Generation and detection of acoustic solitons in crystalline slabs by laser ultrasonics

Acoustic solitons have been recently observed in different systems (Si, Sapphire, MgO, alpha-quartz). Such acoustic waves could lead to sub-picosecond acoustic pulses. In this paper, we report on the formation of acoustic solitons in a GaAs crystalline slab. A short picosecond acoustic pulse is generated by absorption of a femtosecond laser pulse in an aluminum thin film deposited on one side of the slab. This strain pulse travels through the sample up to the opposite side where it is detected by a time delayed laser pulse reflected by an aluminum transducer. We use interferometric detection to measure independently the real and imaginary parts of the relative change in optical reflectivity induced by the acoustic pulse. We find that, at low temperature and with a laser pump pulse energy of 10 nJ, an acoustic soliton clearly separates from the acoustic pulse in GaAs slab. The soliton shape is compared with numerical simulations for different excitation conditions. From the very unique properties of solitons, we infer a soliton pulse duration of about 2.3 ps which corresponds to a spatial extent of only 12 nm.     

Monitoring of temperature variation in the focal region of an ultrasonic transducer

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.     

The measurement of peak acoustic intensity generated by pulsed ultrasonic equipment

The peak acoustic intensity generated by pulsed ultrasonic equipment is calculated from measurement of the transmitted acoustic pulse by a calibrated, small, high frequency transducer. The average acoustic output is then computed from the intensity waveform, the beamwidth and the pulse repetition rate. Alternatively, the acoustic pulse may be measured by an uncalibrated transducer and the intensity is then computed from the average acoustic output.     

Generation and detection of incoherent phonons in picosecond ultrasonics

In picosecond ultrasonics experiments the absorption of a femtosecond laser pulse in a thin metallic transducer is used to generate very short acoustic pulses. These pulses are made of coherent longitudinal waves with a frequency spectrum that can reach 100-200 GHz. The laser pulse absorption gives rise to a heating of the film of a few Kelvin within a typical time of 1 ps. Later on, the heat goes in the substrate through an interface thermal resistance and is diffused by thermal conduction. At very low temperature and in pure crystals the thermal phonons emitted by the heated metallic film can propagate ballistically over large distances and produce a so-called heat pulse. We report on the experimental evidence of the coexistence of the coherent acoustic pulse and the incoherent heat pulse generated and detected by laser ultrasonics.     

Improvement of echo shape in low impedance materials

Multiple internal reflections within an air-backed transducer lead to a deterioration of pulse shape and consequent errors in pulse echo measurements. This effect is particularly pronounced in low impedance materials. It is demonstrated both by calculations and experimentally that using a quarterwave buffer having a specific acoustic impedance higher than either transducer or sample considerably reduces this echo distortion.     

Correction for transducer influence on sound velocity measurements by the pulse echo method

The accuracy of sound velocity measurements calculated by the pulse echo method is influenced markedly by the transducer used. This influence is the result of multiple internal reflections within the transducer and is dependent both on transducer thickness and the impedance mismatch between transducer and sample. Corrections for this effect have been computed for a wide range of sample acoustic impedances and the results were confirmed by experiment. These corrections improve sound velocity measurements, particularly on low impedance materials.     

Broadband terahertz ultrasonic transducer based on a laser-driven piezoelectric semiconductor superlattice

Spectral characteristics of laser-generated acoustic waves in an InGaN/GaN superlattice structure are studied at room temperature. Acoustic vibrations in the structure are excited with a femtosecond laser pulse and detected via transmission of a delayed probe pulse. Seven acoustic modes of the superlattice are detected, with frequencies spanning a range from 0.36 to 2.5 THz. Acoustic waves up to ∼2 THz in frequency are not significantly attenuated within the transducer which indicates excellent interface quality of the superlattice. The findings hold promise for broadband THz acoustic spectroscopy.     

用压电聚合物构成的单极性声脉冲发生器

本文介绍了用阶跃电压的快速下降沿作用在PVDF压电膜上而产生的单极性声脉冲。实验结果表明，声脉冲的波形和脉冲宽度取决于PVDF压电膜的厚度，阶跃电压的下降沿宽度，以及背衬材料和声传播媒质的形状等多个因素。该声脉冲发生器所产生的压力波脉冲的宽度约为50ns，不仅适用于固体中空间电荷的测量，也可以用于其他方面的应用，例如超声医学诊断，对材料的非破坏性的检测等。文章以该声脉冲发生器在压电压力波中的应用为例进行论述，并且所有测量结果都是通过压电压力波测试系统获得的。     

Surface morphological changes induced in catalysts by acoustic waves

An interdigital transducer device equipped with a metal catalyst sample that can be used to enhance catalytic activity by acoustic excitation is investigated. It was found that the propagation of 20 MHz surface acoustic waves of the Rayleigh type on Pt thin film single crystals causes drastic changes in surface morphology. The process of film breaking is observed and it is concluded that a phase shift in the acoustic wave induced by folds and cavities in the sample is responsible for the appearance of cracks. The influence of the change in morphology on catalytic activity and the acoustically induced rate enhancement effect is studied, and it is concluded that these changes are not a significant factor in the observed enhancement.     

