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The results of application of the ultrasonic (US) Doppler method for measuring the elasticity of skeletal muscles lying at a small depth (2–4 cm) from the skin surface are presented. The technique is based on calculating the speed of shear waves excited by a vibrator on the skin’s surface at frequencies of 150–250 Hz. The distribution of shear displacements in an elastic phantom, calculated by the finite element method, was compared with the results of measurements using a miniature accelerometer. The shear modulus of the elastic phantom, measured using the proposed technique, corresponds to values obtained from static measurements.

The measurements of the reflection and transmission coefficients of platinum films with thicknesses of 1–30 nm fabricated on quartz substrates using magnetron sputtering are reported. The measurements were conducted in a rectangular waveguide at frequencies of 9–11 GHz. For a wave falling onto the Pt film from the quartz substrate side (Q‒Pt orientation), the growth of the absorption coefficient (Amax = 0.45) and the presence of a pronounced minimum of the reflection coefficient (Rmin = 0.23) for the 3-nm-thick film have been observed. In films thinner than 10 nm, the values measured are consistent with the calculations performed with the model thickness dependence of conductivity. The specific conductivity of the Pt films as a function of thickness has been calculated using the approximate boundary conditions and the measured reflection coefficients.

We report on the approximate boundary conditions obtained for the problem of calculating the optical coefficients of a system consisting of a dielectric substrate and an inhomogeneous ultrathin metallic film with an arbitrary thickness dependence of the conductivity deposited onto this substrate. The boundary conditions have been derived on the basis of the Picard’s method of successive approximations. Analytical expressions for estimating the error of the calculation of the optical coefficients obtained using the proposed approximate boundary conditions are presented. It is shown that the error increases with frequency and film thickness. The maximum error for 10-nm-thick films is no larger than 10.7% at a frequency of 1 THz. As an example, the complex optical coefficients of a system like the Fabry‒Perot etalon and a metallic film without a substrate with the model thickness dependence of the conductivity have been calculated. The coincidence of the results of the calculations performed by the numerical simulation and using the approximate boundary conditions is shown. The possibility of direct calculation of the average conductivity of a film using the experimental reflectance and transmittance is demonstrated.

A method is proposed for noninvasive assessment of the pulmonary artery pressure based on separation of the aortic and pulmonary components of the second heart sound S2 and determining the delay time between them. The algorithm for separating the aortic and pulmonary components is based on the Wigner–Ville transform. Analysis of the phonocardiograms of 96 patients with pulmonary hypertension (PH) recorded by doctors of the Scientific Research Center of Cardiology using an original set of equipment was carried out. In 53 patients with PH, it was possible to separate S2 signals into components and determine the delays between them. We construct the dependence of delays between the components on pulmonary artery pressure (PAP), measured by right heart catheterization (RHC). For seven patients with PH, the obtained dependences were used to predict the PAP value from the measured time delay. For five patients, the determination coefficient for the predicted PAP values was 0.83, which indicates a fairly good correlation with measurements by the RHC method.

A variational method for separating the signal of the second heart sound into aortic and pulmonary components is proposed based on minimizing the functional using model signals dependent on a number of parameters. The range of variation of these parameters was determined, and the algorithm was tested on model signals and on signals obtained in patients with pulmonary hypertension. An analysis of errors of the method depending on the noise level and the delay time between the components was performed. We demonstrated the efficiency of the algorithm for separating the signals measured in patients and for determining the delay time between the components. The correlation between the delay time and the pulmonary artery pressure was shown, which can serve as the basis for a new method of diagnosis of pulmonary hypertension.

A modified model for the operation of the semilunar valve is proposed, which makes it possible to estimate the spectrum of acoustic oscillations that occur as it closes. The authors have analyzed closing of the pulmonary valve cusps under a drop in pressure in the pulmonary artery and the right ventricle. Filling of the semilunar valve cusps with blood was taken into account, which leads to an increase in its closing time. This time increases in proportion to the square root of the increase in mass. The second part of the article considers elastic vibrations of a valve that is already closed. The vibrations involve both the cusps and walls of the pulmonary artery, as well as the surrounding fluid. The simplest model of this complex oscillatory system can be an elastic stretched membrane immersed in a fluid and excited by a pulsed force action from the side of the closing valve cusps. It is demonstrated that the second heart sound, heard during auscultation, is a train of damped oscillations, the ultimate duration of which is controlled by the hydraulic resistance of the vibrating cusps. Both the closing process and excited vibrations are nonlinear and multimode. The spectrum and duration of vibrations depend on their amplitude and tension of the valves. The average vibration frequency decreases with time. These results can be useful both for understanding the physical features of the process and for developing new diagnostic methods. In particular, increased pressure in the pulmonary artery can be diagnosed by increasing the frequency and decay time of vibrations in the pulmonary component of the second heart sound.

