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 the ray surface, and thereby the phenomenon of external conical refraction. The observations have been made first, with pointlike longitudinal (L) and shear (S) piezoelectric transducers acting as acoustic source and detector; second, with a breaking capillary source with Heaviside time-dependence and a capacitive displacement detector at epicenter; and third, using a laser source and piezoelectric detector. The amplitudes of the signals observed by the piezoelectric sources and detectors in both time and frequency domains show intense focusing in the symmetry direction [0001], side-lobed by diffraction fringes caused by the interferences between collinear rays associated with different wave vectors k. A broadband capacitive displacement signal shows a very sharp deep minimum associated with the arrival of the conical rays, being consistent with the calculation of the dynamic Green's function and the theory of phonon focusing for the medium.