Frequency dependence of tissue attenuation measured by acoustic microscopy

Broadband scanning acoustic microscopy (SAM) has been used to investigate the mechanical properties of sections of tissue with a resolution of around 8 microns. The work reported here extends these results by reporting the frequency dependence of the attenuation coefficient from 100-500 MHz. A discussion of the theory of the measurements is presented. The scanning laser acoustic microscope (SLAM) is used to characterize similar tissue sections at 100 MHz. The data obtained with the two forms of acoustic microscopy are compared with results from the literature.     

Frequency dependence of the acoustic radiation force acting on absorbing cylindrical shells

The frequency dependence of the radiation force function Y(p) for absorbing cylindrical shells suspended in an inviscid fluid in a plane incident sound field is analysed, in relation to the thickness and the content of their interior hollow region. The theory is modified to include the effect of hysteresis type absorption of compressional and shear waves in the material. The results of numerical calculations are presented for two viscoelastic (lucite and phenolic polymer) materials, with the hollow region filled with water or air indicating how damping and change of the interior fluid inside the shell's hollow region affect the acoustic radiation force. The acoustic radiation force acting on cylindrical lucite shells immersed in a high density fluid (in this case mercury) and filled with water in their hollow region, is also studied.     

Frequency dependence of the amplitude of domain-wall oscillations in an acoustic wave field

The velocity of oscillatory motion of domain walls is investigated as a function of the parameters of a magnetic material and an external acoustic field. The dependence of the amplitude of domain-wall oscillations on the frequency of an external acoustic wave is determined. It is found that this dependence exhibits a resonant behavior.     

Frequency dependence of acoustic parameters of freshly excised tissues of Sprague Dawley rats

A modified version of the pulse echo technique was used to measure the velocity of propagation and attenuation of ultrasound in excised tissue of young-adult Sprague-Dawley rats. The measurements were made at ultrasonic frequencies of 1.0, 2.25, 5.0, 7.5 and 10.0 MHz. The temperature of the tissues was monitored continuously to within ± 0.1°C of the ambient temperature. The acoustic parameters were measured in the liver, kidney, cardiac muscle and gastrocnemius muscle. All measurements were carried out in the near field region of the ultrasonic beam. It was observed that the velocity of propagation in tissues showed a slight dispersion with frequency. The attenuation in tissues increased with increasing frequency.     

Frequency dependence of acoustic properties of aqueous glucose solutions in the VHF/UHF range

The bioultrasonic spectroscopy system was employed for measurements of velocity and attenuation coefficient of glucose solutions in the VHF/UHF range. The relation between the slope of the square of velocity and the relaxation parameters, and the relation between the frequency exponent on attenuation coefficient and the relaxation parameters are investigated. In order to carry out numerical calculations, a model for a single relaxation process is employed, wherein the attenuation coefficient is expressed as (A/( 1 + (f/falpha)2) + B)f2 where falpha is the attenuation relaxation frequency, and A and B are constants. The numerical calculations show that the slope of the square of the velocity is determined uniquely by the velocity relaxation frequency fv and v(infinity)2 - v(0)2 where v0 is the zero-frequency velocity and v(infinity) is the infinite-frequency velocity, and that the frequency exponent on the attenuation coefficient is determined uniquely by falpha and A/B. For experimental considerations, the velocities and the attenuation coefficients of 5, 15, and 25% concentration aqueous solutions of glucose were measured in the frequency range 20 to 700 MHz. The data for the 5 and 15% aqueous solutions can be explained using the single relaxation model. However, the data for the 25% aqueous solution suggest the existence of multirelaxation processes.     

Electronic tuning of acoustic resonances in acousto-optic mode lockers

The effect of electronic tuning of acoustic resonances in an acousto-optic mode locker is studied theoretically and experimentally. The tuning is implemented by means of changing a matching inductance connected to the transducer in parallel. Experimental investigations are carried out with a mode locker made of a fused quartz with a lithium niobate transducer. Varying magnitude of the inductor from 0.025 to 0.25 mH has made it possible to retune the acoustic resonance frequency by 0.19 MHz, i.e. wider than the acoustic resonance half-width.     

