Table of contents

In the year 2010 we celebrated the 50th birthday of the laser, and laser-ultrasonics is only slightly younger. For the last few decades, laser-ultrasonics has attracted the attention of an increasing number of academics and engineers. It has motivated research in many directions, often in pluridisciplinary fields; and in parallel, having gained maturity, it has attracted more and more interest from industry.

Over the years laser-ultrasonics has been the topic of several sessions in different conferences dedicated to ultrasonics, optics, photo-acoustics, phonon scattering, material characterization, and non-destructive evaluation and control. With the aim of creating a fruitful forum for the exchange of ideas, national periodic meetings dedicated to laser-ultrasonics have been organized in Asia, North America and in Europe. For example, from 1999, we began holding convivial meetings in France, in which contributions by PhD students are strongly encouraged. Discussions at an international level were initated a few years ago, examining the opportunity for creating a dedicated international conference assembling researchers and end-users with an interest in the field. Under Jean-Pierre Monchalin's instigation, the first International Symposium on Laser-Ultrasonics took place in Montreal in July 2008, and was a great success. We were very honoured to be in charge of the organization of the second meeting of the conference in Bordeaux in July 2010.

The second symposium assembled about 150 attendees from 30 countries. The program included 90 oral contributions, with 17 invited and plenary talks, and 40 contributions with posters. Despite extending the conference from three to four days, the organization of two parallel sessions was necessary. Thanks to the contributions of the participants, the scientific and organizing committees could construct an intensive and attractive scientific program.

The conference tour was a sunny visit to Saint-Emilion vineyard and the gala dinner took place in a wine chateau in the medieval village. We hope this experience will remain in the attendees' memories as a pleasant and convivial time.

We would like to express our thanks to the members of the organizing committee, the scientific committee, and all our generous sponsors, either institutions or companies, for their help in making this event possible. We would also like to thank the scientists involved for their confidence in our organization, and for their contributions.

As unanimously decided by the attendees, the next symposium will be held in Japan, most likely in 2012.

The papers published in this volume of the Journal of Physics: Conference Series provide a collection of state-of-the-art and recent advances in research and applications of laser ultrasonics as presented at this second Symposium.

Bertrand Audoin – Conference Chair Thomas Dehoux – Conference proceedings co-editor Yannick Guillet – Conference proceedings co-editor

Scientific committee

Organizing committee

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

Surface Brillouin scattering of light has been used to measure the angular dependence of the Rayleigh surface acoustic wave (SAW), pseudo surface acoustic wave (PSAW) and longitudinal lateral wave (LLW) speeds in a (100)-oriented single crystal of the ternary semiconductor alloy InAs_0.91Sb_0.09. The wave speed measurements have been used to determine the room temperature values of the elastic constants C₁₁, C₁₂ and C₄₄ of the alloy. A simple and robust fitting procedure has been implemented for recovering the elastic constants, in which the merit function is constructed from explicit secular functions that determine the surface and lateral wave speeds in the [001] and [011] crystallographic directions. In the fitting, relatively larger weighting factors have been assigned to the SAW and PSAW data because of the greater precision with which the surface modes can be measured as compared with the lateral wave.

Non destructive testing has been performed on a thin indium layer deposited on a two inches silicon wafer. Guided waves were generated and studied using a laser ultrasonic setup, and a two-dimensional Fourier transform technique was employed to obtain the dispersion curves. The inverse problem, in other words the determination of the layer thickness and the elastic constants of the substrate, has been solved by means of a feedforward neural network. These parameters were then evaluated simultaneously, the dispersion curves being entirely fitted. The experimental results show a good agreement with the theoretical model. This inversion method was found to be prompt and easy to automate.

Based on bending vibrations model method and also construction of multilayer transducer, which allow the measurements of thermal conductivity for porous layer on Si wafer is proposed. Time dependence of photoacoustic signal under rectangular modulation of exciting light was numerical modeling. The experimental tests were done on the samples of porous silicon on Si wafers as well as for the porous silicon free standing layers, obtained under the same anodization regime. The value of thermal diffusivity of the porous silicon layer correlates well with the value of thermal diffusivity of the porous silicon free layer, determined by TDC method.

