FIG. 3.

(a) Instantaneous velocity boundary condition (Eqs. (15) and (16)) in the x-direction u₁ (◇) and the y-direction w₁ (×) imposed on the substrate surface (y = 0) from x_a to x_b (see Fig. 2) in the absence of amplitude modulation (f_m = 0). The attenuation of the SAW along the substrate surface is approximated as $\text{exp}\hspace{0.17em}{(-x/{\alpha }_{\text{SAW}})}^{-1}$, where ${\alpha }_{\text{SAW}}^{-1}$ is the characteristic SAW attenuation length. (b) Variation in the acoustic velocity in the y-direction w₁ at x = x_a as a function of the number of cycles for different modulation indices: (i) m = 0 (no modulation), (ii) m = 0.25, (iii) m = 0.5, and (iv) m = 0.75; the frequency of the SAW is f_SAW = 30 MHz and the modulation frequency is f_m = 100 kHz.