Efficient phase conjugation via two-photon coherence in an optically densecrystal

We demonstrate efficient phase conjugation in an inhomogeneously broadened rare-earth doped crystal using electromagnetically induced transparency. Th e observed frequency conversion efficiency of the phase conjugate in energy over a backward pump beam is as high as 10% with similar to 1/4 of maximal Raman coherence. This efficient phase conjugation has potential applicatio ns of nonlinear optical processes such as frequency up-conversion and Raman excited spin-echo optical memory. [S1050-2947(99)50704-X].