Doppler-free transitions in four-photon parametric resonance processes

It is shown that the resonance nonlinear susceptibility of four-photon parametric processes in gaseous media can be increased by several orders of magnitude. The increase in the susceptibility is due to suppression of the Doppler broadening of two-photon transitions in strong pump fields when the frequencies of the fields interacting with these transitions can be very different. Under these conditions there is an additional opportunity for increasing the susceptibility by reducing the frequency detuning relative to intermediate levels participating in two-photon transitions.