Exceeding the Manley-Rowe quantum efficiency limit in an optically   pumped THz amplifier
  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. 
Ines Waldmueller, Michael C. Wanke, Weng W. Chow