Its symmetry differs significantly from the crystal third-order symmetry and the SAW sixth-order symmetry natural for (111) silicon. The near-field structure is visible in the center and corresponds to the leading edge of an ( n−1st) pulse that has passed through the focal point. This velocity is close to that of a quasi-longitudinal wave, which, depending on the direction of propagation, is in the range from 8.30 to 9.35 µm/ns. This front travels a distance of 28 µm in 3.46 ns, which corresponds to a velocity value of ~8.1 µm/ns. The dark contours corresponds to the longitudinal leaky SAW (LLSAW), marked by red and green arrows 2, are faintly visible here. One can see a circular front, propagating towards the center from the excitation region (marked by red arrow 1), as well as a front leaving the excitation region (green arrow 1). The left picture corresponds to the probe pulse 3.46 ns delay relative to excitation pulse. For both crystallographic orientations of silicon, the deviation of the group velocity from the isotropic case was less than 5%.Ī similar experiment on a silicon surface (111) is shown in Figure 4b. Calculations ( Figure 3b, right picture) showed that these fronts correspond to Rayleigh waves. The resulting patterns of SAW wave fronts are shown in Figure 3b, left picture. Similar measurements were carried out on silicon with the (111) orientation. Calculations ( Figure 3b, right picture) showed that one of the modes (blue curve) is Rayleigh in the direction and equivalent ones and becomes slow shear in the direction, while another one (red curve) is Rayleigh in the direction and becomes a pseudosurface in the direction. Despite the fact that fronts on Si (001) differ slightly from circles, they were formed by two modes. The normal frequency dispersion associated with the presence of an aluminium film on the silicon surface begins to emerge in the form of contours following the main wavefront at >180 μm from the center.
In Figure 3a, the left picture shows the SAW pattern on the (001) surface where consecutive wavefronts correspond to repetitive laser pulses. Surface waves were detected under point excitation on silicon surfaces (001) and (111). The first beam excited the sample after frequency doubling (wavelength 400 nm, pulse energy 10 3 times less than the radiation resistance threshold of our samples. The laser radiation was split into two beams. The experimental setup was based on the well-known pump–probe technique with a Mira-900 femtosecond laser (160 fs pulse width, 76 MHz repetition rate) as a source ( Figure 1).
The samples were 0.8 mm thick silicon wafers oriented (001) or (111) covered by thermally deposited ~400 nm thick aluminium film.
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