  Thin GaP films can be grown on an exact Si(001) substrate with nearly perfect
lattice match. We perform all-optical pump-probe measurements to investigate
the ultrafast electron-phonon coupling at the buried interface of GaP/Si.
Above-bandgap excitation with a femtosecond laser pulse can induce coherent
longitudinal optical (LO) phonons both in the GaP opverlayer and in the Si
substrate. The coupling of the GaP LO phonons with photoexcited plasma is
reduced significantly with decreasing the GaP layer thickness from 56 to 16 nm
due to the quasi-two-dimensional confinement of the plasma. The same laser
pulse can also generate coherent longitudinal acoustic (LA) phonons in the form
of a strain pulse. The strain induces not only a periodic modulation in the
optical reflectivity as they propagate in the semiconductors, but also a sharp
spike when it arrives at the GaP layer boundaries. The acoustic pulse induced
at the GaP/Si interface is remarkably stronger than that at the Si surface,
suggesting a possible application of the GaP/Si heterostructure as an
opto-acoustic transducer. The amplitude and the phase of the reflectivity
modulation varies with the GaP layer thickness, which can be understood in
terms of the interference caused by the multiple acoustic pulses generated at
the top surface and at the buried interface.
