Intraband radiative transitions can occur in a semiconductor with an artificial periodic structure (superlattice). The "lattice momentum" of the periodic structure makes possible the conservation of momentum during the electronic transition. When the electrons in the band are drifting in an electric field, an intraband population inversion may occur, providing optical wave amplification. Under conditions where the Landau damping of the semiconductor carrier's plasma wave is low, phase-matched coupling may occur between the plasma wave and a Floquet component of the electromagnetic wave and result in a high rate of power transfer from one of the waves to the other. These effects are discussed and analyzed quantum mechanically and suggestions are made with regard to possible device applications (amplifier, modulator) in the infrared regime.