Thermoelastic hologram for focused ultrasound

Gutfeld, R. J. von; Vigliotti, D. R.; Ih, Charles S.; Scott, William R.

doi:10.1063/1.93826

Cited by 13 publications

(6 citation statements)

References 5 publications

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“…In such a situation, a displacement field mimicking an ideal plane wave is often looked for and propagation is adequately described by phase quantities, such as the phase velocity. In non piezoelectric materials, surface acoustic waves can be excited using a focused laser beam [3]. In many instances, the surface can be considered to be excited at a single source point.…”

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confidence: 99%

Subwavelength focusing of surface acoustic waves generated by an annular interdigital transducer

Laude

Gérard

Khelfaoui

et al. 2008

Applied Physics Letters

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We propose and demonstrate experimentally the concept of the annular interdigital transducer that focuses acoustic waves on the surface of a piezoelectric material to a single, diffraction-limited, spot. The shape of the transducing fingers follows the wave surface. Experiments conducted on lithium niobate substrates evidence that the generated surface waves converge to the center of the transducer, producing a spot that shows a large concentration of acoustic energy. This concept is of practical significance to design new intense microacoustic sources, for instance for enhanced acouto-optical interactions.A variety of anisotropic propagation phenomena are observed in solids, that are mainly determined by the symmetries of the rank-four elastic constant tensor [1]. The piezoelectric effect offers a convenient means of generating elastic waves, by transforming electrical to mechanical energy. The wide majority of elastic wave experiments in piezoelectric solids employ planar or interdigitated transducers (IDT) emitting plane waves with well-defined wave vectors [2]. In such a situation, a displacement field mimicking an ideal plane wave is often looked for and propagation is adequately described by phase quantities, such as the phase velocity. In non piezoelectric materials, surface acoustic waves can be excited using a focused laser beam [3]. In many instances, the surface can be considered to be excited at a single source point. In this case, the surface supports outgoing waves that depart from the source and propagation in the far field depends on the angular dispersion of the spatial group velocity. At a distance exceding a few wavelengths, the formed ripple pattern has the shape of the wave surface, obtained by plotting the group velocity as a function of the emission angle. This effect was observed very neatly for surface waves in the experiments of Vines et al. [4] and Sugarawa et al. [5]. Here, we investigate the converse problem: can we construct an extended source using an appropriate IDT that will focus elastic energy to a single point on the surface of a piezoelectric solid?A circular IDT has already been proposed by Day and Koerber [6] for substrates with in-plane isotropy (orthotropy). Since phase and group velocities are equal in this case, the wave surface reduces to a simple circle. A more recent alternative, the so-called focused interdigital transducer (FIDT), makes use of acoustic emission only within a circular arc [7,8]. In this case, the acoustic field is strongly dependent on the angular extent of the arc and on the focal length of the transducer. Another limit comes from the anisotropy of the substrate which leads * D. Gérard is now with the Institut Fresnel, Domaine Universitaire de Saint Jérôme, F-13397 Marseille cedex 20, France. † C. F. Jerez-Hanckes is also with the Centre de Mathématiques Appliquées, CNRS, Ecole polytechnique, F-91128 Palaiseau, France, and EPCOS Sophia Design Center, 1300 Route des Crêtes, F-06560, Sophia-Antipolis, France. to aberrations at the focal point. In t...

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confidence: 99%

Subwavelength focusing of surface acoustic waves generated by an annular interdigital transducer

Laude

Gérard

Khelfaoui

et al. 2008

Applied Physics Letters

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“…A is a constant independent of the position of observation. Equation (1) indicates that in the far field, propagation occurs in the direction θ at the group velocity v g (θ) and that the amplitude decreases with the square root of the distance. The term |h (ψ)| is proportional to the so-called phonon focusing factor, which gives cuspidal points and the caustics when it vanishes.…”

Section: Field Radiated By a Point Sourcementioning

confidence: 99%

“…In such a situation, a displacement field mimiking an ideal plane wave is often looked for and propagation is adequately described by phase quantities, such as the phase velocity. In non piezoelectric materials, surface acoustic waves can be excited via the photoelastic effect, usually using a focused laser beam [1], [2]. In many instances, the surface can be considered to be excited at a single source point.…”

