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

Abstract

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...

Microscale acoustofluidics: Microfluidics driven via acoustics and ultrasonics

“…There are a variety of focusing designs, including elliptical and circular structures, that give different focal patterns of acoustic waves, which in itself can be useful (Shilton et al, 2008). Additionally, Laude et al (2008), using interdigital electrodes that are designed from the slowness curves of the piezoelectric substrate (Auld, 1973), showing an ability to increase the fidelity of the mechanism to focus the acoustic energy to subwavelength scales.…”

FEM analysis of wavelength effects in piezoelectric substrate

“…Meanwhile, the wavelength of A-SAW device, λ is determined by the electrodes' finger spacing, p and they are related by λ = 4 p. The associated frequency, f of the waves propagating with velocity, v is given by f = v / λ. Since the geometry of electrodes is in the shape of comb-circular, the emission of acoustic waves follows the shape of the transducer and confined only in the region of the innermost electrodes [9]. Fig.…”

Laser interferometers in physical acoustics

“…What is really going on in the device? Left hand side: Illustration of an annular IDT (AIDT), designed to focus SAWs into a point [1]. Photograph of the AIDT-device with a measured (f = 72.55 MHz) instantaneous deformation field at the focal region superposed.…”