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 2=1. The first effect is resonance-enhanced three-photon scattering (TPS), which leads to the population of state | b and consequently contributes a term in ba, 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 ba characterized by a linewidth 12(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.