Enhancement of near-resonant degenerate four-wave-mixing signals by excitation of a coupled transition

It is shown that intense collisionless Rayleigh-type extraresonant signals in degenerate four-wave mixing (DFWM) can be obtained when the | a 〉→| b 〉 transition which produces the DFWM signal is coupled to another transition (| a 〉→| d 〉) which is excited near-resonantly by an additional laser beam. Two effects contribute to the Rayleigh-type extraresonant signal at ω₂=ω₁. The first effect is resonance-enhanced three-photon scattering (TPS), which leads to the population of state $| b 〉$ and consequently contributes a term in ρba, the off-diagonal density-matrix element, which is characterized by the linewidth (1T₁)ba, the inverse lifetime of the state $| b 〉$. The second effect derives from the effective lifetime acquired by the ground state $| a 〉$ due to its strong coupling to state $| d 〉$. This leads to a term in ρba characterized by a linewidth 1/2(1T₁)da, which is half the inverse lifetime of the state $| d 〉$. When (1T₁)da(1T₁)ba, the TPS effect dominates and the extraresonant DFWM signal is narrow and intense. When (1T₁)da(1T₁)ba, the signal is less intense and interference between the two effects is evident.