This paper deals with determination of in-plane elastic constants of thin layers deposited on substrates. Modified resonant ultrasound spectroscopy is used to measure resonant spectra before and after layer deposition . These two spectra are compared and changes in the position of the resonant peaks are associated with layer properties. It is shown that for thin layers either the elastic moduli or the surface mass density can be determined, providing the complementary information (the surface mass density for the determination of the moduli, the elastic moduli for the determination of the surface mass density) is known. As an experimental demonstration of this approach, the elastic moduli of diamond-like-carbon film deposited on a silicon substrate and the surface mass density of a thin spray paint on a silicon substrate are determined.

Wide frequency-band Rayleigh waves (~100 MHz) were utilized to characterize the elastic constants of thin Au/Cr films deposited on glass substrates. The Rayleigh waves were excited utilizing laser induced thermoelastic mechanism and detected using a knife-edge technique apparatus. The dispersion of the signals in glass substrates coated with Au/Cr was measured and fitted to theory using a non-linear regression algorithm. From the fitting, the Au films Young modulus and the film thickness were extracted. The results were analyzed with regards to AFM scans performed on the samples and independent thickness measurement done by a dektak³ profiler. Results show a good agreement between the two measurements.

Thermal properties of hydrogenated amorphous carbon (a-C:H) thin-films are measured using an ultrafast optical pump-probe technique. The a-C:H samples were grown in a home-built direct-current (DC) plasma enhanced chemical vapor deposition (PECVD) system with varying hydrogen (H₂) diluents to methane (CH₄) flow-rate ratios. Thermal diffusivities of samples are extracted by comparing thermoreflectance measurements with numerical calculations so that thermal conductivities (k) can be determined. Although the dependence of thermal property on H₂ dilution was not significant, our films show lower k (0.10–0.15 W/mK ± 20%) compared to the results of previous studies.

A laser-ultrasonic approach based on the propagation of surface skimming longitudinal wave (P-wave) and Rayleigh wave (SAW) is investigated for the measurement of texture in moderately thick steel plates. The angular dependence of the P-wave and SAW velocities presents different patterns, both of amplitude of a few % attributed to texture. Moreover, the difference of the P-wave and SAW profiles is shown to be very robust against small path length changes or other detrimental effects. For measurements on large plates, a solution based on the rotation of a generation line using an axicon lens is tested. Texture variations across the width of steel plates and the influence of residual stress are discussed.

A thermo-mechanical coupling finite element (FEM) model is built to simulate frictional heating phenomenon of a crack when the plate containing the crack is excited by intensive ultrasonic waves. In the explicit FEM model, a high-power ultrasonic transducer is included for simulating the phenomena of acoustic chaos, and piezoelectric excitation of the transducer is realized by using a piezoelectric thermal-analogy method. Moreover, the dynamical interaction between both crack faces is modelled by using contact-impact algorithm. In the simulation, Dynamical contact force and relative motion of the crack faces, temperature-rise distribution around the crack, and temperature-rise versus time curves of the crack faces, are quantitatively calculated and analyzed when the plate undergoing chaotic excitation.

A new method to evaluate a crack depth is introduced. Surface acoustic wave generated by Q-switched Nd:YAG laser (wavelength: 532 nm) and detected by frequency-stabilized long pulse laser (wavelength: 1,064 nm) coupled with confocal Fabry-Perot interferometer is used to evaluate a depth of surface-breaking tight crack. When the generated surface acoustic wave propagates through a crack before it is detected, only the lower frequency component is observed at the detection point due to interaction between the broadband surface acoustic wave and shallow crack. Energy of surface acoustic wave penetrates about its one wavelength into the propagation medium; it means that surface acoustic wave with higher frequency component localizes only thin layer from the surface and one with lower frequency component easily travel through cracks if it is shallow. A frequency response analysis technique, as well as an amplitude response analysis, of the surface acoustic wave is developed to quantitatively evaluate the depth of cracks. Several stress corrosion cracks introduced on type 304 stainless steel plates by immersing corrosive solution with tensile stress are prepared to verify the performance of this method. The results demonstrate the error of this depth measurement method is estimated at less than 0.3 mm.

