Alexey M. Lomonosov

Laser-based linear and nonlinear guided elastic waves at surfaces (2D) and wedges (1D).

The characteristic features and applications of linear and nonlinear guided elastic waves propagating along surfaces (2D) and wedges (1D) are discussed. Laser-based excitation, detection, or contact-free analysis of these guided waves with pump-probe methods are reviewed. Determination of material parameters by broadband surface acoustic waves (SAWs) and other applications in nondestructive evaluation (NDE) are considered. The realization of nonlinear SAWs in the form of solitary waves and as shock waves, used for the determination of the fracture strength, is described. The unique properties of dispersion-free wedge waves (WWs) propagating along homogeneous wedges and of dispersive wedge waves observed in the presence of wedge modifications such as tip truncation or coatings are outlined. Theoretical and experimental results on nonlinear wedge waves in isotropic and anisotropic solids are presented.

3. Laser-based surface acoustic waves in materials science

Publisher Summary This chapter discusses the application of laser-based surface acoustic waves (SAWs) in materials science. Laser sources are widely used in investigations by means of SAWs of elastic properties of solids, various films, and coatings on surfaces. These sources are noncontact; they allow a SAW transmitter of desired shape to be created and thus an acoustic beam with predetermined properties to be formed. All laser-based methods of SAW generation in solids can be divided into two groups. The first group is connected with heating of the solid by the absorbed laser radiation and all consequences of this heating, such as thermal expansion, evaporation, and ablation. The second group combines the processes of interaction of the electromagnetic field with the lattice or electronic structure of the solid, including electrostriction, deformation because of carrier density modulation by laser radiation, and nonradiative recombination in semiconductors. The chapter considers thermal laser sources of SAWs, because they are universal and in general more effective. Among thermal methods, it is convenient to distinguish between two limiting cases according to the relation between the absorbed energy density and specific heat of fusion and evaporation of the solid. The low-energy limit is the linear thermoelastic regime of SAW generation. All thermal and elastic parameters are assumed constant. Local pulsed heating causes transient thermal expansion and elastic stresses, which subsequently lead to emission of bulk and surface waves. In the high-energy case, strong evaporation or ablation occurs in the irradiated area.

Depth evaluation of surface-breaking cracks using laser-generated transmitted Rayleigh waves

A theoretical and experimental study of nondestructive evaluation of surface-breaking cracks with linear surface acoustic wave (SAW) pulses is presented. Schwarz–Christoffel conformal mapping was used to introduce a special orthogonal coordinate system that conserves the profile of the cracked surface. The inverse problem for two dimensions has been solved by means of conformal mapping. Thermoelastically generated broadband SAW pulses were employed to study the scattering of linear SAW pulses by a single crack. The surface wave component transmitted through the isolated microcrack was recorded as a function of distance by the cw laser probe-beam-deflection method. The Fourier transform of the transmitted SAW waveforms provides a stationary solution for any frequency. With this procedure the depth of the crack, produced in a separate experiment with a strongly nonlinear SAW pulse in a silica sample, was evaluated.

Sizing of partially closed surface-breaking microcracks with broadband Rayleigh waves

Sizing of cracks in the range of tens of micrometers by laser-generated surface acoustic wave (SAW) pulses with a bandwidth of up to 200 MHz is reported. The radius of a semicircular surface-breaking crack was evaluated using two methods, one based on the experimentally measured reflection coefficient and the other on the frequency dependence of the phase lag of the transmitted wave. These quantities were also simulated numerically by means of the finite differences method and fitted to the experimental ones by varying the assumed size of the crack. In both cases the interaction between crack faces was taken into account by an effective interfacial stiffness parameter and the crack sizes obtained acoustically were compared with those measured by optical microscopy. Nondestructive evaluation was extended to the characterization of real microcracks in the range of tens of micrometers with the laser-based pump-probe technique. The microcracks studied were generated in fused silica by strongly nonlinear SAW p...

