International audienceThis paper presents a method for ultrasonic characterization of porous silicon in which a genetic algorithm based optimization is used to solve the inverse problem. A one dimensional model describing wave propagation through a water immersed sample is used in order to compute transmission spectra. Then, a water immersion wide bandwidth measurement is performed using insertion/substitution method and the spectrum of signals transmitted through the sample is calculated using Fast Fourier Transform. In order to obtain parameters such as thickness, longitudinal wave velocity or density, a genetic algorithm based optimization is used.A validation of the method is performed using aluminum plates with two different thicknesses as references: a good agreement on acoustical parameters can be observed, even in the case where ultrasonic signals overlap.Finally, two samples, i.e. a bulk silicon wafer and a porous silicon layer etched on silicon wafer, are evaluated. A good agreement between retrieved values and theoretical ones is observed. Hypothesis to explain slight discrepancies are proposed

The piezoelectric properties of compositional spread (1 À x)BiFeO 3 -xGaFeO 3 epitaxial thin films are investigated where Ga 3þ substitution for Bi 3þ is attempted in Bi 1Àx Ga x FeO 3 compounds. Ga content x was varied from 0 to 12% (atomic). Ferroelectric characterizations are reported at various length scales. Around 6.5% of Ga content, an enhancement of the effective piezoelectric coefficient d eff 33 is observed together with a change of symmetry of the film. Measured d eff 33 values in 135 nm thick films increased from 25 pm/V for undoped BiFeO 3 to 55 pm/V for 6.5% Ga with no extrinsic contribution from ferroelastic domain rearrangement. V

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