Condition for Doppler-free three-photon resonance in a ladder-type atomic system.

We report the essential condition for three-photon electromagnetically induced absorption (TPEIA) in a Doppler-broadened ladder-type atomic system. When the two coupling lasers operate at different frequencies, we observed Doppler-free TPEIA resonance at a counterintuitive frequency, which is the almost half-frequency detuning of the frequency difference between the two coupling fields. Considering three-photon coherence in a Doppler-broadened ladder-type three-level atomic system, the TPEIA due to the atomic group of non-zero velocity was in good agreement with the calculated TPEIA spectrum under the one-photon resonance condition of all three optical fields. From the results, we found that an atomic group with a proper velocity for ladder-type TPEIA should satisfy the one-photon resonances of all three optical fields.