Impulsive fracture of fused quartz and silicon crystals by nonlinear surface acoustic waves

During nonlinear evolution of laser-generated surface acoustic wave (SAW) pulses the stress increases with distance of propagation, and causes fracturing of brittle materials. This effect was used to evaluate the strength of isotropic fused quartz and anisotropic crystalline silicon with respect to impulsive loading in the nanosecond range and spontaneous cracking without using seed cracks. Crack nucleation and propagation along the surface was studied by optical microscopy and into the depth of the bulk by the focused ion beam technique and confocal microscopy. In fused quartz, fracturing produced characteristic regular patterns at the surface with cracks extending into the bulk at an angle of 30°–35° to the surface in the direction of SAW propagation. On Si(111) surfaces cracks extended at the surface in the 〈110〉 direction and propagated along the weak {111¯} cleavage plane into the bulk. Other crack planes were observed in only a few cases, e.g., at a larger depth and on the Si(100) surfaces. The obs...

Solitary surface acoustic waves and bulk solitons in nanosecond and picosecond laser ultrasonics.

Recent achievements of nonlinear acoustics concerning the realization of solitons and solitary waves in crystals and their surfaces attained by nanosecond and picosecond laser ultrasonics are discussed and compared. The corresponding pump-probe setups are described, which allow an all-optical contact-free excitation and detection of short strain pulses in the broad frequency range between 10 MHz and about 300 GHz. The formation of solitons in the propagating longitudinal strain pulses is investigated for nonlinear media with intrinsic lattice-based dispersion. The excitation of solitary surface acoustic waves is realized by a geometric film-based dispersion effect. Future developments and potential applications of nonlinear nanosecond and picosecond ultrasonics are discussed.

Multimode photoacoustic method for the evaluation of mechanical properties of heteroepitaxial diamond layers

A multimode photoacoustic method was developed for evaluating acoustically thick anisotropic layers, using surface acoustic waves. Such layers support multiple acoustic modes. This complicates the reverse problem, but on the other hand, makes it possible to extract more materials properties. Several mechanical properties of a layer-substrate system, consisting of a 110 μm thick heteroepitaxial chemical vapor deposited diamond layer on Ir/YSZ (yttria-stabilized zirconia)/Si(001), were evaluated, based on two surface acoustic modes. A dispersive and a nondispersive mode measured in two different crystallographic directions were employed to evaluate the three elastic stiffness coefficients C11, C12, C44, and the mass density of the diamond layer. It is demonstrated that accurate elastic moduli can be determined without special sample preparation, employing the layered system as obtained from the heteroepitaxial diamond growth process.

Nonlinear surface acoustic waves : Realization of solitary pulses and fracture

A laser-based technique for the contact-free generation and detection of strongly nonlinear surface acoustic wave (SAW) pulses with amplitudes limited by the materials strength has been developed. The effects of nonlinear propagation of short elastic surface pulses with finite strength in isotropic solids, such as fused quartz, anisotropic solids, such as silicon, and dispersive media were investigated. Solitary surface wave propagation was observed in layered structures for normal and anomalous dispersion. In addition, a SAW-based method for evaluating the critical fracture stress of anisotropic brittle solids, such as single crystal silicon, is introduced.

Laser-based measurement of elastic and mechanical properties of layered polycrystalline silicon structures with projection masks

The Young’s modulus, density, and thickness of a 20.3μm polycrystalline silicon layer deposited on a silicon wafer covered with a ∼2.5-μm-thick silicon-oxide interface layer were measured using projection masks to generate surface acoustic waves (SAWs) with higher harmonics approaching 600MHz. The propagating SAW train was detected with a laser probe-beam-deflection setup. The characteristic strongly nonlinear dispersion effect allowed the simultaneous extraction of several unknown film properties. The dispersion was described theoretically by the boundary element method model. A Young’s modulus of 152GPa, a density of 2.25g∕cm3, and a film thickness of 20.3μm were determined for the polycrystalline silicon film.

Guided acoustic waves propagating at surfaces, interfaces and edges

Surface and interface acoustic waves are two-dimensionally guided waves, as their displacement field is plane-wave like regarding its dependence on the spatial coordinates parallel to the guiding plane, while it decays exponentially along the axis normal to that plane. When propagating at the planar surface or interface of homogeneous media, they are non-dispersive. Another type of non-dispersive acoustic waves which is, however, one-dimensionally guided, has displacement fields localized near the apex of a wedge made of an elastic material. In this short review, their propagation properties are described as well as theoretical and experimental methods which have been used for their analysis. Experimental findings are discussed in comparison with corresponding theoretical work and potential applications of this fascinating type of acoustic waves are presented.