The reflectance, transmittance, and absorption coefficient of ultrathin copper films on a quartz substrate have been measured in a waveguide at frequencies of 9–11 GHz. Films less than 5 nm thick are oxidized almost completely and transparent to microwave radiation. A conducting layer is formed at a film thickness above 5 nm; however, the reflectance increases with thickness in the range of 5–15 nm more slowly than is yielded by calculations using the model conductivity of a continuous film. These results can be explained by the film morphology.

The results of precision measurements of the rate of breakdown development in air at atmospheric pressure in the tip–plane electrode system are presented. It is shown that breakdown begins when the threshold voltage Uthr is reached. The higher its value is, the larger is the radius of the tip electrode; a breakdown development rate of 6.2 mm/ns is practically independent of the radius of the tip electrode and is the same for pulses with peak voltages of 25–30 kV. Visualization of the discharge structure with an aluminum film 4 nm thick showed the presence of a large number of microchannels with sizes ranging from 5 to 20 μm.

Electrostriction mechanism of breakdown in polar liquid caused by high-voltage nanosecond pulse is studied. Nonstationary motion of liquid in the pin-to-plane system exposed to a pulse with peak voltage of 30 kV and 6-ns duration is numerically analyzed. Cavities in which the breakdown can be initiated are formed in the vicinity of the tip electrode. Dimensions of such cavities are estimated, and the conditions for formation are formulated. Profiles of voltage and current pulses are measured for breakdown in water and air. The velocity of breakdown development in air is determined using the time delay of the pulse measured by the shunt of discharge current at several interelectrode distances. The proposed method has limitations in the measurement of breakdown in water due to the effect of displacement current that exceeds the discharge current.

The presence of nanoparticles lowers the levels of ultrasound (US) intensity needed to achieve the therapeutic effect and improves the contrast between healthy and pathological tissues. Here, we evaluate the role of two main mechanisms that contribute to the US-induced heating of aqueous suspensions of biodegradable nanoparticles (NPs) of mesoporous silicon prepared by electrochemical etching of heavily boron-doped crystalline silicon wafers in a hydrofluoric acid solution. The first mechanism is associated with an increase of the attenuation of US in the presence of NPs due to additional scattering and viscous dissipation, which was numerically simulated and compared to the experimental data. The second mechanism is caused by acoustic cavitation leading to intense bubble collapse and energy release in the vicinity of NPs. This effect is found to be pronounced for as-called Janus NPs produced via a nano-stopper technique, which allow us to prepare mesoporous NPs with hydrophobic inner pore walls and hydrophilic external surface. Such Janus-like NPs trap air inside the pores when dispersed in water. The precise measurement of the heating dynamics in situ enabled us to detect the excessive heat production by Janus-like NPs over their completely hydrophilic counterparts. The excessive heat is attributed to the high intensity cavitation in the suspension of Janus-like NPs. The present work elicits the potential of specifically designed Janus-like mesoporous silicon NPs in the field of nanotheranostics based on ultrasound radiation.

Optical properties of silver nanofilms on its thickness and mechanical deformations in visible and infrared ranges are studied theoretically. The deformation of the film during its elongation leads to a rearrangement of the structure of a surface layer and the appearance of dislocations. It is shown that 9% elongation is crucial for the six monolayer silver film. Mechanical deformations change the electronic structure of atoms in a film, which leads to a significant change in their optical properties. Stretching of the film shifts the absorption peak to the long wavelength region and leads to a slight decrease in absorption. The effects are explained by the significant transformation of the electron structure of deformed silver nanofilms.

A numerical analysis of the liquid motion in a spherical capacitor under the applied voltage pulse with a duration of several nanoseconds and a peak value to 300 kV is performed. A system of hydrodynamics equations written in the general form in the spherical coordinate system and the Tait equation of state are used to simulate the liquid motion. The electrostrictive pressure is calculated for a liquid in the form of a dipole dielectric.

We present the results of our theoretical study of quantum conductance properties of ultrathin silver nanofilms. As the result of our ab initio calculations the quantum size effect in the dependence of electronic conductivity of silver nanofilms on its thickness was found. We revealed the changes in the electronic structure of silver ultrathin nanofilm leading to a change of its conductance properties, which explains the change in the conductive characteristics of ultrathin films observed before in several experimental works. Also we found the emergence of dielectric nature of silver nanofilm conductivity due to changes in its band structure.

Specific features of the interaction of terahertz radiation with nanometer-thick chromium films deposited on quartz substrates are analyzed. Optical coefficients of chromium films with thicknesses ranging from 1.5 to 40 nm are measured in a frequency range of 0.25–1.1 THz. Pulsed spectroscopic measurements employ a picosecond oscillator. A maximum absorption coefficient of 43% is measured at a film thickness of 10 nm. Theoretical calculations of optical coefficients are performed with the aid of expressions that take into account a phenomenological dependence of conductivity on film thickness. It is demonstrated that the conductivity of thick chromium films (20–40 nm) is less than the conductivity of bulk metal by an order of magnitude.

