Mathieu Edely

Ultrafast acousto-optic mode conversion in optically birefringent ferroelectrics

The ability to generate efficient giga–terahertz coherent acoustic phonons with femtosecond laser makes acousto-optics a promising candidate for ultrafast light processing, which faces electronic device limits intrinsic to complementary metal oxide semiconductor technology. Modern acousto-optic devices, including optical mode conversion process between ordinary and extraordinary light waves (and vice versa), remain limited to the megahertz range. Here, using coherent acoustic waves generated at tens of gigahertz frequency by a femtosecond laser pulse, we reveal the mode conversion process and show its efficiency in ferroelectric materials such as BiFeO3 and LiNbO3. Further to the experimental evidence, we provide a complete theoretical support to this all-optical ultrafast mechanism mediated by acousto-optic interaction. By allowing the manipulation of light polarization with gigahertz coherent acoustic phonons, our results provide a novel route for the development of next-generation photonic-based devices and highlight new capabilities in using ferroelectrics in modern photonics.

Laser ultrasonics evaluation and testing of coated HTR nuclear fuel

Laser ultrasonics was applied to the manufacturing control of the quality and integrity (no failure) of coated spherical particles designed for High Temperature Reactors (HTR). The coating of the nuclear fuel kernel is designed to prevent from the diffusion of fission products outside the particle during reactor operation. The quality assessment of the coating layers is of major importance. Using laser ultrasonics, we determined the vibration eigenmodes of dummy HTR particles. The vibration spectrum of a HTR particle provides a non-destructive method of evaluating some important mechanical parameters of the coating. Moreover, without damaging the particle, the laser ultrasonics technique allows to test the presence of a crack in the SiC layer, through the observation of the particle vibration spectrum, which is significantly changed, compared to that of a defect-free particle. We applied nanosecond acoustic pulses, i.e., high frequency laser-generated ultrasound, to measure the acoustic velocity of longitudinal waves the SiC layer. This technique provides an alternative method of evaluation of the Young modulus of the SiC layer. We measure the velocity of surface acoustic waves (SAW) on a pyrocarbon layer cross-section and we demonstrated that the anisotropy of the internal pyrocarbon layer can be evaluated by laser ultrasonics.

Generation of acoustical phonons by femtosecond laser pulses in GaAs in the presence of external electric field

Experimental results on the generation and the detection by fs laser pulses of the acoustical phonons at frequencies from tens to hundreds GHz in GaAs in the presence of external electric field are presented. The influence of the magnitude and the direction of the applied electric field on the parameters of ps ultrasound is investigated. Results obtained in GaAs and low‐temperature GaAs are compared. The experimental opportunities to discriminate the acoustical phonons produced by laser‐induced inverse piezoelectrical effect and the acoustical phonons due to the thermoelastic mechanism and to the mechanism of electron‐phonon deformation potential are discussed. This study was supported by ANR project BLAN06‐3‐136284.

On the optical generation and detection of high frequency ultrasounds: thermal and non‐thermal processes

We report on the results of the generation and detection by femtosecond laser pulses of the acoustic waves at frequencies of tens to hundreds GHz in semiconductors and in oxides compounds exhibiting phase transition. We focus first on the generation mechanisms involved to achieve opto‐acoustic transformations. Particular attention will be paid to the cases where classical thermal effects (thermoelastic coupling) drive the mechanism of generation of acoustic phonons and those where non‐thermal effects become significant and sometimes dominant sources of acoustic phonons field. In the latter cases, we will especially focus on the phonons generation based on photo‐induced modifications of microscopic internal electric fields (potential deformation) and also on the use of photoexcited carriers dynamics (carriers recombination) as a tuning parameter of the photo‐generated ultrasounds spectrum. Secondly, we give the examples demonstrating that the choice of optical frequency for ultrasound detection influences ...

Opto‐acousto‐optic evaluation of the physical properties of nanoporous materials

The porous materials with the characteristic dimensions of the pores from few nanometers up to a few hundred of nanometers find applications in microelectronic industry (as low‐k materials), in photovoltaics and for developing of effective chemical sensors. When the pores are ordered in a spatially periodic structure, these systems present photonic and/or phononic properties which are of a prime interest in applied optics and telecommunication (light and/or phonons spectrum control). Here we report how the methods of picosecond laser ultrasonics based on the generation and detection by lasers of the acoustic waves with frequencies in the band of 10 GHz ‐ 1 THz (with the lengths of hundreds of nanometers down to few nanometers) are applied for the evaluation of the mechanical and optical properties of these materials.

Thermo-Optical Properties of Nd0.3Sm0.7NiO3 Ceramic

Nd0.3Sm0.7NiO3 ceramic has been synthesised using a sol-gel method and annealing at oxygen pressure. Nd0.3Sm0.7NiO3 exhibits a reversible metal insulator phase transition which is, in the infrared range, responsible of a thermal-optical contrast. Samples are structurally characterized by X-ray diffraction and thermogravimetric analysis. The Nd0.3Sm0.7NiO3 thermal – optical contrast is characterized in the mid-infrared range, using reflectance and thermal-optical measurements.