Recent advances in the amplification of surface elastic waves in layered structures

The emergence of surface elastic waves as an important area of applied research has been largely the result of the successful application of well known principles of physical acoustics rather than the discovery of new phenomena. A number of factors have contributed to the rapid growth of interest in this field, among which may be noted the recent demonstration of terminal gain in an acoustoelectric surface wave amplifier at 100 MHz, the considerable reduction of two port insertion losses, and the recognition that planar acoustic systems offer significant advantages in sophistica- ted signal processing devices such as transversal filters. Most laboratories currently engaged in surface wave work had already developed the necessary facilities and skills during a period devoted to exploring the potential of bulk elastic waves. The single crystal materials of most interest for surface waves, generally low acoustic loss strongly piezoelectric insulators, are also those that have formed the basis for much of the bulk wave work. In certain respects, then, the renais- sance for surface waves came at a most propitious time, being able to take advantage of the experi- ence gained from several years of bulk wave activity. In retrospect, it may be surprising that bulk waves were favored first or that the interest in surface waves developed so slowly after efficient transduction with the interdigital comb structure was first demonstrated. It is generally agreed that much of the experimental research being carried out at the present time would have been possible, in principle, at any time within the last decade. This applies, for example, in all of the experiments involving bulk materials where a demonstration of feasibility rather than a reduction to useful prac- tice was the principal objective.