We investigate ultrafast harmonic generation (HG) in Si: B, driven by intense pump pulses with fields reaching sim100\phantomrule{0.28em0ex}kV\phantomrule{0.16em0ex}cm^-1 and a carrier frequency of 300 GHz, at 4 K and 300 K, both experimentally and theoretically. We report several findings concerning the nonlinear charge carrier dynamics in intense sub-THz fields: (i) Harmonics of order up to $n=9$ are observed at room temperature, while at low temperature we can resolve harmonics reaching at least $n=11$. The susceptibility per charge carrier at moderate field strength is as high as for charge carriers in graphene, considered to be one of the materials with the strongest sub-THz nonlinear response.
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