  Ultrafast carrier and phonon dynamics at the buried heterointerface of
GaP/Si(001) are investigated by means of two-color pump-probe reflectivity
measurements. The carrier-induced reflectivity signal exhibits a resonant
enhancement at pump-photon energies of 1.4 eV, which can be assigned to an
optical transition between electronic interface states. The transient
reflectivity is modulated by a coherent oscillation at 2 THz, whose amplitude
also becomes maximum at 1.4 eV. The observed resonant behavior of the phonon
mode in combination with a characteristic wavelength-dependence of, both, its
frequency and initial phase, strongly indicate that the 2-THz mode is a
difference-combination mode of a GaP-like and a Si-like phonon at the
heterointerface and that this second-order scattering process can be enhanced
by a double resonance involving the interfacial electronic states.
