  Using a microscopic theory based on the Maxwell-semiconductor Bloch
equations, we investigate the possibility of an optically-assisted
electrically-driven THz quantum cascade laser. Whereas in optical conversion
schemes the power conversion efficiency is limited by the Manley-Rowe relation,
the proposed optically-assisted scheme can achieve higher efficiency by
coherently recovering the optical pump energy. Furthermore, due to quantum
coherence effects the detrimental effects of scattering are mitigated.
