  High-order nonlinear light-matter interactions in gases enable generation of
x-ray and attosecond light pulses, metrology, and spectroscopy. Optical
nonlinearities in solid-state materials are particularly interesting for
combining optical and electronic functions for high-bandwidth information
processing. Third-order nonlinear optical processes in silicon have been used
to process optical signals with greater than 1 GHz bandwidths. Fundamental
physical processes for a Si-based optical modulator in the THz bandwidth range,
however, have not yet been explored. Here we demonstrate ultrafast phononic
modulation of the optical index of Si by irradiation with intense few-cycle
femtosecond pulses. The anisotropic reflectivity modulation by the resonant
Raman susceptibility at the fundamental frequency of the longitudinal optical
(LO) phonon of Si (15.6 THz) generates a frequency comb up to 7th-order. All
optical >100 THz frequency comb generation is realized by harnessing the
coherent atomic motion of the Si crystalline lattice at its highest mechanical
frequency.