FEM calculation of an acoustic field in a sonochemical reactor

The spatial distribution of the acoustic amplitude in a sonochemical reactor has been numerically calculated using the finite element method (FEM). In the FEM program, the acoustic field in a sonochemical reactor is coupled with the vibration of the reactor's wall. The present calculations have revealed that the thin (thick) glass or stainless steel wall is nearly a free (rigid) boundary and that the glass wall is freer than the stainless steel wall. The influence of the attenuation coefficient of ultrasound on the acoustic field has also been studied in order to see the effect of bubbles on the acoustic field. As the attenuation coefficient increases, the vibration amplitude of the reactor's wall becomes smaller and the acoustic emission from the vibrating wall becomes weaker. The qualitative feature of the spatial pattern of sonochemiluminescence from an aqueous luminol solution has been reproduced by the calculation when the attenuation coefficient is in the range of 0.5-5m(-1). When the attenuation coefficient is less than about 0.05 m(-1), the standing wave pattern of an acoustic field in the liquid is very complex due to the acoustic emission from the vibrating wall. The present calculations have also revealed that some stripes of pressure antinodes have also been disconnected when the radius of the transducer is much smaller than the side length of the vibrating plate. The dependence of the acoustic field on the liquid height is also discussed.     

Coherent control of ultrahigh-frequency acoustic resonances in photonic crystal fibers

Ultrahigh frequency acoustic resonances (approximately 2 GHz) trapped within the glass core (approximately 1 microm diameter) of a photonic crystal fiber are selectively excited through electrostriction using laser pulses of duration 100 ps and energy 500 pJ. Using precisely timed sequences of such driving pulses, we achieve coherent control of the acoustic resonances by constructive or destructive interference, demonstrating both enhancement and suppression of the vibrations. A sequence of 27 resonantly-timed pulses provides a 100-fold increase in the amplitude of the vibrational mode. The results are explained and interpreted using a semianalytical theory, and supported by precise numerical simulations of the complex light-matter interaction.     

基于高次体波谐振的弹性板厚度测量方法研究

利用压电片在弹性板中激发出的高次体波谐振,提出一种评估弹性板厚度变化的测量方法。根据该层状结构的高次谐振的频率随弹性板厚度变化的关系,给出测量方法的理论分析和数值计算。计算结果表明,并联谐振频率和并联谐振频率间隔与弹性板厚度的偏差量相关。其中,并联谐振频率随厚度偏差量呈线性变化,并具有周期性跳变规律。研究这种跳变规律,能提升高次谐振模式测量板厚的精度。此外,通过对数值仿真算例和实验测量比较,验证了本测量方法的可行性。     

Experimental observation of nonlinear ion - acoustic resonances in bounded plasma

The response of well matched Nb point-contract junctions, with a relatively small ratio of barrier and noise energy, has been measured at 35 GHz and is compared with theories on Josephson tunneling in the presence of strong fluctuations.     

Position dependence of average electron velocity in a submicrometer GaAs channel

The Monte Carlo method has been applied to obtain the average electron velocity at different positions of a submicrometer GaAs channel in the presence of a position independent electric field. Velocity-distance curves are presented for channel lengths of 0.1, 0.2, and 0.5 m and for lattice temperatures of 300 and 77 K. The curves show significant effects of collisions and boundary conditions.     

Frequency modulation spectroscopy of coherent dark resonances of multi-level atoms in a magnetic field

The resonances of coherent population trapping (CPT) excited by a frequency-modulated (FM) field at the Zeeman sublevels during the transition F _g = 2 → F _e = 1 of the ⁸⁷Rb D ₁-absorption line were studied theoretically. The influence of the nonlinear Zeeman effect on the structure of the observed resonances was considered. The spectra of CPT resonances were calculated for different values of magnetic field induction and compared with the experimental data.     

Frequency dependence of the magnetoelectric effect in a magnetostrictive-piezoelectric heterostructure

The frequency dependence of the magnetoelectric effect in a magnetostrictive-piezoelectric heterostructure is theoretically studied by solving combined magnetic, elastic, and electric equations with boundary conditions. Both the mechanical coupling coefficient and the losses of the magnetostrictive and piezoelectric phases are taken into account. The numerical result indicates that the magnetoelectric coefficient and the resonance frequency are determined by the mechanical coupling coefficient, losses, and geometric parameters. Moreover, at the electromechanical resonance frequency, the module of the magnetoelectric coefficient is mostly contributed by the imaginary part. The relationship between the real and the imaginary parts of the magnetoelectric coefficient fit well to the Cole-Cole circle. The magnetostrictive-piezoelectric heterostructure has a great potential application as miniature and no-secondary coil solid-state transformers.     