We applied a commercial ultrasonic clinical diagnostic system for studies of human biceps. The investigated area was visualized in B-mode at a frequency of 8 MHz. We selected 1 cm and 2.5 cm depths for shear wave excitation. On these depths, the focused ultrasonic wave caused the acoustical radiation force. Due to nonlinear mechanism of excitation, a shear wave arose. The results we have obtained show that the biceps have the shear moduli of the order of 10 kPa. The loaded biceps demonstrated the nonlinear behavior better pronounced for the volunteer with smaller body mass index (BMI). As the load on the biceps increases, the shear modulus measured along the muscle fibers grows. The observed growth was stronger for the shear modulus of the short head. The shear modulus, measured in the direction across the fibers of the biceps, does not depend on the magnitude of the applied load and remains at the unloaded value. In 1 minute after load is removed the biceps tend to relax and its shear moduli turn their initial values.We applied a commercial ultrasonic clinical diagnostic system for studies of human biceps. The investigated area was visualized in B-mode at a frequency of 8 MHz. We selected 1 cm and 2.5 cm depths for shear wave excitation. On these depths, the focused ultrasonic wave caused the acoustical radiation force. Due to nonlinear mechanism of excitation, a shear wave arose. The results we have obtained show that the biceps have the shear moduli of the order of 10 kPa. The loaded biceps demonstrated the nonlinear behavior better pronounced for the volunteer with smaller body mass index (BMI). As the load on the biceps increases, the shear modulus measured along the muscle fibers grows. The observed growth was stronger for the shear modulus of the short head. The shear modulus, measured in the direction across the fibers of the biceps, does not depend on the magnitude of the applied load and remains at the unloaded value. In 1 minute after load is removed the biceps tend to relax and its shear moduli turn their i...

The spectral characteristics (reflection, transmission, and absorption coefficients) of thin chromium films on silica substrates have been measured using a pulsed source of terahertz radiation. The spectra of optical coefficients were obtained in a frequency range of 0.25–1.1 THz. Dependences of the optical coefficients on the metal film thickness at 1 THz were constructed. The maximum absorption coefficient (43%) was observed at a film thickness of 10 nm.

The development of novel electromagnetic materials for various applications largely depends on ferrite composites with optimal combination of constituent elements. The evaluation of their dielectric and magnetic properties is important in understanding the physical properties and potential for specific use. In this work, permittivity and permeability spectra of composite materials based on granulated soft magnetic Mn-Zn and Ni-Zn ferrites in paraffin, polyvinyl acetate glue and silicone sealant matrices were investigated for different ferrite phase concentrations and ferrite granule sizes. Mn-Zn ferrites are characterized by a relatively large conductivity and their composites show unusual behavior for the real part of the permittivity which can be essentially higher than the values observed for bulk ferrite samples, especially at low frequencies. The permittivity spectra also demonstrate a strong frequency dispersion which is not seen for bulk ceramic ferrites. Ni-Zn ferrite composites with very large electrical resistance do not show these effects and their microwave properties are consistent with the behavior of the constituent phases. The observed permittivity enhancement is explained on the basis of the conductive properties of the ferrite granules and the equivalent capacitance effect. The effective permittivity calculated from an equivalent capacitor circuit gives a qualitative agreement with the experimental data at lower frequencies. However, the origin of the frequency dispersion remains unclear. The developed composites have a potential for applications in broadband absorbers since they combine magnetic and electric losses along with a strong frequency dispersion of the permittivity.

The current contrast agents utilized in ultrasound (US) imaging are based on microbubbles which suffer from a short lifetime in systemic circulation. The present study introduces a new type of contrast agent for US imaging based on bioresorbable Janus nanoparticles (NPs) that are able to generate microbubbles in situ under US radiation for extended time. The Janus NPs are based on porous silicon (PSi) that was modified via a nanostopper technique. The technique was exploited to prepare PSi NPs which had hydrophobic pore walls (inner face), while the external surfaces of the NPs (outer face) were hydrophilic. As a consequence, when dispersed in an aqueous solution, the Janus NPs contained a substantial amount of air trapped in their nanopores. The specific experimental setup was developed to prove that these nano air seeds were indeed acting as nuclei for microbubble growth during US radiation. Using the setup, the cavitation thresholds of the Janus NPs were compared to their completely hydrophilic counterparts by detecting the subharmonic signals from the microbubbles. These experiments and the numerical simulations of the bubble dynamics demonstrated that the Janus NPs generated microbubbles with a radii of 1.1 μm. Furthermore, the microbubbles generated by the NPs were detected with a conventional medical ultrasound imaging device. Long systemic circulation time was ensured by grafting the NPs with two different PEG polymers, which did not affect adversely the microbubble generation. The present findings represent an important landmark in the development of ultrasound contrast agents which possess the properties for both diagnostics and therapy.

