Nikolay Chigarev

Laser generation and detection of longitudinal and shear acoustic waves in a diamond anvil cell

Laser ultrasonics in a point-source-point-receiver configuration is applied for the evaluation of elastic properties of nontransparent materials in a diamond anvil cell at high pressures. Measurement of both longitudinal and shear acoustic wave velocities in an iron foil at pressures up to 23 GPa does not require any information in addition to the one obtained by all-optical pump-probe technique.

All-optical probing of the nonlinear acoustics of a crack.

Experiments with an all-optical method for the study of the nonlinear acoustics of cracks in solids are reported. Nonlinear acoustic waves are initiated by the absorption of radiation from a pair of laser beams intensity modulated at two different frequencies. The detection of acoustic waves at mixed frequencies, absent in the frequency spectrum of the heating lasers, by optical interferometry or deflectometry provides unambiguous evidence of the elastic nonlinearity of the crack. The high contrast in crack imaging achieved by remote optical monitoring of the nonlinear acoustic processes is due to the strong dependence of the efficiency of optoacoustic conversion on the state of the crack. The highest acoustic nonlinearity is observed in the transitional state of the crack, which is intermediate between the open and the closed ones.

Nonlinear frequency-mixing photoacoustic imaging of a crack: Theory

One-dimensional theory of the nonlinear frequency-mixing photoacoustic crack imaging is developed. This imaging can be realized through the excitation of the crack by two laser beams independently modulated in intensity at two very different frequencies ωH⪢ωL and the detection of the components of photoacoustic spectrum at frequencies ωH±nωL with an integer n. It is predicted that the high contrast of this imaging can be caused by strong dependence of the efficiency of photoacoustic conversion on the mechanical state of the crack, i.e., on whether the crack is open or is at least partially closed due to the contacts between the crack faces. The theory relates earlier experimental observation of the large number of the side-lobes ωH±nωL to strong bimodular nonlinearity of the crack. In response to sinusoidal modulation of pump laser intensity at low frequency ωL the rigidity of the crack varies in a strongly nonsinusoidal manner through abrupt jumps between its value corresponding to an open soft state of ...

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.

Nonlinear frequency-mixing photoacoustic imaging of a crack

We present a technique for nonlinear photoacoustic imaging of cracks by laser excitation with intensity modulation at two fundamental frequencies combined with detection at mixed frequencies. By exploiting the strong dependence of the photoacoustic emission efficiency on the state—open or closed—of the contacts between the crack faces, remarkably enhanced image contrast is observed, ∼20 times higher than in linear photoacoustic images at the highest of the fundamental frequencies.

Probing of laser-induced crack modulation by laser-monitored surface waves and surface skimming bulk waves.

All-optical monitoring of the nonlinear motion of a surface-breaking crack is reported. Crack closing is induced by quasi-continuous laser heating, while Rayleigh surface acoustic pulses and bulk longitudinal surface skimming acoustic pulses are also generated and detected by lasers. By exploiting the strong dependence of the acoustic pulses reflection and transmission efficiency on the state-open or closed-of the contacts between the crack faces, the parametric modulation of ultrasonic pulses is achieved. It is observed that bulk acoustic waves skimming along the surface can be more sensitive to crack motion than Rayleigh surface waves.

Probing of laser-induced crack closure by pulsed laser-generated acoustic waves

The monitoring and characterization of laser-heated crack by the laser ultrasonics technique are reported. In comparison with existing studies, where the Rayleigh and bulk skimming waves were generated by laser-induced line source, the point source is used here. Crack closure by thermoelastic stresses modifies the propagation paths of the acoustic rays from a point source to a point receiver. Thus, the arrival times of the acoustic waves contain useful information on the state of crack closure induced by a particular level of laser heating. An important dependence of the detected signals on the initial width/state of the crack and a presence of local necks/narrowings in the crack are revealed. It is demonstrated that the mode conversion of the incident skimming longitudinal bulk waves into the transmitted Rayleigh waves is very sensitive to imperfectness of cracks closure. The proposed interpretation of the laser-ultrasonics experimental observations is supported by atomic force microscopy measurements.

Analysis of ultrasonic echoes induced by pulsed laser action on an iron film in a diamond anvil cell

The results of laser ultrasonic experiments with iron film under high pressure are described. In addition to skimming and bulk acoustic modes observed and analyzed in a previous study [1], the detection of “mixed modes” or head waves is reported. For these modes, the acoustic disturbance propagates along a part of its ray trajectory between the excitation and detection point as a skimming wave on the diamond/iron interface and along another part of its ray trajectory as a bulk wave in iron. Experimentally measured arrival times of the head waves were found to be in a good agreement with the theory.

Revealing sub-μm and μm-scale textures in H2O ice at megabar pressures by time-domain Brillouin scattering

The time-domain Brillouin scattering technique, also known as picosecond ultrasonic interferometry, allows monitoring of the propagation of coherent acoustic pulses, having lengths ranging from nanometres to fractions of a micrometre, in samples with dimension of less than a micrometre to tens of micrometres. In this study, we applied this technique to depth-profiling of a polycrystalline aggregate of ice compressed in a diamond anvil cell to megabar pressures. The method allowed examination of the characteristic dimensions of ice texturing in the direction normal to the diamond anvil surfaces with sub-micrometre spatial resolution via time-resolved measurements of the propagation velocity of the acoustic pulses travelling in the compressed sample. The achieved imaging of ice in depth and in one of the lateral directions indicates the feasibility of three-dimensional imaging and quantitative characterisation of the acoustical, optical and acousto-optical properties of transparent polycrystalline aggregates in a diamond anvil cell with tens of nanometres in-depth resolution and a lateral spatial resolution controlled by pump laser pulses focusing, which could approach hundreds of nanometres.

Surface motion induced by laser action on opaque anisotropic crystals

Analytical solutions for the acoustic wave equations obtained by temporal Fourier and spatial Laplace transformations directly provide a description of the motion of the crystal surface caused by the spatially distributed laser heating of a semi-infinite crystal. Evaluation of the acoustic field in the bulk of the material is not needed here. In general, all three acoustic modes are excited due to the laser-induced thermoelastic effect and contribute to each of the three components of the transient surface displacement. Numerical simulations of the surface displacement as a function of time and crystal surface orientation are performed with the use of the analytical formulae derived in the case of a hexagonal crystal, for which only two modes are excited. The formulae obtained make it possible to optimize the orientation of the surface of the crystal in order to improve the efficiency of the excitation of the in-plane motion of the surface.