Enhancement of four-wave mixing and line narrowing by use of quantum coherence in an optically dense double-Lambda solid

We have demonstrated enhanced nondegenerate four-wave mixing by use of a re sonant, probe in a double-A system consisting of an optically dense spectra l hole-burning solid. The observed probe diffraction efficiency is similar to 16% in amplitude at 6 K which is higher than for an off-resonant probe i n a A-type scheme. We have also observed two-photon coherence line narrowin g, which has potential application to high-resolution spectroscopy. (C) 199 9 Optical Society of America.