FOCUSING OF FAST TRANSVERSE-MODES IN (001) SILICON AT ULTRASONIC FREQUENCIES

This paper presents observations of the focusing of fast transverse (FT) ultrasonic waves in a (001) oriented, disk-shaped silicon single crystal. These modes are almost perfectly shear horizontally (SH) polarized and were absent from earlier reported observations of focusing of ultrasonic waves based on axisymmetric excitation and sensing. In an experiment the FT modes are generated and detected at room temperature by two small [100] polarized PZT piezoelectric shear transducers. The source transducer is fixed on the bottom surface of the specimen and the detector scans the top surface in the [100] direction along lines that intersect the [010] axis at various distances from epicenter. The observed focusing pattern indicates a strong concentration of the FT mode flux in a narrow band about the (100) plane containing the source. Because of the specific way in which the monopolar source acts, the radiated acoustic flux pattern breaks the fourfold symmetry associated with cubic media. While it shows a strong concentration of FT flux toward the (100) plane, it suppresses FT modes propagating very near the (010) plane passing through the source. The spatial variation of the Fourier components of the detected signal at approximately 2, 6, and 8 MHz has been examined, and there is good accountability for this variation on the basis of the computed frequency domain elastodynamic Green's function for silicon.