5 pagesInternational audienceWe present an all-optical method to investigate the GHz dynamics of the elastic contact between a single metallic nanoparticle and a substrate. A resonant excitation mechanism driven by the 82-MHz Dirac comb of the femtosecond oscillator is associated with femtosecond pump-probe experiments performed in a transient reflectivity configuration. This scheme allows us not only to detect the known breathing mode of the nanoparticle but also to unravel the existence of an axial oscillation of the nanoparticle through an intrinsic common-path interferometer.We measured the eigenfrequency and the lifetime of this vertical motion, which are related to the contact stiffness and hysteresis, and to the acoustic leakage at the nanoparticle-substrate interface. A modeling of the axial oscillation in the framework of classical adhesion theories predicts a simple power law dependence of the axial eigenfrequency with respect to the breathing mode frequency. Measurements performed for single particles with radii ranging from 60 to 700 nm are in strong agreement with this prediction
The picosecond ultrasonic technique is applied for the non-invasive evaluation of sound velocity at a submicron scale in living onion cells. Velocity and attenuation of hypersound in cells are measured by a femtosecond laser pump-probe technique. A nanometric co-polymer layer deposited between the cell and the substrate has been used to improve the photoacoustic signal. Comparison of the measured signals with the photoacoustic responses calculated according to thermoelastic generation mechanism and reflectometric detection shows high sensitivity to the cell adhesion on substrate. Measurements achieved in different vegetal cells illustrate the sensitivity of the technique. In addition to single cell imaging with the high lateral resolution provided by optics (ie ≈1µm), the sensitivity of the measurements to cell compressibility suggests promising perspectives in the field of biology
We present experimental and calculational results demonstrating the thermoelastic generation of shear acoustic waves using femtosecond laser pulses in submicrometric isotropic aluminum films. We show that the generation of the shear waves is correlated to the reduction of the width of the optoacoustic source on the surface. The presence of shear waves is related to acoustic diffraction and acoustic mode conversion at the thin film interfaces.
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