Contact laser-ultrasonic evaluation (CLUE) provides ultrasonic testing with sharp probe ultrasonic pulse. It makes it possible to enhance spatial resolution at relatively lower frequency band than piezoelectric ultrasonic testing. The advantage of CLUE is the possibility to distinguish the acoustic wave reflection at the soft or rigid boundary. This feature can be effectively used for the investigation of disbonding in laminate structures. The sample consisting of three aluminum sheets glued with polymer layers is tested with CLUE. The results of back-reflection coefficient measurement are compared with the numerical simulation. The signatures from regular area and area with disbonded layer are compared. The sensitivity and resolution of testing is discussed.

This paper explores some effects that occur when using laser ultrasound to scan defective samples. Surface defects can often propagate at an angle to the surface; however, for calibration, slots machined normal to the surface of the sample are typically used. Several interesting angle-dependent effects are observed when Rayleigh waves interact with angled surface defects, and are explored here using measurements and models for a scanning laser detector (SLD) or scanning laser line source (SLLS) scanned across the defect. Reflection and transmission coefficients are calculated for different crack angles and lengths. Additionally, interesting angle-dependent effects are observed in the Rayleigh wave amplitude and frequency enhancements in the near field when using SLD or SLLS.

Disc-shaped specimen of nickel of about 30 mm diameter and 2.25 millimeters thick was prepared by the high pressure torsion method. The produced equivalent shear strain increased linearly from a minimum at the centre to the periphery. The cold working process multiplied the dislocations and led to the grain fragmentation. The saturation of the grain refinement was approached at the circumference of the specimen yielding a narrow distribution of the grain size with a mean value of about two hundred nanometers. The refinement of the microstructure changes the elastic properties of the material such as hardness, strength and has an influence on the attenuation and phase velocity of the ultrasound. The paper reports the measurements of the attenuation of the longitudinal wave in severely plastically deformed nickel with polycrystalline and ultrafine microstructure.

During normal usage components are subject to stresses that while not sufficient to cause fracture cause fatigue, which gradually weakens the component. Linear ultrasonic methods have been shown to be poor at detecting fatigue. However, there is evidence that the accumulation of damage gives the material a nonlinear elastic response that can be probed by ultrasound. By measuring the change in a material's nonlinear properties a measure of the fatigue can be obtained. Several methods of detecting material nonlinearity using acoustic waves have been proposed. The collinear mixing technique is used here. By measuring the velocity change of a probe wave due to the induced stress from a second pump wave, a measure of the nonlinearity is obtained. By generating the probe wave and detecting both waves using laser ultrasound techniques we gain the benefits of high spatial and temporal resolution. This is important when investigating the nonlinear response of a material as there is evidence that the microstructure affects the nonlinear response of a material. The change in nonlinearity over a region of a specimen (aluminium) has been monitored over several fatigue levels to investigate any relation. Early stage results are given with a discussion on the development of the technique.

Non destructive testing of civil engineering infrastructures is becoming of primary importance for their diagnosis, residual time life estimation and/or structural health monitoring. A particularity of civil engineering application is the large size of the survey zones and the expected low cost of inspection. In this context non contact ultrasonics may offer the possibility to built robots that can automatically scan large areas (or eventually be integrated in moving vehicles) to recover mechanical properties of material or to perform imagery for geometrical information recovery. In this paper we present two possible applications of in situ laser ultrasonics : one is the detection of voids in tendon duct with the impact echo method, the other is the use of surface waves to recover mechanical properties of the first centimetres of concrete structures (here after called cover concrete).

