Cross-coupled phase conjugation through two-frequency four-wave mixing in dyes

Two-frequency four-wave mixing in a dye is demonstrated, in which degenerate four-wave mixing is performed simultaneously within the absorption and emission frequency spectra, with two separate sets of beams. The coupling between the two bands gives rise to backward output waves at both frequencies, which are proportional to the phase conjugate of a single probe which can be at either of the two frequencies. Forward-propagating waves which are not conjugated are also generated at both frequencies. The frequency shifted outputs are tunable over the bandwidth of the spectrum in which they lie. A theoretical analysis is performed which is based on a four-level system model of the ground and first excited singlet states in a dye. Resulting are coupled equations, which are solved to yield the efficiencies of the various outputs. Experiments give results in good agreement with the theory.