Section: Introductionmentioning

confidence: 99%

P4L-3 Anisotropic Wave-Surface Shaped Annular Interdigital Transducer

Laude

Gérard

Khelfaoui

et al. 2007

2007 IEEE Ultrasonics Symposium Proceedings

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Interdigital transducers (IDT) are widely used to generate surface acoustic waves directly on piezoelectric materials. However, in most applications, the generating fingers are straight, giving rise to the emission of plane waves. One notable exception is the circular IDT proposed by Day and Koerber for isotropic substrates [IEEE Trans. Sonics and Ultrason. SU-18, 461 (1972)]. More recently, the focused interdigital transducer (FIDT) has been used to obtain high intensity generation at the focal spot. The FIDT uses surface wave emission inside a circular arc for concentrating acoustic energy at its focus. However, the anisotropy of the substrate can lead to aberrations at the focal point. We investigate the problem of constructing an extended source that will focus elastic energy to a single point on the surface of a piezoelectric crystal. On the surface of a piezoelectric solid that is mechanically excited at a single point, concentric waves originate and form in the far field a ripple pattern that follows the shape of the wave surface, obtained by plotting the group velocity as a function of the emission angle. We conversely propose the concept of an annular interdigital transducer (AIDT), in which the shape of the fingers follows the wave surface. The surface acoustic waves generated by an AIDT are expected to converge to the center of the transducer, producing a spot that is limited in resolution by diffraction only. Experiments have been conducted on Y and Z cut lithium niobate (LiNbO3). AIDTs operating at a resonance frequency of 75 MHz have been constructed. Electrical measurements show that despite anisotropy in-phase emission at all angles is obtained for Rayleigh waves. In addition, spatial maps of the displacements at the surface have been obtained using a heterodyne optical probe, showing an important focusing of surface acoustic waves in the center of the device. The measured displacement fields at resonance show surface ripples converging to a spot at the center of the transducer. This result is promising for several applications including intense microacoustic sources.

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“…A ring focused beam has been used to detect a 1 mm deep, 0.1 mm width, electro-discharge machined (EDM) slot in aluminium, but this measurement required the defect length to be larger than the ring diameter [21]. Sample surface deposits or screens can also be employed to shape the laser footprint on the sample surface, changing the frequency bandwidth of the generated ultrasound waves and creating a focus [23][24][25]. However, lasers are not always viable for regular machine testing in a factory setting due to their high costs and potentially serious safety implications.…”

Section: Introductionmentioning

confidence: 99%

Focused Rayleigh wave EMAT for characterisation of surface-breaking defects

Thring

Yang

Edwards

2016

NDT & E International

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Developments towards higher resolution and the ability to detect small defects are bringing a step-change in non-destructive testing. This paper presents a new method for increasing resolution, using a focused electromagnetic acoustic transducer (EMAT) optimised to generate Rayleigh waves at 2 MHz. This high frequency allows detection of mm-depth defects, and the focusing allows sizing of much shorter defects than is possible when using standard EMATs. The focusing behaviour and the aperture angle effect are analysed using laser vibrometry and finite element modeling, showing that a reduced aperture shifts the focal point from the designed value and increases the focal depth. The dual-EMAT has excellent signal to noise ratio (up to 30 dB) and has been used in single shot mode to image a variety of surface-breaking defects, including detecting and positioning a pair of real defects in an aluminium billet sample, and a machined defect of 2 mm length, 0.2 mm width, and 1.5 mm depth, giving an upper limit on the defect length of 2.1±0.5 mm. The results can be used to design an EMAT with optimised focal behaviour for defect detection.

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Thermoelastic hologram for focused ultrasound

Cited by 13 publications

References 5 publications

Subwavelength focusing of surface acoustic waves generated by an annular interdigital transducer

Subwavelength focusing of surface acoustic waves generated by an annular interdigital transducer

P4L-3 Anisotropic Wave-Surface Shaped Annular Interdigital Transducer

Focused Rayleigh wave EMAT for characterisation of surface-breaking defects

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