Non-destructive testing techniques are developed to secure reliability of aerospace vehicles used repetitively. In the case of cracks caused by thermal stress on walls in combustion chambers of liquid-fuel rockets, it is examined by ultrasonic waves visualization technique developed in AIST. The technique is composed with non-contact ultrasonic generation by pulsed-laser scanning, piezoelectric transducer for the ultrasonic detection, and image reconstruction processing. It enables detection of defects by visualization of ultrasonic waves scattered by the defects. In NIMS, the condition of the detection by the visualization is investigated using computer simulation for ultrasonic propagation that has capability of fast 3-D calculation. The simulation technique is based on finite-difference method and two-step elastic wave equations. It is reported about the investigation by the calculation, and shows availability of the simulation for the ultrasonic testing technique of the wall cracks.

Femtosecond optical amplifiers typically have high nonlinear spectral dispersion breaking relationships of the spectral phases of femtosecond pulse. It is necessary to use some additional dispersive device for controlling of spectral amplitude and phases of amplified laser pulses. We discuss the investigation of light dispersive delay lines based on the light-sound interaction in crystals. The light dispersive delay lines geometry uses the effect of strong elastic, photoelastic and optic anisotropy in the TeO₂ single crystal. Different experimental delay lines were designed and tested. The problem of femtosecond laser beam angular chirp compensation was solved also. The experiments were performed at the OPCPA femtosecond laser system.

Laser ultrasonic technique has been successfully applied to a sample embedded in a transparent pressure transmitting medium to measure sound velocity at high pressures in a diamond anvil cell. Shear and longitudinal acoustic wave velocities are measured both for an opaque sample, Fe, and a transparent pressure transmitting medium, KBr, at pressures ranging from 0.3–24.8 GPa. The small sample size used in this experiment for sound velocity measurements suggests great potential for using this technique to evaluate the elastic properties of solids at pressures approaching 100 GPa and for other samples embedded in soft, transparent pressure transmitting media such as KBr or argon.

Laser-induced lattice deformation generating acoustic pulses has been studied by time-resolved X-ray diffraction method using a brilliant synchrotron radiation source. We constructed a time-resolved X-ray diffraction system equipped with a synchronized femtosecond pulsed laser at an undulator beamline of SPring-8 facility, and observed laser-induced acoustic pulse in a semiconductor wafer of GaAs, where the laser irradiation causes initial strain of expansion with a response time of around 200 ps. High resolution X-ray diffractometry in asymmetric configuration combined with laser-pump X-ray probe method revealed that the strain is formed with coherent longitudinal acoustic phonons and lattice expansion along the surface normal. The laser power dependence shows the saturation of the lattice expansion ratio with the lengthened relaxation time for high power excitation.

We assume that a coherent phonon generation was found out in structures with a single graphitized layer under picosecond laser excitation by means of the Pump/Probe method. The thermal component of the response was calculated that allowed us to define the photothermal coefficient of the graphitized layer as well as its thermal conductivity from the experimental results. The thermal-conductivity coefficient of the graphitized layer appears to grow with the thickness decrease. This makes it possible to assume that the microstructure of the graphitized layer produced during annealing depends on its thickness.

A pump-probe picosecond technique is used to reveal the opto-acoustic response of micrometric carbon fibers. Two carbon fibers having the hexagonal symmetry are experimentally studied. A dedicated experimental setup allows to generate acoustic waves that propagate in a cross section of the fiber. The elastic properties in the transverse direction of a single carbon fiber are thus evaluated. Complex optical refractive index is also measured. The results provide interesting perpectives for the non-destructive evaluation of elastic and optical properties at a micron scale.

Laser based impulsive stimulated scattering or transient grating excitation in a heterodyne diffraction scheme is a powerful method to extract information about different relaxing properties from different signal contributions. Longitudinal acoustic waves are detected simultaneously with thermal expansion and thermal diffusion. Careful fitting of the time-domain density response at different temperatures makes it possible to obtain the various relaxing physical parameters, and to construct Arrhenius plots for the respective relaxation processes. In this work we focus on the influence of the specific heat capacity C on the slower part of the density response function S_ρ(t), and, inversely, on the possibility to extract from experimental S_ρ(t) data the relaxation behaviour C(ω). The specific heat capacity is relevant for both the initially rising part of the impulsive stimulated scattering signal (together with the time and frequency dependent thermal expansion γ(t)), and for the thermal diffusion dominated decrease of the signal at later times after the excitation. By simulating S_ρ(t) data in different scenarios, we address the feasibility of unravelling the impulse response functions C(t) and γ(t) (and via Fourier transform also C(ω) and γ(ω)) by careful fitting of the signal. This approach offers a unique possibility to extend the 100 kHz bandwidth of current dynamic calorimetric techniques determining C(ω) (photopyroelectric spectroscopy) to the sub-GHz range.

