FOCUSING OF QUASI-TRANSVERSE MODES IN ZINC AT ULTRASONIC FREQUENCIES

The intense focusing of quasi-transverse (QT) modes along the symmetry axis of zinc is observed by various methods at ultrasonic frequencies . The focusing is associated with a conical point on the QT sheet of t he ray surface, and thereby the phenomenon of external conical refract ion. The observations have been made first, with pointlike longitudina l (L) and shear (S) piezoelectric transducers acting as acoustic sourc e and detector; second, with a breaking capillary source with Heavisid e time-dependence and a capacitive displacement detector at epicenter; and third, using a laser source and piezoelectric detector. The ampli tudes of the signals observed by the piezoelectric sources and detecto rs in both time and frequency domains show intense focusing in the sym metry direction [0001], side-lobed by diffraction fringes caused by th e interferences between collinear rays associated with different wave vectors k. A broadband capacitive displacement signal shows a very sha rp deep minimum associated with the arrival of the conical rays, being consistent with the calculation of the dynamic Green's function and t he theory of phonon focusing for the medium.