Analysis of excitation and coherent amplitude enhancement of surface acoustic waves by the phase velocity scanning method

We present a general theoretical formulation for the characteristics of surface acoustic waves (SAW) generated by the phase velocity scanning (PVS) method that employs a scanning single laser beam (SSB) or a scanning interference fringes (SIF). In the SSB approach, a broad band SAW pulse is generated and its amplitude is coherently enhanced when the laser scanning velocity V is equal to the phase velocity upsilon(R) of the SAW. The amplitude of the SAW follows a resonance curve represented by a sinc function of the scanning velocity V, but different spatial frequency components in the SSB significantly suppress the side lobes of the resonance curve. In the SIF approach, the scanning velocity upsilon(f) of the fringes is determined by the intersection angle and the frequency difference omega(a) of the laser beams. A narrow band tone burst of SAW with frequencies higher than 100 MHz can be excited. The SAW frequency omega depends upon a characteristic time t*, defined as a propagation time of the SAW across the laser beam spot. The SAW frequency omega is identical to the frequency difference omega(a) when the laser pulse width T is longer than the characteristic time t*. But, the SAW frequency omega is determined as a product k(f)upsilon(R) of the wave number of the SIF and the SAW velocity when the laser pulse width is shorter than the characteristic time. Precise frequency measurement provided by the amplitude enhancement effect and the narrow frequency bandwidth in the SIF approach make the PVS method particularly promising for the noncontact SAW velocity measurement