A non-contact method with a laser-ultrasonic technique for measuring two-dimensional temperature distribution on a material surface is presented. The method consists of a laser-ultrasonic measurement of a one-dimensional temperature distribution on a material surface and its two-dimensional area mapping. The surface temperature is basically determined from a temperature dependence of the velocity of the surface acoustic wave (SAW) propagating on a material surface. One-dimensional surface temperature distributions are determined by an inverse analysis consisting of a SAW measurement and a finite difference calculation. To obtain a two-dimensional distribution of surface temperature on a material surface, SAW measurements within the area of a square on the surface are performed by a pulsed laser scanning with a galvanometer system. The inverse analysis is then applied to each of the SAW data to determine the surface temperature distribution in a certain direction, and the obtained one-dimensional distributions are combined to construct a two-dimensional distribution of surface temperature. It has been demonstrated from the experiment with a heated aluminum plate that the temperature distributions of the area of a square on the aluminium surface determined by the ultrasonic method almost agree with those measured using an infrared camera.

This paper describes the application of Impulse Stimulated Thermal Scattering (ISTS) for measurement of surface acoustic wave (SAW) velocity and thermal diffusion along a free surface of a strongly anisotropic material. The motivation for this work stems from the study of thermoelastic properties of individual phases of ferroelastics; experimental results were obtained on a single crystal in the austenitic phase a Cu-Al-Ni alloy (bcc single crystal having elastic anisotropy factor of about 12). The measured SAW velocities in specific directions are in a good agreement with the values calculated for the elastic constants obtained by other ultrasonic methods. Similarly, the evaluated thermal diffusivity coefficient (22 ± 2).10⁻⁶ m²/s of the austenite is consistent with the data in the literature. The proposed approach has also a potential for characterization of thin films grown on anisotropic substrates.

Time Resolved Pump Probe (TRPP) technique has been implemented to study the thermal and mechanical properties of Ge₂Sb₂Te₅ (GST) film deposited on a silicon substrate. According to the knowledge of the thermal properties of the GST layer, the temperature dependant Thermal Boundary Resistance (TBR) at the metal-GST interface is evaluated. Measuring the acoustic oscillation and more particularly its damping leads to characterize the adhesion at the metal – GST interface. This quantity can be efficiently related to the temperature dependent TBR in the 25°C – 400°C range. The TBR increases with temperature and follows the changes of the crystalline structure of materials. A linear relation between the acoustic reflection coefficient and the logarithm of the thermal boundary resistance is found.

There are numerous techniques for the optical detection of ultrasound but many of these are of very limited application on optically rough surfaces. Techniques that work on rough surfaces are often complicated, inefficient and expensive. In this paper a new technique for the optical detection of ultrasound on rough surfaces is presented. It is a variation on the knife-edge (KE) technique but adapted for rough surfaces by replacing the knife-edge with a speckle correlated spatial filter (SCSF) which is implemented using a spatial light modulator (SLM). This technique is conceptually and practically simple and may be implemented using relatively low cost components. It is also relatively general and may also be applied to the detection of small displacements in many circumstances.