Acoustic Imaging Frequency Dynamics of Ferroelectric Domains by Atomic Force Microscopy

We report the acoustic imaging frequency dynamics of ferroelectric domains by low-frequency acoustic probe microscopy based on the commercial atomic force microscopy. It is found that ferroelectric domain could be firstly visualized at lower frequency down to 0.5 kHz by AFM-based acoustic microscopy. The frequency-dependent acoustic signal revealed a strong acoustic response in the frequency range from 7kHz to 10 kHz, and reached maximum at 8.1 kHz. The acoustic contrast mechanism can be ascribed to the different elastic response of ferroelectric microstructures to local elastic stress fields, which is induced by the acoustic wave transmitting in the sample when the piezoelectric transducer is vibrating and exciting acoustic wave under ac electric fields due to normal piezoelectric effects.     

利用PVDF传感器检测激光超声的实验研究

简要介绍了激光超声技术以及声表面波的基本特点、激光超声产生和接收的基本原理及激光超声技术的应用。概述了聚偏二氟乙烯（PVDF）压电薄膜材料的结构、性质和应用，以及薄膜压电性产生的机理。对PVDF换能器的设计思路和实验方法进行了简单讨论。具体实验采用脉冲激光器激发声表面波，利用PVDF传感器接收实验信号，调试实验信号，得出波形，并对实验现象作出初步分析。证实了该实验装置应用于激光超声无损检测的可行性与可靠性。     

Generation and reception of short acoustic pulses by multilayer piezoelectric transducers

By means of the Fourier transformation, a formula is obtained for calculating the pulse response of a transducer when a square pulse of a given duration is supplied to its input and the excited “sound” is reflected from the free flat end of the acoustic line and returns to the transducer. For a frequency of 100 MHz, real lithium niobate transducers that are connected through intermediate layers to the acoustic line made of fused quartz and to a rear load with a given acoustic impedance are considered. The pulse response is calculated for the transducers with different values of the sublayer thickness, rear acoustic load, transmission line impedance, and original pulse duration. The results of the calculations are compared with the experiment.     

Semicylindrical acoustic transducer from a dielectric elastomer film with compliant electrodes

A semicylindrical acoustic transducer was constructed using a dielectric elastomer film with compliant electrodes that is an electroactive polymer composed of a polyurethane elastomer base and polyethylene dioxythiophene/polystyrene sulfonate electrodes. The use of this dielectric elastomer is advantageous because polyurethane is a common material that keeps its shape without any rigid frame. Because the dielectric elastomer films are essentially incompressible, electric-field-induced thickness changes are usually translated into much larger changes of the film area and side length. Here it is proposed that this change in side length can be utilized for sound generation when the film is bent into a semicylindrical shape. Accordingly, a semicylindrical acoustic transducer was fabricated using a film of thickness of 300 μm and its acoustic characteristics were investigated. The transducer can be operated at low applied voltages by reducing the film thickness, as long as the film is thick enough to generate sufficient force to overcome sound radiation impedance. The second harmonic distortion of the transducer was also investigated as a function of the ratio of the direct current bias voltage to the alternating current audio signal amplitude.     

Evaluation of excimer laser ablation of thin Cr film on glass substrate by analysing acoustic emission

Excimer laser ablation of a thin Cr film on a glass substrate was evaluated through detecting acoustic emission arising from the ablation process and observing surface morphology. In the experiment, an acoustic emission transducer was coupled to the sample to acquire acoustic emissions of laser–material interaction and an oscilloscope was used to record the signals. The patterned areas were examined by using an optical microscope. Characteristics of the acoustic emissions were studied through applying a range of signal analysis tools. Several features (e.g., average power, RMS) showed a clear linkage with the ablation mechanism of thin Cr film when varying the fluence of the laser source. Moreover, the damage to the glass substrate was well discriminated by the extracted features. In sum, evaluation of acoustic emissions not only provided a tool to study the ablation mechanisms of the thin metal film but also addressed a monitoring strategy for excimer laser micromachining.     

Transient acoustic field radiated by a thickness—shear mode transducer

The acoustic field generated by a thickness—shear mode transducer is theoretically analysed by an approximate model to find the origin of the longitudinal pulse in the signal received by the same transducer. It is believed to arise from the edge longitudinal wave.     

