Narrow resonances of two-photon absorption of super-narrow pulses in a gas

A scheme to generate oppositely travelling super-narrow pulses is proposed. These resonant peaks are produced when the sum of frequencies of two synchronized modes coincides with the transition frequency. Their widths are not determined by the pulse duration but rather by the homogeneous width of the two-photon transition. The resonance intensity turns out to be the same as that at the single-frequency condition with the same power.