The microstructure of a material influences the characteristics of a component such as its strength and stiffness. A previously described laser ultrasonic technique known as spatially resolved acoustic spectroscopy (SRAS) can image surface microstructure, using the local surface acoustic wave (SAW) velocity as a contrast mechanism. The technique is robust and tolerant of acoustic aberrations. Compared to other existing methods such as electron backscattered diffraction, SRAS is completely non-contact, non-destructive (as samples do not need to be polished and sectioned), fast, and is capable of inspecting very large components. The SAW velocity, propagating in multiple directions, can in theory be used to determine the crystallographic orientation of grains. SRAS can be implemented by using a fixed grating period with a broadband laser excitation source; the velocity is determined by analysing the measured frequency spectrum. Experimental results acquired using this "frequency spectrum SRAS" (f-SRAS) method are presented. The instrumentation has been improved such that velocity data can be acquired at 1000 points per second. The results are illustrated as velocity maps of material microstructure in two orthogonal directions. We compare velocities measured in multiple propagation direction with those predicted by the numerical model, for several cubic crystals of known orientations.

The displacement response of piezoelectric PZT thick films fabricated by means of electrophoretic deposition and laid down an alumina substrate is investigated using coherent optical detection. According to thickness properties determined by electrical impedance measurements, the film presents a resonance around 40 MHz. Other resonance peaks are observed that correspond to eigen modes of the film substrate couple structure. Uniformity of the response of the integrated structure is studied across the surface of the sample when excited by either a continuous or impulse electrical voltage. Results on the amplitude of the detected signal versus the frequency and the input excitation voltage are reported. The optical detection used in these experiments is complementary to conventional techniques of characterization of piezoelectric devices such as electrical impedance measurements and allows getting information on the displacement response of the device.

We report the first experimental evidence for the resonant excitation of coherent high-frequency acoustic phonons in semiconducting doping superstructures by far-infrared laser radiation. After a grating-coupled delta-doped silicon doping superlattice is illuminated with ~1 kW/mm² nanosecond-pulsed 246 GHz laser radiation, a delayed nanosecond pulse is detected by a superconducting bolometer at a time corresponding to the appropriate time-of-flight for ballistic longitudinal acoustic phonons across the (100) silicon substrate. The absorbed phonon power density in the microbolometer is observed to be ~10 μW/mm², in agreement with theory. The phonon pulse duration also matches the laser pulse duration. The absence of any delayed transverse acoustic phonon signal by the superconducting bolometer is particularly striking and implies there is little or no incoherent phonon generation occurring in the process.

The generation of ultrasound by a collapsing single cavitation bubble in a strongly absorbing liquid illuminated with a moderate power CW laser is described. The ultrasound shock wave is detected with hydrophone and interferometric device. To obtain a stronger pulse it is necessary to adjust a liquid absorption and a beam diameter. Their influence can be qualitatively understood with a simple model.

The purpose of this paper is to study the effect of a non-normal optical penetration due to an obliquely incident laser source. It is shown that the loss of symmetry due to such a penetration influences specific bulk acoustic modes. For a given detection position, increase in shear wave amplitude is obtained by orienting the incident laser source.

An alternative theoretical solution is proposed to predict ultrasound excited by a laser line source in a homogeneous, isotropic cylinder coated with a homogeneous and isotropic material. The spectrum and transient displacement are in agreement to author's previous model for the cylinder without coating. Numerical results are obtained for copper on aluminium and vice versa. It is found for the surface waves that the high frequency component travels faster for hard coatings with velocities faster than that of the substrate, and vice versa for soft coatings.

A laser ultrasonics system was setup to measure the directivity (angular dependence pattern) of the amplitude of ultrasonic waves generated in aluminum samples. A pulsed Nd:YAG laser operating at 1064 nm optical wavelength, with typical pulse width (FWHM) of 8 ns, and energy per pulse of 450 mJ, was used to generate the ultrasound waves in the samples. The laser detection system was a Mach-Zehnder interferometer with typical noise-limited resolution of 0.25 nm (rms), frequency range from 50 kHz to 20 MHz, and measurement range from −75 nm/V to +75 nm/V. Two different optical spot sizes of the Nd:YAG laser were used to generate waves in the ablation regime: one was focused and the other was unfocused. Using the obtained data, the directivity graphics were drawn and compared with the theoretical curves, showing a good agreement. The experiments showed the directivity as a function of the optical spot size. For a point ultrasonic source (or focused optical spot), the directivity shows that the longitudinal waves present considerable amplitude in all directions. For a larger ultrasonic source (or an unfocused optical spot) the directivity shows that the longitudinal waves are generated with the higher amplitudes inside angles around ±10°.

