  Optically excited electron-hole pairs, driven by a strong terahertz (THz)
field, create high-sidebands in the optical spectrum. The sideband spectrum
exhibits a 'plateau' up to a cutoff of 3.17Up, where Up is the ponderomotive
energy. This cutoff is determined, semi-classically, from the maximum kinetic
energy an electron-hole pair can gain from the THz field along a closed
trajectory. A full quantum treatment reveals a second, classically forbidden,
plateau with a cutoff of 8Up, the maximum kinetic energy an electron-hole pair
can gain from the THz field along an open trajectory. The second plateau
appears because a spatially separated electron and hole can still recombine if
the classical excursion is within the coherence length of the electron-hole
wavefunction or, equivalently, the coherence time is longer than the excursion
time (half the THz field period). This effect broadens the range of materials
and excitation conditions where high-sideband generations can occur, thereby
providing a wealth of novel systems for ultrafast electro-optical applications.
