In Figure 3, the long time scale reflectivity oscillation is plotted for the epilayer(I), DQW(II), single quantum well (III) and the blue LED structure(IV) at 3.29 eV (below the GaN bandgap). Surprisingly, the oscillation amplitude abruptly decreases within one cycle of scillation at a critical time which appears to scale with the thickness of the GaN layer in each sample. In the inset of Figure 3, the slope of the GaN layer thickness vs. the critical time is very close of the known value of the sound velocity in GaN. Figure 3b shows results when the probe laser energy is changed to 3.44 eV which is above the GaN bandgap. The coherent phonon oscillation appears only within the time that the sound wave travels the absorption depth of the GaN layer. The amplitude of the reflectivity oscillation decays exponentially with a decay time of 24.2 ps corresponding to a penetration depth in GaN (-0.17 μιη = 24.2 ps x 7000 m/s). Even more amazing is that the oscillation reappears at 260 ps for epilayer(I) and 340 ps ps for DQW (II). These are twice the critcal time for the oscillations to disappear when the photon energy is 3.29 eV (below the GaN bandgap energy). This shows that the “echo” of the acoustic wave reflected on the GaN/sapphire interface gives acoustic wave oscillation when the wave reaches the GaN absorption region.