This paper presents a method to reconstruct thermal conductivity depth profile of a layered medium using noisy photothermal data. The method tries to obtain an accurate reconstruction of discontinuous profile using particle swarm optimization (PSO) algorithm and total variation (TV) regularization. The reconstructions of different thermal conductivity profiles have been tested on simulated photothermal data. The simulation results show that the method can find accurately the locations of discontinuities, and the reconstructed profiles are in agreement with the original ones. Moreover, the results also show the method has good robustness and anti-noise capability.

We report on (to our knowledge) the first remote contactless photoacoustic imaging with short excitation pulses on semitransparent solid polymer samples for material inspection. In this work solid semitransparent samples are excited with pulses from a short pulse laser. The local absorption of the electromagnetic radiation leads to generation of broadband ultrasonic waves. Ultrasonic waves arriving on the sample surface are detected with a confocal Fabry-Pérot interferometer. After data acquisition the absorbed energy density is reconstructed by utilizing an F-SAFT algorithm. The work shows the potential of photoacoustic imaging on material inspection of semitransparent solid materials.

The development of nanometre sized ultrasonic transducers is important in both biological and industrial applications. The small size can be important in its own right or necessary in order to generate acoustic waves with nanometric wavelengths. Potential applications of nanotransducers range from embedded sensors through to sub optical wavelength acoustic imaging. In this paper, we show the generation and detection of ultra high frequency acoustic waves using nanometre scaled optical ultrasonic transducers. The optical and mechanical properties of these devices have been modelled using finite element modelling (FEM) and analytical techniques. The models allow the fine tuning of the design parameters to enhance both the acoustic and optical performance of the transducers. The devices were fabricated by evaporating the required metal and transparent layers onto a substrate, and then surface patterning of the device was created by laser machining or photolithography, thus allowing close comparison between model and experiment. We discuss the transducer design process and the effect of the coating parameters and how these affect the operating frequency and efficiency of the devices. We discuss the possibility of using molecular self assembly to produce even smaller devices.

The use of low power modulated laser diode systems has previously been established as a suitable method for non-destructive laser generation of ultrasound. Using a quasi-continuous optical excitation amplified by an erbium-doped fibre amplifier (EDFA) allows flexible generation of ultrasonic waves, offering control of further parameters such as the frequency content or signal shape. In addition, pseudo-random binary sequences (PRBS) can be used to improve the detected impulse response. Here we compare two sequences, the m-sequence and the Golay code, and discuss the advantages and practical limits of their application with laser diode based optical excitation of ultrasound.

The propagation of ultra-short sound pulses in water has been studied using an ultrafast opto-acoustic technique. A pulse time-of-flight technique for measuring the depths of deep channels in Si-based nanostructures was demonstrated. We report in these proof-of-concept ultrasonic experiments how spatial profile information of nanostructures can be acquired, where sound pulses propagate down narrow channels in patterned nanostructures. We have been able to detect acoustic echoes for sound propagating along a channel as narrow as 35 nm with depth to width ratios exceeding 10:1.

In the present work we have realized experimentally the laser optoacoustic method for the measurements of local volume fraction of air pores (porosity P) of isotropic composite materials. It is based on measurements of phase velocities of thermooptically excited longitudinal ultrasonic waves in composite samples in the frequency range 0.5÷50 MHz. The local porosity (lateral resolution 1÷2 mm) is determined using the dependence of phase velocity of longitudinal acoustic wave on porosity for porous metals and theoretical calculation of phase velocity in a composite with the two-phase medium model. A number of aluminum alloy (silumin) matrix composite samples reinforced by SiC particles of a different mass concentration with the mean particle size of 14 mm was investigated. The samples were disks with the diameter d = 40 mm and the porosity in the center and in the periphery area of each sample was determined. The increase of mass concentration of the filler SiC leads to the growth of P. The results coincide within relative inaccuracy of 2–3% with the gravimetrical measurements of average <P> value, the porosity in the center of each sample was slightly higher than in the periphery.