We theoretically and experimentally substantiate the method of detecting microcalcifications in mammary gland tissue. Calcium salts accumulate in soft tissues, primarily forming clusters of individual microparticles. We study the motion of solid microparticles distributed in a viscoelastic medium. Displacement of particles is caused by the radiation force occurring as a consequence of energy scattering and absorption of an ultrasound beam focused in the particle region. The radiation force acts over the course of 200 μs, after which the medium with distributed particles relaxes to the initial state. Motion of the medium is tracked by the cross-correlation method with short probing pulses following at a frequency of 5 kHz. The presence of solid microparticles leads to a change in the character of motion of the medium after pulsed ultrasound action. The amplitude and duration of displacements increases in comparison to the homogeneous medium, and the motion character itself becomes significantly complicated.

We consider the possibility of using a thermoacoustic detector (TAD) for recording of high-power pulse radiation at frequencies of 0.55, 0.68, and 0.87 THz. Electromagnetic wave is transformed into an acoustic wave in a structure consisting of a 10-nm thick chromium film deposited on a quartz substrate and a layer of the immersion liquid that is in contact with the film. It is shown that for the pulse of microsecond duration (3–10 μs) the waveform detected by the thermoacoustic detector is matched with high accuracy to the derivative of the terahertz pulse profile. For recording of electromagnetic radiation in the 0.5–0.9 THz frequency range it is possible to employ the simplified design of TAD, in which a transparent quartz substrate is in contact with a layer of water or ethanol.

The optical properties of Ag ultrathin films in dependence of their thickness are studied theoretically in a wavelength range 0.3 - 10 μm. The extinction coefficient ( k ) and refractive index ( n ) for thin Ag films with smooth surface structure are calculated with software package VASP. It was found the effect of growth of extinction coefficient and shift of its peak into long wavelength range with the thickness increasing. The effect is explained by the significant increasing of the surface electron states. Refractive index n is increased with the wavelength growth and attains saturation value n s at the wavelength λ s . The thicker the films the higher the magnitude of n s and the larger the wavelength λ s . Our results of calculations of k(λ) are in a good agreement with experimental data from ref.[25]. The difference in magnitudes of n obtained experimentally and theoretically can be explained by the formation of Ag nanoclusters on the surface of sputtered film.

Mechanical properties of freestanding Au-Mn nanowires and Au-Mn nanowire on a Cu (110) substrate are studied with ab initio theoretical approach. The calculations were carried out using the software package Vienna Ab-initio Simulation Package (VASP), which is based on the density functional theory (DFT). It was shown that the breaking force (0.45nN) as well as the interatomic distance at a breaking point in bimetallic nanowire (3.0 A) are higher than in one component Au wire (0.4 nN and 2.6A respectively). Relative elongation of 15 % results in a fracture of bimetallic nanowire. We studied the mechanical response of the nanojunction in a form of three-atomic Au chain aligned vertically between two pyramidal gold electrodes and demonstrated that the breaking of nanocontact depends only the interaction between Au atoms in the chain and dependents slightly on the structure and properties of the atomic structure of the electrodes.

Ultrathin (1–10 nm) Cu and Au films were prepared on the silicon and quartz substrates by magnetron sputtering at room temperature. We measured the transmission coefficient of the films at a wavelength of 3cm and analyzed a surface morphology of these films. It was shown that the films with thicknesses less than 7.5 nm (Au) and 3 nm (Cu) are almost transparent for microwaves. This effect is explained by quick oxidation of Cu and the complex surface morphology of nanometer thick films. The Au film morphology is evolved with increasing average Au thickness d from hemispherical islands initially (1.0 nm d d =10 nm).

The effects of ultrasonic cavitation and heat deposition in aqueous suspensions of mesoporous silicon nanoparticles were experimentally investigated using an original setup enabling precise temperature control and registration of cavitation by harmonics analysis of the transmitted signal. The obtained results reveal significant lowering of cavitation thresholds and heating of suspensions as compared to pure water.

