechanisms that contribute to the optomechanical coupling, namely the bulk photoelastic effect and the deformations of the interfaces due to the acoustic strain. The coupling strength is estimated by two different methods, modulation of the photonic mode frequency during one period of the acoustic oscillations and calculation of the optomechanical coupling rate. We discuss the important roles of the symmetry and degeneracy of the modes to discriminate which ones are not able to interact efficiently. Calculations in Si and GaAs crystals at different optical wavelengths emphasize the dependence of the photoelastic contribution to the optomechanical interaction as a function of material and wavelength, especially owing to the significant variation of the photoelastic coefficients near the semiconductor band gap.

Low temperature exchange bias in [DyFe(2)/YFe(2)] superlattices: effect of the thermo-magnetic preparation.

The effect of the thermo-magnetic preparation on exchange bias is investigated in an exchange-coupled [3 nm DyFe(2)/12 nmYFe(2)](22) superlattice. X-ray magnetic circular dichroism (XMCD) experiments at low temperature reveal that exchange bias originates from the quenched DyFe(2) magnetization, biasing the unpinned YFe(2) reversal. This quenched configuration can be tailored by changing the cooling field or the magnetic preparation at 300 K before zero-field cooling. Changing the amplitude of the cooling field induces interface domain walls and tends to modify the orientation of the pinning moments at the interfaces. This results in the observation of single loops and in the continuous variation of the bias field as a function of the cooling field. A specific magnetic preparation (field cycling) at 300 K induces different remanent states with lateral domains in the pinning layer, which remain unchanged at low temperature after zero-field cooling and behave independently. This gives rise to combined loops, whose shape reflects the domain populations.

Factors influencing the transmission and incidence of tropical theileriosis (Theileria annulata infection of cattle) in Morocco.

A longitudinal epidemiological investigation of tropical theileriosis was carried out in an endemic region of Morocco during the 1991 disease season. This involved approximately 220 cross-bred cattle on 15 farms. Data were collected on the frequency of existing infections, the size of the vector tick population and the incidences of new infections and clinical disease, and these were then analysed using statistical models. The prevalence of subclinically infected carriers of Theileria annulata, the number of adult Hyalomma detritum, the vector, and the probability of becoming newly infected with T. annulata increased significantly with the age of cattle, although the age effect on new infections may be a result of increased tick numbers on older animals. The probability of clinical disease in newly infected cattle was not significantly influenced by age or by the number of adult ticks, but was significantly positively associated with the cattle population on the farm. The number of H. detritum nymphs counted on cattle in the autumn was related significantly to the previous adult tick count on the same animal.