In this paper we show the benefits of a quasi balanced fringe hopping CFPI (confocal Fabry-Pérot interferometer) with broadband CMRR (common mode rejection ratio) for remote ultrasound detection. Ultrasonic information in general lies in the phase modulation of laser light which in this case is demodulated by using the CFPI at a certain working point on a fringe. By hopping from the positive to the negative slope on the same fringe the detected ultrasonic signals are inverted. In contrary interference signals like crosstalk from the generation, ghosts, or noise correlated to pulse laser excitation are not influenced and hence get rejected by subtracting the signals from both slopes. Hence, a minimum of two measurements is needed for common mode rejection. The fringe hopping from the positive to the negative slope is done by changing the distance of the CFPI mirrors with a precise piezoelectric-stack and a fast high resolution digital controller. As only one photo-detector with a transimpedance-amplifier is needed a high CMRR can be accomplished which is not affected by the symmetry of the fringe but only by pulse to pulse energy fluctuations of the generation laser. We show that with fringe hopping and averaging the signal to noise ratio increases much faster than with averaging without fringe hopping. This is due to the correlation of the quasi-noise with the generation cycle.

We describe an application of dynamic holography in rubidium vapour for laser ultrasound detection. This medium combines good signal to noise ratio with high cut-off frequency of approximately 1MHz.

A non-destructive laser ultrasonic surface acoustic wave technique has been demonstrated to quantitatively evaluate the elastic response of human dental enamel. We demonstrate the system performance by measuring surface acoustic wave velocity in sound and demineralised enamel. In addition, progressive measurements were made to monitor the change in the enamel elasticity during a two week remineralisation process. The results are presented and they confirm the efficacy, as well as illuminating the progress, of the treatment.

The measurement of the mechanical properties of single biological cells with a nanometer depth resolution using only coherent light is proposed. A pump-probe set-up based on an ultrafast laser (100 fs pulses) is used to excite and detect acoustic frequencies in the GHz range. Experiments are performed on single fixed mouse MC3T3 cells adhering on titanium alloy substrate. Using two different probe wavelengths, the contributions to the optical detection resulting from the cell interface displacements and from interactions between acoustic waves and the laser light are identified. Semi-analytical calculations allow the determination of acoustic celerities and thicknesses in cells thinner than 150 nm.

Biological objects are exquisitely sensitive to temperature variations and their mechanical characterization is often a challenge when using the picosecond ultrasonics technique. To reduce the laser-induced temperature rise, we place single biological cells on a thin metal transducer and we focus the laser beam that generates the acoustic waves at frequencies ≤ 150 GHz on the rear side of the transducer. The acoustic waves propagate through the transducer and are partially transmitted to the cell to create the so-called Brillouin oscillations. The frequency of these oscillations provides a direct measurement of the sound velocity. The simultaneous measurement of the acoustic reflection coefficient at the transducer/cell interface allows the determination of both the density and the compressibility of the cell.

Adhesive bonding of composites laminates is highly efficient but is not used for joining primary aircraft structures, since there is presently no nondestructive inspection technique to ensure the quality of the bond. We are developing a technique based on the propagation of high amplitude ultrasonic waves to evaluate the adhesive bond strength. Large amplitude compression waves are generated by a short pulse powerful laser under water confinement and are converted after reflection by the assembly back surface into tensile waves. The resulting tensile stresses can cause a delamination inside the laminates or at the bond interfaces. The adhesion strength is evaluated by increasing the laser pulse energy until disbond. A good bond is unaffected by a certain level of stress whereas a weaker one is damaged. The method is shown completely non invasive throughout the whole composite assembly. The sample back surface velocity is measured by an optical interferometer and used to estimate stress history inside the sample. The depth and size of the disbonds are revealed by a post-test inspection by the well established laser-ultrasonic technique. Experimental results show that the proposed method is able to differentiate weak bond from strong bonds and to estimate quantitatively their bond strength.