Doppler-free transitions induced by strong double-frequency optical excitation

The possibilities to achieve Doppler-free transitions influenced by two strong radiations with different frequencies are discussed. Derived expressions show that two-photon absorption ceases to be Doppler-free with the intensity increase of equal-frequency counterpropagation light beams. On the contrary, absorption of distinct-frequency counterpropagating waves and moreover copropagating Stokes light scattering appears to be Doppler-free under a proper pump field intensity. Applications of these induced Doppler-free transitions to resonant nonlinear optics of gas-phase media are considered. The importance of the new features resulting from the growth of both the interacting fields is stressed.