Shear waves with finite amplitude in a one-dimensional resonator in the form of a layer of a rubber-like medium with a rigid plate of finite mass at the upper surface of the layer are investigated. The lower boundary of the layer oscillates according to a harmonic law with a preset acceleration. The equation of motion for particles in a resonator is determined using a model of a medium with a single relaxation time and cubical dependence of the shear modulus on deformation. The amplitude and form of shear waves in a resonator are calculated numerically by the finite difference method at shifted grids. Resonance curves are obtained at different acceleration amplitudes at the lower boundary of a layer. It is demonstrated that, as the oscillation amplitude in the resonator grows, the value of the resonance frequency increases and the shape of the resonance curve becomes asymmetrical. At sufficiently large amplitudes, a bistability region is observed. Measurements were conducted with a resonator, where a layer with the thickness of 15 mm was manufactured of a rubber-like polymer called plastisol. The shear modulus of the polymer at small deformations and the nonlinearity coefficient were determined according to the experimental dependence of mechanical stress on shear deformation. Oscillation amplitudes in the resonator attained values when the maximum shear deformations in the layer were 0.4–0.6, which provided an opportunity to observe nonlinear effects. Measured dependences of the resonance frequency on the oscillation amplitude corresponded to the calculated ones that were obtained at a smaller value of the nonlinear coefficient.

A modification of the finite-element method is proposed for calculating shear standing waves in a resonator filled with an incompressible elastic medium with allowance for the finite dimensions of the resonator and inhomogeneities of the shear modulus. Resonance curves are calculated for resonators with inhomogeneities in the form of cavities and elastic inclusions. Numerical calculations are compared with experimental data.

Standing shear waves arising in layered media the shear modulus of which varies in a stepwise manner at the plain boundaries between the layers are considered. A general solution is obtained for the shear wave amplitudes in a resonator with an N-layer structure the lower boundary of which performs harmonic vibrations while a finite-mass plate is attached to the upper boundary. Results of calculations and measurements are presented for a resonator with a structure in which nondeformable metal layers alternate with elastic rubberlike polymer layers. It is shown that the resonance frequencies of such a resonator can be controlled by changing the number of layers and their thicknesses. It is demonstrated, both experimentally and theoretically, that, from the resonance curve of a resonator with a two-layer structure, it is possible to determine the shear modulus of one of the layers under the condition that the elasticity of the other layer is known. The method of separation into a finite number of layers is used to analyze the resonance characteristics of a one-dimensional resonator filled with a rubberlike medium the properties of which continuously vary in the direction perpendicular to the shear displacements. The choice of the number of layers depending on the type of inhomogeneity is analyzed.

Standing shear waves in a plane-parallel rubberlike layer fixed without slippage between two rigid plates with finite masses are investigated. The lower plate, which underlies the layer, oscillates in the direction parallel to its surface under an external harmonic force, whereas the upper plate freely overlies the layer. It is shown both theoretically and experimentally that such a system exhibits resonances at frequencies the values of which depend on the mass of the free plate and the shear modulus of the layer. The shapes of the resonance curves are calculated and measured for different values of parameters of the layer and different masses of the upper plate. From the measured resonance curves, it is possible to determine the dynamic shear modulus and the shear viscosity of the rubberlike material.

We present the results of testing a new thermoacoustic sensor designed to detect microwave pulses having durations from 3 to 120 ns at wavelengths of 0.8 and 3 cm. Operation of the sensor is based on the effect of generation of acoustic signals during absorption of microwave pulses in a radiotransparent substrate-absorber-liquid layered structure . A thin nanometer-thick film deposited on a substrate is used as an absorber. Microwaves are converted to an acoustic pulse in the film and the adjacent liquid. The pulse is received by a wideband acoustic receiver and then recorded by a digital oscilloscope. It is shown that for a pulse duration of 120 ns, the shape of the signal recorded by the thermoacoustic sensor completely corresponds to the signal of a tube-diode detector of microwave pulses. The response of the thermoacoustic sensor to shorter pulses (3 and 5 ns long) is a pulse with a duration of 18 ns which is determined by a limited frequency band of the acoustic receiver.

Propagation of a probing pulse through a thermal inhomogeneity produced in the medium as a result of heating by a focused ultrasonic beam is considered. The wave equation is used to numerically calculate the waveform of the pulse arriving at the receiver whose focus coincides with that of the radiator. Calculations are performed for a pulse focused at the center of the inhomogeneity and crossing it in the transverse direction. The results of numerical simulation are compared with experimental data. Conditions under which the diffraction effects should be taken into account in calculating the pulse delay at the receiver are formulated.

A new method is proposed for measuring temperature in the focal region of a high-intensity ultrasonic emitter on the assumption of axially symmetric temperature field. This method is based on solving the integral equation relating the temperature of the medium to the delay times of the probe ultrasound pulse intersecting the heated region in the transverse direction at different distances from the beam axis. The accuracy of the algorithm for calculating temperature with allowance for the finite experimental data set is analyzed. The calculation results are compared with the experimental data.

The operating principle and design of an uncooled noise-immune high-power microwave pulse sensor is described. The sensor operation is based on the effect of acoustic signal generation when microwave pulses are absorbed in a layer structure, in which a thin metal nanometer-thick film is used as an absorber. The sensor is placed in a free space and intended to detect microwave pulses with ∼10- to 100-ns durations in a 10- to 300-GHz frequency band with a pulse repetition rate of up to 5 kHz. For 10-ns-long pulses, the sensor sensitivity is 0.5 V/mJ.

