  We report on the intrinsic optical generation and detection of coherent
acoustic phonons at (001)-oriented bulk Si and GaP without metallic phonon
transducer structures. Photoexcitation by a 3.1-eV laser pulse generates a
normal strain pulse within the $\sim$100-nm penetration depth in both
semiconductors. The subsequent propagation of the strain pulse into the bulk is
detected with a delayed optical probe as a periodic modulation of the optical
reflectivity. Our theoretical model explains quantitatively the generation of
the acoustic pulse via the deformation potential electron-phonon coupling and
detection in terms of the spatially and temporally dependent photoelastic
effect for both semiconductors. Comparison with our theoretical model reveals
that the experimental strain pulses have finite build-up times of 1.2 and 0.4
ps for GaP and Si, which are comparable with the time required for the
photoexcited electrons to transfer to the lowest X valley through intervalley
scattering. The deformation potential coupling related to the acoustic pulse
generation for GaP is estimated to be twice as strong as that for Si from our
experiments, in agreement with a previous theoretical prediction.
