  Optical dual-pulse pumping actively creates quantum-mechanical superposition
of the electronic and phononic states in a bulk solid. We here made transient
reflectivity measurements in an n-GaAs using a pair of relative-phase-locked
femtosecond pulses and found characteristic interference fringes. This is a
result of quantum-path interference peculiar to the dual-pulse excitation as
indicated by theoretical calculation. Our observation reveals that the pathway
of coherent phonon generation in the n-GaAs is impulsive stimulated Raman
scattering at the displaced potential due to the surface-charge field, even
though the photon energy lies in the opaque region.