Two-fluid model description of isovector giant resonances in fast rotating nuclei

Isovector giant resonances of arbitrary multipolarity in fast rotating nuclei are studied by solving the inviscid two-fluid equation of relative motion in a rotating frame of reference. Both Coriolis and centrifugal forces are taken into account. The resulting expressions display in a quite simple way general features of giant multipole resonances of fast rotating nuclei, in addition to a good agreement with other calculations for the giant dipole resonance. Typical values for the resonance energies and their fragmentation due to nuclear deformation and rotation are given. In particular, enormously large resonance splitting should occur in the superdeformed states.     

Excitation of autoionization resonances in collisions of helium atoms with fast ions

A theoretical model of collisions between fast ions and atoms is proposed which describes the effect of projectiles on the excitation of autoionization resonances. The model takes into account the change in the binding energy of electrons in a target atom induced by the field of a projectile, the effect of the field of the atom on the kinematics of the ion scattering, as well as the effect of the intermediate (1snl)¹ L states on the two-electron excitation mechanism. The charge dependence of the excitation cross section of the (2s ²)¹ S and (2s2p)¹ P resonances is found to be weaker than in the first order of the perturbation theory and is in qualitative agreement with experimental data. The reasons for the emergence of such a charge dependence are analyzed.     

Statistics of resonances and time reversal reconstruction in aluminum acoustic chaotic cavities

The statistical properties of wave propagation in classical chaotic systems are of fundamental interest in physics and are the basis for diagnostic tools in materials science. The statistical properties depend in particular also on the presence of time reversal invariance in the system, which can be verified independently by time reversal reconstruction experiments. As a model system to test the combination of statistical properties with the ability to perform time reversal reconstruction we investigated chaotic systems with time reversal invariance using ultrasonic waves in aluminum cavities. After excitation of the samples with a short acoustic pulse the reverberation responses were recorded and analyzed. In the analysis of the spectral density of the recorded responses we explicitly included the fact that not all resonances are detected. Reversibility of the excited wave dynamics in the cavity after a time delay was studied by reconstruction of the excitation pulse in time reversal experiments. The statistical properties of resonance frequencies in the cavities were obtained from the reverberant responses. The distribution of the transmission intensities displays random division of intensity between cavity waves in narrow frequency bands. The distribution of frequency spacing between neighboring cavity resonances and the spectral rigidity agree with the predictions for the Gaussian Orthogonal Ensemble. This agreement is achieved if a fraction of typically 25 percent of resonances is not detected in the experiment. The normalized amplitude of the pulse that is reconstructed in the time reversal experiments decays exponentially with the time delay between the original excitation pulse and the end of the reversed oscillation track. The exponential behavior exists for time delays longer than the inverse of the nearest neighbor resonance spacing.     

All-optical control of gigahertz acoustic resonances by forward stimulated interpolarization scattering in a photonic crystal fiber

We report the observation of a novel nonlinear optoacoustic phenomenon, that we name forward stimulated interpolarization scattering. When two frequency-offset laser signals are colaunched into orthogonally polarized guided modes of a birefringent small-core (1.8 μm diameter) photonic crystal fiber, a pattern of axially moving polarization fringes is produced, with a velocity and spacing that depends on the frequency offset. At values of frequency offset in the few-GHz range, the pattern of moving fringes can perfectly match the phase velocity and axial wavelength (3.9 mm) of the torsional-radial acoustic mode tightly guided in the core. An intense optoacoustic interaction ensues, leading to efficient nonlinear exchange of power from the higher frequency (pump) mode to the orthogonally polarized lower frequency (Stokes) mode. A full-vectorial theory is developed to explain the observations.     

Frequency separation of surface acoustic waves in layered structures with acoustic metamaterials

We show theoretically that in elastic layered structures containing an upper layer of smoothly varied thickness and a substrate of a highly dispersive metametarial it is possible to significantly enhance spatial frequency separation of surface acoustic waves. Theory of Love surface acoustic waves propagation in waveguides with varied thickness, taking into account mutual modes coupling, is built. Appropriate structure of metamatererial with resonant frequency dependence of material parameters, making frequency separation effective, is provided. Efficiency of spatial frequency separation and modes coupling is calculated for various metamaterial parameters and wave frequencies.