114 research outputs found
STRUCTURAL DIAGNOSTICS OF GEOMATERIALS BY LASER ULTRASONIC SPECTROSCOPY
Abstract The possibilities of laser ultrasonic echoscopy for detection and diagnostics of rock structural imperfections are considered. It is shown, that calculating with the help of fast Fourier transformation spectra of the signals, arrived in a sample and multiple scattered, and carrying out a corresponding frequency filtration, it is possible to obtaine environment reflaction factor and on its basis to construct the sample structure image.The examples of forming images of structural changes under different influences on the rock samples are presented. The ferruginous quartzite samples were influenced by pulse electromagnetic field, the Karelian marble samples -thermal field
Resonant properties of a nonlinear dissipative layer excited by a vibrating boundary: Q-factor and frequency response
Noninvasive laser-induced photoacoustic tomography for structural and functional in vivo imaging of the brain
Imaging techniques based on optical contrast analysis can be used to visualize dynamic and functional properties of the nervous system via optical signals resulting from changes in blood volume, oxygen consumption and cellular swelling associated with brain physiology and pathology. Here we report in vivo noninvasive transdermal and transcranial imaging of the structure and function of rat brains by means of laser-induced photoacoustic tomography (PAT). The advantage of PAT over pure optical imaging is that it retains intrinsic optical contrast characteristics while taking advantage of the diffraction-limited high spatial resolution of ultrasound. We accurately mapped rat brain structures, with and without lesions, and functional cerebral hemodynamic changes in cortical blood vessels around the whisker-barrel cortex in response to whisker stimulation. We also imaged hyperoxia- and hypoxia-induced cerebral hemodynamic changes. This neuroimaging modality holds promise for applications in neurophysiology, neuropathology and neurotherapy
Temporal fluctuations of waves in weakly nonlinear disordered media
We consider the multiple scattering of a scalar wave in a disordered medium
with a weak nonlinearity of Kerr type. The perturbation theory, developed to
calculate the temporal autocorrelation function of scattered wave, fails at
short correlation times. A self-consistent calculation shows that for
nonlinearities exceeding a certain threshold value, the multiple-scattering
speckle pattern becomes unstable and exhibits spontaneous fluctuations even in
the absence of scatterer motion. The instability is due to a distributed
feedback in the system "coherent wave + nonlinear disordered medium". The
feedback is provided by the multiple scattering. The development of instability
is independent of the sign of nonlinearity.Comment: RevTeX, 15 pages (including 5 figures), accepted for publication in
Phys. Rev.
Noninvasive laser-induced photoacoustic tomography for structural and functional in vivo imaging of the brain
Determination of optical parameters of pulp suspensions by time-resolved detection of photoacoustic signals and total diffuse reflectance measurements
Inhomogeneous Burgers Equation with Modular Nonlinearity : Excitation and Evolution of High-Intensity Waves
Solutions to an inhomogeneous partial differential equation of the second-order like Burgers equation are derived. Instead of the common quadratically nonlinear term, this equation contains the term with modular nonlinearity. This model describes the excitation of elastic waves in dissipative media differently reacting to tensile and compressive stresses. The equation is linear for the functions, preserving the sign. Nonlinear effects are manifested only to alternating functions. The solution for the periodic signal is found. The processes of generation of fundamental and higher harmonics are studied. The stationary wave profile is constructed. For one special kind of right-hand-side of the "modular" equation the solution in the form of S-wave is pointed out which is a bipolar single pulse.open access</p
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