  The coherent modulation of electronic and vibrational nonlinearities in atoms
and molecular gases by intense few-cycle pulses has been used for high-harmonic
generation in the soft X-ray and attosecond regime, as well as for Raman
frequency combs that span multiple octaves from the Terahertz to Petahertz
frequency regions. In principle, similar high-order nonlinear processes can be
excited efficiently in solids and liquids on account of their high nonlinear
polarizability densities. In this paper, we demonstrate the phononic modulation
of the optical index of Si and GaAs for excitation and probing near their
direct band gaps, respectively at ~3.4 eV and ~3.0 eV. The large amplitude
coherent longitudinal optical polarization due to the excitation of
longitudinal optical (LO) phonon of Si (001) and LO phonon-plasmon coupled
modes in GaAs (001) excited by 10-fs laser pulses induces effective amplitude
and phase modulation of the reflected probe light. The combined action of the
amplitude and phase modulation in Si and GaAs generates phonon frequency combs
with more than 100 and 60 THz bandwidth, respectively.
