C. Rossignol

In Vitro picosecond ultrasonics in a single cell

Ultrasonics signals at frequencies 5.7±0.1 and 6.8±0.1GHz are measured in two organelles of a single vegetal cell in vitro with a picosecond ultrasonic technique. Using standard values for cell optical index, ultrasound velocities of 1.6±0.1 and 2.0±0.1μm∕ns are measured from several signals recorded in the vacuole and in the nucleus of a single Allium cepa cell, respectively. A 1μm lateral and 0.25μm depth resolution is attained.

Acoustic waves generated by a laser line pulse in a transversely isotropic cylinder

The acoustic field of a homogeneous and transversely isotropic cylinder generated by a laser line pulse in either ablation or thermoelastic regime is obtained theoretically. A two-dimensional Fourier transform is used to calculate the acoustic displacement at the cylinder surface. Experimental and theoretical normal displacements under either regime are obtained and compared for aluminum cylinders. Very good agreements are observed in the time, shape, and relative amplitude (i) of the cylindrical Rayleigh waves with different roundtrips and (ii) of the various longitudinal and transverse bulk waves propagating through the cylinder or reflected at the free circular surface.

Picosecond acoustics in vegetal cells: Non-invasive in vitro measurements at a sub-cell scale

A 100 fs laser pulse passes through a single transparent cell and is absorbed at the surface of a metallic substrate. Picosecond acoustic waves are generated and propagate through the cell in contact with the metal. Interaction of the high frequency acoustic pulse with a probe laser light gives rise to Brillouin oscillations. The measurements are thus made with lasers for both the opto-acoustic generation and the acousto-optic detection, and acoustic frequencies as high as 11 GHz can be detected, as reported in this paper. The technique offers perspectives for single cell imaging. The in-plane resolution is limited by the pump and probe spot sizes, i.e. approximately 1 microm, and the in-depth resolution is provided by the acoustic frequencies, typically in the GHz range. The effect of the technique on cell safety is discussed. Experiments achieved in vegetal cells illustrate the reproducibility and sensitivity of the measurements. The acoustic responses of cell organelles are significantly different. The results support the potentialities of the hypersonic non-invasive technique in the fields of bio-engineering and medicine.

Acoustic waves generated by a laser point source in an isotropic cylinder

The acoustic field of a homogeneous and isotropic cylinder generated by a laser point source in either ablation or thermoelastic regime is obtained theoretically. A three-dimensional Fourier transform is used to calculate the acoustic displacement at the cylinder surface. Experimental waveforms were measured and analyzed for both regimes. Theoretical normal displacements under either regime are calculated and compared to the experimental signals for aluminum cylinders. Very good agreements are observed in the arrival time, shape, and relative amplitude (i) of the cylindrical Rayleigh waves with different round trips, and (ii) of the various longitudinal and transverse bulk waves propagating through the cylinder or reflected at the free circular surface.

Optoacoustic response of a single submicronic gold particle revealed by the picosecond ultrasonics technique

The optoacoustic response of a single submicron (430 nm) gold particle embedded in a silica thin film is experimentally revealed by femtosecond pump-probe experiments. A semianalytical model is developed to calculate the transient reflectivity accounting for optical index changes in both media and for particle and film surface displacements. The displacement of the particle-film interface turns out to be the major contribution to the measured signal. The amplitude of the acoustical component of the transient reflectivity is modulated by the depth at which the particle is buried in the film.

Probing single-cell mechanics with picosecond ultrasonics

The mechanical properties of cells play a key role in several fundamental biological processes, such as migration, proliferation, differentiation and tissue morphogenesis. The complexity of the inner cell composition and the intricate meshwork formed by transmembrane cell-substrate interactions demands a non-invasive technique to probe cell mechanics and cell adhesion at a subcell scale. In this paper we review the use of laser-generated GHz acoustic waves--a technique called picosecond ultrasonics (PU)--to probe the mechanical properties of single cells. We first describe applications to vegetal cells and biomimetic systems. We show how these systems can be used as simple models to understand more complex animal cells. We then present an opto-acoustic bio-transducer designed for in vivo measurements in physiological conditions. We illustrate the use of this transducer through the simultaneous probing of the density and compressibility of Allium cepa cells. Finally, we demonstrate that this technique can quantify animal-cell adhesion on metallic surfaces by analyzing the acoustic pulses reflected off the cell-metal interface. This innovative approach allows investigating quantitatively cell mechanics without fluorescent labels or mechanical contact to the cell.

Surface displacement measured by beam distortion detection technique: Application to picosecond ultrasonics

A sensitive technique of surface displacement measurement without interferometry is proposed for the goals of picosecond ultrasonics. Simple description of detection mechanism is provided on the basis of paraxial approximation of light diffraction. Test experiments with gold and tungsten layers have been performed and analyzed. The efficiency of the technique is compared with interferometry and reflectometry methods.

Effect of laser pulse duration in picosecond ultrasonics

An optical grating has been introduced in a picosecond ultrasonics experiment, in order to vary continuously the duration of the laser beam pulse from 0.1to150ps. The evolution of the measured signal has been observed and analyzed through the comparison with a theoretical approach based on a two-temperature model. The latter allows matching the acoustic echoes together with the thermal background and the coincidence peak, for each pulse duration and at any time scale. The broadening of the acoustic echoes and the disappearing of its Brillouin component, along with the diminishing of the thermal coincidence peak, have been demonstrated when increasing the pulse duration. For a constant incident pulse energy, the efficiency of acoustic generation is optimum for the shortest pulses. Nevertheless, for longer pulses designed to obtain thermal conditions below the ablation threshold, acoustic generation could be enhanced.

Identification of laser generated acoustic waves in the two-dimensional transient response of cylinders

The published model [Appl. Phys. Lett. 82, 4379-4381 (2003)] for the two-dimensional transient wave propagation in a cylinder is modified to avoid the inherited integration of the numerical inverse scheme. The Fourier series expansion is introduced for one spatial coordinate to resolve the transient response problem: theoretical radial displacements in either the ablation or the thermoelastic regime are obtained with little numerical noise and short computation time. The normal mode expansion method fails to deliver results with the same accuracy. Acoustic waves are fully identified by the ray trajectory analysis. These identified waves are further verified on the experimental results observed with the laser ultrasonic technique.

Acoustic waves generated by a laser line pulse in cylinders; Application to the elastic constants measurement

A model is proposed to predict acoustic waves generated in a transversely isotropic cylinder by a laser line pulse extended in beamwidth and time duration, and an application to elastic constants measurement is presented. Documented good agreements are observed in the comparison of experimental and theoretical normal displacements for aluminum cylinders under either ablation or thermoelastic generation. Bulk waves are identified and processed for the elastic constants measurement. The effects of source beamwidth and time duration on wave forms and on the elastic constants measurement are predicted by numerical simulations. For nondestructive evaluation applications using bulk waves, a radius of 0.3 mm appears as a minimum limit for the sample size using a laser source of 0.1 mm beamwidth and 20 ns time duration. Elastic constants of aluminum rods are experimentally measured with very good accuracy.