Anisotropic wave-surface shaped annular interdigital transducer

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.