  We study theoretically the nonlinear optical properties of a semiconductor
quantum well (QW) irradiated by a two-mode electromagnetic wave consisting of a
strong resonant dressing field and a weak off-resonant driving field. In the
considered strongly coupled electron-field system, the dressing field opens
dynamic Stark gaps in the electron energy spectrum of the QW, whereas the
driving field induces electron oscillations in the QW plane. Since the gapped
electron spectrum restricts the amplitude of the oscillations, the emission of
a frequency comb from the QW appears. Therefore, the doubly-driven QW operates
as a nonlinear optical element which can be used, particularly, for optically
controlled generation of terahertz radiation.