The method and results of measuring the shear elastic modulus of a rubberlike polymer by the deformation of a plane elastic layer are described. For shear deformations not exceeding 0.5 of the layer thickness, the shear modulus is constant and its value is in agreement with the value determined by pressing a rigid ball against the polymer layer. For deformations exceeding 0.5 of the layer thickness, the stress-strain dependence becomes nonlinear. The coefficient of shear viscosity is determined from the shear wave form generated by focused ultrasound in a homogeneous polymer sample.

A method for remote monitoring of temperature in the focal region of a high-intensity ultrasonic transducer is described. Results of measurements and theoretical simulation are presented. The measurements were conducted on a polymer sample with thermophysical and acoustic parameters close to the properties of a soft biological tissue. The sample was heated by a focused piezoelectric transducer with different values of radiation power. The delay of a probe pulse transmitted through the heated region perpendicularly to the axis of the intense ultrasonic beam was detected. The local character of temperature measurements was provided by focusing the probe pulse at the heated region. The application of an additional transducer installed confocally with the probing one provided an opportunity to enhance the precision of measurements. An analysis was conducted on the basis of a numerical solution of the heat conduction equation. The function of thermal sources in the heat conduction equation was calculated according to the results of measuring the pressure distribution in the focal region of the heating transducer. The experimental data obtained agree well with the results of simulation and demonstrate a fundamental possibility of using the proposed ultrasonic technique for remote temperature measurements.

We study the features of excitation of acoustic waves by high-power microwave pulses in thin metal films bordering on liquid. Aluminum films with thicknesses 1–10 nm deposited onto a quartz substrate were used in experiments. It is shown theoretically that the absorption coefficient of microwaves is maximum for film thickness from 2 to 3 nm and the value of this maximum is determined by the dielectric permittivity of the bordering liquid. Theoretical calculations and experiments are performed for water and ethyl alcohol. The sound generation in a layered system quartz-aluminum film-liquid is analyzed with the help of the step-by-step approach. At the first step, microwave energy is absorbed in the film and heat is released. Then heat almost instantly diffuses into a liquid whose thermal expansion creates an acoustic signal. Profiles of acoustic signals excited in aluminum films by microwave pulses with a 5-ns duration and an energy of up to 1 mJ are experimentally detected. The most efficient transduction was observed for an aluminum film 3.5 nm thick.

Standing shear waves in a resonator in the form of layer of gel-like medium placed between two rigid plates are studied. The bottom plate is fixed to the vibrator and oscillates in the horizontal direction with a preset amplitude. Two rubber threads attached to the upper plate can displace the plate by a specified value in the horizontal direction. The change in the tension of the threads creates an additional static deformation of the elastic layer resulting in the effective shear elasticity increase. The measured static stress-strain dependence of the elastic layer can be described by the cubic parabola. We measured the dependences of the resonance frequency on the static deformation of the layer. For static deformations of the layer less than 0.3 h (h—is the layer thickness), the resonance frequency increases linearly, that can be explained by a linear growth of the elastic force of rubber threads. In the deformation range of 0.3-1 h, an additional shift of the resonance frequency caused by the nonline...

A laser optoacoustic imaging system (LOIS) uses time-resolved detection of laser-induced pressure profiles in tissue in order to reconstruct images of the tissue based on distribution of acoustic sources. Laser illumination with short pulses generates distribution of acoustic sources that accurately replicates the distribution of absorbed optical energy. The complex spatial profile of heterogeneous distribution of acoustic sources can be represented in the frequency domain by a wide spectrum of ultrasound ranging from tens of kilohertz to tens of megahertz. Therefore, LOIS requires a unique acoustic detector operating simultaneously within a wide range of ultrasonic frequencies. Physical principles of an array of ultrawide-band ultrasonic transducers used in LOIS designed for imaging tumors in the depth of tissue are described. The performance characteristics of the transducer array were modeled and compared with experiments performed in gel phantoms resembling optical and acoustic properties of human tissue with small tumors. The amplitude and the spectrum of laser-induced ultrasound pulses were measured in order to determine the transducer sensitivity and the level of thermal noises within the entire ultrasonic band of detection. Spatial resolution of optoacoustic images obtained with an array of piezoelectric transducers and its transient directivity pattern within the field of view are described. The detector design considerations essential for obtaining high-quality optoacoustic images are presented.

