Resonant four-photon parametric oscillation in the presence of a strong field due to a neighboring transition

An analysis is made of generation of the sum frequency ω₃ = 2ω₁ + ω₂ in the case when the pump frequency coincides with that of a two-photon transition 2ω₁ = ω₂₁, and the frequency ω₃ is tuned near the resonance line of an atom. Under conditions of resonance with the frequency of the generated field, the parametric process has some characteristic features associated with a strong competition between effects involving increases in the nonlinear susceptibility, absorption, and changes in the phase matching. Moreover, in the presence of a fairly high-intensity field due to a neighboring transition at the frequency ω₂, a nonlinear Raman process (which changes the characteristics of the medium at the frequency ω₃) begins to play an important part. It is found that the highest parametric oscillation powers over a wide spectral range may be achieved by appropriately varying the intensity of the field due to the neighboring transition, the atom concentration, or the interaction length.