Parametric instability of a standing gravity–capillary wave is observed in conditions of irradiation of a surface of a liquid by a plane ultrasound beam modulated by amplitude. A threshold value of ultrasound intensity for excitation of the parametric oscillations of the surface is determined, and the frequency response of the process is measured. A mechanism of instability based on sound radiation pressure applied to a liquid surface curved with menisci is suggested.

The nonlinear pressure profile at the focus of an ultrasonic beam radiated into water at a frequency of 2.0 MHz and having a shock front with amplitude of 50 MPa is reconstructed for the first time using Raman lidar signals. This is possible due to the relationship between the degree of deformation of the O–H stretching vibration band in the Raman spectrum of water, the amplitude of elastic scattering, and the pressure at the sounding site. The lidar profile is in good agreement with a PVDF hydrophone profile.

It has been demonstrated for the first time that pulsed laser Raman spectroscopy can be used for diagnostics of a local acoustic pressure profile with a peak pressure drop of 50 MPa and a carrier frequency of 2.0 MHz in the focus of an ultrasound beam propagating in water. A 527-nm 10-ns laser pulse has been focused into the waist of the ultrasound beam at an angle of 90°. Backscattered photons have been recorded in a gated spectrum analyzer. It has been found that the Raman spectra at the times corresponding to the maximum and minimum acoustic pressures are significantly different. This feature has been used for point-to-point reconstruction of the acoustic pressure profile; for this purpose, the delay between the ultrasound and laser pulses is consequently increased with a step of 50 ns. It has been shown that, within the measurement error, the resulting changes in the position of the center of the stretching OH vibration band of water molecules in the Raman spectrum reproduce the acoustic pressure profile directly measured using a PVDF hydrophone at the laser sensing point. The results obtained can be used to develop a new method for remote diagnostics of the time profile of acoustic pressure and monitoring the local dynamic of the compression-tension processes in water up to critical pressures corresponding to the cavitation rupture, when the use of the hydrophone can lead to its damage.

For the first time, nanoparticles are observed that result from ablation of a bulk solid-state sample in water as its surface is affected by high-intensity focused ultrasound pulses with a carrier frequency of 1.8 MHz and peak-to-peak acoustic pressure of 50 MPa in the focus. As photo and video recordings show, formation of ablation particles and their ejection into water under these extreme conditions is of local and spallation-explosive character. Size and mass distributions of the ablation particles measured using a laser particle analyzer reveal that ultrasound ablation results in formation of nanometer-sized particles among others. The size of these particles mostly ranges between 20 and 60 nm with the distribution maximum at 35 nm.

Synchronous heating of two local regions of an absorbing medium by phase conjugate ultrasound beams focused on them has been experimentally demonstrated. A polymeric biological tissue phantom with two small air cavities scattering sound has been used as the medium irradiated by a 5-MHz “probe” ultrasound beam. The scattered field is incident on a parametric device for ultrasonic wave phase conjugation. The conjugate and amplified field is self-adaptive focused on scatterers and heats the medium owing to the absorption of the ultrasonic energy. In this case, these regions are heated by about 5°C in 70 s. Only an insignificant increase in the temperature owing to the heat conduction effect is observed in the remaining volume of the phantom. The implemented effect can be used in medical applications of phase conjugate ultrasound beams.

Experimental results on heating a biological tissue phantom using phase conjugate ultrasonic beams are presented. A polymer with acoustic properties close to those of biological tissues is used as the phantom. Heating is measured by thermocouples. The advantages of the proposed method of heating via wave phase conjugation include self-targeting of an ultrasonic beam on the region of heating and the weak effect the sample’s position or movement has on the heating parameters within a certain region.

Local heating of a liquid-like medium by means of self-adaptive focusing of a high-power phase conjugate ultrasound wave has been experimentally implemented. A sample of organic silicon polymer with a reflecting inhomogeneity was placed in water and exposed to a plane ultrasound beam so as to make the reflected acoustic field partially to enter the phase conjugation device. A parametrically amplified and conjugate wave was self-adaptively focused back to the inhomogeneity. Thermocouple measurements revealed ultrasound heating, which was localized near the inhomogeneity and depended weakly on its position within the aperture of the incident ultrasound beam. Themaximum heating value was about 6.5°C in 120 s with a frequency of ultrasound of 4.75MHz and a mean power of phase conjugate wave of about 0.1W.

A possibility of using a parametric ultrasound phase conjugation system with a grooved ferrite element for hyperthermal heating of a biological tissue phantom is experimentally investigated. The grooved surface of the ferrite element improves the quality of phase conjugation. With focused conjugate beams, this is supposed to lead to an increase in the ultrasound intensity and accordingly to more effective heating of the medium due to ultrasound absorption. The results of the investigation prove this supposition: ultrasound heating by 8.0°C in about 80 s is obtained for a sample with an inserted thermocouple placed between a focused piezoelectric transducer, which radiated a test wave with a frequency of 5.0MHz, and a system for phase conjugation and amplification of ultrasound.

The local heating of an absorbing medium by an ultrasonic beam with a conjugate wave front has been experimentally demonstrated. Plastisol, which is a polymeric material close in acoustic properties to biological tissue, is used as the medium. An ultrasonic heating of 7.2°C has been obtained in a time of about 100 s when the sample equipped with a thermocouple is placed between a focused piezoelectric transducer emitting a “probe wave” with a frequency of 5.0 MHz and a system that reverses the ultrasonic wave front with amplification. The characteristic features of heating by ultrasonic beams with the conjugate front, as well as the prospects of applications of this effect in medicine and other fields, have been discussed.