We propose a method for detection of the envelope of a microwave nanosecond pulse, which is based on the effect of generation of sound pulses resulting from absorption of electromagnetic waves. We show that to reproduce exactly the envelope shape of a short microwave pulse, it is necessary that the wave be absorbed in a layer of thickness much less than the product of the sound speed and the pulse duration. In searching for conditions ensuring this, we study the process of thermoacoustic generation in thin metal films deposited on quartz substrates. Measurements performed at a wavelength of 8 mm show that the absorption coefficient is maximum if the aluminum-film thickness is 22–25 A. The maximum absorption coefficient amounts to 49% if the wave is incident from the quartz-substrate side. It is important that in this case, the wave reflection coefficient does not exceed 44%, whereas an aluminum plate reflects almost completely (99.8%) the incident-wave power. The phenomena observed in thin metal films are explained theoretically in terms of the anomalous skin effect. An experimental setup for a study of generation of ultrasonic signals by powerful microwave pulses is described.

We studied standing shear waves in anisotropic resonator represented by a rectangular parallelepiped (layer) fixed without slipping between two wooden plates of finite mass. The viscoelastic layer with edges of 70 mm × 40 mm × 15 mm was made of a rubber-like polymer plastisol with rubber bands inside. The bands were placed vertical between the top and the bottom plate. Mechanical properties of the plastisol itself were carefully measured previously. It was found that plastisol shows a cubic nonlinear behavior, i.e. the stress-strain curve could be represented as: σ = μe + βμe3, where e stands for shear strain and σ is an applied shear stress. The value of shear modulus μ depends on frequency and was found to be several kilopascals which is common for such soft solids. Nonlinear parameter β is frequency dependent too and varies in range from tenths to unity at 1-100 Hz frequency range, decreasing with frequency growth. Stretching the rubber bands inside the layer leads to change of elastic properties in re...

Electrical breakdown of heterogeneous dielectrics in the form of rock samples was studied. The studies were conducted in an inhomogeneous field using the scheme conical tip — plane. The high-voltage generator of 10 ns pulses duration with a slope of 3 ns rise and peak voltage up to 300 kV was employed. Forms of incident and reflected voltage pulses can be recorded by a coaxial meter, which allowed characterizing breakdown dynamical parameters of the samples. A model of development of the breakdown based on electrostrictive mechanism was proposed. The dynamics of motion of electrons and ions in the micro-cracks formed by high-voltage pulses with nanosecond fronts near impurities in the materials with developed impurity phase was investigated numerically. Characteristic times of microgap crossing and maximum electron energy were determined. For the breakdown field, maximum electron energy can reach tens of electron volts, which causes ionization of atoms of the substance including deep levels. The role of space charge in the electron dynamics was also studied.

An experimental setup and a technique for measuring the transient period before a stationary cavitation in a liquid by the evolution of the cavitation noise spectrum are described. The time dependences of harmonic amplitudes both near the radiator and outside the cavitation region are presented. From the form of these dependences, the characteristic transient periods preceding the stage of a fully developed cavitation in water and in transformer oil are calculated. A formal scheme for describing the cavitation region is proposed.

Summary form only given. Results of measurements of electromagnetic pulses with 3 to 10 mcs duration at frequencies 0.55-0.87 THz by the thermoacoustic detector are presented. Operation of the detector is based on effect of acoustic signal generation when electromagnetic pulses are absorbed in the layered structure: radiotransparent substrate-absorber-immersion liquid. Thin metallic film of nanometer thickness sputtered onto the quartz substrate is used as an absorber. The transformation of electromagnetic radiation into acoustic pulse is performed in the film and in the liquid contacting with the film. Acoustic pulse is detected by the broadband acoustic transducer and registered with a digital oscilloscope. It is shown that for the pulse of microsecond duration the signal waveform detected by the thermoacoustic detector is completely matched to the derivative of a profile of terahertz pulse. When water and ethanol are used as immersion liquids the presence of additional film absorber is not compulsory because these liquids absorb effectively the electromagnetic radiation in frequency range 0.5-1 THz.

We study finite-amplitude shear waves in one-dimensional resonator represented by a layer of rubber-like medium with inhomogeneities in the form of through holes made on the side face. The holes are parallel to the bases and perpendicular to the direction of vibrations. Two different configurations of the resonator: with holes at the bottom and at the top are studied. A rigid plate of finite mass is fixed on the upper surface. The lower boundary of the layer oscillates harmonically with a given acceleration. The equation of motion of particles in the resonator was found using the model of medium with one relaxation time, and a cubic dependence of the shear modulus of deformation. The measurements were performed in a resonator in the form of a rectangular parallelepiped of 15 mm thickness made of a rubber-like polymer plastisol. The linear shear modulus and shear viscosity of the polymer at the first resonant frequency were determined using the finite element method. The amplitudes of the oscillations in the resonator reached a point where the maximum shear strain in the resonator is 0.4 - 0.6, making it possible to observe nonlinear effects. The evolution of the resonance curves at different amplitudes of acceleration was investigated. A harmonic analysis of the acceleration profiles of the upper boundary was performed. The dependence of nonlinear effects on the holes position was studied.