Dispersion of an acoustic pulse passing through a large-grained polycrystalline film

Propagation of a short acoustic pulse through a polycrystalline film comprised of large randomly oriented elastically anisotropic grains is analyzed theoretically. For average grain size much larger than the film thickness, a short acoustic pulse launched normally into the film will traverse each grain in a time determined by the acoustic slowness in the direction normal to the film, which will depend on the local grain orientation. A typical measurement averages over a large number of grains resulting in the broadening of the composite output pulse. The resulting pulse shape is characterized by distinct features related to stationary values of the directionally dependent acoustic slowness of the crystalline material. Maxima and minima in the slowness yield discontinuities in the pulse shape, while saddle points yield logarithmic singularities. For cubic and hexagonal crystals, power law singularities result from cones of directions in which the slowness is a maximum or minimum. Numerical results, taking into account Gaussian broadening of the input pulse, are presented for thin film materials commonly encountered in picosecond ultrasonic experiments, such as copper, gold, and aluminum.     

Wideband high-frequency line-focus PVDF transducer for materials characterization

This paper presents the design, fabrication, operating characteristics and applications of a wideband, high-frequency, line-focus beam transducer we constructed using a 9 microm thick piezoelectric polyvinylidene fluoride (PVDF) film. This transducer possesses a focal length of 2.38 mm and an aperture angle of 84 degrees. The frequency spectrum of the signal measured at the focal point indicates that the transducer has a wide frequency response which extends from 10 MHz to over 100 MHz. When compensated for the frequency-dependent attenuation of the coupling medium, the operational frequency exceeds 150 MHz. The transducer can be operated in a time-resolved pulse mode or in a radio-frequency (rf) tone burst mode. An application of the transducer to determine the anisotropic elastic property of a silicon wafer is demonstrated. The phase velocities of surface acoustic waves (SAW) propagating along various directions on the (001) surface of cubic silicon are measured and compared to computed values.     

Development of an accelerometer-based underwater acoustic intensity sensor

An underwater acoustic intensity sensor is described. This sensor derives acoustic intensity from simultaneous, co-located measurement of the acoustic pressure and one component of the acoustic particle acceleration vector. The sensor consists of a pressure transducer in the form of a hollow piezoceramic cylinder and a pair of miniature accelerometers mounted inside the cylinder. Since this sensor derives acoustic intensity from measurement of acoustic pressure and acoustic particle acceleration, it is called a p-a intensity probe. The sensor is ballasted to be nearly neutrally buoyant. It is desirable for the accelerometers to measure only the rigid body motion of the assembled probe and for the effective centers of the pressure sensor and accelerometer to be coincident. This is achieved by symmetric disposition of a pair of accelerometers inside the ceramic cylinder. The response of the intensity probe is determined by comparison with a reference hydrophone in a predominantly reactive acoustic field.     

Thermal melting and ablation of silicon by femtosecond laser radiation

The space-time dynamics of thermal melting, subsurface cavitation, spallative ablation, and fragmentation ablation of the silicon surface excited by single IR femtosecond laser pulses is studied by timeresolved optical reflection microscopy. This dynamics is revealed by monitoring picosecond and (sub)nanosecond oscillations of probe pulse reflection, which is modulated by picosecond acoustic reverberations in the dynamically growing surface melt subjected to ablation and having another acoustic impedance, and by optical interference between the probe pulse replicas reflected by the spalled layer surface and the layer retained on the target surface. The acoustic reverberation periods change during the growth and ablation of the surface melt film, which makes it possible to quantitatively estimate the contributions of these processes to the thermal dynamics of the material surface. The results on the thermal dynamics of laser excitation are supported by dynamic measurements of the ablation parameters using noncontact ultrasonic diagnostics, scanning electron microscopy, atomic force microscopy, and optical interference microscopy of the modified regions appearing on the silicon surface after ablation.     

Ultrafast optical probe of coherent acoustic phonons in Co_2MnAl Heusler film

In this work, pronounced oscillations in the time-resolved reflectivity of Heusler alloy Co_2MnAl films which are epitaxially grown on Ga As substrates are observed and investigated as a function of film thickness, probe wavelength,external magnetic field and temperature. Our results suggest that the oscillation response at 24.5 GHz results from the coherent phonon generation in Co_2MnAl film and can be explained by a propagating strain pulse model. From the probe wavelength dependent oscillation frequency, a sound velocity of(3.85±0.1)×10~3m/s at 800 nm for the epitaxial Co_2MnAl film is determined at room temperature. The detected coherent acoustic phonon generation in Co_2MnAl reported in this work provides a valuable reference for exploring the high-speed magnetization manipulation via magnetoelastic coupling for future spintronic devices based on Heusler alloy films.