Acoustical imaging in complex media (e.g., biological tissue) can be affected by phase aberrations introduced in a wave during propagation. Wave phase conjugation (WPC) of ultrasound is known for its ability to compensate for phase distortions due to inhomogeneity of the propagation medium, and it can be used for improvement of acoustical imaging under these conditions. In a nonlinear medium harmonics are generated during propagation of an intense beam of ultrasound, and this principle is used in tissue harmonic imaging. The parametric method of WPC permits phase conjugation of a selected frequency component of the probe beam. In this way the peculiarities of WPC can be combined with advantages of harmonic imaging. Automated WPC-focusing of the conjugated second-harmonic component of a focused nonlinear probe beam is studied experimentally and theoretically for the case of a homogeneous medium, and experimentally for a medium with pseudo-random inhomogeneities. The generated conjugate wave can also be sufficiently intense to generate higher-order harmonics, which display enhanced focusing. Improvement of a C-scan harmonic imaging system operating in an inhomogeneous medium is provided as an example.

The effect of phase conjugation for the second harmonic of a focused ultrasonic beam was investigated experimentally and by numerical simulation. An ultrasonic pulse with the carrier frequency f=3 MHz was emitted into water and focused at a point between the source and the phase conjugating system. The phase conjugation for the second harmonic of the incident wave (2f=6 MHz) was performed in a magnetostrictive ceramic as a result of the parametric interaction of the incident wave with the pumping magnetic field (the pumping frequency was fp=4f=12 MHz). The axial and focal distributions of sound pressure in the incident and conjugated beams were measured using a broadband PVDF membrane hydrophone. The corresponding calculations were performed by solving numerically the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation allowing for the nonlinearity, diffraction, and thermoviscous absorption. The results of measurements agreed well with the calculations and showed that the field of a conjugate wave adequately reproduces the field of the second harmonic of the incident wave. A certain advantage of focusing with the phase conjugation for the second harmonic was demonstrated in comparison with the operation at the doubled frequency of the incident wave. The results of this study can serve as a basis for the utilization of the phase conjugation of harmonics in ultrasonic tomography and nondestructive testing.

Wave phase conjugation of the second-harmonic component generated nonlinearly in a focused beam of ultrasound is investigated experimentally and theoretically. The incident field in this case is radiated from an extended volume of the fluid between the acoustic source and the phase conjugation system. A tone burst of frequency f=3 MHz was radiated into water and focused at a point midway between the source and the conjugator. Phase conjugation of the second harmonic 2f was performed inside a magnetostrictive ceramic modulated by a magnetic pump field at frequency 4f. The conjugate beam at frequency 2f reproduces quite accurately the incident second-harmonic beam everywhere between the focal plane and the conjugator. The agreement deteriorates somewhat between the focal plane and the acoustic source, because it is mainly in this region where second-harmonic generation occurs. Experimental observations are supported by analytical and numerical results. Phase conjugation using the nonlinearly generated second harmonic possesses some advantages over conventional phase conjugation of the sound beam at the source frequency. The obtained results may provide a basis for applications employing phase conjugation of harmonics in acoustic imaging and nondestructive evaluation, such as second harmonic imaging in tissue. [Work supported by RFBR, CRDF, and ONR.]

The criterion, known from optics, of wave phase conjugation quality is accommodated for acoustical experiment. An incident focused sound beam at 5 MHz is propagated towards a supercritical magnetoelastic conjugator through an introduced random phase layer, and the conjugate beam propagates backwards through the layer to the source. 1D and 2D distributions of wave field are measured for the conjugate beam of finite amplitude using a membrane hydrophone. The maximum sound intensity of 640 W/cm2 and acoustic power of 4 W are registered in the focal maximum of the conjugate wave. The quality of WPC is calculated from the ratio between the power in the central maximum and the total power of the conjugate beam. The method is applied to compare two operation modes of a supercritical parametric conjugator. It is shown that in the linear mode the conjugator provides higher quality than in the nonlinear (0.54 compared to 0.24).

A shift of the band envelope of liquid water stretching vibrations in the field of an ultrasonic acoustic pulse is experimentally observed; this shift is indicative of modification of the water cluster structure in the acoustic field. Simple evaluations show that the observed shift can be caused by a change in the average distance between oxygen atoms in a molecular cluster by about 0.05 Å.

The propagation of a nonlinear phase-conjugate ultrasonic wave through a layer introducing random phase aberrations is studied experimentally. The wave is generated by an overthreshold parametric phase-conjugating ultrasonic amplifier. It is shown that, with the extent of nonlinearity achieved for the conjugate wave, the phase locking of harmonics is retained and, as a consequence, a compensation of the distortions introduced by the layer takes place. The possibility of an automatic focusing of a nonlinear phase-conjugate wave propagating in an inhomogeneous medium is demonstrated, which is important for practical applications.

The results are given of a study on the remote generation of shear waves in a rubber-like medium as a result of the radiation pressure arising on the absorption of an intense single compression pulse. In order to enhance the effect, non-linear conditions were used in which the absorption was increased as a result of the formation of a shock front in the time profile of the acoustic pulse. In the experiment, intense acoustic pulses were excited by means of a focused optoacoustic converter. The shear displacements were recorded in a gelatinous sample by an optical method. The theoretical estimates obtained for the shear amplitude an in good agreement with the experimentally recorded values.