Highly efficient four-wave mixing in a coherent six-level system in ultraslow propagation regime

We propose a scheme to obtain a highly efficient four-wave mixing (FWM) in a coherent six-level tripod system by using a double-dark resonance and multiphoton destructive interference induced transparency. We show both analytically and numerically that the optical-wave mixing in such a system with a pump wave and an internally generated FWM wave mediated by electromagnetically induced transparency and hence propagating with matched ultraslow group velocities can result in a significant enhancement of FWM conversion efficiency. This interesting scheme may be used to generate coherent short-wavelength radiation in dense optical media with very low pump-wave intensity.