Aqueous suspensions of porous silicon nanoparticles (NPs) with average size ∼100 nm and concentration ∼1 g/L undergo significant heating as compared with pure water under therapeutic ultrasonic (US) irradiation with frequencies of 1–2.5 MHz and intensities of 1–20 W/cm2. This effect is explained by taking into account the efficient absorption of US energy by NPs. The observed US-induced heating of biodegradable NPs is promising for applications in ultrasonic hyperthermia of tumors.

Poor penetration of macromolecular drugs and genes in tumor cells substantially limits efficacy and safety of cancer chemo‐ and biotherapy. Cavitation induced by interaction of ultrasound with nanoparticles accumulated in tumors due to the enhanced permeability and retention effect may provide efficient drug and gene delivery in tumor cells. Biodegradable polymer (PLGA) air‐filled nanoparticles manufactured in our laboratory were used for enhancement of drug and gene delivery in vitro and in vivo in human breast, prostate, or ovarian tumors of nude mice (two tumors in each animal). Our specially designed confocal ultrasound system allowed for irradiation and cavitation detection in vitro and in vivo. It was found that the nanoparticles substantially decrease cavitation threshold and increase cavitation activity in vitro and in vivo. Moreover, we obtained three‐fold increase of tumor cell transfection in vivo compared to non‐irradiated control tumors of same mice. Substantial tumor necrosis was achieved when macromolecular anticancer drug anti‐IGFBP‐2 was used in combination with the nanoparticles and ultrasound. Our results indicate that biodegradable polymer nanoparticles and ultrasound can be used for enhancement of drug and gene delivery in tumors. [These studies were supported by the DoD Prostate Cancer Research Program (Grant No. W81XWH‐04‐1‐0247) and Breast Cancer Research Program (Grant No. DAMD17‐01‐1‐0416).]

Ultrasonic excitation of shear waves is important for elasticity imaging of biological tissue. In the shear wave elasticity imaging shear strain is remotely induced in tissue by the radiation force of focused modulated ultrasound. Previous study has shown that tone bursts of millisecond duration at megahertz frequencies with intensities and exposures typical for diagnostic ultrasound may induce bulk radiation force of the order of 0.01 N/cm3 and shear displacements of the order of 10 μ. In this work, new nonlinear mechanisms are discussed that provide detectable shear displacements by means of single submicrosecond pulses. Two new possibilities are discussed. Significantly enhanced radiation force and greater shear displacement can be produced in tissue by single acoustic videopulse with nonzero momentum. Another possibility is based on the enhanced nonlinear absorption which occurs after the shock is formed in the temporal profile of the pulse. Nonlinear mathematical models describing the process of generation of shear waves using single pulses are developed. Experimentally, single focused pulses are excited by high‐power laser radiation inside an optoacoustic cell. Shear displacements are detected in a sample of gel using optical scattering. Results of modeling and experiments are in good agreement. [Work supported by CRDF and RFBR.]

Biological tissues have a specific frequency dependence of the ultrasonic absorption and sound‐speed dispersion. It makes fruitful the diagnostics of tissues by studying these parameters. It is necessary to carry out the spectroscopic investigation in a wide frequency band in a real time. Pulsed laser ultrasonic spectroscopy is the most useful technique to solve this problem. The method of wideband acoustic spectroscopy with laser thermo‐optical source is proposed. A Q‐switched YAG‐Nd laser was explored to excite the short acoustic transients (pulse duration 12 ns, amplitude up to 8 MPa) by an especially produced thermo‐optical generator. The acoustic transients transmitted through the medium under the testing were detected by a wideband transducer (frequency band 2–350 MHz). The absorption of ultrasound was calculated by FFT. The absorption in glycerol, acetic acid, and butanediol was investigated in a frequency band 2–50 MHz. The spectra of relaxation times were calculated. [Work supported by RFBR and CRDF.]

Nonlinear phenomena are known to be significant after the wave of moderate amplitude passes a long distance through a weakly dissipative medium. Such ‘‘accumulative’’ effects are studied exhaustively by nonlinear wave physics. In contrast, the ‘‘local’’ nonlinearities are studied incompletely. Such nonlinear behavior can be demonstrated by a piston immersed in liquid and vibrating with high amplitude. This piston subjected to a harmonic load can radiate not only the fundamental frequency but high‐order harmonics as well. Moreover, reaction to high‐power radiation can create an additional nonlinear resistance to a piston motion. Local nonlinear phenomena are expessed clearly at vibration velocities comparable with the sound speed of the surrounding medium. Such a case can be realized using liquids containing gas bubbles where sound velocity is much less than one in a pure liquid. This work is devoted to the theoretical calculation of the temporal and spectral characteristics of a nonlinear wave radiated by...