  We propose a novel picture of high-harmonic generation (HHG) in solids based
on the concept of temporally changing band structures. To demonstrate the
utility of this picture, we focus on the high-order sideband generation (HSG)
caused by strong terahertz (THz) and weak near-infrared (NIR) light in the
context of pump-probe spectroscopy. We find that the NIR frequency dependence
of the HSG indicates the existence of new energy levels (sub-bands) around the
band-gap energy, which have multiple frequencies of THz light. This sub-band
picture explains why the HSG intensity becomes a non-monotonic function of the
THz light amplitude. The present analysis not only reveals the origin of the
plateau structure in HHG spectra, but also provides a connection to other
high-field phenomena.
